ESTRO 38 Abstract book

S993 ESTRO 38

locoregional breast planning, a more complex treatment volume. Material and Methods Two approaches were compared: VMAT partial arcs (without avoidance sectors) and VMAT with avoidance sectors. The latter was set in such a way that the arc arrangement was mimicking the tangential technique, thus allowing an optimal sparing of the heart and contralateral OARs. An avoidance arc of about 40 degrees was chosen for each patient based on particular clinical objectives. The same arc trajectory and same dose objectives were applied for both. The two approaches were evaluated based on DVH for both PTVs and OARs for 15 locoregional left breast patients. Homogeneity and conformity indices for PTVs, V5 for OARs as measure for low dose and number of monitor units (MU) were also compared. Results VMAT with avoidance sectors demonstrates a significant reduction of the low dose for almost all OARs with major impact on the contralateral OARs. From the preliminary results, we found a mean volume reduction for the V5 of 18 % for the contralateral lung, 14 % for the contralateral breast, 4 % for the heart, 3 % for the body-PTV (healthy tissues) and 0.4 % for the ipsilateral lung. The trade-off was a slight increase of the V20 to the ipsilateral lung by 2 % for the plan with avoidance sectors as expected due to the more tangential-like arc arrangement. However, the two approaches were able to fulfill the same initial dose objectives. The PTV criteria were met with both techniques, the V95 for the total PTV (breast + regional lymph node) were about 95 %. The mean homogeneity index was 0.96 and 0.94 for the complete partial arcs and avoidance sectors, respectively. The mean conformity index was 0.88 and 0.87, respectively. The mean number of MU were similar between the two approaches. Conclusion The avoidance sector strategy shows significant improvement in decreasing the low-dose-bath to OARs while keeping an acceptable PTV coverage in agreement with the ICRU criteria. We encourage the use of VMAT with avoidance sectors as a solution to reduce possible late toxicities and secondary cancer induction in locoregional breast treatments. However, in the planning process, a complex anatomy and/or specific dosimetric objectives would require careful evaluation in the choice of the approach to use. EP-1832 Dosimetric comparison of IMRT for early-stage glottic cancers with and without air cavity in the PTV D. Asher 1 , W. Amestoy 1 , M. Studenski 1 , S. Samuels 1 , M. Abramowitz 1 , L. Freedman 1 , N. Elsayyad 1 , M. Samuels 1 1 University of Miami, Radiation Oncology, Miami, USA Purpose or Objective For early-stage glottic cancers, intensity-modulated radiation therapy (IMRT) has been shown to have comparable local control to 3D-conformal radiotherapy with the advantage of decreased dose to the carotid arteries. The planning target volume (PTV) for early glottic cancers typically includes the entire larynx, plus a 3-5 mm uniform margin. The air cavity within the larynx creates a challenge for the inverse optimization process as the software attempts to “build up” dose within the air. This unnecessary attempt at dose build-up in air can lead to hot spots within the rest of the PTV and surrounding soft tissue. We hypothesized that removal of the air from the PTV would decrease hot spots and allow for a more homogeneous plan while still maintaining adequate We analyzed 20 consecutive patients with early-stage glottic cancer, T1-2N0, who received IMRT at our institution from April 2015, to December, 2016. Each patient received 63-65.25 Gy in 2.25 Gy per fraction. Two coverage of the PTV. Material and Methods

We have demonstrated that the KBPs for multiple BMs was dosimetrically equivalent to the CLs with a single time of optimization, even if the number of BMs included in the model training group was different from that in the validation group. EP-1830 Dosimetric comparison of planning techniques in Radiosurgery for Arteriovenous Malformation R. Chauhan 1 , V. Mhatre 1 , K. Talapatra 1 , P. Chadha 1 , P. Shree 1 , A. Balasubramanium 2 , A. Kumar 1 1 Kokilaben Dhirubhai Ambani Hospital, Radiation Oncology, Mumbai, India ; 2 Yashoda Hospital, Neurosurgery, Hydrabad, India Purpose or Objective To investigate the dosimetric indices employing different techniques such as Static Conformal Fields(SCF), Intensity -Modulated Radiourgery (IMRS), Dynamic Conformal Arc (DCA), Volumetric Modulated Arc Therapy (VMAT) . Material and Methods Fifteen patients of Arteriovenous Malformationas were include in the study of which 2 patients were grade 1 , 7 patients were grade 2 and 6 patients were grade 3 by Spetzler Martin grading system. Thirteen out of 15 patients were unruptured and 2 were ruptured for which partial embolisation was done followed by radiosurgery . Range of target volume (0.7cc to 12.28cc Mean=7.94cc). A plan each of SCF, DCA , IMRS and VMAT were generated for each patient using MLC 2.5mm. For every patient fixed dose of 22Gy prescribed at 80%. All plans were compared using standard dosimetric indices. Dosimetric comparison includes target coverage, conformity index(CI), homogenity index(HI), gradient index(GI) . In addition maximum doses to OARs i.e Brainstem. Results Paddick CI for SCF is (0.58±0.08), DCA Paddick CI (0.580±0.09),IMRT Paddick CI (0.66±0.09) and VMAT Paddick CI (0.68±0.01), best Paddick CI observed in VMAT, P<0.05 . HI for SCF is (1.05±0.04), HI for DCA (1.046±0.04), HI for IMRT (1.044±0.03), HI for VMAT (1.043±0.03) best HI observed in VMAT . Primary goal is to achieve 99% of target coverage by 1760cGY, minimum dose to target is 17.5GY to V99%, VMAT had improved coverage but no significant difference, P>0.05. Best GI observed in DCA(3.13±0.5), GI for SCF is (3.22±0.55), GI for IMRT is (4.34±1.16), GI for SCF is (3.45±0.64), P<0.05. Dose Heterogeneity Index for SCF is (0.079±0.01), for DCA is (0.08±0.01), for IMRT (0.108±0.02), for VMAT DHI is (0.08±0.01), P<0.01. The dose received by 1 cc volume of brainstem were (5.33±5 )Gy , (5.72±5)Gy , (5.85±5)Gy and (5.08±4)Gy for SCF , DCA , IMRS . VMAT plans respectively Conclusion We have found that VMAT has shown better coverage as compared to other plans and it spares OARs , It is better in terms of 1 cc volume dose to the brainstem . This dosimetric comparison provides insight into selection of the right modality for the current treatment scenario . EP-1831 Avoidance sector strategy to reduce healthy tissue dose in locoregional breast planning with VMAT S. Thengumpallil 1 , J. Bezard 1 , K.H. Spruijt 1 , N. Peguret 1 , C. Vrieling 1 1 Clinique des Grangettes, Radiation Oncolgy, Geneve, Switzerland Purpose or Objective The benefit of the avoidance sector strategy for breast only VMAT radiotherapy planning has been proven to reduce significantly the healthy tissue dose levels (Fogliata A. Br J Radiol. 2017). Our aim is to explore whether this strategy leads to similar results in