Electron Radiotherapy Past, Present & Future

John A. Antolak, Ph.D., Mayo Clinic, Rochester MN Kenneth R. Hogstrom, LSU, Baton Rouge, LA Learning Objectives

1. Learn about the history of electron radiotherapy that is relevant to current practice. 2. Understand current technology for generating electron beams and measuring their dose distributions. 3. Understand general principles for planning electron radiotherapy 4. Be able to describe how electron beams can be used in special procedures such as total skin electron irradiation and intraoperative treatments. 5. Understand how treatment planning systems can accurately calculate dose distributions for electron beams. 6. Learn about new developments in electron radiotherapy that may be common in the near future. Outline

 History, KRH  Machines & Dosimetry, JAA  Impact of heterogeneities, KRH  Principles of electron planning, KRH  Special Procedures, JAA  Electron Dose Calculations, JAA  Looking to the Future, JAA History of Electron Therapy

Kenneth R. Hogstrom, Ph.D.

History of Electron Therapy Clinical Utility Electron beams have been successfully used in numerous sites that are located within 6 cm of the surface: – Scalp – Head (Ear, Eye, Eyelid, Nose, Temple, Parotid, …) – Craniospinal Irradiation for Medulloblastoma – Posterior Chest Wall (Paraspinal Muscles Sarcomas) – Breast (IMC, Lumpectomy Boost, & Postmastectomy) – Extremities (Arms & Legs) – Total Skin Electron Irradiation – Intraoperative Electron Therapy

History of Electron Therapy History of Electron Therapy Accelerator Technology • Van de Graaff Accelerators (late 1930s)  E<3 MeV for mycosis fungoides and other skin cancers • Betatrons (late 1940s)  6

History of Electron Therapy History of Electron Therapy Dose Calculation & Measurement Technology • Electron Transport and Dose Calculations  ICRU 35 (1984) and Use of Fermi-Eyges Theory (1980s)  Monte Carlo: EGS4, BEAM, DOSXYZ (1985-1995) • Treatment Planning  CT-Based Planning: GE Target TPS (1981)  Pencil-beam Dose Calculations: GE Target (1983)  3D Treatment Planning Systems (late 1990s) • Dose Measurement Protocols  AAPM TG Reports 21, 25, 51, & 70 • Bolus Electron Conformal Therapy (2000s)

History of Electron Therapy Electron Beam Therapy Impediments to Clinical Use

• Inadequate Education  Administrators and Radiation Oncologists  Medical Physicists and Medical Dosimetrists  Vendors

• Competing Technologies

• Lack of Commercially Available Technology

History of Electron Therapy Competing Technology Examples • Helical TomoTherapy  Scalp  Chest wall  Craniospinal  Parotid

• Proton Therapy  Craniospinal, Parotid

• HDR Brachytherapy  Lumpectomy boost  Intraoperative therapy

History of Electron Therapy Competing Technologies Postmastectomy Chest Wall- TomoTherapy

Ashenafi et al 2010

• Tradeoffs  TomoTherapy improved PTV dose homogeneity (cosmesis)  Electrons have lower risks of secondary cancers (8% vs. 3%)

History of Electron Therapy Competing Technologies Craniospinal Irradiation- Proton Therapy

Mu et al 2005

• Risks of secondary cancers  Photon: 20%  IMXT: 30%  Electron: 21%  History of Electron TherapyIMET: 15%  IMPT: 4% Leveling the Playing Field Missing Integrated Technologies • Fixed Beam Therapy  Lack of treatment planning tools for modeling treatments  Updated pencil beam (redefinition) algorithm  Treatment planning tools for energy segmentation  Retractable eMLC for treatment delivery • Electron Arc Therapy  Treatment Planning Systems (2º & 3º collimator design, energy segmentation, bolus design, dose & MU calcs)  Treatment Machine (dynamic use of MLC)  Tools for constructing treatment aids 3º skin collimation • Total Skin Electron Irradiation  Configuration of beam delivery system  Treatment stand

History of Electron Therapy Leveling the Playing Field: Missing Technologies- Treatment Planning Tools

(1) Bolus scatter plate not modeled

(2) Surface dose calculation inaccurate

(3) Backscatter dose calculation inaccurate Tapley et al 1976 (4) Eyeshield not modeled

(5) Skin collimation not modeled

(6) Penumbra calculation inaccurate

Squamous Cell Carcinoma PTV=Red Volume Question #1: Currently, commercially available electron beam treatment planning technologies include a. Modeling of eye blocks b. Energy segmentation c. Electron arc therapy d. Pencil beam redefinition algorithm e. Bolus electron conformal therapy

History of Electron Therapy Question #1: Currently, commercially available electron beam treatment planning technologies include e. Bolus electron conformal therapy

A third party vendor offers a bolus electron conformal therapy design and fabrication service compatible with many commercial planning systems.

