Therapeutic Electron Beams: Clinical Dosimetry and AAPM TG-70

Dimitris Mihailidis, Ph.D . Charleston Radiation Therapy Consultants THANK YOU

PROGRAM DIRECTORS OUTLINE

Objectives General characteristics of electron beams Current calibration protocols Measurement of CA %DDs in water Output factors Non-water phantoms: Relative measurements Small and irregular field dosimetry Extended treatment distances Electron algorithms Some data and examples Objectives

Address the issues that currently influence the dosimetry of clinical electron beams due to TG- 51 protocol. Describe a detail procedure of converting measured depth-ionization curves with ion chambers into depth-dose curves Describe the necessary conversion factors.

Present details of AAPM TG-70. (Gerbi et al. Med. Phys. July 2009, issue!)

G T Objectives

Describe the use of water equivalent phantoms for relative electron dosimetry. Describe the concept of lateral buildup ratio (LBR) for small and irregularly shaped electrons fields. Give some clinical examples. TG-25 General Characteristics of Electron %DDs and isodoses definitions….

Practical Range

Therapeutic Range ICRU 35 (1984) As Energy increases

Surface dose (at 0.5mm) ↑.

dmax ↑ for lower Es and stops at moderate/high Es.

Beam penetration (d 90 , d 50 and Rp) ↑.

Distance between d 90 -d10 ↑. x-ray contamination ↑. As beam energy increases….

Hogstrom (2003) As beam energy increases….

10x10cm 2

iction onstr ose c Isod As field size decreases

Surface dose (at 0.5mm) ↑.

dmax and d 90 ↓ (shift to surface).

Distance between d 90 -d10 ↑.

Rp unchanged. As field size decreases….

9 MeV 20 MeV

Hogstrom (2003) Important reports to have

TG25 (Clinical electron beam dosimetry). TG51 (protocol for clinical dosimetry for high energy photon and electron beams). TG69 (Radiographic Films for MV Beam Dosimetry). TG39 (The calibration and use of planeparallel ionization chambers for dosimetry of electron beams) . TG53 (Quality assurance for clinical radiotherapy treatment planning). TG106 (Accelerator beam data commissioning and procedures. TG-70 (just published) Current calibration protocols Current calibration protocols

TG-51 of AAPM (1999). TRS-398 of IAEA (2003). “Absorbed Dose determination in External Beam Radiotherapy: An International Code of Practice based on Standards of Absorbed Dose to Water” Code of practice of IPEM (2006). “Code of practice for electron dosimetry of radiotherapy beams with initial energy from 2 to 50 MeV based on air kerma calibration ” ossible) . Co w , ref w air 60 D   

ρ N L q    k watertoair restricted SPRs. = w , Q D N at oneat point, d

Main objectives of MainTG51 objectives Dose to water is closer to the dose to tissue to dose the to is water closer to Dose Absorbed dose is more robust than the AirKerma than robust doseAbsorbed the more is Std. 1. Incorporate the new absorbed dose standard absorbed dose new the 1. Incorporate (as as much p the 2.formalism calibration Simplify Defines Defines Dose Proposes to do two things: Makes use of realistic Because of TG51 (and other recent dosimetry data), do we need to modify TG25 ?

YES! TG70 was charged! EBeam Quality Specification Electron Beam Quality Specification

New beam quality specifier*: R50 (depth in water at which dose falls to 50% of maximum for large field size (>10x10 cm 2) (TG51).

First find I50 (50% of ionization maximum).

