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Ch 10. A System of Dosimetric Calculations

Ch 10. A System of Dosimetric Calculations. The physics of Radiation Therapy, pp. 200 - 224. Introduction Dose Calculation Parameters Collimator Scatter Factor Phantom Scatter Factor Tissue-Phantom and Tissue-Maximum Ratios Practical Application Accelerator Calculations Irregular Fields

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Ch 10. A System of Dosimetric Calculations

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  1. Ch 10. A System of Dosimetric Calculations The physics of Radiation Therapy, pp. 200 - 224

  2. Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field

  3. Introduction • Limitation of using TARs, SAR & Percent depth dose for calculating absorbed dose in a patient • Percent depth dose is suitable for SSD treatment technique. • Tissue-air-ratios (TAR) suitable for SAD treatment technique, but limited to energies no higher than Co-60. • Increase the size of chamber build-up cap • Material of build-up cap is different from phantoms • Overcome the limitation of the TAR • Tissue-Phantom Ratio (TPR) • Tissue-Maximum Ratio (TMR)

  4. Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field

  5. Reference field 1.0 SAD ↓Sc Build-up cap Reference field → Field Size AIR Collimator Scatter Factor (Sc) • The beam output measured in air depends on the field size • Field size ↑; output ↑; collimator scatter ↑ • “Output factor” • Definition • The ratio of the output in air for a given field to that for a reference field (10 x 10 cm) • Direct measurement

  6. Phantom Scatter Factor (Sp) • The change in scatter radiation originating in the phantom reference depth as the field side is change • Definition • The ratio of the dose rate for a given field at a reference depth (e.g. depth of Dmax) to the dose rate at the same depth of the reference field size (10 x 10 cm), with the same collimator opening • Related to the change in the volume of the phantom irradiated

  7. Phantom Scatter Factor (Sp) • Indirect measuring Sp #1 • through backscatter factor (BSF) • BSF can be accurately measured for the photon beam (e.g. 60Co and 4 MV)

  8. 1.0 ↓Sc,p Reference field → Field Size Reference field SAD Reference depth PHANTOM Phantom Scatter Factor (Sp) • Indirect measuring Sp #2 • Through total scatter factor (Sp) • Contains both the collimator and phantom scatter

  9. S S d t0 × × rd rd Dd Dt0 Tissue-Phantom and Tissue-Maximum Ratios • Definition of TPR • The ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, usually 5 cm

  10. Tissue-Phantom and Tissue-Maximum Ratios • Definition of TMR • The ratio of the dose at a given point in phantom to the close at the same point at the reference depth of maximum dose • Special case of TPR • Adopted the point of central axis Dmax as a fixed reference depth

  11. Properties of TMR Like TAR, TMR is independent of SSD, increases with energy and field size. TMR 3030 1010 00 Is caused entirely by the primary beam Depth in water TMR data for 10 MV x-ray beams

  12. Tissue-Phantom and Tissue-Maximum Ratios • Relationship between TMR and effective linear attenuation coefficient (μ) • Obtain the effective linear attenuation coefficient (μ) • Plotting μ(determined from TMR data) as a function field size • Extrapolating it back to 0 × 0 field size

  13. Tissue-Phantom and Tissue-Maximum Ratios • Relationship between TMR and percent depth dose (P) • Relationship between TMR and TAR

  14. Tissue-Phantom and Tissue-Maximum Ratios

  15. Scatter-Maximum Ratio (SMR) • Definition • The ratio of the scattered dose at a given point in phantom to the effective primary dose at the same point at thereference depth of maximum dose • For Co-60 γ-rays • SMRs are approximately at the same as SARs • For higher energies • SMRs should be calculated from TMR

  16. Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field

  17. Practical Applications (accelerator calculations) • Machine are usually calibrate to deliver • 1 rad per MU • At reference depth (t0) • For a reference field size (10 × 10 cm) • Source to calibration point distance of SCD

  18. Dose / MU =K dose rate under calibration conditions field size changed  Sc (rc) Sp(r)  (SSD factor) distance (SSD) changed  PDD(d,r)/100 depth changed Practical Applications (accelerator calculations) SSD technique: MU = TD / (Dose / MU)

