Abstract Book

S1074

ESTRO 37

Conclusion Coverage of the CTV by the voxel-wise minimum dose is closely related to coverage of the PTV in the nominal dose evaluation and the V95 criteria used for the PTV can be applied approximately 1:1 to the voxel-wise minimum dose. EP-1975 Comparison of Field in Field and Tangential Wedged Beams techniques in radiotherapy of breast cancer M. Yarahmadi 1 , A. Haghparast 2 , B. Faramarzi 2 , Z. Saalehi 1 1 Kurdistan University of Medical Sciences, Medical physics, Sanandaj, Iran Islamic Republic of 2 Kermanshah University of Medical Sciences, Medical physics, kermanshah, Iran Islamic Republic of Purpose or Objective In this study dose distribution of the chest wall in post- mastectomy breast cancer patients were evaluated and compared in the tangential wedged beams (TWB) and field-in-field (FIF) plans. Material and Methods 36 patients with left-sided breast cancer were enrolled in this study. The FIF and TWB plans were generated for each patient to compare dosimetric parameters of the chest wall. The maximum dose (D max ), the mean dose (D mean ), the homogeneity index (HI), the conformity index (CI) and the uniformity index (UI) were defined and used for comparison of the dosimetric parameters of the planning target volume (PTV) in both the FIF and the TWB plans. D mean and the percentage of volumes receiving at least 10, 20, 30 and 40 Gy of the left lung and 5, 10, 20, 25 and 30 Gy of the heart were used to compare the dosimetric results of the organs at risk. All statistical analysis was performed using the SPSS version 20 software. Results The FIF plan had significantly lower HI (p = 0.000) than TWB plan, which means that FIF plan was better than TWB plan in the PTV. The V 10lung (25.28±5.91 vs. 27.19±6.22), V 40lung (15.36±4.35 vs. 18.37±4.42), V 10heart (11.34±4.40 vs. 14.06±4.31) and V 30heart (8.15±3.75 vs. 10.94±3.94) were significantly lower in the FIF plan than in the TWB plan. D mean heart (5.08±1.84 vs. 6.39±1.95) and D mean left lung (10.50±2.51 vs. 11.70±2.69) with (p=0.000), were significantly lower in the FIF plan than in the TWB plan. Also the Monitor Unit (MU) was significantly lower in the FIF plan than the TWB plan (227.76 vs. 323.59). The comparison of plan Dose Volume Histograms (DVH) curves for PTV and OARs of a patient for FIF and TWB plans are shown in Figure 1. Fig1. An example of Dose Volume Histograms (DVH) curves for PTV and OARs of a patient; comparison of FIF and TWB plan.

PTV planned treatments. The purpose if this work was to determine which robustness evaluation methods and which criteria are suitable for transition to a PTV-less treatment plan evaluation procedure that is generally applicable to photon treatments as well as to proton treatments. Material and Methods To obtain a clinically feasible workflow for evaluation of multiple scenario doses, including visual inspections, the relatively high number of scenario doses needed to be replaced by just a few evaluation dose distributions from which risk of under-dosage of the CTV and over-dosage of critical structures could be determined. Included were worst scenario dose, voxel-wise worst dose and voxel- wise mean dose. Determined metrics for the CTV were the V95 of these doses, as well as the average V95 of the scenario doses (). Acceptance criteria for these metrics were then determined in a calibration procedure by comparison to clinical PTV evaluations for 18 HN IMRT/VMAT treatment plans, to obtain consistency in the pass ratio with the PTV evaluations. Results The PTV V95>98% criterion corresponded 1:1 to the CTV V95>98% for the voxel-wise minimum dose, while for the average V95 of the scenario doses it corresponded to > 99.8%. For the high-dose CTV the same number of plans passed for PTV as well as for evaluation and (except for one plan) for the voxel-wise minimum evaluation. The low-dose target showed lower pass rates for and voxel-wise minimum dose compared to the PTV evaluation (Table 1). This indicates that the criteria were more strict for the low-dose CTV compared to the PTV evaluation and could be slightly relaxed to accept the same plans. Found differences, however, were caused by regions where dose was reduced by only about 1Gy, and would only need minor adjustments to bring the pass rate in agreement with the PTV evaluation. Visual inspection showed generally good agreements between PTV evaluation and voxel-wise minimum dose to the CTV (Figure 1).

Conclusion Using the FIF plan significantly reduced the dose volume of the left lung and heart in chest wall radiotherapy of post-mastectomy patients compared using the TWB plan. Therefore the FIF plan is recommended for post- mastectomy radiotherapy.