= − ≤ ≤ R50 .1 029 I50 06.0 cm for 2 I 50 10 cm

= − ≥ R50 .1 059 I50 37.0 cm forI 50 10 cm

*(TG25, beam quality specifier: (Ep)0) ) ) ) MeV (     MeV MeV p ( ( d R 2 50 2

(IPEM 2003) (IPEM 50 I R − 1 )     040 022 0 .0 .0 cm (Harder 1968) (Harder E ( + + = 23.0 50 d 50 I R E

(Rogers 1996) (Rogers − 935 059 .1 .2 50 R + + 271 818 .1 656 .0 Energy at depth at Energy .0 = = = 0 Practical range: p 0 Harder’s valid! Eq. still E E R Mean energy at surface of phantom : ofphantom surface Mean at energy Mean energy depth: at or ? ) MeV

(Harder 1968) (     p d R − 1     0 E Energy at depth at Energy Mean energy at depth: at energy Mean = d E Harder’s Eq.still valid! ? ) )

(IPEM (IPEM 2003) MeV ( MeV ( 2 50 2 50 R I ) 022 .0 cm 040 .0 ( + + 23.0 50 50 R I − (Rogers 1996) 50 059 935 .2 .1 R + + 271 .1 818 656 .0 .0 = = = p 0 0 R Practical Practical range: E E Mean energy surfaceat of phantom : Energy at surface and at Energy range practical or ) ips MeV (     p d R − 1     0 E = d E ) )

MeV MeV (

Comment ( 50 50 R R Harder’s valid! Eq. still 40.2 33.2 = = 0 0 E E Mean energy depth: at of Use of TG25): (as Produce values within 0.4 MeV 0.4 values within Produce previous relationsh of or Measurements of %DDs Phantoms and detectors

WATER for relative measurements (%DDs, OFs, etc) such as large data collection . Plastic (non-Water) for limited relative measurements of clinical setups (%DDs, OF, etc). Detectors (cylindrical, pp, diodes, film) Cylindrical chambers in water Placement of ion chamber in water

TG106, (2008) Step 1 : Shift chamber deeper by 0.5 rcav % depth ionization or dose dose or or ionization ionization depth depth % % Step 2 : Correct reading for Pion, Ppol

= ⋅ ⋅ M (d) M raw (d) Pion (d) Ppol (d)

• Apply TG51 requirements for Pion and Ppol (For %DD: measure at I50 and Rp depths)

M (d) %di (d) = 100 M (Imax ) Step 3 : Use realistic SPRs water/air

From Burns 1996 (in water)

w + ( )+ ( )2 + (z )  L  a b ln R c ln R d R   ()= 50 50 50   R50 , z ρ 1+ e()()()ln R + f ln R 2 + g ln R 3 + h()z   air 50 50 50 R50 Where:

a = 1.0752 b = - 0.50867 c = 0.088670 d = - 0.08402 e = - 0.42806 f = 0.064627 g = 0.003085 h = - 0.12460

These coefficients give an rms deviation of 0.4% and a max.

deviation of 1.0% for z/R 50 between 0.02 and 1.1. The max. deviation increases to 1.7% if z/R 50 values up to 1.2 are considered. Step 4 : a) Computation of %DD

b) Correct for Pfl and Pwall

 w L ⋅ ⋅   (R50 ,d) Pfl (Ed ) Pwall (d) .  ρ  %dd (d) = %id (d)× air w w w  L    (R ,d )⋅ P (E )⋅ P (d )  ρ  50 max fl dmax wall max  air Errors associated with SPRs

Fitted SPRs vs. individually calculated Error: % of Max dose= %error of the fitted SPRs vs. individually calculated values.

Rogers has shown that the %DD can be

Rogers, 2004 determined to within 1% of the dose at dmax .

(Rogers 2004) Reminder : For electrons = × Prepl Pgr Pfl

Pfl components

Pgr: addressed with chamber shift

Pfl: Use Table V in TG-25 Pfl from TG25 Inner diameter (mm) Ed (MeV) 3 5 6 7 2 0.977 0.962 0.956 0.949 3 0.978 0.966 0.959 0.952 5 0.982 0.971 0.965 0.960 7 0.986 0.977 0.972 0.967 . 10 0.990 0.985 0.981 0.978 15 0.995 0.992 0.991 0.990 20 0.997 0.996 0.995 0.995 = ( − ) *where Ed E0 1 d / R p ? show change up wall for MeV.6 : make sure your chamber chamber sure your make :