Reference: Kim MM, Kudchadker RJ, Kanke JE, Zhang S, and Perkins GH 2012 Bolus electron conformal therapy for the treatment of recurrent inflammatory breast cancer: a case report,Medical Dosimetry 37 208-213

History of Electron Therapy Machines & Dosimetry

John A. Antolak, Ph.D. Linac Head

Karzmark, C. J., Nunan, C.S., Tanabe, E: Medical Electron Accelerators, McGraw Hill, 1992. Machines & Dosimetry Electron Mode

Karzmark, C. J., Nunan, C.S., Tanabe, E: Medical Electron Accelerators, McGraw Hill, 1992. Machines & Dosimetry Dual Scattering Foils

Machines & Dosimetry Electron Collimation Systems  Applicators with Inserts  Variable Trimmers  Intracavitary Cones  Intraoperative radiotherapy cones  Intraoral Cones  Transvaginal cones

Machines & Dosimetry Applicators (Cones)

 Figure showing general principles of applicator design

Machines & Dosimetry Dosimetry EGS: Basic Simulation Tool

http://www2.slac.stanford.edu/vvc/egs/basicsimtool.html

Machines & Dosimetry 10 MeV Electrons in Water

Machines & Dosimetry 10-20 MeV Electrons in Water

Machines & Dosimetry % Depth Dose

AAPM TG-25 Report (1991)

Machines & Dosimetry Side-scatter Equilibrium

Side-scatter equilibrium exists if the electron fluence scattered away from a small area in the vicinity of a point is replaced by electrons scattering into that area.

In-scatter Out-scatter Machines & Dosimetry Side-Scatter Equilibrium (homogeneous phantom) All of the electrons that can Some of the electrons that can reach the point of interest are reach the point of interest are let through blocked

Side-scatter equilibrium exists! Side-scatter equilibrium does not exist!

R R 100 100 Reference geometry Reduced field size

Machines & Dosimetry Depth Dose Energy Dependence (6-20 MeV)  As energy increases

 Surface dose (Ds) increases (70%-90%)

 Therapeutic depth (R90) increases

 Dose falloff (R10-R90) increases

 Practical range (Rp) increases  Bremsstrahlung dose (Dx) increases  Small variations due to method of beam flattening and collimation

Machines & Dosimetry % Depth Dose Field Size Dependence  As field size decreases  Therapeutic depth (R90) decreases E=9 MeV  Surface dose (DS) increases  Practical range (Rp) remains constant  Decrease in R90 less significant at lower energies  Increase of DS more significant at lower energies

Machines & Dosimetry % Depth Dose Field Size Dependence  As field size decreases  Therapeutic depth (R90) decreases  Surface dose (DS) increases  Practical range (Rp) remains constant  Decrease in R90 more significant at higher energies  Increase of DS almost E=20 MeV insignificant at higher energies

Machines & Dosimetry % Depth Dose Applicator Dependence

20 MeV 10x10 Insert

Machines & Dosimetry Square-Root Rule % Depth Dose of Rectangular Field

1 2 %DD L,W (d) = [%DD L,L (d)×%DDW ,W (d)]

L,W €  %DD (d) is percent dose at depth d for rectangular field of dimensions L by W  Note: the resultant %DD curve must be normalized

such that its Dmax=100%

Machines & Dosimetry % Depth Dose SSD Dependence  Inverse Square- small impact due to:  SSD>110 cm is seldom used  Dose is superficial (d<10 cm).  Ex: at R90 (d=6 cm)  %DD(110-cm SSD)≈%DD(100-cm SSD)*1.01  Collimator Scatter- occasional impact due to:  Electrons scattered from collimating edges being at large angles and propagating out of field at extended SSD  Result is lowering of surface dose and deepening of R90. 20 MeV Measured %DD: 110-cm SSD (dashed) Calculated %DD: %DD at 100-cm SSD multiplied by inverse-square factor

Machines & Dosimetry Off-Axis Dose (Penumbra) Field Size Dependence

 Penumbra is the edge of the beam for which there is not side- scatter equilibrium  Penumbra width is a measure of penumbra shape  P90-10=distance from 90% to 10% OAR  P80-20=distance from 80% to 20% OAR  Penumbra width remains constant as field size increases once there is side-scatter equilibrium on central axis.

Machines & Dosimetry Off-Axis Dose Energy & Depth Dependence

Penumbra at surface is sharper at higher energies

Penumbra width

at depth of R90 increases with energy Penumbra increases

quickly for depth < R90

Penumbra constant (or getting smaller)

for depth > R90

Machines & Dosimetry Off-Axis Dose SSD

Dependence Penumbra width at surface increases in proportion to air gap (distance from final collimating device to 6 MeV, 100-cm SSD patient, SSD-SCD)

Penumbra width at

depth of R90 increases significantly for lower electron energies 6 MeV, 110-cm SSD

Machines & Dosimetry Off-Axis Dose SSD

Dependence Penumbra width at surface increases in proportion to air gap (distance from final collimating device to patient, SSD-SCD)

16 MeV, 100-cm SSD Penumbra width at

depth of R90 increases little for higher electron energies

16 MeV, 110-cmMachines SSD & Dosimetry Question #2: For a cylindrical ion chamber with its center at depth d and radius r, the effective point of measurement is at depth a. d b. d – 0.5*r c. d – 0.6*r d. d + 0.5*r e. d + 0.6*r

Machines & Dosimetry Answer #2: For a cylindrical ion chamber with its center at depth d and radius r, the effective point of measurement is at depth b. d – 0.5*r

Reference: Gerbi, B. J., J. A. Antolak, et al. (2009). "Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25." Medical Physics 36(7): 3239-3279.

Machines & Dosimetry Question #3: When measuring percent depth ionization with a cylindrical ion chamber, the following corrections are recommended when converting to percent depth dose. a. Effective point of measurement. b. Effective point of measurement, stopping power ratio. c. Effective point of measurement, stopping power ratio, fluence correction. d. Stopping power ratio, fluence correction. e. Fluence correction.

