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# 17 chap 13 treatment planning iii

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### 17 chap 13 treatment planning iii

1. 1. Chapter 13 Treatment Planning IIIField shaping, Skin dose, and Field separation 1
2. 2. Why do we need field shaping?To minimize the dose to normal tissues outsidethe targetMethods: Blocks, Independent Jaws, and MLCs 2
3. 3. 13.1 Field Blocks - Block thicknessThe number of HVLs (n) neededto achieve ‘t’ transmission is: n 1 t 2For example, to achieve 5%transmission, the number of HVLsneeded is: n 1 1 0.05 2 20 or 2 n 20 or n log 20 / log 2 4.32 For Megavoltage photon beams, typically 7.5 cm of cerrobend is needed to achieve transmission < 5%. 3
4. 4. 13.1 Field Blocks - Block thicknessA primary beam transmission of 5% through the block isconsidered acceptable.Block thickness calculation (example: lead, = 11.36 g/cm3):n: number of half-value layers needed1/2n = 0.05n = log 20/ log 2 = 4.32 (between 4.5 to 5 HVL)Table 13.1: about 6.5 cm for 6MV (HVL ~ 1.4 cm)Most commonly use: Cerrobend ( = 9.4 g/cm3)Thickness ~ Thickness of lead (Table 13.1)*1.21In MV range, ~ 7.5 cm in thickness commonly used. 4
5. 5. 13.1 Field Blocks - Block divergenceTo minimize the penumbra at the blockedge, the block is shaped or taperedfollowing the geometric divergence of thebeam. 5
6. 6. Source-to-block-tray distance Source-to-film distance 13.2 Field Shaping – block cutter6
7. 7. 13.2 Field Shaping – independent jawsUsed for matching fields, beam splitting(without changing the isocenter)Benefit: no divergenceCaution: MU calculation should be basedon points in the open portion of the field,taking into account the beam flatness atthe point of reference. Machine central axis Effective beam central axis 7
8. 8. 13.2 Field Shaping – multileaf collimator (MLC) Used to replace Cerrobend blocks, (a) Avoids collision between the blocking tray & the couch (b) Avoids block exceeding the weight limitation of the tray (c) Delivers intensity modulated beam 8
9. 9. 13.2 Field Shaping – multileaf collimator (MLC) Benefits: (a) Easy control and modification (b) Reduce mistakes (record & verify) (c) Eliminates the need for mold room activity (d) Avoids the need for block storage (e) Avoids entry into the room, lifting difficulties (f) Time saving for a treatment with large number of fields Limitations: (a) Limitation of leaf span (Varian: ~14.5 cm) (b) Mantle field; kidney blocks (island) (c) Scalloping effect on field edge 9
10. 10. 13.2 Field Shaping – multileaf collimator (MLC) MLC block Scalloping effect 10
11. 11. 13.3 Skin Dose Percent depth dose 15 MV orthovoltage 6 MV depth in water Skin sparing is an important feature of the Megavoltage beam. 11
12. 12. 13.3 Skin Dose – electron contaminationSkin dose is the result of e- contamination of the incident beam +back scattered radiation from the medium.Contaminated electrons arise from photon interactions in the aircolumn between the machine head and patient surface; in themachine head (flattening filter, collimators); and any objects in thebeam (wedge, blocking tray). Megavoltage beams produce an initial electronic buildup with depth → reduced dose at the surface Higher energy → the effect of skin sparing more pronounced 12
13. 13. 13.3 Skin Dose – Measurement of Dose Distribution in Build-Up region The size of the dosimeter along the beam direction should be as small as possible. Ion Chambers: Extrapolation chambers (electrode spacing ~ μm ) Parallel-Plate Chambers with adequate guard ring TLDs: Thin layers (<0.5 mm) of TLDs (thermoluminescent dosimeter) 13
14. 14. 13.3 Skin Dose – skin sparing as a function of photon energy Higher photon energy, more skin sparing Depth (mm) Co-60 4 MV 10 MV 25 MV SSD=80 cm 80 cm 100 cm 100 cm 0 18.0 14.0 12.0 17.0 ? 1 70.5 57.0 30.0 28.0 2 90.0 74.0 46.0 39.5 3 98.0 84.0 55.0 47.0 4 100.0 90.0 63.0 54.5 5 100.0 94.0 72.0 60.5 6 - 96.5 76.0 66.0 8 - 99.5 84.0 73.0 10 - 100.0 91.0 79.0 15 - - 97.0 88.5 20 - - 98.0 95.0 25 - - 100.0 99.0 30 - - - 100.0Table 13.2 build-up dose distribution in polystyrene for a 10x10 cm field 14
15. 15. 13.3 Skin Dose – Effect of absorber-skin distance Fig 13.6 10 MV photons, d 15x15 cm field sizeAs the absorber (e.g. blocking tray) moves away from the skin, theelectrons generated in the absorber are more likely to scatter laterally outof the beam, thus reducing the dose to the skin, increasing skin sparing. 15