50 R (Buckley&Rogers 2006) What about P about What

(Johansson 1978) for electron beams lowZand thinwalled Recent data

: : =1: introduces less than 2% error at depthsby at error 2% than less introduces publishedresponse to its data. comparing Recommendation

wall wall

P to 2.5% 0.5 cm at

P chambers p-p chambers in water Markus and Capintec have shown strong

point of measurement for correction factors). TG-39

(Buckley&Rogers 2006) for ppall chambers. PS033 (usePS033 cases. for most chambers except corrections are considered negligible mostin

=0 =1 gr fl wall

Use ofUse p-p chambers for %DD Front window thickness should(12mm) be taken into account. P P P :

wall depth dependence for NACP02, Roos, Markus and Capintec PS033. P Recent data • In general, front of window is NEW: Prepl for pp chambers

(Wang & Rogers 2010) Buckley&Rogers: p-p chambers

Recommendation: Compare your chamber’s response with data from literature to be <2%

Know your chamber! diodes in water . taking large taking (TG25)

Use ofUse diodes for %DD amount of data. of amount (get specs from manufacturer). from specs (get chambers with measured curves Diodes specifically designed for electron beams. designed specificallyelectron Diodes for diodesbefore for program QC a Have The effective point of measurement is dyeis the Theeffective pointof measurement Validate%DDs diodes with against measured Use of diodes for %DD

1) Look at new TG106 (Das 2008)

2) Look at AAPM Summer School 2009 book, Chapter 12 (Mihailidis, 2009) Output factors Definition

( ( ) ) ()() = D& d max ra , ra , SSD treat Se d max ra , ra , SSD ()() D& d max r0 , r0 , SSD nom ra: treatment field at SSD treat

r0: reference / cal field at SSD nom Non-water phantoms Relative measurements Use of nonwater phantoms

Water substitutes should mimic water across the whole electron energy range

mainly in stopping and scattering powers thus, both the electron density and the effective atomic number should be matched to water in practice for some phantoms, this is difficult to achieve (due to the carbon in plastics)

offtheshelf material can have large variations in density and scattering power Must be careful in using these materials

SO: USE WATER WHENEVER POSSIBLE! Use of nonwater phantoms discussion of corrections

Depths need to be scaled. Chamber readings need to be multiplied by an appropriate fluenceratio correction. Stoppingpower ratios should be taken at the scaled depth. Charge storage effects should be kept in mind using polystyrene and PMMA. Corrections needed

 R w  d = d ρ = d  50  Depth correction: w med eff med  med   R50 

Corrected dose (Ding 1997): max = max [( ρ ) ]w φ w Dw (d w ) Dmed (d med ) L / coll med med

Electron fluence correction factor Depth correction factors

Material Mass density Recommended effective -3 ρρρ (g cm ) density, eff Water* 1.00 1.00 Clear polystyrene* 1.045 0.975 High-impact polystyrene 1.055 0.99 (white)* Electron solid water* 1.04 1.00 Polymethylmethacrylate 1.18 1.115 (PMMA)* Epoxy resin water 1.02 0.98 substitute, photon formulation**

*from TG-25, Table VII, p. 84. **From IPEM 2003, p. 2945. Output in non-water phantom

φ max w  (dmed ,r) max = max ⋅ med Se,w (dw ,r) Se,med (dmed ,r) w φ(dmax,r )  med 0 med

φ w med : Use data from Ding 1997.

NOTE: Since corrections appear as RATIOS might be neglegible. CHECK it! Measurements PDDs in nonwater Phantoms

The SSD and field size are not to be scaled.

Chamber must be positioned with its effective point of measurement at the equivalentscaled depth in the nonwater phantom.