Machines & Dosimetry Answer #3: When measuring percent depth ionization with a cylindrical ion chamber, the following corrections are recommended when converting to percent depth dose. c. Effective point of measurement, stopping power ratio, fluence correction.

Reference: Gerbi, B. J., J. A. Antolak, et al. (2009). "Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25." Medical Physics 36(7): 3239-3279.

Machines & Dosimetry Impact of Patient Heterogeneity on Dose Distribution

Kenneth R. Hogstrom, Ph.D. Influence of Patient Anatomy on Electron Dose Distributions How do electron dose distributions in patients differ from those in water?

Impact of Patient Heterogeneity Influence of Patient Anatomy on Electron Dose Distributions • Patient Surface  Oblique incidence  Irregular Surface

• Internal Heterogeneities  Bone  Air  Lung

Impact of Patient Heterogeneity Effects of Oblique Incidence on Electron Dose Distribution

Penumbra • decreases for surfaces closer to source

• increases for surface further from sources

Impact of Patient Heterogeneity Effect of Oblique Incidence on Electron Dose Distribution

Ekstrand and Dixon (1982) Depth Dose

• Surface dose (Ds) increases

• Depth of maximum dose (R100) decreases

• Maximum dose (Dmax) increases

• Therapeutic depth (R90) decreases

• Depth of maximum penetration (Rp) increases • Effects become more severe as angle from ⊥ increases

Impact of Patient Heterogeneity Effect of Irregular Surface on Electron Dose Distribution

13 MeV, 10 x 10 cm2, 100 cm SSD

Depression (e.g. ear canal, surgical defect) • Increased dose in shadow of depression • Decreased dose around its periphery

Impact of Patient Heterogeneity Hot spot in inner ear due to external ear and ear canal

Impact of Patient Heterogeneity Effect of Irregular Surface on Electron Dose Distribution 13 MeV, 10 x 10 cm2, 100 cm SSD 17 MeV, 7.3 x 6.8 cm2,100 cm SSD

Protrusion (e.g. nose, ear) • Decreased dose in shadow of protrusion • Increased dose around its periphery

Impact of Patient Heterogeneity Effects of Bone on Electron Dose Distribution

• Therapeutic dose contours (80%-90%) shift toward the surface due to increased stopping power of bone.

• Hot spots lateral to bone and cold spots under bone have small effect (< 5%).

Impact of Patient Heterogeneity Effects of Bone on Electron Dose Distribution

• Hot spots between spinous process; cold spots under spinous process

Impact of Patient Heterogeneity Effects of Bone on Electron Dose Distribution

• Small increase in dose to upstream tissue due to backscatter (< 4%) • Small increase in dose in bone due to multiple Coulomb scattering (< 7%)

Impact of Patient Heterogeneity Effects of Air on Electron Dose Distribution

• Influence of Air Cavities  Dose falloff region penetrates deeper  Hot/cold spots can become significant (as much as 20%)

Impact of Patient Heterogeneity Effects of Lung on Electron Dose Distribution Assume lung =water Properly account for lung

Dose penetration in lung can be 3-4 times that of unit density tissue.

Impact of Patient Heterogeneity Summary Impact of Patient Heterogeneity on Dose Distribution • Key effects of patient anatomy (heterogenity) on the dose distribution include  Isodose shifting due to bone, air, & lung  Hot/cold spots due to loss of side-scatter equilibrium resulting from irregular surfaces and edges of internal air cavities

• Effect of patient anatomy (heterogeneity) must be accounted for in electron beam planning to ensure:  Adequate electron energy, i.e. no geographical miss of PTV in depth  Adequate dose homogeneity in PTV, i.e. minimal hot/cold spots  Minimal dose to critical structures underlying PTV

Impact of Patient Heterogeneity Question #4: The most significant impact of compact bone in electron beam therapy planning is a. Increased upstream dose due to electron backscatter from bone b. Increased dose in bone due to increased multiple Coulomb scattering c. Increased dose lateral to bone due to multiple Coulomb scattering from bone d. Shifting of the 90% dose contour due to increased linear stopping power in bone e. Increased dose in bone due to increased bremsstrahlung energy loss

Impact of Patient Heterogeneity Answer #4: The most significant impact of compact bone in electron beam therapy planning is d. Shifting of the 90% dose contour due to increased linear stopping power in bone The most important impact of bone is its shifting the 90% isodose surface toward the bone, which can result in underdosing the PTV distal to the bone, e.g. treating the retromolar trigone behind the mandible. The other responses, all result in increased dose around the bone, but are not clinically significant.

Reference: Hogstrom KR 2004 Electron beam therapy: dosimetry, planning, and techniques Principles and Practice of Radiation Oncology ed C Perez et al (Baltimore, MD: Lippincott, Williams, & Wilkins) pp 252-282 Impact of Patient Heterogeneity Question #5: The most significant impact of patient heterogeneity on lung dose in electron therapy of the post mastectomy chest wall is a. Increased penetration in lung due to low lung density b. Scatter from an irregularly shaped chest wall surface c. Scatter from closely spaced ribs d. Scatter from the mediastinum in the IMC field to lung e. Increased dose in region of where electron beams of differing energy abut

Impact of Patient Heterogeneity Answer #5: The most significant impact of patient heterogeneity on lung dose in electron therapy of the post mastectomy chest wall is a. Increased penetration in lung due to low lung density The low density of lung (0.25-0.33) results in isodose surfaces penetrating 3-4 times deeper. The 90%-40% dose falloff in depth is approximately 1 cm, which corresponds to a 3-4 cm penetration in lung. Also, an extra 1.0 MeV of beam energy, if not compensated using bolus, gives an extra 0.5 cm of penetration in water, which corresponds to a 1.5-2 cm of penetration in lung.

