The impact of dose prescription on treatment volume Summer Chaudhari 1 st year medical physics resident University of Minnesota Medical Center Advisor: Dr. Patrick Higgins NCCAAPM chapter meeting April 24, 29
Purpose of Study Previously published literature has stressed the importance of: prescribing recording reporting radiation therapy treatment doses and their treatment volumes clearly and unambiguously so that they can be easily tracked worldwide. One of the recommendations of ICRU 5 and its supplement ICRU 62 is to prescribe to a point inside the planning target volume (ICRU reference point) instead of prescribing to the periphery of the PTV (an isodose line). Our study evaluated the effects on the dose distribution of prescribing to the ICRU reference point versus prescribing to an isodose line.
Introduction: Dose Specification ICRU 5 states: The most significant difference found in the literature appears to be the location where one prescribes and delivers the dose and how one reports the full treatment.
Introduction: ICRU reference point The ICRU Reference Point is selected according to the following criteria: Must be clinically relevant and representative of dose throughout the PTV Should be easy to define in a clear and unambiguous way The point should be chosen where the dose can be accurately determined The point should not lie in a region where there is a steep dose gradient These recommendations are fulfilled if the ICRU reference point is: Always at the center (central part) of the PTV When possible, at the intersection of the beam axis
Methods Pinnacle treatment planning system Cases: Phantom: Drew 2 cm and 8 cm spherical diameter targets on 3 cm 3 phantom 4-field box Clinical: 4-field prostate 1-field stereotactic body radiotherapy (SBRT) treatment plans Different combinations of: Block margin (,.5 and 1. cm) Prescriptions (ICRU or scaled) 2 mm dose calculation grid
Phantom Plans
Methods Prescription Criteria: Prescription 1 - ICRU: 1% dose to isocenter Prescription 2 - Scaled: Pick an isodose line so that 95% volume gets 1% dose Our clinic tries to meet the following treatment planning goals (using ICRU): 95% target volume gets 95% dose
Methods Evaluations: The effect of different block margins (,.5 and 1 cm) on doses and volumes The effect of different dose prescription (ICRU vs scaled) on normal tissue volumes Plots: Calculated each plan and plotted dose vs. volume for comparison
Overview Plots: DVH plots for phantom and clinical plans Comparison of 2 different prescription methods Prescription 1 - ICRU: 1% dose to point Prescription 2 - Scaled: 95% volume gets 1% dose Different scale factors were applied to these plans in order to meet the prescription criteria Our clinic goals: 95% planning target volume gets 95% dose
Baseline case: 8 cm sphere 8 cm PTV only Isodose Volume vs Dose 3 Volume (cm3) 25 2 15 1 cm Block ICRU cm Block 89% Rx line.5 cm Block ICRU.5 cm Block 96% Rx line 1 cm Block ICRU 5 1 cm Block 97.5% Rx line 95% dose 5 6 7 8 9 1 11 12 Isodose line (%)
Baseline case: 8 cm sphere 8 cm Case Isodose Volume vs Dose cm Block ICRU cm Block 89% Rx line.5 cm Block ICRU Volume (cm3) 14 12 1 8 6 4 2.5 cm Block 96% Rx line 1 cm Block ICRU 1 cm Block 97.5% Rx line 4 5 6 7 8 9 1 11 12 Isodose line (%)
Baseline Case: 2 cm sphere 2 cm PTV only 3.5 3 Volume (cm3) 2.5 2 1.5 1 cm Block ICRU cm Block 76% Rx line.5 cm Block ICRU.5 cm Block 96.7% Rx line 1 cm Block ICRU.5 1 cm Block 99% Rx line 95% dose 4 5 6 7 8 9 1 11 12 13 14 Isodose line (%)