φ w Correct for electron fluence med PDDs in non-water phantoms • Cylindrical chambers   w L w w ⋅   (R50 , d ) Pfl (E med )  ρ  d φ w med w = med × air × med (d ) %dd w (d ) %di med (d )   w φ w (d med ) L w w ⋅ med max   (R50 , d max ) Pfl (E med ) ρ dmax   air

• pp chambers: Pfl=1

• Film XV2 (TG25) Large energy dependence Dosimetry of small and irregular fields Dose determination in small/irregular fields

Inherent problems in dosimetry of small electron fields

depth of dmax becomes shallower. the output factor may be significantly different than the cone factor if the field size is small enough for lateral scatter equilibrium (LSE).

isodose coverage is reduced in all directions as the field shrinks. SquareRoot Method

For rectangular fields of X & Y dimensions:

= × 2/1 %dd (d,rX ,Y ) [% dd (d,rX ,X ) %dd (d,rY ,Y )]

= × 2/1 Se (d m ,rX ,Y ) [Se (d m ,rX ,X ) Se (d m ,rY ,Y )]

(Hogstrom (1981) & Mills (1982)) Approximate irregular shapes with rectangles

Hogstrom (2000) Comparison of the percent difference between measured and calculated output factors for common methods of calculation in the literature. The measured and calculated output factors were for cutout shields that were shaped as squares, rectangles, circles, ellipses, and arbitrary shapes used in the clinic

Calculation method Percent difference (≤±%) Equivalent square 2.7*, 5.9*, 3.0* Square root 3.0*, 2.3*, 4.6*, 3.0* One-dimensional 3.0*, 2.0*, 2.1* Pencil beam 2.7*, 2.0* Sector integration 3.0, 1.5, 1.0*

(Jursinic 1997) Lateral Buildup Ratios (LBRs) Model (Khan et al. 1999) When is special dosimetry required?

When the minimum field dimension is less than the minimum radius of a circular field that produces lateral scatter equilibrium (LSE) = Req 88.0 E p 0, or ≅ (Khan&Higgins 1999) a 58.1 E p 0, where = + + 2 E p 0, 22.0 98.1 R p .0 0025 R p Do you remember what it has been?

From Lax&Brahme (1980) we have been using the criterion for LSE:

E R ≅ 0 eq 5.2 where (also in Khan’s book) = E0 E0 or = E0 E p 0, Pencil beam…

Broad field-same surface fluence as pencil beam  2  exp  − r   σ 2   r (z) Due to multiple d (r, z) = D∞ ,0( z) ⋅ p πσ 2 Coulomb Scattering r (z) Khan et al. (1998) LBR definition σ 2 ( ) LBRs are related to r d mean square radial spread of pencil beam (independent of field size). DEFINITION of LBR: D(d,r ) Φ (r , E) LBR (d,r) = x ⋅ i ∞ Φ D(d,r∞ ) i (r, E)

rx: small field (e.g., 2 cm radius) 2 r∞: broad field (e.g., 20x20 cm ) Φ(…): incident fluence (normalize at 0.5 mm depth)

(Khan et al. 1998) Measured PDDs normalized at 0.5 mm depth (surface)

Diode data 1: 2 cm 2: 2.8 cm 3: 3.7 cm 4: 5.8 cm 5: 20x20 cm 2

Khan et al. (1998) σ 2 ( ) Determine r d

Take a small circular field rx=2 cm diam. to measure PDDs and ratio those to PDDs from a 20x20 cm 2 ,normalized both at 0.5 mm, to determine LBRs.

2 σ 2 = rx r (d)  1  with LBR<1 ln  −  1 LBR (rx ,d)

(Khan et al. 1998) Sigma vs. field size and energy

LBR min Khan et al. (1998) In which applicator to use the rx field?

6 MeV 12 MeV

It does NOT matter!

20 MeV 2 cm diam. insert in 10x6, 10x10 and 20x20 cm2 applicators For irregularly shaped fields…

Sector integration summing the pencil beam contributions.

∆θ n − r 2 LBR (d , r ) = 1 − ( ) exp[ i ] eff π ∑ σ 2 2 i =1 r (d )

∆∆∆θ x O

(Khan&Higgins 1999) Can we predict dmax for small fields?

Circular fields as example, within ±0.5% of maximum dose.