Kenneth R. Hogstrom, Ph.D.

Principles of Electron Beam Treatment Planning • Selection of Beam Energy • Selection of Beam Direction • Collimating Techniques • Bolus Techniques • Field Abutment Techniques

Principles of Electron Beam Treatment Planning Selection of Beam Energy • Beam energy should be selected to ensure that:

– R90>maximum depth of PTV

– Rp

– Ep,o(MeV)≈3.3 x R90(cm)

– Ep,o(MeV)≈2.0 x Rp(cm) • Therefore, to estimate beam energy:

– Ep,o(MeV)>3.3 x maximum depth in cm of PTV

– Ep,o(MeV)<2.0 x minimum depth in cm of CS • Actual beam energy may differ due to: Example: max depth of PTV=3 cm; min • Field-size dependence of %DD depth of cord =6 cm) • Patient heterogeneity ⇒9.9 MeV

Principles of Electron Beam Treatment Planning Selection of Beam Direction

• Generally, electron beam should be incident ⟂ to skin surface to ensure: – Maximum penetration of therapeutic depth – Most uniform penumbra width

Principles of Electron Beam Treatment Planning Collimating Techniques

• Collimator Thickness • Design of Aperture (PTV-Portal Margin) • Skin Collimation • Utility of Small Blocks • Internal Collimation

Principles of Electron Beam Treatment Planning Design of Aperture: Basic Rule for Target-Portal Margin

Boundary within PTV Beam edge defined should be contained by collimator

2 Ep,0 = 14.8 MeV 10x10 cm

Principles of Electron Beam Treatment Planning

Skin Collimation: Basic Rules for Collimator Thickness

tPb (mm) = 1/2 Ep,o (MeV) + 1

tCerrobend = 1.2 tlead

Examples: 8 MeV → 5 mm Pb → 6 mm Cerrobend 20 MeV → 11 mm Pb → 13 mm Cerrobend

Principles of Electron Beam Treatment Planning Utility of Skin Collimation Clinical Indications

• Small Fields • Protection of Critical Structures • Under Bolus • Electron Arc Therapy

Principles of Electron Beam Treatment Planning Utility of Skin Collimation: Small Fields

6x6 cm2

3x3 cm2 3x3 cm2

• Restores penumbra enlarged by air gap. • This is particularly important for small fields.

Principles of Electron Beam Treatment Planning Utility of Skin Collimation (Large Blocks): Protection of Critical Structures

• Example: Maximum protection of eyes

Principles of Electron Beam Treatment Planning Electron Collimation: Utility/Futility of Small Blocks 7 MeV 15 MeV 15 x 15 cm 2 , 100 cm SSD 15 x 15 cm 2 , 100 cm SSD

Pb Pb 2.5 cm 2.5 cm

7 cm 7 cm

2.5 cm 2.5 cm Pb Pb

Little or no benefit if air gap present

Principles of Electron Beam Treatment Planning Utility of Skin Collimation(Small Blocks) Protection of Critical Structures • Useful for protecting superficial structures • Ex: lens, cornea in treatment of retinoblastoma)

Principles of Electron Beam Treatment Planning Utility of Skin Collimation: Under Bolus

• Restores penumbra under bolus

Principles of Electron Beam Treatment Planning Utility of Skin Collimation: Arc Electron Therapy

• Restores penumbra for electron arc 10 MeV treatments 100 cm SAD 55 cm SCD W=5 cm PMMA ρ=15 cm θ=90°

Principles of Electron Beam Treatment Planning Electron Collimation: Internal Collimation

• Used for:  Protection of eye in treatment of eyelid tumors  Intraoral stents in head and neck treatments

• Problems with:  Penetration due to insufficient thickness  Increased dose due to backscattered electrons

Principles of Electron Beam Treatment Planning Eye Shields: X-ray Lead Eye Shield Unsuitable

Eye Shield %Transmission Incident Energy Type Surface 6 mm Medium* 5.7 MeV 35% 22% 7.1 MeV 60% 37% 125 kVp x-rays 5% 4% Gold (Large)** 5.7 MeV 50% 36% *black plastic coated lead eyeshieldsPrinciples from of Electron Ace Medical Beam Supply Treatment Planning **gold-plated lead eyeshields Electron Collimation: Tungsten Electron Eye Shield

9 MeV - Tungsten

Tungsten rather than lead eye shields should be used for 6-9 MeV electrons.

Principles of Electron Beam Treatment Planning Internal Electron Collimation: Attenuation of Back-scattered Electron Dose

Lambert and Klevenhagen (1982)

Energy at Increase in HVL of Back- Interface* Dose scattered Dose

14 MeV 35% 6 mm 10 MeV 45% 5 mm 6 MeV 55% 4 mm * Energy at Interface (MeV) = Initial Energy (MeV) - Depth (cm) * 2 MeV/cm

Principles of Electron Beam Treatment Planning Electron Bolus: Definition

• A specifically shaped material, which is usually tissue equivalent and which is normally placed either in direct contact with the patients surface, close to the patients surface, or inside a body cavity. • The material is designed to provide extra scattering or energy degradation of the electron beam. • Its purpose is usually to shape the dose distribution to conform to the target volume and/or to provide a more uniform dose inside the target volume.

