ESTRO 36 Abstract Book

S551 ESTRO 36 2017 _______________________________________________________________________________________________

Poster: RTT track: Image guided radiotherapy and verification protocols

This study showed that VMAT treatment plans were relatively robust during the treatment course. In this patient cohort small changes in dose to OAR were not significant, despite a reduction in PTV. PO-1008 Feasibility of stereotactic ablative radiotherapy for locally-advanced non-small cell lung cancer K. Woodford 1 , V. Panettieri 1 , T. Tran Le 1 , S. Senthi 1 1 The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia Purpose or Objective Stereotactic ablative radiotherapy (SABR) has enabled a curative treatment for elderly patients or those with significant comorbidities diagnosed with early-stage non- small cell lung carcinoma (NSCLC) who would have otherwise gone untreated. As a result population-based survival has improved. If SABR could be utilized in the treatment of locally-advanced NSCLC in the same way, the public health impact would be greater, as twice as many patients are diagnosed with advanced disease. We assessed the feasibility of SABR for locally-advanced NSCLC. Material and Methods Twenty three patients with N2 and/or N3 locally-advanced lung cancer were retrospectively replanned. Targets and organs-at-risk (OAR) were delineated using 4DCT and replanned with RapidArc delivery (AcurosXB Vn13.6). Three planning approaches were assessed; conventional approach (1.0cm ITV to PTV expansion, prescribed to 100%); SABR approach (0.5cm ITV to PTV expansion, prescribed to 80%) and a hybrid approach (0.5cm ITV to PTV expansion, prescribed to 100%). We assessed the feasibility of three dose regimes, with PTV doses all having a biologic equivalence of 60Gy in 30 fractions (α/β=10). The planning aim was to determine the least number of fractions to deliver an acceptable plan. Acceptable was defined as ≥95% target coverage by the prescribed dose whilst maintaining the OAR tolerances below. Marginally acceptable was defined as 90-95% target coverage with lung V20 <30% and other OAR tolerances met. Descriptive statistics were used. We assessed doses to the PRVs (2mm expansion) of each OAR to determine the IGRT requirements for each strategy. Results Fourteen patients had N2 involvement whilst nine had N3 involvement. Mean ITV size was 207.7cc (range 31- 706.1cc). The hybrid approach generated acceptable plans in 48% of patients (11/23), while the conventional and SABR approaches achieved 26% (6/23) and 4% (1/23) respectively. If acceptable was defined by >90% target coverage by the required dose and lung V20 was less than 30%, 70% (16/23) of patients had acceptable plans with the hybrid approach. Those that failed the hybrid approach did so due to poor PTV coverage (n=5) or unacceptable lung dose (n=2). Of the 18 patients who had an acceptable plan generated (regardless of planning approach), one was achieved with the 8-fraction regime, with the remaining needing the 12-fraction regime. OAR PRV max doses were 2-3.5% over the OAR dose for the conventional and hybrid approaches and 6% for the SABR approach, highlighting the need for IGRT. Conclusion SABR was feasible for approximately half of the locally- advanced NSCLC patients we assessed and for almost all of these cases only a 12-fraction scheme was feasible. If the alternative to SABR is no treatment at all, compromises to tumour coverage or OAR tolerances may be acceptable, increasing feasibility. This data will inform a phase I study testing the safety of SABR for locally advanced NSCLC.

PO-1009 Evaluation of setup margins using cone-beam CT for prostate and pelvic nodes irradiation A. Van Nunen 1 , T. Budiharto 1 , B. De Vocht 1 , D. Schuring 1 1 Catharina Ziekenhuis, Radiotherapie, Eindhoven, The Netherlands Purpose or Objective In 2014 radiotherapy for prostate and pelvic nodes was introduced in the Catharina hospital. For this tumour site, CBCT is used for position verification. Due to variation in prostate position in relation to lymph nodes, large setup margins are required to deliver the correct target dose to both volumes. A CTV-PTV margin of 1 cm is used for both prostate and lymph nodes. The aim of this study was to evaluate the required setup margins using different correction and registration strategies. Material and Methods CBCT-scans of 20 patients were included in this study. 220 scans were analysed retrospectively. Patients were treated with an offline SAL correction protocol with an initial action level of 10 mm and a maximum number of 3 measurements. When large day-to-day variations were observed, an online correction protocol was performed. All CBCT-scans were registered automatically using a grey value, seed or bone match algorithm of the XVI software (Elekta, Crawley, UK). For these automatic matches either a clipbox containing bony structures and the entire PTV, a mask consisting of the prostate or a mask consisting of lymph nodes CTV was used (figure 1). Registration of the lymph node area was performed to determine the correlation between bony anatomy and the position of the pelvic lymph nodes. For all these registrations all translations, rotations and table corrections were collected. From these results the random and systematic setup errors were determined. The required setup margins were then calculated using the margin recipe M = 2.5Σ+0.7σ (Σ: systematic error, σ: random error).

Results There was a large correlation between bony structures and lymph nodes in all directions (correlation coefficient > 0.82). Correlation between bony structures and the prostate position was large in lateral direction and small in longitudinal and vertical direction due to large variation in rectal filling. This resulted in larger margins in this direction. The required setup margins are summarised in Table 1. In this margin calculation, we did not account for