ESTRO 2020 Abstract Book

S803 ESTRO 2020

Results The minimum dose requirement of the PTV (Dmean = 40.05 Gy (100%), D95% = 38.05 Gy (95%), D2% = 42.85 Gy (107%)) is not achieved by a conventional reference plan and all mixed constellations. PTV dose coverage problems mainly occur in 60:40 and primary constellations with use of 15 MV photons. Both mixing ratios are far superior to the conventional technique in PTV dose coverage, conformity and homogeneity. Combinations of 80:20, whose 3D-CRT base plan consists of only 6 MV photons (wedge constellation and VMAT energy play a secondary role), provide the best possible PTV dose coverage and homogeneity. A higher weighting of the VMAT technique using the ratio 60:40 can only slightly improve the conformity. Regarding mixed techniques, the ratio of 80:20 guaranteed better OAR protection. Anyhow, conventional 3D-CRT can assert itself in terms of OAR protection versus mixing techniques. Only in exceptional cases do combinations of 80:20 provide more optimal values (especially regarding heart and contralateral breast).

Purpose or Objective To assess which Volumetric Modulated Arc therapy modality could be suitable for postmastectomy implants CTVs adjuvant radiotherapy (aRT) according the ESTRO- ACROP guidelines in cases including the supraclavicular (SVC) and the internal mammary chain (IMC). Material and Methods Since these guidelines have been published, in our institution 25 advanced breast cancer patients (pts) treated with postmastectomy immediate breast reconstruction (IBRi) have been studied for aRT planning. Single postmastectomy right CTVs in 4 pts and left in 6 pts were included togheter to 15 advanced postmastectomy CTVs (7 right and 8 left) requiring the SVC (5) and IMCiaRT (10). Postmastectomy implants CTVs were delineated according the ESTRO-ACROP consensus guidelines with a 0.5 cm margin to the posterior -medio-lateral sides to obtain the IBRi- PTV. The implant was considered as an organ at risk as the controlateral breast. The prescribed total dose was 50 Gy , 2 Gy/25 frs to the IBR PTV and IMC PTV and 48 Gy, 1.92 Gy/25 frs to supraclavicular nodes using a SIB modality delivery. For each case, three VMAT modalities were generated: hybrid-VMAT (h-VMAT) , multi arc (m-VMAT) and dual arc-VMAT (d-VMAT) plans and then compared on the basis of DVHs for PTVs coverages in terms of D95%, hot spots ( D ≥ 108%), Conformity Index, Homogeneity index and OARs constraints The m-VMAT consisted of 3-4 small 6MV photon beams arcs of 50°-60° degree ( start from 200°-220° in right side and from 300°- 310° in left side). The d-VMAT plan provided a dual-arc combination of 6-10 MV photon beams (for left from 170° to 340°; for right from 60° to -195° ). The h-VMAT plan included two delivery steps. The first step consisted of 3- 6 sliding-window IMRT 6-10 MV photon beams delivering more than 1 half of the prescribed dose to spare the ipsilateral lung, the implant and the heart. In the second step the remaining dose was delivered using a d-VMAT (from 170°to 340°for left; from 230° to 180 for right). Results In cases of single IBRi-PTV the m-VMAT resulted an optimal solution in terms of PTV coverage showing a D95 = 98% vs 85% vs 83% for d-VMAT and h-VMAT respectively (p < 0.002 ), CI ( p < 0.003) and HI ( p < 0.002) for both right or left sided PTVs. In scenarios with nodal irradiation , the d- VMAT resulted in the best dose coverage for implant and supraclavicular area in right sided PTVs (p < 0.003). Dosimetry by h-VMAT improved also for IMC ( D95= 95% vs 85% , p < 0.002) and left sided PTVs. Moreover, with h- VMAT a significant low dose region involvement was achieved with sparing of ipsilateral lung ( V5 = 60% vs 75% , p < 0.01) and heart ( V17 = 8% vs 12% p < 0.03). Conclusion This study shows that combining different VMAT modalities each other, a customized satisfactory dosimetry to irradiate the ESTRO-ACROP defined iBRi-PTV in different scenarios are suitable in order to spare the underlying implant. PO-1493 VMAT half-body palliative irradiation in widespread complex pelvic bone metastatic cancers G. Lazzari 1 , D. Becci 2 , A. Bruno 3 , M.G. Leo 3 , A. Terlizzi 3 , D. Mola 3 , G. Silvano 1 1 Azienda Ospedaliera SS. Annunziata Presidio Osped, Radiology, Taranto, Italy ; 2 Azienda Ospedaliera SS. Annunziata Presidio Osped, Fisica Sanitaria Department, Taranto, Italy ; 3 Azienda Ospedaliera SS. Annunziata Presidio Osped, Department of Fisica Sanitaria, Taranto, Italy Purpose or Objective Fractionated half-body irradiation (HBIRT) for widespread and symptomatic bone metastatic cancers has shown a fast and effective palliative role using a schedule of 15 Gy/5Fr/5 days . The most common feature to treat consists of wide volumes involving the bone pelvis ± the

Conclusion Compared to the conventional tangential field arrangement, the mixing technique can generate more conformal and homogeneous dose distributions and reduce underdoses as well as dose maximums. The ratio 80:20 is superior to the 60:40 and conventional 3D-CRT technique in PTV dose coverage, homogeneity and OAR doses (some cases). When using the mixing concept, a ratio 80:20 and constellations with the sole use of 6 MV photons as the 3D- CRT base plan should be preferred. Base plan wedge constellation and VMAT energy are of secondary importance. PO-1492 VMAT modalities scenarios in ESTRO-ACROP consensus guidelines postmastectomy implants radiotherapy G. Lazzari 1 , M.G. Monis 1 , G. Porrazzo 1 , M. Clarizio 1 , D. Becci 2 , A. Bruno 2 , G. De Zisa 2 , D. Mola 2 , G. Silvano 1 1 Azienda Ospedaliera SS. Annunziata Presidio Osped, Radiology, Taranto, Italy ; 2 Azienda Ospedaliera SS. Annunziata Presidio Osped, Fisica Sanitaria Department, Taranto, Italy