1. Part VIII:Medical Exposures in Radiotherapy Lecture 6: Determination of dose to the patient in Radiotherapy II IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources Module 3 Optimization of Protection for Medical Exposures

2. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 2 Scope To understand the use of the factors for estimation of treatment timeTo understand the use of the factors for estimation of treatment time To familiarize with the calculation of treatment time for a single field To familiarize with the calculation of treatment time for a single field To familiarize with the calculation of treatment time for multiple beams and for rotation beamTo familiarize with the calculation of treatment time for multiple beams and for rotation beam To understand the dose prescription and reporting guidelines of ICRUTo understand the dose prescription and reporting guidelines of ICRU

3. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 3 This lecture covers……… Corrections to be applied to the calibration dose rate to estimate the dose rate for the treatment conditionCorrections to be applied to the calibration dose rate to estimate the dose rate for the treatment condition Calculation of treatment time / MU for single fieldCalculation of treatment time / MU for single field Calculation of treatment time / MU for multiple fieldsCalculation of treatment time / MU for multiple fields Use of TPR and PDD for estimation of treatment timeUse of TPR and PDD for estimation of treatment time Treatment time calculation for extended SSDTreatment time calculation for extended SSD Recommendations of ICRU for reporting doseRecommendations of ICRU for reporting dose

4. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 4 Put your Plan into action How long should the beam be ON? How to determine the Treatment time? The Basic equation is Treatment Time = Dose per Beam per fraction Dose Rate at that point

5. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 5 There are two basic quantities to be determined 1. Dose per beam per fraction 2. Dose rate at the point for that beam Where do you get these from? 1. Dose Prescription 2. Treatment plan 3. Calibration conditions of your teletherapy unit

6. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 6 Prescription Example: Tumour dose 6000 cGy in 30 fractions to be delivered to: a point, usually the centre of the tumour or the isocentre (100%) OR an isodose line covering the tumour volume e.g. 95%

7. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 7 Treatment Plan - Many ways to plan Technique :- SSD or SADTechnique :- SSD or SAD Number of BeamsNumber of Beams – single, parallel opposed, three field, etc Beam weightBeam weight –Weighted at isocentre or at D max (given dose) Beam modifiersBeam modifiers –Wedges, shielding, compensators etc.

8. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 8 Calibration - different methods / protocols Output stated at D max for a reference field size (10 x 10 cm 2 )Output stated at D max for a reference field size (10 x 10 cm 2 ) Output stated at a reference depth d ref for reference field size (10 x 10 cm 2 )Output stated at a reference depth d ref for reference field size (10 x 10 cm 2 )

9. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 9 How to get the dose per beam per fraction? First part of the question; How much dose per fraction? This is obtained from the prescription Prescription (e.g.): Tumour dose (TD) 6000 cGy in 30 fractions (N) to 1. a point, may be the centre of tumour or isocentre i.e to 100% (Recommended by ICRU) 2. an isodose line covering the tumor e.g. 95%

10. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 10 Dose per fraction Case 1- To a point at a depth Dose (f) === 200cGy Case 2 – To an isodose line covering the tumour Dose (f) = = = 210.5cGyDose (f) = = = 210.5cGy –TD is the Tumour Dose TD N 6000cGy 30 TD N x % isodose 6000 x 100 30 x 95

11. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 11 How to get the dose per beam per fraction? Second part of the question; How much dose per beam? This is obtained from the beam weights Dose (b,f) = Total Weight(W) Dose (f) x w (b)

12. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 12 Dose rate at the point for the beam Calibration condition to treatment condition 1. Correct for Field size D (d ref,S ref ) d ref Ref Field Size (S ref= 10 x 10 cm 2 )Treatment Field Size (S=15 x 15cm 2 ) d ref D (d ref, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S)

13. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 13 Calibration condition to treatment condition 2. Calibration depth to Treatment depth If calibration is at reference depth use TPR... D (d ref,S ref ) d ref Ref Field Size (S ref =10 x 10 cm 2 )Treatment Field Size (s) d D (d, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S) x TPR (d,s,Q)

14. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 14 Calibration condition to treatment condition 2. Calibration depth to Treatment depth If calibration is at D max depth use TMR…. D (d ref,S ref ) d ref Ref Field Size (S ref =10 x 10 cm 2 )Treatment Field Size (s) d D (d, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S) x TMR (d,s,Q)

15. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 15 Correction for beam modifiers 3. Correction for shielding (if applicable) D (d ref,S ref ) d ref Ref Field Size (S ref =10 x 10 cm 2 ) Treatment Field Size (S) d D (d, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S) x TPR (d,S,Q) x T f Shielding block Tray Where T f is the shielding tray factor and S is the equivalent square of the shielded field

16. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 16 Correction for beam modifiers 3. Correction for Wedge (if applicable) D (d ref,S ref ) d ref Ref Field Size (S ref =10 x 10 cm 2 ) Treatment Field Size (s) d D (d, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S) x TPR (d,S,Q) x T f x W f Shielding block Tray Wedge Where W f is the Wedge factor

17. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 17 Treatment at extended SSD Use of large field sizes require treatment at extended SSD.Use of large field sizes require treatment at extended SSD. Points to rememberPoints to remember –Output decreases - inverse square law correction to be applied –Field size corrected by similar triangle

18. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 18 Extended SSD treatment D (d ref,S ref ) d ref Ref Field Size (S ref =10 x 10 cm 2 ) Treatment Field Size (s) d D (d, s) D (d ref,S) = D (d ref, S ref ) x RDF ( S) x TPR (d,s,Q) x T f x W f x (f/f 1 ) 2 SAD=f=100cm SAD=f 1 =120cm

19. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 19 And thus the basic equation... Treatment Time = becomes Trt. Time = = Dose per Beam per fraction Dose Rate at that point D (d ref, S ref ) x RDF ( S) x TPR (d,s,Q) x S f x W f D (d,S) Dose (b,f) Dose (f) x w (b) / Total Wt

20. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 20 Treatment time calculation with Percentage depth dose Treatment Time = Treatment Time = becomes becomes Trt. Time = = Dose per Beam per fraction Dose Rate at that point D (d ref, S ref ) x RDF ( S) x PDD (d,f,s,Q) x S f x W f x Total Wt Dose (d ref,S) Dose (b,f) Dose (f) x100 x w (b) / Total Wt Remember, PPD should be corrected for Extended SSD

21. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 21 Try this example Dose per beam per fraction = 3000/15=200cGy Dose rate at 4cm = 150 x 1.024 x 1 x.965 x 1.06 =157.2 cGy/min Treatment Time = 200/157.2 = 1.29 minutes Dose per beam per fraction Dose rate at that point Calculate Treatment time to deliver 3000 cGy to 4 cm depth in15 # by single direct field 15 x 15 cm 2, no wedge but shielding included single direct field 15 x 15 cm 2, no wedge but shielding included (RDF=1.024, W f =1, S f =.965, TPR (4cm) =1.06, D (ref) =150cGy/minute) Treatment Time =

22. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 22 Try this now - one step further Calculate the treatment time to deliver 5000cGy to a tumour volume covered by a 95% isodose line by SAD technique with 60 Co beam for the plan shown here. Beam output 150cGy/min at D maxCalculate the treatment time to deliver 5000cGy to a tumour volume covered by a 95% isodose line by SAD technique with 60 Co beam for the plan shown here. Beam output 150cGy/min at D max 10cm 16cm 6 x 10 cm 2 Beam Wt. 40% 6 x 10 cm 2 30 o Wedge Beam Wt 30% 6 x 10 cm 2 30 o Wedge Beam Wt 30%

23. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 23 Points to note Technique used : SADTechnique used : SAD –To use TPR or TMR Dose calibrated at D maxDose calibrated at D max –TMR to be used for calculation Collect the data and tabulateCollect the data and tabulate

24. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 24 Treatment time table

25. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 25 How to calculate the treatment time for rotational beam? Points to Note:-Points to Note:- –Rotational or ARC techniques are isocentric –TPR (TMR) is used for calculation –For a rotational beam or arc technique, the depth of treatment varies constantly –Average TPR (TMR) is to be determined

26. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 26 Determination of average TPR(TMR) Obtain the depth for every 10 or 20 degree intervalObtain the depth for every 10 or 20 degree interval Obtain the TPR for each depthObtain the TPR for each depth Estimate the average TPR or TMREstimate the average TPR or TMR

27. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 27 And thus the basic equation for rotation therapy is... Trt. Time = = Where TPR AVG =  n TPR (d i, s,q) D (d ref, S ref ) x RDF ( S) x TPR (avg) D (d,S) Dose (b,f) i=1 Dose (f) x w (b) / Total Wt

28. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 28 Dose Prescription/Reporting Dose Reporting always toDose Reporting always to Dose at or near center of PTVDose at or near center of PTV Max/Min dose in PTVMax/Min dose in PTV Prescribing dose to a reference pointPrescribing dose to a reference point Minimum dose point Maximum dose point ICRU Reference point – generally the beam meeting point.

29. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 29 Methods of prescribing and reporting dose in EBT Dose at the periphery of the PTV e.g. isodose line or isodose surface encompassing the PTVDose at the periphery of the PTV e.g. isodose line or isodose surface encompassing the PTV Average dose in the PTVAverage dose in the PTV Dose in or at the central parts of PTV and on or close to the central axis of the beam(s) – ICRU reference pointDose in or at the central parts of PTV and on or close to the central axis of the beam(s) – ICRU reference point

30. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 30 ICRU Reference Point - significance PointPoint –Clinically relevant & representative of dose in PTV –Easy to define unambiguously –Where dose can be determined accurately –Where there is no steep dose gradient LocatedLocated –At or near centre of PTV –Near central axis of beam(s) –Sometimes not possible at centre of PTV Select meaningful pointSelect meaningful point

31. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 31 ICRU recommendation on reporting dose variation within PTV ICRU recommends that as a minimum requirement, the maximum and minimum dose to the Planning Target Volume (PTV) shall be reported together with the dose at the ICRU reference point.ICRU recommends that as a minimum requirement, the maximum and minimum dose to the Planning Target Volume (PTV) shall be reported together with the dose at the ICRU reference point.

32. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 32 ICRU Reference Point – some examples Single field Parallel Opposed PairWedge Pair

33. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 33 ICRU Reference Point – some more examples 3 Field 4 Field

34. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 34 Summary To obtain the dose rate during treatment the output has to be corrected for field size, depth, attenuators and SSDTo obtain the dose rate during treatment the output has to be corrected for field size, depth, attenuators and SSD TPR or TMR is used for calculation of treatment timeTPR or TMR is used for calculation of treatment time PDD could be used for treatment time estimationPDD could be used for treatment time estimation

35. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 35 Summary For extended SSD treatments, the output should be corrected for Inverse square lawFor extended SSD treatments, the output should be corrected for Inverse square law Dose prescription and reporting should be to ICRU reference point where tumour cell density usually is highDose prescription and reporting should be to ICRU reference point where tumour cell density usually is high

36. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 36 Try these questions What are the basic parameters required for treatment time calculation?What are the basic parameters required for treatment time calculation? What are the corrections required for output to correct from calibration condition to treatment condition?What are the corrections required for output to correct from calibration condition to treatment condition? Why TMR/TPR is preferred for calculation of rotational treatments/Why TMR/TPR is preferred for calculation of rotational treatments/ What are the significances of ICRU reference point?What are the significances of ICRU reference point?

37. Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II Slide 37 Where to find more information? The Physics of RadiologyThe Physics of Radiology –H.E Johns & J R Cunningham The Physics of Radio TherapyThe Physics of Radio Therapy –Faiz M Khan The Modern Technology of Radiation Oncology, edited by J. Van DykThe Modern Technology of Radiation Oncology, edited by J. Van Dyk International Commission on Radiation Units and Measurements Report No. 50 and 62International Commission on Radiation Units and Measurements Report No. 50 and 62