ESTRO 36 Abstract Book

S896 ESTRO 36 2017 _______________________________________________________________________________________________

for each considered OAR (e.g. D 2 ) and TVs (e.g D 50 ). Dose constraints were also defined according to the tumor site (e.g. D mean Parotid < 30 Gy). Two levels of warning were considered: • red flag: a 10% deviation of the clinical indicator relative to the planned value (e.g. for the parotid ΔD mean (cumulated)>10% D mean (planned)) AND a violation of a dose constraint (e.g. for the parotid D mean (cumulated) >30 Gy) , V 5 orange flag: a 10% deviation of the clinical indicator relative to the dose constraint (e.g. for the parotid ΔD mean (cumulated) >3 Gy). Both adaptive software evaluated the dose to TVs using deformed PTVs. This approach is questionable because the PTV corresponds to a geometrical (not anatomical) safety margin. Therefore, we reported the dose on rigidly registered PTVs. Results Deformed contours were judged acceptable for all H&N and lung cases. However, registrations failed for most pelvic cases, for which large anatomical deformations occurred (see figure 1). Consequently, pelvic cases were excluded. •

Dose calculation of both analytical engines were in good agreement with TomoPen (around 1.5% mean difference on PTV D 50 ). Results are reported in Table 1. For TVs, only 6 flags (out of 62 patients) were reported for the rigidly registered PTV, which was considered as the only relevant volume. The flags reported for lung cases were irrelevant because of the blurring of the tumor density leading to large dose calculation deviations. For the H&N case, the red flag was rejected after analysis (wrong doses in part of the PTV out of the external contour). For the OARs, one H&N was flagged (true flag) with an increase of 11% of the mean parotid dose that exceeded the dose constraint (30 Gy).

Conclusion CBCT images for a head and neck VMAT treatment provide accurate dose calculation in adaptive radiotherapy, making them suitable for the assessment of possible changes over the original treatment planning for all the calibration curves analyzed. EP-1669 Assessment of the clinical value of off-line adaptive strategies for tomotherapy treatments D. Dumont 1 , X. Geets 2 , M. Coevoet 2 , E. Sterpin 1 1 Université catholique de Louvain, MIRO, woluwe-saint- lambert, Belgium 2 Cliniques Universitaires Saint-Luc, radiotherapy, woluwe-saint-lambert, Belgium Purpose or Objective This study assessed the clinical potential of offline adaptive strategies based on the dose computed on daily MVCTs (Tomotherapy). We defined clinical indicators that were subsequently used to identify the percentage of plans that should have been adapted due to significant dose deviations to TVs or OARs. Only the consistency of the initial plan throughout the treatment was addressed. Thus, dose was reported to constants TVs and deformed OARs. Material and Methods Cumulative doses were calculated from daily MVCT for 41 lung, 50 prostate and 21 H&N patients, using research versions of off-line adaptive solutions from Accuray and 21 st century Oncology. All deformed contours were checked by an experienced radiation oncologist, while all dose calculations were crosschecked using our in-house Monte Carlo model (TomoPen). The clinical indicators were the DVH metrics used during the treatment planning

Conclusion Considering a constant PTV, the impact of treatment adaptation on the quality of delivered plans is minor for the included patients. The conclusion might be different for pelvic cases due to the larger anatomical deformations. Conclusions might also differ for an adapted PTV, but such strategy must address clinical considerations before implementation. EP-1670 Couch shifts in NAL protocols: ¿Which is the optimal threshold? A. Camarasa 1 , V. Hernández 1 1 Hospital Universitari Sant Joan de Reus, Servei de Protecció Radiològica i Física Mèdica, Reus, Spain Purpose or Objective The NAL protocols applied to patient positioning in treatments evaluated by CBCT use a threshold regarding couch shifts. If the CBCT demands shifts over the threshold, the patient must be moved, while shifts below the threshold remain as residual errors. The aims of this