ESTRO 36 Abstract Book

S897 ESTRO 36 2017 _______________________________________________________________________________________________

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.

subjective and depends on the importance given to both factors. We showed that using a threshold <2 mm doesn’t effectively reduce the total uncertainty. We believe that a threshold of 3 or 4 mm is adequate, keeping the positioning uncertainty below 1 mm and a reasonable clinical workload. EP-1671 Calculation of the skin dose-of- the-day during Tomotherapy for head and neck cancer patients M. Branchini 1,2 , C. Fiorino 1 , M. Mori 1 , I. Dell'Oca 3 , M.G. Cattaneo 1 , L. Perna 1 , N.G. Di Muzio 3 , R. Calandrino 1 , S. Broggi 1 1 San Raffaele Scientific Institute, Medical Physics, Milan, Italy 2 IRCCS Istituto Oncologico Veneto, Medical Physics, Padova, Italy 3 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy Purpose or Objective Late fibrosis is known to depend on the severity of acute skin toxicity; an increase of skin dose during RT due to anatomy deformation may translate into an increased risk of acute toxicity, suggesting a potential benefit from planning adaptation to counteract this effect. Within this scenario, current study started to explore a previously suggested method for dose-of-the-day calculation in quantifying changes of the skin dose during Tomotherapy (HT) for head and neck (HN) cancer. Material and Methods Planning CTs of 9 HN patients treated with HT (SIB: 54/66/69 Gy/30fr or sequential boost: 54/66.6-70.2Gy in 37-39 fr) were deformable registered to MVCT images acquired at the 15 th fraction (processed with anisotropic diffusion filter) using a constrained intensity-based algorithm (MIM software). At the same day, a diagnostic kVCT was acquired with patient in treatment position (CT15) and taken as reference. The original HT plans were recalculated on both the resulting deformed images (CTdef) and CT15 using the DQA (dose quality assurance) HT module. In order to validate the method in computing the dose-of-the-day of the skin, the superficial layers (SL) of the body with thickness of 2, 3 and 5 mm (as a surrogate of the skin dose distribution: SL2,SL3,SL5) were considered in the body cranial-caudal extension corresponding to the high-dose PTV. The SL V95%, V97%, V98%, V100%, V102%, V105% and V107% of the prescribed PTV dose (i.e: likely to correlate with skin toxicity) were extracted for CT15 and CTdef and compared. In addition, trendlines’ R 2 of the graphs with Vd% of CT15 vs CTdef were computed to assess correlation between the twos. Then, as a first example of clinical application, skin dose differences between fraction 15 and planning (V95%- V107% of SL) were retrospectively analyzed for 8 patients treated with SIB. Results The differences between SL2/SL3/SL5 V95%-V107% in CT15 and CTdef were very small (<1%/1cc Figure 1). The correlation between SL DVHs parameters estimated on CT15 and CTdef was high (mean R 2 =0.91), with higher correlation for lower doses (i.e.: V95%, R 2 : 0.97, 0.98 and 0.99 for SL2, SL3 and SL5, respectively). When looking to the changes during HT, small average differences between planned vs dose-of-the-day values of SL V95%- V107% were found (< 2 cc), excepting one patient (out of 8) who showed a much more relevant difference between the planned skin dose and the delivered dose at fr 15 (V102%=7cc for SL5, Figure 2).

Conclusion A compromise between the patient uncertainty positioning and the associated workload is needed. The optimization of the threshold used for couch shifts is