ESTRO 2020 Abstract Book

S1053 ESTRO 2020

Results Figure 1 shows a typical output from Philips' geometric QA analysis algorithm, with colours inverted for clarity. Iso- contours of systemic distortion are drawn on top of MR- images of the phantom at the isocentre, ± 6 cm, ± 13 cm, and ± 20 cm.

PO-1888 Dose effects of interfraction target shifts in irradiation of prostate with two partial VMAT arcs. M. Admiraal 1 , A. De la Fuente 1 , J. Van der Himst 1 , S. Virginia 1 1 Amsterdam UMC, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Full arc VMAT treatment plans for prostate cancer treatment are known to be dosimetrically robust for setup on the target using implanted gold fiducials to account for day to day variations of the prostate position [1]. This study aims to assess the dose robustness for a VMAT treatment technique consisting of two partial arcs instead of full arcs. Material and Methods 20 prostate cancer patients were included in the study. 7 patients were treated with a simultaneous integrated boost (SIB) to the CTV of 20 x 3.1 Gy, while delivering at least 20 x 3.0 Gy to the PTV. 10 patients were treated with a homogeneous PTV dose of 20 x 3.0 Gy and 3 with a PTV dose of 19 x 3.4 Gy. Normalization was applied to the PTV to achieve D98%=95%. All radiation plans consisted of two partial VMAT arcs excluding the dorsal angles between 145° and 215° to limit the dose to the rectum. Prostate and rectum motion within the pelvic region was simulated by manually shifting the prostate and rectum contours by 1 cm in either one of 4 directions (cranial, caudal, dorsal and ventral). The isocenter of the treatment plan was shifted accordingly to mimic setup on the PTV. With this, dose re-calculation of the treatment plan was performed on the initial planning CT scan to evaluate the dosimetric effect of these 4 new geometries. Dose to the target (D98% and D1cc) and hotspots in the rectum (D1cc) were evaluated and compared with the clinical treatment plan. Results For 79 out of 80 plans, the dose coverage to the prostate was robust within the clinically tolerated accuracy of 3%. In 1 plan dose coverage to the PTV volume dropped 3,4% (patient 20, figure 1). Homogeneity was affected slightly by prostate shifts, showing a systematic increase of D1cc of about 2% in the prostate for all shifts in ventral direction. For all simulated shifts, the D1cc to the PTV stayed below the plan restriction of 107%. The high dose to the rectum (D1cc) was only slightly influenced by shifts of the prostate, showing both increases and decreases with a maximum difference of 1 Gy.

Figure 2 shows distance from the centre point to the 2mm iso-contour in all 54 QA scans, at the isocentre. The same kind of plot was made for slices at other positions. Each week is plotted in one shade of colour, and each set of measurements within a week has a different line style. The graphs were used to estimate whether a slice passed the QA, as an iso-contour in each slice is compared with an ellipse of vendor-prescribed dimensions. The 2mm contour was compared to the three central slices, the 3mm contour to the ± 13 cm slices, and the 5mm contour to the ± 20 cm slices. Only two slices, a month apart, did not pass the QA. They were deemed intermittent fluctuations, as subsequent scans passed the QA.

Conclusion The geometric QA was found to be highly reproducible over a six-week period, with only two scans, over a month apart, showing non-passable anomalies in only one slice each. Graphs as shown in Figure 2 were found to be useful for estimating whether there were significant deviations from typical scanner performance over time, and which scans required more detailed evaluation. Further verification, using the External Laser Positioning System (ELPS), which is used in positioning patients for MRCAT, is required. Geometric QA using a 3D phantom, the abdominal coil, and the sequences used for MRCAT is on the agenda.