ESTRO 38 Abstract book

S353 ESTRO 38

may present a solution to this, and warrant referral of this subgroup of breast cancer patients for protons. Material and Methods Five patients with left-sided breast cancer, and visually confirmed pectus excavatum, were included in the study. Three treatment techniques were compared: Standard two-field tangential plans (forward planned using dynamic wedges and field-in-field techniques, plus occasional use of low-weight patch fields from other angles), VMAT with two arcs with separate isocenters, and IMPT using two or three fields (see figure 1). All treatment planning was done in the Eclipse TPS vs 13.7 (Varian Medical Systems). For all plans, the objective was to cover the PTV (whole breast plus IMN) within 95-107% of prescription dose 50Gy, with dose limits to whole heart (V40Gy<5%, V20Gy<10%), left anterior descending coronary artery (LAD, V20Gy=0%, V10Gy<5%) and lung (V20Gy<25%, mean dose <18Gy). Results Target coverage was generally better in proton plans than in photon plans – see figure 2a – and was more consistent between patients for protons than for photons. For none of the patients could the minimum target dose of 95% be achieved for the entire PTV, however for proton plans V90%>99% could be achieved for all patients, and V95% was always above 93%. For organs at risk, proton plans could achieve doses below the dose limit levels for all patients and for all considered OAR. In figure 2b dose to LAD is shown for all plans. For whole heart, the mean[range] V40Gy for the five patients was 1.2[0;5.6] for protons, 1.2[0.2;3.1] for VMAT and 9.2[0.8;16.7] tangential photon plans. The mean[range] V20Gy was 1.7[0.7;2.3], 13.6[8.4;27.4] and 16.4[4.2;28.2] for protons, VMAT and tangential plans. Mean heart dose was 1.6[0.9;3.1]Gy, 11.4[7.5;15.1]Gy and 9.2[3.6;14.4]. For LAD, V20Gy was 0 for all proton plans and V10Gy was 1.4[0;3.7]. For VMAT photon plans LAD V20Gy was 6.2[0;19.3], and V10Gy was 46.5[16.9;80.8]. For tangential photon plans V20Gy was 50.2[23.4;67.2] and V10Gy was 56.3[28.8;75.5]. For lung, the mean[range] V20Gy for the five patients was 15.6[14.3;17.7] for protons, 30.9[25.8;39.4] for VMAT and 34.7[24.3.51] for tangential photon plans. The mean[range] mean lung dose was 7.6[7;9.6]Gy, 17.2[15.6;21]Gy and 17.3[12.3;24.4]Gy.

Conclusion For patients with left sided breast cancer, and the challenging anatomy pectus excavatum, proton therapy could advantageously be used to achieve adequate target coverage without compromising doses to organs at risk, for all patients included in this study. This could not be achieved for any of the patients using photon therapy with