Outline

 Field shaping Treatment planning III: ◦ Custom blocks field shaping, skin dose, and ◦ Use of independent jaws field separation ◦ Multileaf collimators  Skin dose Chapter 13 ◦ Maintaining skin sparing of MV beams F. M. Khan “The Physics of Radiation Therapy”  Filed separation techniques ◦ Geometric ◦ Dosimetric

Field shaping Field blocks

n  The shaping of a treatment field is  1  1     0.05 dictated by two factors:  2  2n ◦ Complete coverage of the tumor with 1 2n   20 prescription dose, including local and distal 0.05 disease log 20 n   4.32 ◦ Dose to normal tissue should be minimized; log 2 vital organ tolerance observed  Typical figure of merit: primary beam transmission of 5% or less through a  Thickness of a block should be large enough to transmit blocked region only 5% of the primary photon beam  n is the number of HVL’s, thickness depends on the beam quality

Custom blocks Custom blocks

 The blocks should be shaped  Custom blocks are made of so that their sides follow the melted Cerrobend based geometric divergence of the on shapes cut from beam, minimizing the block Styrofoam transmission penumbra ◦ “Positive” or “negative” (partial transmission of the beam at the edges of the  Mounted on a block tray in block) the accelerator head  Straight blocks are used for  Transmission is ~3.5% beams with large geometric penumbra (60Co source) Styrofoam block cutter

1 Independent jaws Multileaf collimators

 Used for rectangular filed  MLC consists of a large blocking (asymmetric number of collimating blocks (leaves) that can be driven fields) automatically, independent of  Transmission ~1% each other  The effect on the isodose  Can generate field of almost distribution is very close any shape to that of a custom block  Typical MLC consists of 40 ◦ Close agreement as well as pairs (80 leaves) the tilt of the isodose curves ◦ Made of tungsten toward the blocked edge ◦ Each leaf 1 cm wide at the isocenter ◦ Thickness 6 to 7.5 cm Comparison of isodose distribution with half ◦ Leaf transmission ~2% the beam blocked by an independent jaw ◦ Interleaf transmission ~3% versus a block on a tray

Multileaf collimators Multileaf collimators

 Adjustable collimator systems are designed to follow the beam divergence (“focused” design)  Difficult to implement with independent leaves  Leaf edges of different design; most common – rounded edge  Single and double-focused The penumbra at any position is • Micro-MLC’s for stereotactic treatments: leaf within 1–3 mm of that obtained (in both x and y directions) width is 2 mm at the isocenter with a focused system or for alloy systems • Interleaf leakage is minimized with tongue- blocks with divergent sides and-groove design “Basic applications of multileaf collimators”, Report of TG-50, AAPM 2001

Multileaf collimators  Custom blocks ◦ Cerrobend  MLC produces larger penumbra than lung blocks used when either jaws or custom blocks (leaf edges MLC’s may not of different design) be able to cover all  MLC is ideally suited for multi-field critical treatments, and complex multi-field structures treatments (IMRT) adequately  Limitation of filed shaping: cannot produce blocked islands

2 Measurement of dose distribution in the Skin dose build-up region  Skin sparing is highly desirable feature of  Due to steep dose gradient MV beams 100 the accurate measurement is difficult  Secondary electron contamination of 80 60  Extrapolation chamber is photon beams may reduce this effect the best tool, but is not 40 Monte Carlo

 (%) PDD Extrapolation chamber readily available, and is Sources of secondary electrons: beam 20 Parallel plate chamber 0.125 cc cylindrical chamber difficult to use shaping equipment and air 0 0.0 0.5 1.0 1.5 2.0  Next best is plane-parallel  Contamination depends on several Depth (cm) plate chamber 6 MV beam, FS=10x10 factors: photon energy, field size, SSD,  TLD can be used for angle of incidence measuring skin dose due to their small thickness

Skin dose Skin dose

 The block tray (shadow tray) increases the secondary electron scatter and thus the skin dose  The amount of skin dose depends on the distance of the tray from the skin  It is best to have the  Skin dose decreases with increasing photon energy Figure 13.6.Effect of Lucites hadow tray on dose buildup for 10-MV x-rays. Percent depth dose tray as far from the  For high-energy beams significant sparing is achieved for distribution is plotted for various tray to surface distances (d). 10-MV x-rays, tray thickness =1.5 patient as possible subcutaneous layers g/cm2, field size =15 x15 cm, SSD =100 cm, and source to diaphragm distance =50 cm.

