ESTRO 36 Abstract Book

S910 ESTRO 36 _______________________________________________________________________________________________

Conclusion A compromise between the patient uncertainty positioning and the associated workload is needed. The optimization of the threshold used for couch shifts is 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 The calculation of the skin dose-of-the-day using planning CT-to-MVCT DIR is sufficiently reliable. The method was proven to be able of pointing out early superficial overdosing, to inform adaptive strategies. Preliminary results suggest that clinically relevant changes at half treatment should occur in a minority of patients, reinforcing the utility of our approach to select patients who may really benefit from adaptive replanning.

Electronic Poster: Physics track: CT Imaging for treatment preparation

EP-1672 Dual energy CT for improved proton stopping power estimation in head and neck cancer patients V. Taasti 1 , L. Muren 1 , K. Jensen 2 , J. Petersen 1 , J. Thygesen 3 , A. Tietze 4 , C. Grau 2 , D. Hansen 1 1 Aarhus University Hospital, Dept. of Medical Physics, Aarhus, Denmark 2 Aarhus University Hospital, Dept. of Oncology, Aarhus, Denmark 3 Aarhus University Hospital, Dept. of Clinical Engineering, Aarhus, Denmark 4 Aarhus University Hospital, Dept. of Neuroradiology, Aarhus, Denmark Purpose or Objective Pre-clinical and phantom studies have established that dual energy CT (DECT) improves estimation of the proton stopping power ratio (SPR) compared to single energy CT (SECT), leading to increased accuracy in treatment planning dose calculations. However, proton SPR estimation using DECT vs. SECT has only been compared in a single study of tumours in the cranial region with limited anatomical variations, with inconclusive results. We have therefore initiated a clinical imaging study of proton SPR estimation in the head and neck region comparing DECT and SECT. The aim of this study was to investigate if SPR differences between the two CT modalities were found when evaluating heterogeneous tissues of the head and neck region. Material and Methods