Principles of Electron Beam Treatment Planning Electron Bolus: Clinical Indications • Increased Surface Dose  Low energy electron beams  electron arc beams • Homogenize Dose Distribution  Irregular surface anatomy  Surgical defects • Sparing of Distal Structures  Critical structures  Normal tissue Principles of Electron Beam Treatment Planning Electron Bolus: Basic Rules for Clinical Use • Tissue-like Material – Wax, water, SuperFlab, plastic sheets, ... • Close to the Skin Surface • No Sharp Edges in Field – Extend sharp edges outside field • Verify Intent – In-vivo dosimetry (TLD) – CT-based dose calculation with bolus

Principles of Electron Beam Treatment Planning Electron Bolus for Craniospinal Irradiation

Principles of Electron Beam Treatment Planning Electron Bolus: Basic Rule for Clinical Use: Place Bolus Close to Skin Surface.

If bolus is placed too far from surface, scattered electrons can increase penumbra and decrease maximum dose.

Principles of Electron Beam Treatment Planning Electron Bolus: Basic Rules for Clinical Use

• No Sharp Edges in Field

Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Low Energy Electron Beams

1/4 Lucite(PMMA)=0.7 cm water Ds decreases with decreasing energy. Bolus for total scalp irradiation

Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Electron Arc Therapy

• Electron arc therapy lowers surface dose relative to that of a fixed beam • Surface dose can be increased by using surface bolus

Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Electron Arc Therapy

Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Irregular Patient Surface (Ear)

Water bolus in ear canal (R) reduces dose to inner ear from 160% to 110% (Morrison et al. 1995) Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Irregular Patient Surface (Nose)

• Nose creates hot spots lateral to nose and a cold spot under nose. • Nasal passages creates a cold spot in septum, location of cancer.

Principles of Electron Beam Treatment Planning Clinical Indications for Electron Bolus: Irregular Patient Surface (Nose)

Rule of Thumb: Make the patient as much like a bucket of water as possible. Principles of Electron Beam Treatment Planning Clinical Indications for Bolus Electron Conformal Therapy (ECT): Variable Depth of PTV • Variable Depth of PTV  Post mastectomy chest wall  Paraspinal muscles  Parotid  Nose GTV  Ear 68.2 Gy (90%) line  Temple • Bolus Spares Distal Structures CTV  Lung 55.8 Gy  Salivary glands line  Brain

Principles of Electron Beam Treatment Planning Clinical Indications for Bolus ECT: Variable Depth of Target Volume (Chest Wall) Recurrence at CW-IMC Junction Plane at y = -0.35 cm 10 cm Hot Spot: 56.5

50 .0

45 .0 40 .0

30 .0

20 .0

Prescription: 50 Gy @ 100% or 45 Gy @ 90%

Principles of Electron Beam Treatment Planning Clinical Indications for Bolus ECT: Head & Neck (Parotid Gland) Distal Surface Proximal Surface

Principles of Electron Beam Treatment Planning Clinical Indications for Bolus ECT: L Temple & Upper Neck (Mixed Beam Plan)

9 MeV Bolus ECT + 6 MV IMRT

Principles of Electron Beam Treatment Planning Field Abutment: Ideal Criteria

• Broad and matched penumbra • Overlap at 50% dose ratios – usually, but not always, edge of light fields • Virtual source in same position for both fields.

Principles of Electron Beam Treatment Planning Field Abutment Classification Scheme

Three possible beam configurations of adjacent radiation beams in order of increasing overlapping problems. Figure 7.3, ICRU-35.

a. Common virtual source position

b. Parallel central axes c. Converging central axes

Principles of Electron Beam Treatment Planning Field Abutment Craniospinal Irradiation Technique

• X-ray field edge feathered to match electron penumbra. • Extended SSD used to broaden electron beam penumbra. • Two posterior electron fields abutted along common edge to give uniform dose.

Principles of Electron Beam Treatment Planning Field Abutment Chest Wall: Junction Hot Spot

Principles of Electron Beam Treatment Planning Field Abutment Chest Wall: Junction Shift Note improved dose homogeneity

Principles of Electron Beam Treatment Planning Recommendations for Patient Treatment Planning • 3D treatment planning system is required • CT data set for defining heterogeneous patient • Dose algorithm should accurately calculate absolute dose (dose/MU), accounting for beam geometry, patient heterogeneity, and treatment aids • TPS should model all treatment parameters: – Energy, angle, SSD, and collimating geometry (applicator + insert) – Special collimation (skin collimation, eye shields, internal collimation) – Bolus (uniform thickness, missing tissue, conformal) – Special treatment techniques (arc therapy)

Principles of Electron Beam Treatment Planning Summary: Electron Beam Treatment Planning

• For optimal electron therapy, consider:  Properties of dose distributions  Effects of patient anatomy  Proper utilization of collimation  Proper utilization of electron bolus  Methods of field abutment  Proper utilization of 3D treatment planning system

Principles of Electron Beam Treatment Planning Question #6: Consider a PTV having a 5-cm diameter circular cross section and a maximum depth of 4 cm. For the distal 90% dose surface to contain the PTV, which of the following are the best initial estimates for beam energy (Ep,o) and field size? a. 12 MeV, 5-cm diameter field b. 12 MeV, 6-cm diameter field c. 12 MeV, 7-cm diameter field d. 13 MeV, 6-cm diameter field e. 13 MeV, 7-cm diameter field