Skin dose Electron filters

 The larger is the field size, the more secondary electrons are emitted from the collimator and air  Skin sparing is  Materials with medium Z produce less forward scattered significantly reduced electrons, Z=50 (tin) is the best Figure 13.7.Percent surface dose as a function of field size. 60Co, Theratron 80, source to surface for the larger field  Electron filters can be used when skin dose becomes distance(SSD) =80 cm, source to diaphragm excessive: distance (SDD) =59 cm. 4 MV, Clinac 4, SSD =80 sizes cm. 10 MV, LMR 13, SSD =100 cm, SDD =50 cm. ◦ Large field size is large 60Co and 4-MV. ◦ The block tray distance is 15-20 cm from the skin

3 Electron filters Oblique incidence

 The angle of  Tin filter should face incidence of a beam the patient surface has an effect on skin  The thickness of filter dose and the depth should be equal to of dmax the range of  High energy photon secondary electrons beams generate (0.9 mm of tin for secondary electrons Co-60) in the air around Figure 13.10.The use of electron range them surface (ERS) to determine surface dose buildup at point P. A:Perpendicular beam incidence. B:Oblique beam incidence. C:Tangential beam incidence.

Oblique incidence Separation of adjacent fields

 Obliquity factor: a  In some cases there is a need for ration of doses at a treatments involving adjacent fields point on CAX in ◦ Hodgkin’s disease (lymphoma) phantom for oblique to ◦ Craniospinal fields in treatment of normal incidence medulloblastoma  For tangential beams ◦ Some head and neck treatment fields maximum skin dose can be estimated as  Problem: when photon fields are placed (entrance dose – in next to one another the divergence normal incidence): causes hot spots at depths and cold areas

Figure 13.11.Obliquity factor at the surface 1 near the surface plotted as a function of beam angle for various %Skin dose  100%  entrance dose energy beams. Jackson formula for tangential 2 beam incidence is based on Equation 13.1.

Separation of adjacent fields Methods of field separation

 Various techniques used for  Two basic approaches: geometric and field matching: dosimetric ◦ A: Angling the beams away from each other so that the  In geometric approach fields are joined at 50% two beams abut and are isodose line, producing 100% at the junction aligned vertically point ◦ B: Fields separated at the skin surface. The junction point is ◦ The lateral dose distribution at the junction depth at a depth where dose is can be more or less uniform, depending on the uniform across the junction interfield scatter contribution ◦ C: Isocentric split-beam  technique for head and neck In dosimetric approach the goal is to produce a tumors composite isodose distribution which is uniform ◦ D: Craniospinal irradiation at the desired depth using penumbra generators

4 Separation of adjacent fields Separation of adjacent fields  An example gap calculation

Figure 13.13. Geometry of two adjacent beams, separated by a distance S1+S2 on the = 5cm surface and junctioning at depth d. = 10cm

 Finding distances S1 and S2 from similar SSD1 for para-aortic = 95cm, SSD2 for pelvis = 98cm triangles, obtain the total filed separation: Matching fields at 8cm depth gives: d d S  S  S  1 L  1 L  5cm  10cm  1 2 2 1 2 2 gap   8cm    8cm  1.24cm SSD1 SSD2  95cm   98cm 