33.0 rx ≅ ( 67.0 67.1 − 2 ) r ≤ R dmax .0 174 rx E p 0, .0 0625 E p 0, x eq

For broad field Req ≅ 67.0 dmax 46.0 E p 0, rx ≥ Req

Relative applicator Compute output factor: Output Factor = × × S e (d , r j ) S e LBR eff (d , r j ) % dd ∞ (d , r∞ ) (Khan&Higgins 1999) Data vs. predictions for small circular fields dmax

(Khan&Higgins 1999) Data vs. predictions for small circular fields

Dose MU

(Khan et al. 1998) Treatments at extended distances Electron beam features at extended SSDs

Differences in PDDs resulting from InvSq. effect are small because electrons do not penetrate that deep and because the significant increase of penumbra width with SSD restricts the SSD to 115 cm or less in clinical practice (Hogstrom 2003).

The depth of dmax at extended SSD is a complex function of field size and beam energy. For small field sizes, the dmax depth changes little. As the field size increases (>10x10 cm 2), the

dmax depth increases very slowly for electron energies below 12 MeV. For higher energies, the dmax depth increases with increasing SSD (Das et al. 1995, Cygler et al. 1997). However, this effect is clinically insignificant since higher

electron energies have broad dmax ranges. In addition, beam flatness decreases and penumbra width increases at extended SSD, an effect more pronounced at smaller field sizes and lower electron energies. (also in Khan’s book) Treatment planning systems (without Monte Carlo) differ in their ability to accurately depict the effects of extended SSD and individual institutions should investigate the limitations of their planning systems before use on patients.

Use TG25 recommendations. Beam output at extended distances

The Effective-SSD(SSDeff ) method How to determine SSDeff

For every applicator and energy plot:

= − Gap: g SSD ext SSD nom

Io: Rdg at SSD nom

I: Rdg at SSD ext

SSD : extended SSD SSD eff ext (typically <115 cm)

SSD nom : 100 cm

Typically dmax (TG25 & in Khan’s book) Data from Siemens Primus

7 MeV 14 MeV

CONE INSERT dmax S SSD eff CONE INSERT dmax S SSD eff

SIZE SIZE [cm] [cm] SIZE SIZE [cm] [cm]

10x10 2x2 0.9 0.791 33.1 10x10 2x2 1.2 0.893 58.7 " 3x3 1.2 0.870 46.6 " 3x3 1.6 0.918 66.4 " 4x4 1.4 0.957 55.4 " 4x4 2.0 0.944 62.5 " 6x6 1.4 1.001 61.5 " 6x6 2.6 0.987 71.5 " 8x8 1.6 0.998 73.6 " 8x8 2.9 0.995 79.1 5cm 5cm 1.5 0.817 57.3 5cm 5cm 2.7 0.922 82.0 10x10 10x10 1.6 1.000 83.9 10x10 10x10 2.9 1.000 94.2 15x15 15x15 1.6 0.998 99.4 15x15 15x15 2.9 0.984 103.5 20x20 20x20 1.6 1.001 100.4 20x20 20x20 2.9 0.957 104.6 25x25 25x25 1.6 0.992 106.0 25x25 25x25 2.9 0.957 106.9

(Mihailidis, et al.) Weak dependence of SSDeff on applicator size (Varian 2100 Data)

Insert Beam Energy (MeV) size (cm 2)

6 9 12 16 20

4 46.2 61.1 72.5 76.4 76.6 6 62.2 74.7 80.2 81.8 80.6 8 77.6 83.8 83.2 83.6 82.7 10 82.9 85.9 86.8 85.5 83.8 15 90.7 90.9 90.1 90.0 89.7 20 90.0 91.8 91.4 90.9 92.0 25 90.7 91.9 91.0 92.5 93.3

Average (Roback et al. 1995) Output at extended distances

= × Se (d m , SSD ext ) Se (d m,SSD nom ) GF

2  SSD + d  GF =  eff max  with  + +   SSD eff dmax g 

D Monitor Units: MU = prescripti on S e (d m , SSD ext )

(Khan et al. 1998) Acknowledgment

Faiz Khan. Dave Rogers. Ken Hogstrom. THANK YOU!