ESTRO 36 Abstract Book

S257 ESTRO 36 2017 _______________________________________________________________________________________________

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 (r s ). DVH metrics per fraction (D x , absolute V x and relative V x ; x:5- 100% 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±94cm 3 vs. 217±119 cm 3 ; 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% r s 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% r s percentiles: 0.91-1.00 vs. 0.09- 0.61; however, for the whole bladder, differences were smaller between 25% and 75% r s percentiles: (0.93, 1.00) vs. (0.23, 0.71) for pelvic and prostate RT, respectively.

Conclusion The availability of a real-time 4D dose accumulation based treatment assessment tool allows to assess the quality of the delivered dose during progress of the treatment course and to take appropriate actions, as for example, plan adaptation, in cases of significant deviations. It is foreseen to extend this study for a full treatment course of a broader population of patients. OC-0489 Variation in bladder volume and associated spatial dose metrics in prostate and pelvic radiotherapy O. Casares-Magaz 1 , V. Moiseenko 2 , A. Hopper 2 , N.J. Pettersson 2 , M. Thor 3 , R. Knopp 2 , J.O. Deasy 3 , L.P. Muren 1 , J. Einck 2 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 CBCT- based 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

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.