1 / 37

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

dborum
Download Presentation

Ch 10. A System of Dosimetric Calculations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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!

More Related