ESTRO 2020 Abstract book

S1104 ESTRO 2020

PO‐1885 Margin evaluation and Plan‐of‐the‐day exploration for rectal tumors E. Kneepkens 1 , K. Waizy 1 , E. Hagelaar 1 , J. Cnossen 1 , J. Theuws 1 , P. Van Haaren 1 1 Catharina Hospital Eindhoven, Radiotherapy, Eindhoven, The Netherlands Purpose or Objective Due to bladder and rectum filling variability, PTV margins required for rectal tumors locally reach up to several cm. However, the required margins are often not applied clinically, because the risk of recurrence after neoadjuvant treatment is relatively low and because a balance between OAR sparing and target coverage is desired. Following the introduction of daily CBCT for rectal cancer patients, we evaluated whether our current clinical margins could be reduced and whether a plan-of-the-day (POTD) approach would be feasible. Material and Methods CBCT scans of 10 patients treated with short-course RT (5 x 5 Gy, all fractions) and 5 patients with long-course RT (25 x 2 Gy, 13/25 fractions) were evaluated. Online matching of CBCT-images was performed on bony anatomy. Retrospectively, the coverage of the mesorectum and relevant lymph node regions by the PTV was evaluated by two observers (1 RTT, 1 medical physicist resident). The PTV margins varied in the ventral part of the upper mesorectum (UM, cranial to the base of the bladder), which was 2 cm for the PTV large and 1.5 cm for the PTV small . Dorsal and lateral margins were 0.7 cm, cranial and caudal margins were 1 cm. Additionally, for each fraction a selection was made out of 5 POTD contours with a ventral margin for the UM varying from -1 to +3 cm (Fig. 1).

PO‐1884 Matching time optimization in tangential breast RTT: CBCT and EPI comparison A. Lastrucci 1 , L. Marciello 1 , L. Fedeli 1 , E. Serventi 1 , Y. Wandael 2 , M. Carpentieri 1 , S. Segnini 1 , A. Aiazzi 1 , M.A. Spediacci 1 , M. Betti 1 , S. Marzano 1 , R. Ricci 1 1 USL Toscana Centro, Radiotherapy, Prato, Italy ; 2 A.O.U. Careggi, Radiotherapy, Florence, Italy Purpose or Objective Acceleration in breast cancer treatment might become the new standard. As fraction dose rises, the importance of correct positioning increases. Increasing interval between positioning and actual treatment reduces precision. We therefore compare the matching time in Cone-Beam Computed Tomography (CBCT) technique and Electronic Portal Image (EPI) technique. CBCT and EPI show insignificant variation in their ability to detect setup error in the tangential breast radiotherapy. Material and Methods Used Linac is Versa HD (Elekta Oncology Systems Crawley UK). EPI were acquired with iViewGT System and the matching were performed with MOSAIQ version 2.64.278; CBCT were acquired and matched with XVI Volume View System. Parameters of the CBCT protocol were: 210° of rotation (start angle 190°/100° right/left breast), collimator S20 and filter F1, 100 kV, 400 total frames acquired, Nominal Scan dose 1,0 mGy, CTDIw 4,33 mGy. The protocol was designed for breast imaging. EPI were acquired considering anterior-posterior and lateral projections, with 2 monitor units for each one. Four radiotherapy operators were enrolled in this study, each one match both CBCT and EPI; 12 patients treated for breast cancer with tangential beams have been enrolled. Matching time for each image and for each operator were recorded. Results Gantry rotation plus scan time were 110/112 s for CBCT and EPI respectively. Matching time, considering all enrolled operators, in CBCT was 71±32 s, 29-183 s (mean±SD and range) and 157±45 s, 56-248 s (mean±SD and range) for EPI. Box plot of matching time for each operator was reported in Figure 1.

Figure 1: Box plots

Conclusion CBCT images show same grey values as simulation CT images, in particular clips are always visible and of special importance in high density or voluminous breasts; moreover, CBCT is a 3D imaging technique and allow both rotation and translation in patient alignment. CBCT and EPI images were done for each enrolled patient and matching was performed from four operators. The mean CBCT matching time is around half than EPI imaging, reducing time between positioning and treatment phases and consequently, reducing also the probability of alinement mismatching. Considering the above results, in clinical practice in breast tangential radiotherapy the use of CBCT is preferably with respect to EPI images.

Results PTV small

did not cover the CTV for on average 23% resp. 16% of the fractions for the short-course patients, and the long- course patients. There was a large variability among patients, ranging 0-60% of the fractions in both groups. For 2/5 long-course patients and 6/10 short-course patients, PTV small always covered the CTV. PTV large only improved coverage for 3/115 evaluated fractions. Whenever the CTV was not fully covered, it was at the ventral part of the UM.