Principles of Electron Beam Treatment Planning Answer #6: Consider a PTV having a 5- cm diameter circular cross section and a maximum depth of 4 cm. For the distal 90% dose surface to contain the PTV, which of the following are the best initial estimates for beam energy (Ep,o) and field size? e. 13 MeV, 7-cm diameter field

Ep,o(MeV) ≈ 3.3 x 4 cm = 13.2 ⇒ 13 MeV Field diameter ≈ 5 cm + 1 cm (L border) +1 cm (R border) = 7 cm

Reference: Hogstrom KR 2004 Electron beam therapy: dosimetry, planning, and techniques Principles and Practice of Radiation Oncology ed C Perez et al (Baltimore, MD: Lippincott, Williams, & Wilkins) pp 252-282 Principles of Electron Beam Treatment Planning Question #7: Which of the following is not a clinical basis for skin collimation? a. Sharpening penumbra at ends of treatment area in arc therapy of chest wall b. Improvement of dose in abutted electron beams in fixed beam therapy of chest wall c. Protection of eye in treatment of nose d. Restoration of penumbra under bolus e. Small field for treating eyelid

Principles of Electron Beam Treatment Planning Answer #7: Which of the following is not a clinical basis for skin collimation? b. Improvement of dose in abutted electron beams in fixed beam therapy of chest wall In answers a, c, d, and e, skin collimation sharpens the penumbra resulting in improved sparing of normal tissue and structures outside the treatment area. In answer e, skin collimation sharpens the penumbra inside the treatment region making error in placement of the edges of abutting fields leading to larger dose variations (increased or decreased dose) in the abutment region. Reference: Hogstrom KR 2004 Electron beam therapy: dosimetry, planning, and techniques Principles and Practice of Radiation Oncology ed C Perez et al (Baltimore, MD: Lippincott, Williams, & Wilkins) pp 252-282 Question #8: Which of the following is the most appropriate use of uniform thickness bolus? a. Placing around nose to remove surface irregularity and increase dose to septum b. Placing in ear canal to protect inner ear from increased dose without bolus c. Placing on chest wall to increase surface dose when using low energy electron beams or arc electron therapy d. Placing on chest wall to conform 90% dose surface to distal chest wall PTV surface e. Placing on lateral head to conform 90% dose surface to distal parotid PTV surface

Principles of Electron Beam Treatment Planning Answer #8: Which of the following is the most appropriate use of uniform thickness bolus? c. Placing on chest wall to increase surface dose when using low energy electron beams or arc electron therapy Both low energy electron beams and arc electron beams have a low surface dose that can be increased by uniform thickness bolus for chest wall treatments. Answers a and b require shaped bolus shaped to the nose or ear, respectively, to remove surface irregularities. Answers d and e require shaped bolus used in electron conformal therapy. Reference: Hogstrom KR 2004 Electron beam therapy: dosimetry, planning, and techniques Principles and Practice of Radiation Oncology ed C Perez et al (Baltimore, MD: Lippincott, Williams, & Wilkins) pp 252-282 Principles of Electron Beam Treatment Planning Electron Special Procedures

John A Antolak, Ph.D. Electron Special Procedures  TSEI  Total Limb  Intraoperative electrons  Total scalp  Craniospinal  Electron arc

Electron Special Procedures Treatment of Mycosis Fungoides (cutaneous T-cell lymphomas)

 First described and named by Alibert (Paris, 1806)  First x-ray treatments by Sholtz (Berlin, 1902), but there were severe side effects  First electron treatment by Trump (Boston, 1952) with Van de Graaff generator @ 2.5 MeV  Stanford University Medical Linear Accelerator adapted in 1957

Electron Special Procedures Total Skin Electron Irradiation  AAPM Report #23, Total Skin Electron Therapy: Technique and Dosimetry (1988)  Antolak, J. A. and K. R. Hogstrom (1998). “Multiple scattering theory for total skin electron beam design." Medical Physics 25(6): 851-859.  Antolak, J. A., J. H. Cundiff, et al. (1998). "Utilization of thermoluminescent dosimetry in total skin electron beam radiotherapy of mycosis fungoides." International Journal of Radiation Oncology, Biology, Physics 40(1): 101-108.

Electron Special Procedures TSEI Stanford Technique

1st day of cycle 2nd day of cycle

laser

ANT POST RPO LPO LAO RAO

Electron Special Procedures TSEI Treatment Positions (note: disposable paper gown, thickness < 0.005 g cm-2, acceptable)

ant r ant obl

MD Anderson TechniqueElectron Special Procedures Stanford Dual-Beam Technique

Electron Special Procedures Stanford Dual-Beam Dosimetry Karzmark CJ, et al. Radiology 74:633-644(1960)

Electron Special Procedures Monitor unit calculation 50% of dose delivered with beam up/down Total Prescribed Dose Prescribed Skin Dose per Treatment Cycle = Number of Cycles

Prescribed Skin Dose per Treatment Cycle Dose per Field per Treatment Cycle = × 0.5 2.8

cGy = current TSEI Calibration Factor MU measured quantity: Dose per Field per Treatment Cycle ratio of average skin dose for all 6 MU = positions (12 fields, equal cGy MU weights) to the given dose to the calibration point for one patient position (2 fields, up/down)

Electron Special Procedures Total Limb Electron Irradiation Diagnoses

 Diffuse Large Cell Lymphoma  Right lower leg  Kaposis sarcoma  Right lower leg  Recurrent malignant melanoma  Right arm  Malignant melanoma  Right upper leg

Electron Special Procedures 6-Field Extremity Technique  6 large fields with flash  Wooden, K. K., K. R. Hogstrom, et al. (1996). "Whole-limb irradiation of the lower calf using a six-field electron technique." Medical Dosimetry 21(4): 211-218.