Example 1: Prostate Prostate Case - PTV only 3 25 Volume (cm3) 2 15 1 5 cm Block ICRU cm Block 68% Rx line.5 cm Block ICRU.5 cm Block 9% Rx line 1 cm Block ICRU 1 cm Block 96.5% Rx line 95% dose 4 6 8 1 12 14 16 Isodose line (%)
Example 2: SBRT small sized target SBRT Target PTV only Isodose Volume vs Dose Case 1 Volume (cm**3) 5 45 4 35 3 25 2 15 1 5 cm block ICRU cm block 71.8% Rx line.5 cm block ICRU.5 cm block 81.5% Rx line 1 cm block ICRU 1 cm block 9% Rx line 95% dose 4 6 8 1 12 14 16 Percent Isodose
Example 3: SBRT Large sized target SBRT Target PTV only Isodose Volume vs Dose Case 11 Volume (cm**3) 2 18 16 14 12 cm block ICRU 1 cm block 82% Rx line 8.5 cm block ICRU 6.5 cm block 94% Rx line 4 1 cm block ICRU 1 cm block 98% Rx line 2 95% dose 4 5 6 7 8 9 1 11 12 13 Percent Isodose
Results average 11 patients Average 11 cases Volume (cm**3) 2 18 16 14 12 1 8 6 4 2 cm block ICRU cm block 88.5% Rx line.5 cm block ICRU.5 cm block 95.2% Rx line 1 cm block ICRU 1 cm block 97% Rx line 95% dose 4 5 6 7 8 9 1 11 12 13 Percent Isodose
Conclusions Advantages of the ICRU definition of prescription: Can meet the dose prescription criteria using adequate block margins Can reduce the hot spot doses by as much as 3% as compared to the isodose prescription Uniformity in dose reporting can be achieved
Summary If the ICRU reference point is used as the dose prescription point, the dose to the healthy tissue will be lower as compared to prescribing to an isodose line. Prescribing to the ICRU reference point will make it more feasible to record the dose in a way that follows the ICRU recommendations on dose prescription, recording and reporting.
Thank you! Questions?
Example 2: SBRTsmall sized target SBRT Targe t Case 1 Isodose Volume vs Dose cm block ICRU Volume (cm**3) 6 5 4 3 2 1 cm block 71.8% Rx line.5 cm block ICRU.5 cm block 81.5% Rx line 1 cm block ICRU 1 cm block 9% Rx line 4 6 8 1 12 14 Percent Isodose
Example 1: Prostate Prostate Case Volume (cm3) 4 35 3 25 2 15 1 5 cm Block ICRU cm Block 68% Rx line.5 cm Block ICRU.5 cm Block 9% Rx line 1 cm Block ICRU 1 cm Block 96.5% Rx line 2 4 6 8 1 12 14 16 Isodose line (%)
Example 3: SBRT Large sized target SBRT Target Case 11 Isodose Volume vs Dose Volume (cm**3) 3 25 2 15 1 5 cm block ICRU cm block 82% Rx line.5 cm block ICRU.5 cm block 94% Rx line 1 cm block ICRU 1 cm block 98% Rx line 2 4 6 8 1 12 14 Percent Isodose
Example 3: SBRT Large sized target SBRT Target Case 11 Isodose Volume vs Dose 6 5 Volume (cm**3) 4 3 2 1 cm block ICRU cm block 82% Rx line.5 cm block ICRU.5 cm block 94% Rx line 1 cm blo ck ICRU 1 cm block 98% Rx line 2 4 6 8 1 12 14 Percent Isodose
Baseline case: 8 cm sphere 8 cm Case Isodose Volume vs Dose cm Block ICRU cm Block 89% Rx line Volume (cm3) 35 3 25 2 15 1.5 cm Block ICRU.5 cm Block 96% Rx line 1 cm Block ICRU 1 cm Block 97.5% Rx line 5 4 5 6 7 8 9 1 11 12 Isodose line (%)
Baseline case: 2 cm sphere 2 cm Case Isodose Volume vs Dose cm Block ICRU Volume (cm3) 14 12 1 8 6 4 2 2 4 6 8 1 12 14 Isodose line (%) cm Block 76% Rx line.5 cm Block ICRU.5 cm Block 96.7% Rx line 1 cm Block ICRU 1 cm Block 99% Rx line
Baseline case: 2 cm sphere Volume (cm3) 1 9 8 7 6 5 4 3 2 1 2 cm Case Isodose Volume vs Dose 2 4 6 8 1 12 14 Isodose line (%) cm Block ICRU cm Block 76% Rx line.5 cm Block ICRU.5 cm Block 96.7% Rx line 1 cm Block ICRU 1 cm Block 99% Rx line