OC-0489: Variation in bladder volume and associated spatial dose metrics in prostate and pelvic radiotherapy

S260 ESTRO 36 _______________________________________________________________________________________________ OC-0489 Variation in bladder volume and associated spatial dose metrics in prostate and pelvic radiotherapy O. Casares-Magaz1, V. Moiseenko2, A. Hopper2, N.J. Pettersson2, M. Thor3, R. Knopp2, J.O. Deasy3, L.P. Muren1, J. Einck2 1 Aarhus University Hospital - Aarhus University, Medical Physics, Aarhus, Denmark 2 University of California San Diego, Radiation Medicine and Applied Sciences, San Diego, USA 3 Memorial Sloan Kettering Cancer Center, Medical Physics, New York, USA Purpose or Objective The bladder displays considerable inter-fractional changes during a course of radiotherapy (RT) which leads to differences between delivered and planned dose/volume metrics. The aim of this study was to compare planned with actually delivered spatial bladder dose distributions for patients receiving RT for prostate cancer with a full bladder/empty rectum protocol, by using daily on-board cone-beam CT (CBCT) and to assess impact of concomitantly treating the pelvic lymph nodes. Material and Methods Twenty-five prostate cancer patients (fifteen cases receiving local prostate irradiation and ten cases also receiving pelvic node irradiation) received daily CBCTbased image-guidance RT (81 Gy in 45 fractions) adhering to full bladder and empty rectum protocol. For each patient, 8-9 CBCTs were registered to the planning CT using the clinically applied patient set-up (translations). Bladder was segmented on each CBCT and approved by a radiation oncologist. Bladder shells were extracted using a 3mm inner margin, and bladder shell quadrants were created using axial and coronal planes drawn through the center of mass of the bladder. Dose/volume histograms (DVHs) were extracted for bladder, bladder shell (BS), as well as anterior (A), posterior (P), superior (S), inferior (I), A/I, A/S, P/I, P/S sectors of the BS in each planning CT and CBCT. Differences in DVH metric between the planned and the delivered were calculated, and the association between DVH metrics and bladder volume was evaluated using the Spearman rank correlation coefficient (rs). DVH metrics per fraction (Dx, absolute Vx and relative Vx; x:5100% in 5% steps) were calculated for all bladder sectors and compared between the two groups of patients. Results Bladder volumes varied considerably during RT, with a coefficient of variation ranged between 14% to 54% across treatment. Lower bladder volumes were found for patients receiving pelvic RT compared to patients treated locally (population mean±SD: 173±94cm3 vs. 217±119 cm3; p<0.01). At the anterior and superior part of the bladder, positive associations were found between DVH metrics and bladder volume for pelvic node irradiation fractions, while negative associations were found for prostate alone fractions, 25% and 75% rs percentiles: (0.74, 0.93) and (0.78, 0.96) of S and A/S sectors for pelvic RT vs. (-0.79, 0.43) and (-0.80, -0.40) of S and A/S sectors for prostate RT across all Vx metrics (Fig. 2). Similar trend was found for the BS 25% and 75% rs percentiles: 0.91-1.00 vs. 0.090.61; however, for the whole bladder, differences were smaller between 25% and 75% rs percentiles: (0.93, 1.00) vs. (0.23, 0.71) for pelvic and prostate RT, respectively.

Conclusion CBCT-based bladder analysis exhibits significant volume changes along RT course even under full bladder daily image-guided RT protocol. Larger bladder volumes meant higher delivered doses to the superior and anterior bladder subsectors in pelvic node irradiation, but reduced overall delivered doses for prostate treatment. OC-0490 A robust and fast planning approach for adaptive MR-guided treatment of pancreatic cancer O. Bohoudi1, A. Bruynzeel1, B. Slotman1, S. Senan1, F. Lagerwaard1, M.A. Palacios1 1 VUMC, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective In May 2016, we implemented stereotactic MR-guided adaptive radiation therapy (SMART) using the MRIdian system (Viewray) for locally advanced pancreatic cancer. Interfractional changes in the anatomy of adjacent organsat-risk (OARs) make daily online plan adaptation desirable. The main challenge of online plan adaptation is the requirement that it must be performed fast while the patient remains in treatment position. We evaluated an in-house developed re-planning strategy, which is currently in clinical use. Material and Methods Before use of SMART, robust baseline IMRT plans for online re-optimization are first produced with the MRIdian planning system (ViewRay). The same planning software is available at the treatment console for plan adaption. The target structure used for optimization is defined as PTVopt (GTV+3mm minus OARs). OAR contours are then spatially partitioned in separate OAR portions located within 1, 2 and 3cm from the PTVOPT surface, thereby allowing direct control over the spatial dose distribution (Fig. 1). The optimization process relies on a model which predicts OAR dose as a function of distance from PTVOPT, and generates optimization objectives to achieve a robust baseline plan for daily adaption. For daily SMART, physicians only re-