16. 16. 13.3 Skin Dose – Effect of field size Fig. 13.7 Depth dose curve large field size Small field sizeIncreased field size Increased contaminated electrons decreased skin sparing 16
17. 17. 13.3 Skin Dose – electron filter (?) Electron filter Tray only Depth dose curve Shadow tray only Metal filter With metal filter Open beam 17
18. 18. 13.3 Skin Dose – electron sparing at oblique incidence Incident electron dmax dmax Normal oblique incidence incidence Electron energy deposition electron track length.In a given increased Increasedslab, decreased electron surface dose,increased skin sparing tracklength decreased dmaxobliquity 18
19. 19. 13.3 Skin Dose – summarySkin dose increases when :(1) Field size increases (increased electron emission from the collimator and air)(Fig. 13.6, Khan)(2) SSD decreases, especially in larger field sizes(3) An acrylic tray placed in the beam (beam spoiler to eliminate skin sparing)(4) Blocked field vs. Open field: skin dose increases(5) Blocked field vs. MLC field: skin dose increases(6) Angle of incidence increases 19
20. 20. 13.4 separation of Adjacent Fields separation d Adjacent ‘angled’ fields to cover a Adjacent fields (with a skin separation) large treatment area to deliver uniform dose at depth d 20
21. 21. 13.4 separation of Adjacent Fields Orthogonal split beams to avoid beam divergence at the match line. 21
22. 22. 13.4 separation of Adjacent Fields ‘Penumbra spoiler’ to blur the penumbra to prevent spinal cord injury that may arise from setup uncertainty 22
23. 23. 13.4 separation of Adjacent Fields – methods of field separation SSD1 cold region SSD2 S1 S2 L1 L2 d Uniform dose S1 L1 1 S2 L2 1 Hot region d 2 SSD1 d 2 SSD2 Field separation L1 d L2 d S S1 S 2 on the skin: 2 SSD1 2 SSD2 23
24. 24. L1 L2 SSD1 SSD2SSD1 Ideal match S S L1 d L2 SSD2 Uniform dose, no hot/cold regions 24
25. 25. Three-field overlap S=S1-S2 If the gap S1+S2 is increased by S, there will be a cold spot at the midline d S1 S2 d’ d midline cord S’More conveniently,use the same L andSSD for all 4 fields, To avoid hot spot at S d d(ideal match) but the spinal cord, onetruncate the 2nd pair can increase the gap S dwith independent jaws S1+S2 by S’. 25
26. 26. Example: A patient is treated with parallel-opposed mantle andparaortic fields of lengths 30 and 15 cm, respectively. Calculate (a)the gap required on the surface for the beams to intersect at a midlinedepth of 10 cm and (b) the gap required to just eliminate the three-field overlap on the cord assumed to be at a depth 15 cm from theanterior surface, given SSD = 100 cm for all fields.(a) Think of two beams incident from the same side:S1 = L1/2 * d/SSD1 = 30/2 * 10/100 = 1.5 cmS2 = L2/2 * d/SSD2 = 15/2 * 10/100 = 0.75 cmTotal gap = S1 + S2 = 1.5 +0.75 = 2.3 cm(b) Think of three-field overlap:ΔS = S1 - S2 = 0.75 cmΔS’ = ΔS * (d’-d)/ d = 0.75 * (15-10) /10 = 0.4 cmNew gap required = S1 + S2 + ΔS’ = 2.3 + 0.4 = 2.7 cm 26
27. 27. S1+S2 S1+S2+ S Uniform dose Three-field Cold spot overlap S1+S2+ S’ Relatively uniform dose Spinal cord sparing 27
28. 28. 13.4 separation of Adjacent Fields – craniospinal fields dTechnique A: L d s S 2 SSD L Divergent fields 28
29. 29. 13.4 separation of Adjacent Fields – craniospinal fieldsTechnique B: SSD L1 More convenient to use independent jaw to eliminate beam divergence: 1. No couch rotation. θcoll 2. Moving junction to smear out 1 L1 2 hot/cold spot at junction coll tan SSD SAD L2 1 L2 2 couch tan SAD 29 θcouch
30. 30. 13.4 separation of Adjacent Fields – guidelines for field matching1. The site of matching should be over an area that does not contain tumor or critical organs.2. If the tumor is superficial at the junction, field separation is not needed provided the hot region below does not exceed the tolerance of normal tissues or critical organs.3. Beam splitter or beam tilting can be used to eliminate beam divergence.4. For deep-seated tumors, the fields are separated at the skin so that the junction point lies at the appropriate depth.5. Move the junction few times during the course of treatment is desirable to smear out the dose distribution at junction.6. A field-matching technique must be verified by dose distribution. Light field and dose in penumbra region must be accurate. 30