17 TREATMENT OF BREAST CANCER WITH LYMPH NODES BY VOLUMETRIC ARC THERAPY (VMAT): PRETREATMENT AND IN VIVO DOSIMETRY

Introduction: The aim of this study is to clinically implement the COMPASS 3D dosimetry and the MOSFET for pretreatment quality assurance and in vivo dosimetry of breasts cancers treatments by volumetric modulated arc therapy (VMAT). Methods: For this study, 10 patients treated with VMAT technique for breast cancer with lymph nodes were randomly selected. The treatment plans were verified using the COMPASS system with MatriXXEvolution and the collapse convolution superposition algorithm v3.1.6. This system allows on one hand independent verification of dosimetry with a 3D calculation using the patient’s scan, and on the other hand 3D verification of machines dose delivery. In both cases, histograms dose volumes and 3D gamma index were used, in order to evaluate the dose difference between the measurement/ calculation of the COMPASS and the treatment planning system calculation (Eclipse AAA v10.0.28, VARIAN®). For each patient treated with this technique, in vivo dosimetry with MOSFETs was performed during the first session. The detectors were calibrated for measurement of skin dose. Results: Regarding target volumes (lymph nodes and breast), deviations of the mean dose in the calculation of Eclipse TPS and COMPASS, respectively for the 3D independent calculation and measurement is 0.44 ± 1.83% and −0.72 ± 3.07%; and similarly for the parameters D1 (cGy) 3D independent calculation and measurement are −0.37 ± 0.68% et −0.70 ± 1.18%. Significant differences were found on D99 (cGy) parameter, 3D independent calculation: 11.32 ± 22.11%, and for the measurement: −8.69 ± 24.68%. These differences were well within the clinically tolerance level of ±5%, when analyses are done with the breast tumor volume reframed 5 mm from skin. For risk organs (heart, homolateral and contralateral lung), deviations of the mean dose in the calculation of Eclipse TPS and COMPASS for 3D independent calculation are −2.6 ± 1.92 and for measurement are −2 ± 5.6%. Average 3D gammas for target volumes and organ at risks of patients were less than 0.4. Finally, differences between the dose calculated by the TPS and measurement of MOSFETs are −0.23 ± 2.51%. These very good findings result from the fact that measurement are done at the depth of about 3 mm under the skin. Conclusion: The results of this study show that the COMPASS with MatriXXEvolution system and MOSFETs can be used for quality assurance of VMAT treatments in breast cancer.