ESTRO 38 Abstract book

S1089 ESTRO 38

a prescribed dose of 40Gy/15 fr. CTV and PTV were defined according to national guidelines (including a 5mm crop from body external contour); no additional PTV margins were added toward the body contour in order to avoid the inclusion of skin and the consequent delivery of high skin dose, expected in the HT mode. The changes of the dose distributions during treatment were weekly assessed in 3/15 fractions by dose-of-the-day recalculation on daily MVCT using the DQA TomoTherapy software. MVCTs were rigidly registered to the planning CT by matching chest bones position, according to our clinical procedure. CTV contours were then deformable registered from planning CT to the daily anatomy on MVCT, using a constrained-intensity-based algorithm (MIM software). Finally, after CTV fine-tuning manual correction, contours were cropped 5mm from MVCT body contour, consistently with the original definition. CTV DVHs during therapy were then obtained: for each patient, mean/min values of V32, V36, V38, V40, V43.2 over the three considered fractions were extracted (corresponding to 80%,90%,95%,100% and 105% of the prescribed dose) and compared against planning values. Results On average, the minimum values of V36 and V38 during therapy were 98.3% (92.8.3-99.9 %) and 97.1% (88.9-99.8 %) respectively against 100% at planning. For 3/11 pts (27%) there was an evidence of relevant local underdosing of CTV due to anatomical changes, with minimum values of V38 ranging between 95.4% and 88.9%. For two of these pts, local dose reduction >20% were visible: looking at V36, mean/min values for these pts were: 95.4/92.8 and 96.8/93.4 against planned values equal to 100%. The worst case is shown in Figure 1 (planning CTV in red, daily MVCT CTV in purple, dose delivered at fraction 4). The corresponding DVHs for the three fractions are compared against the planning DVH in Figure 2.

values respectively of 0,171 and 0,140 for Hybrid algorithm were obtained for AD and for D 2 , while values of -0,164 and 0,041 were obtained for Biomechanical algorithm. Patient by patient analysis reveals higher correlation if volume of bladder has more limited variation like in the case of patient 3.

Conclusion Dose warping protocol was applied for dose accumulation in the bladder during radiotherapy for pelvic cancers. Particular attention was addressed to the QA of the DIR involved for dose warping. Only deformable vector fields with a sufficient value of DSC or HD were considered, but random variations of Bladder volume shows poor or absent correlation with “real” dose received in terms of AD and D 2 compared to the original value of planning; an exception is represented by patients in which the volume variation of Bladder is more similar compared to the initial value on pCT. EP-1993 Evidence of CTV underdosing due to anatomical changes during breast Helical Tomotherapy P. Mangili 1 , M. Mori 1 , A. Fodor 2 , B. Longobardi 1 , P. Signorotto 1 , M. Pasetti 2 , F. Zerbetto 2 , N. Di Muzio 2 , R. Calandrino 1 , C. Fiorino 1 1 San Raffaele Scientific Institute, Medical Physics, Milan, Italy ; 2 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy Purpose or Objective Several planning studies claimed Helical Tomotherapy (HT) as a valid technique to optimize breast cancer radiotherapy due to potential improvements in dose homogeneity within PTV and sparing of Organs at risk (OARs), especially in case of concave-shaped PTVs. However, no studies dealt with the robustness of the delivered dose. In current study we quantified the dosimetry effects of anatomical changes on the delivered dose to breast CTV during post-operative HT. Material and Methods Eleven patients (pts) previously treated with TomoDirect (TD) with post-operative radiotherapy to the whole breast were considered; HT treatment plans were optimized with the Tomotherapy planning station (Accuray Inc.) to deliver

Conclusion Although HT provides an optimal planned dose distribution, our results showed that breast HT is clinically associated to potentially dangerous underdosing of CTV during delivery, mostly due to relevant anatomical changes. Improved planning strategies need to be developed to improve delivery robustness and are actually under investigation.