Separation of adjacent fields Figure 13.15.Geometric separation of fields with all  A high-dose region of the four beams intersecting at three-field overlap is midpoint. Adjacent field sizes: 30 x30 cm and 15 x15 cm; created when bigger fields source to surface distance (SSD) =100 cm; diverge into opposing anteroposterior thickness =20 smaller fields cm; 4-MV x-ray beams; each beam weighted 100 at its  The maximum overlap depth of Dmax. region is at the surface A: Field separation at surface =2.3 cm. A three-field overlap exists in this case because L1 SSD1 DS  S1  Sd  0 if  the fields have different sizes L2 SSD2 but the same SSD.  Overlap can be avoided by B: The adjacent field separation increased to 3 cm adjusting SSD’s of adjacent to eliminate three-field overlap on the surface. Figure 13.14.Two pairs of parallel opposed fields. fields C: Field separation adjusted Adjacent fields are separated on the surface so that they all join at a point on the midline.  Another approach: increase 2.7 cm to eliminate three-field A:Ideal geometry in which there is no three-fields overlap at the cord at a 15- overlap. B:Arrangement in which there are two of DS (produces cold spots) cm depth from anterior regions(shaded) of three-fields overlap.

Orthogonal field junctions Orthogonal field junctions

 An arrangement in which the central  Figure 13.16. axes of the adjacent fields are ◦ A: A general diagram orthogonal showing the separation of orthogonal fields.  A geometrical method of field ◦ B: An example of separation with orthogonal fields used for craniospinal irradiation. d ◦ C: A lateral view of B, S  1 L 2 illustrating the geometry of SSD orthogonal field separation.  d is the depth of field junction

5 Orthogonal field junctions Orthogonal field junctions Figure 13.17. Craniospinal irradiation technique. ◦ A:Patient setup showing Styrofoam blocks and Alpha Cradle mold to provide stable position for abdomen, chest, and head. ◦ B: Lateral view of fields showing cranial field rotated to align with the diverging border of the spinal field. ◦ C: Couch rotated to provide match between the spinal field and the diverging border of the cranial field. ◦ D: Elimination of cranial field  An example of orthogonal field divergence by using an independent jaw as a beam splitter. This provides junctions. The AP field (red) is an alternative to couch rotation in C. the s.clav or yoke.  The is abutted to a LT LAT and RT LAT (blue)

Guidelines for field matching Guidelines for field matching 1. The site of field matching should be chosen over 3. For deep-seated tumors, the fields may be separated on an area that does not contain tumor or a critical the skin surface so that the junction point lies at the structure midline. Care must be taken in regard to a critical structure near the junction region. 2. If the tumor is superficial at the junction site, the 4. It is not necessary anatomically to reproduce the line of fields should not be separated because a cold field matching every day because variation in its location spot on the tumor will risk recurrence. will only smear the junction point, which is desirable. For ◦ They will overlap at depth, which may be clinically the same reason some advocate moving the junction site acceptable, provided the excessive dosage delivered to two or three times during a treatment course. the underlying tissues does not exceed their tolerance. 5. A field-matching technique must be verified by actual ◦ In the case of a superficial tumor with a critical organ isodose distributions before it is adopted for general located at depth, one may abut the fields at the surface clinical use. In addition, beam alignment with the light field but eliminate beam divergence using a beam splitter or and the accuracy of isodose curves in the penumbra by tilting the beams region are essential prerequisites.

Key points Key points

 Thickness of lead required to give 5% primary beam  Dose at the surface or in the buildup region is best transmission is 4.3 half-value layer measured with an extrapolation or a plane-parallel chamber  Half-beam blocking gives rise to tilting of the isodose curves toward the blocked edge. This effect is due to  Surface dose depends on beam energy, field size, SSD, and tray to surface distance missing electron and photon scatter from the blocked part of the field into the open part of the field  Electron filters are medium-atomic-number absorbers (Z~50) that reduce the surface dose by scattering  Physical penumbra with MLC is wider than that with the contaminant electrons more than generating them collimator jaws or Cerrobend blocks  Surface dose increases with increasing angle of obliquity  Surface dose in MV beams is predominantly due to the  Separation of adjacent fields, when needed, may be electron contamination of the incident photon beam accomplished geometrically. Hot and cold spots in the resultant dose distribution must be assessed by viewing composite isodose curves

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