ESTRO 36 Abstract Book

S909 ESTRO 36 _______________________________________________________________________________________________

were the DVH metrics used during the treatment planning for each considered OAR (e.g. D 2 , V 5 ) 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) 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. •

threshold, the patient must be moved, while shifts below the threshold remain as residual errors. The aims of this study are: (1) to determine the relation between the treatment positioning uncertainty and the corresponding workload, and (2) to obtain the optimal threshold for couch shifts in prostate treatments. Material and Methods The quadratic sum of the uncertainties associated with patient positioning is calculated. If the proposed shifts remain below the threshold, the uncertainties are related to the CBCT matching procedure and to the distribution of residual errors. If the shifts are over the threshold, the uncertainties are due to the couch movement accuracy and, again, the CBCT matching procedure. The relationship between treatment positioning uncertainty and workload was optimized using the threshold for couch shifts as an independent variable. Partial uncertainties were computed based on 811 CBCT clinical cases, together with the historical QA matching results from OBI’s equipment and measurements from Varian’s couch accuracy. The total positioning uncertainty with K = 2 was calculated for VMAT treatments delivered in 28 sessions with daily CBCT. The workload was estimated from the probability of couch shifts, which was derived from the statistics of the 811 clinical cases. Results The positioning uncertainty and the probability of couch shifts as a function of the chosen threshold are shown in Figure 1. As expected, if a high threshold is used (greater than 12 mm) the workload is minimized but uncertainty is stabilized at an excessively high value. On the contrary, if a very low threshold is used, i.e. between 0 and 2 mm, the probability of couch shifts is very high (between 97% and 100%). In this case, interestingly, the total uncertainty is not significantly reduced due the contribution of the remaining factors. Thus, the chosen threshold should be between 2 and 12 mm. To facilitate the determination of the optimal threshold, the derivations of both functions are shown in Figure 2. It can be observed that uncertainty has a maximum increase when the threshold is raised from 5 to 8 mm. However, if the same procedure is applied to the probability distribution of couch shifts, the maximum decrease takes place for a threshold between 4 and 5 mm.

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 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