Electron Special Procedures 8-Field Extremity Technique  8 large fields with flash

Electron Special Procedures Example Treatment Setup Positions R Arm/ Melanoma

Supine: Prone: Dorsal Ventral field Electron Special Procedures field Total Arm Couch Top

Adjust to length of arm: Note position of thumb)

Rotate for arm to be parallel to isocentric axis

Electron Special Procedures Intraoperative Electron Radiotherapy Advantages and Disadvantages (Owens and Graves, 1991) Advantages Disadvantages  Spares skin,  Requires surgical subcutaneous tissues, procedures: anesthesia, and abdominal wall. postop pain, potential surgical complications.  Allows confinement of  Only single irradiation dose to disease sparing dose possible. nearby tissues.  Puts some normal tissue  Does not preclude at risk of injury. postop radiotherapy.  Is expensive in personnel,  Does not interfere with scheduling, and chemotherapy equipment.

Electron Special Procedures Intraoperative Radiotherapy Modalities

 Treatment modalities used for IORT include  Electron beams  kVp x-ray beams  Brachytherapy using HDR or implants  Electron dose distal to tumor can be eliminated using lead sheets to stop electrons, protecting distal and adjacent structures.  Thus, electron beams can offer the advantage of very small exit dose (x-ray contamination).

Electron Special Procedures Room Requirements for IORT

 Operating room for initial surgical procedure  Sterile irradiation facility that can accommodate needs of IORT irradiation  Post irradiation operating room for closing of wound

Electron Special Procedures Options for IORT Facilities

 Dedicated IORT/OR Suite (M D Anderson, Mayo)  Patient operated in room housing linac  Patient moved from surgical area to linac for IORT  Mobile IORT Linac in OR (Univ Louisville)  Patient operated in surgical OR  Mobile linac in surgical area rolled into OR  OR adjacent to RT Treatment Room (Mass General, Med College of Ohio)  Patient operated in surgical OR  Patient transported to sterile linac in adjacent room  Totally separate OR and IORT facilities  Patient operated in surgical OR  Patient transported from OR to RT clinic to sterile treatment room Electron Special Procedures M D Anderson Cancer Center IORT Suite

 6-15 MeV

Electron Special Procedures Example of Soft Docking System: Laser Guided (Hogstrom et al. 1990)

Electron Special Procedures Mobile Electron Beam IORT System- Mobetron 4-12 MeV

Electron Special Procedures Properties of IORT Electron Cones  Shapes  Circular  Rectangular  Squircle (half square-half circle)  Ends  0º-30º bevel  Material  Able to be sterilized  Able to shield surrounding material from scattered electrons  Thin as possible to allow minimal tumor-normal tissue clearance

Electron Special Procedures Properties of IORT Electron Cones (continued)

 Typical materials  Lucite, stainless steel, chrome-plated brass  Able to view irradiated volume  Direct visual viewing  Mirror reflector  Camera  Cones have differing alignment templates to allow manual docking

Electron Special Procedures IORT Electron Dose Distributions

12 MeV 12-cm ∅, 0º cone

12 MeV 12-cm ∅, 30º beveled cone

Electron Special Procedures Treatment Delivery (continued)  Visual verification of treatment field  Target volume in field of view  Critical structures avoided  Treatment field free of blood  All personnel evacuated from room  Deliver radiation as rapidly as possible  High dose rate option useful (e.g. 600-1000 MU/min)  Patient monitoring  Visual monitoring of patient  Blood pressure and pulse  Pulse and breathing using esophageal stethoscope

Electron Special Procedures Treatment Results Stomach (Abe et al. 1991)

 228 Patients  No distant metastasis  Surgical resection vs. IORT (28-35 Gy)  Results  5-y survival rates based on serosal invasion (+/-)  Surgery: 89% for s(-)__51% for s(+)  IORT: 94% for s(-)__60% for s(+)  5-y survival rates based on lymph node metastasis  Surgery: 97% for n0__67% for n1__32% for n2,n3  IORT: 100% for n0__64% for n1__51% for n2,n3  Conclusion  IORT is able to improve survival of patients with serosal invasion or with n2 or n3 lymph node metastases

Electron Special Procedures Treatment Results Pancreas (Abe et al. 1991)

 103 Patients  Inoperable tumor due to vessel involvement or retroperitoneal invasion  No liver involvement or distant metastases  Arms  Control (41 patients), Operation alone  IORT (25-40 Gy)  Operation +EBRT (55-60Gy in 1.6-1.8Gy fractions)  IORT(10-25Gy)+EBRT(35-50Gy in 1.6-1.8Gy fractions)  Results (median survival time)  Operation alone: 5.5 months  IORT alone: 5.5 months  Operation + EBRT: 9 months  IORT +EBRT: 12 months

Electron Special Procedures Total Scalp Irradiation Electron + X-ray Technique

 Clinical Indications  Treatment Objectives  Electron + X-ray Technique  Treatment Planning  Dose Distribution  Treatment Setup  Treatment Verification

Electron Special Procedures Clinical Indications

 Cutaneous tumors with widespread involvement of scalp and forehead:  Lymphoma  Melanoma  Angiosarcoma

Electron Special Procedures Total Scalp Irradiation Treatment Objectives

 To provide a uniform dose distribution (±10%) to the entire scalp and target volume.  To keep brain dose as low as possible.  To provide a simple and reproducible treatment.

