ESTRO 36 Abstract Book

S822 ESTRO 36 2017 _______________________________________________________________________________________________

was used allowing to better define the achievable mean dose to organs at risk (OAR) during inverse planning step (Moore et al., Int. J. Radiation Oncology Biol. 2011). This model integrates the overlap volume between the OAR and the PTV. The aim of the present study was to evaluate application of this model adapted to our own data to anticipate the lower dose achievable to the parotid glands (PGs) and its impact in the inter-operator variability in head and neck (H&N) cancer treatment planning. Material and Methods Twenty patients treated for locally advanced H&N cancer were used to generate the predictive model (PM). Three (70/63/56 Gy) and two (60/54 Gy) dose levels VMAT simultaneously integrated boost treatment plans were generated using Pinnacle v.9.10 (Philips) Treatment Planning System. To test the PM, 10 additional cases were planned with and without the PM (8 patients with 3 levels of prescribed dose 70/63/56 Gy and 2 patients with 2 levels of prescribed dose 60/54 Gy). In a second time, 12 operators with different treatment planning experience performed a treatment planning on the same patient, with and without the PM. Doses to PTVs, PGs, spinal cord PRV, indexes of conformity (CI), homogeneity (HI) and the number of Monitor Units (MU) were compared. Results Table 1 shows the results for 10 treatment plans with and without the PM. On average, mean doses (Dmean) to PGs decreased of 5.3 Gy [-15.4 Gy; +2.6 Gy] using the model. CI and maximal dose to the spinal cord PRV were similar with both methods. However, plans obtained using the PM show less dose homogeneity into PTV (for middle dose PTV, HI increase by 18% with PM) and had more MU: +13% on average [-3.1%; +32.6%], indicating an increase of plans complexity. Figure 1 shows DVH for homolateral and controlateral PGs with and without PM for the treatment planning generated by 12 operators. With PM use, the dispersion of the data were lower, demonstrating a decrease in inter-operator variability: standard deviation for mean dose delivered to homolateral PG decrease from 2.16 Gy to 1.19 Gy and for contralateral PG from 2.89 Gy This study showed the utility of a PM to reduce the dose received by the PGs in H&N treatment planning (Dmean decreased of 5.3 Gy). The suggested model guides to the lowest achievable Dmean to the PGs at the beginning of treatment planning step. Integrating this method in the treatment planning workflow reduces significantly the inter-operator treatment planning variability and could potentially allow to a time reduction in treatment planning. EP-1548 Dose to risk organs in deep inspiration breath hold non-coplanar VMAT for lung cancer radiotherapy M. Josipovic 1 , G. Persson 1 , J. Bangsgaard 1 , L. Specht 1 , M. Aznar 1 1 The Finsen Center - Rigshospitalet, Dept. of Oncology- Section of Radiotherapy, Copenhagen, Denmar Purpose or Objective Radiotherapy (RT) for locally advanced lung cancer has a high burden on dose to risk organs (OAR), such as lung, heart and oesophagus. Different strategies have been used to decrease the dose to OAR, such as volumetric modulated arc therapy (VMAT) and deep inspiration breath hold (DIBH). In this study we investigated VMAT combined with DIBH and non-coplanar (NC) treatment delivery. Material and Methods Patients with central lung tumours were selected from a cohort treated in a DIBH RT trial. VMAT plans were made clinically in both free breathing (FB) and DIBH and consisted of two coplanar (C) partial or full arcs. For NC plans we aimed for RT delivery within 4-6 DIBHs of 20 s, as for the clinically delivered plans. Therefore an approach to 0.78 Gy. Conclusion

similar to butterfly VMAT was chosen, with two either 120º or 240º arcs at couch 0º, depending on clinical choice of partial or full arcs, and two 60º arcs at couch 90º. FB arc geometry was kept in DIBH, for both C and NC plans. Dose to OAR was compared between FB C, FB NC, DIBH C and DIBH NC plans. Results Twelve patients were included, five had central right and seven central left tumours. Total lung volume in DIBH increased by median 48% (range 20-82%) compared to FB. As expected DIBH reduced both mean lung dose (MLD) and lung V20 (median 2.2 Gy and 4.1%). NC VMAT plan decreased MLD and V20 compared to C VMAT with similar amount in both FB and DIBH (median ~0.25 Gy and ~1.5%). Figure shows impact of techniques on dose to ipsi- and contralateral lung. In right sided tumours, both MLD, lung V20 & V40 were smaller compared to left sided tumours. Mean heart dose (MHD) and heart V50 decreased with DIBH. NC VMAT had the opposite effect, since the two arcs delivered at couch 90° could often not avoid dose entrance through the heart. As anticipated, MHD, heart V50 and heart D2 (minimum dose to the hottest 2% of the heart) were largest in left sided tumours. However, in this small patients group, the observed heart dose parameters were much lower than clinically applied constraints. Still, trying to spare the heart may have resulted in larger lung doses in patients with left sided tumours in all plans (median MLD differences 0.5-2.5 Gy). Mean oesophagus dose (MED) increased in DIBH, but was not affected by NC technique in either FB or DIBH. However, MED is not a clinically used constraint. Oesophagus V66 was smallest in DIHB C, but in none of the plans it reached close to its limit of 1cm3 (national guidelines’ constraint). Dose to spinal cord was reduced with both NC and DIBH, with DIBH NC offering the best sparing. See Table for details on dose parameters and clinically used constraints.