  19. Practical Applications (accelerator calculations ) example 1 SSD technique: Machine: 4 MV photons Calibration conditions: SSD = 100 cm, dmax = 1 cm, field size = 10  10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SSD = 100 cm, d = 10 cm, field size = 15  15 cm2, Sc(1515)=1.020, Sp(1515)=1.010, %DD=65.1, TD = 200 cGy. Dose/MU = 1  1.02  1.01  65.1 × 1 = 67.07 MU = 200  100 / 67.07 = 298

  20. Practical Applications (accelerator calculations) example 2 SSD technique: Machine: 4 MV photons Calibration conditions: SSD = 100 cm, dmax = 1 cm, field size = 10  10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SSD = 120 cm, d = 10 cm, field size = 15  15 cm2, Sc(12.512.5)=1.010, Sp(1515)=1.010, %DD=66.7, TD = 200 cGy. Dose/MU = 1  1.01  1.01  [(100+1)/(120+1)]2  66.7 = 47.4 MU = 200  100 / 47.4 = 422

  21. Dose / MU =K dose rate under calibration conditions field size changed  Sc (rc) Sp(rd)  (SAD factor) distance (SSD) changed  TMR(d,rd) depth changed Practical Applications (accelerator calculations) SAD technique: MU = ID / (Dose / MU)

  22. Practical Applications (accelerator calculation) example 3 SAD technique: Machine: 4 MV photons Calibration conditions: SCD = 100 cm, dmax = 1 cm, field size = 10  10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SAD = 100 cm, d = 8 cm, field size = 6  6 cm2, Sc(66)=0.970, Sp(66)=0.990, TMR(8, 66)=0.787, TD = 200 cGy. Dose/MU = 1  0.970  0.990  0.787 × 1 = 0.756 MU = 200 / 0.756 = 265

  23. Irregular fields • Calculation of percent depth dose for an irregular field • Obtain average SMR by using Clarkson type integration • SMR(d,rd) is then converted to TMR(d,rd) • TMR(d,rd) may be converted into percent depth dose • Final expression

  24. Computer Program • Data permanently stored in this computer program • A table of SMRs as functions of radii of circular fields • The off-axis ratios (Kp) • The following data are provided for a particular patient • Contour point • outline of the irregular field • The coordinate (x,y) of the point of calculation • Reference point • Patient measurements • Patient thickness at various points of interest • SSDs • Source to film distance

  25. As soon as a given area reaches its prescribed dose, it is shielded during subsequent treatments

  26. Asymmetric Fields • Independent jaw • Field center positioned away from the true central axis of the beam • Parameters changes after a field is collimated asymmetrically • Collimator scatter • Phantom scatter • Off-axis beam quality • Beam-flattening filters – greater beam hardening close to the central axis can be assumed equal to symmetrical field

  27. SAD technique: SSD technique: Dose / MU =K Dose / MU =K  Sc (rc) Sp(rd)  Sc (rc) Sp(r)  (SAD factor)  (SSD factor)  TMR(d,rd)  PDD(d,r)/100  OARd(x)  OARd(x) MU = ID / (Dose / MU) MU = TD / (Dose / MU) Practical Applications (asymmetric fields)

  28. Asymmetric Fields • OARd(x) • off-axis ratio at depth d • data are derived from cross-beam profiles • measured at a number of depths in a phantom • for the largest field available (e.g. 40 ×40 cm)

  29. Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field • Point under a block

  30. Irregular Fields • Clarkson’s technique is not practical for routine manual or computerized calculation • Time consuming • Considerable amount of input data

  31. ×1 ×1 ×2 ×2 ×1 ×1 Irregular Fields • Approximate method • Reasonably accurate calculations for most blocked field • Effective field • blocked field • Sc • Collimator field • unblocked field, defined by the collimator • Percent depth dose, TMR, Sp

  32. a d b Q ＋ c P Point Off-Axis • Clarkson’s technique is also not practical for manual calculation • Day’s method • PPD can be calculated at any point within the medium using the central axis data

  33. 2a 2d 2b 2b a d b Q 2a 2d ＋ c P 2c 2c Point Off-Axis • The dose at depth d along the axis through Q

  34. c c a b a c b ● Point Outside the Field

  35. c a ● b ‘t’ is the block transmission factor. Points under a block

  36. Thanks for your attention!

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