Electron Special Procedures Total Scalp Irradiation Treatment Planning Challenges

 Topology of the head  Depth variation of the target volume  Close proximity of brain to the scalp  Impact of bone on dose distribution

Electron Special Procedures Total Scalp Irradiation Treatment Options

 6-field electron beam technique (Able et al 1991)  Electron + 6MV x-ray technique (Akazawa et al 1989; Tung et al 1993)  Tomotherapy  Nomos (Locke et al 2002)  TomoTherapy HI-ART II (Orton et al 2005)

Electron Special Procedures Total Scalp Irradiation 6-Field Electron Technique (Able et al.)

Electron Special Procedures Total Scalp Irradiation Electron + X-ray Technique (Tung et al.)

Electron Special Procedures Total Scalp: Electron + X-ray Technique Beam Arrangements

 Beams 2,4  Electron beams  6-9 MeV  100-cm SSD  Beam weight=100%

 Beams 1,3  6 MV X-ray beams  100-cm SSD  Beam weight = 60%

Electron Special Procedures Total Scalp: Electron + X-ray Technique Custom Bolus (6-mm thick) Wax Bolus Patient Prone with Fabrication Bolus

Electron Special Procedures Clinical Indications

 Pediatric brain tumors that have a tendency to seed along the pathways of cerebrospinal fluid:  Medulloblastoma  Malignant ependymoma  Germinoma  Infratenrotial glioblastoma

(Maor et al. 1985)

Electron Special Procedures Treatment Volumes

 Whole Brain  Base of Brain  Spinal Theca

Electron Special Procedures Craniospinal

Electron Special Procedures Craniospinal Electron & Photon Beam Technique

 Brain: Parallel-opposed lateral photon beams  Spine/Electrons: True posterior electron beam consisting of 1 or 2 fields  Spine/Photons: 6 MV x- rays from posterior fields:  1 field (180°)  2 fields (180°±30°)  3 fields (180°, 180°±30°).

Electron Special Procedures Comparison of Spinal-field Dose Distributions Photon & Electron Beams

 CT Lower Neck

Electron Special Procedures Comparison of Spinal-field Dose Distributions Photon & Electron Beams

 CT Thorax

Electron Special Procedures Comparison of Spinal-field Dose Distributions Photon & Electron Beams

 CT Abdomen

Electron Special Procedures Electron Bolus for Craniospinal Irradiation

Electron Special Procedures Question #9: For total skin electron irradiation using a standard 6-position 12-field (Stanford) technique, the ideal 90% width of the electron beam at the patient plane is a. 40-45 cm. b. 50-55 cm. c. 60-65 cm. d. 70-75 cm. e. 80-85 cm.

Electron Special Procedures Answer #9: For total skin electron irradiation using a standard 6-position 12-field (Stanford) technique, the ideal 90% width of the electron beam at the patient plane is c. 60-65 cm.

Narrower widths make it very difficult to obtain good superior-inferior uniformity, and larger widths scatter more electrons laterally, reducing the dose per MU for the beam. Reference: Antolak, J. A. and K. R. Hogstrom (1998). "Multiple scattering theory for total skin electron beam design." Medical Physics 25(6): 851-859.

Electron Special Procedures Electron Dose Calculations

John A. Antolak, Ph.D. Early Approaches

 Isodose Tables  Diffusion

Electron Dose Calculations Pencil-Beam Algorithms

 Hogstrom algorithm  Lax algorithm  PBRA

Electron Dose Calculations Monte Carlo Algorithms

 VMC  MMC  EGS

Electron Dose Calculations Question #10: For a Monte Carlo based dose algorithm, to reduce the average statistical uncertainty in the high dose region from 3% to 1% requires a. the same calculation time. b. 3 times longer calculation time. c. 6 times longer calculation time. d. 9 times longer calculation time. e. 12 times longer calculation time.

Electron Dose Calculations Answer #10: For a Monte Carlo based dose algorithm, to reduce the average statistical uncertainty in the high dose region from 3% to 1% requires d. 9 times longer calculation time.

Reference: Popple, R. A., R. Weinberg, et al. (2006). "Comprehensive evaluation of a commercial macro Monte Carlo electron dose calculation implementation using a standard verification data set." Medical Physics 33(6): 1540-1551. (and many others)

Electron Dose Calculations Question #11: The primary limitation of the Hogstrom pencil- beam algorithm is due to which approximation? a. small-angle scattering b. central-axis c. range straggling d. 2D geometry e. large-angle scattering

Electron Dose Calculations Answer #11: The primary limitation of the Hogstrom pencil- beam algorithm is due to which approximation? b. central-axis

Reference: Hogstrom, K. R. and P. R. Almond (2006). "Review of electron beam therapy physics." Physics in Medicine and Biology 51(13): R455-489. Reference: Hogstrom, K. R. and R. E. Steadham (1996). Electron Beam Dose Computation. Teletherapy: Present and Future. J. Palta and T. R. Mackie. Madison, WI, Advanced Medical Publishing: 137-174. Reference: Brahme, A. (1985). "Current algorithms for computed electron beam dose planning." Radiother Oncol 3(4): 347-362.

Electron Dose Calculations Looking to the Future

 eMLC  Electron arc  Untapped potential  MET, MERT  With and without IMRT