ESTRO 36 Abstract Book

S445 ESTRO 36 2017 _______________________________________________________________________________________________

house multicriterial optimizer to generate input parameters for automated plan generation in Multiplan, including patient-specific parameters to maximally control integral dose. Plan comparisons were made for 15 patients. Both for automatic and manual planning, the goal was to deliver a single fraction of 12 Gy, with planning priorities PTV V100% ≥ 98%, Brainstem Dmax < 12.5 Gy, while at the same time keeping the integral dose as small as possible. For un-biased plan quality comparisons, AUTOplans were generated such that the resulting CK treatment time was similar to that for the corresponding MANplan. Results AUTOplans were comparable to manual MANplans in terms of PTV coverage (AUTO: 99.4 ± 0.5 %, MAN = 99.1 ± 0.5 %, p=0.1) and treatment time (AUTO = 39.5 ± 4.7 min, MAN = 38.9 ± 5.9 min, p=0.3). On average, the brainstem D2%, D1cc and Dmean were very similar, i.e. 9.5 vs. 9.6, 8.6 vs. 8.5, and 2.0 vs. 2.2 Gy for the AUTO- and MANplans, respectively (p>0.2). Patient volumes receiving more than 1, 2, 4, and 6 Gy were highly reduced in the AUTOplans for the majority of patients, as visible in figure 1 (upper), with average reductions of 26.0% (SD= 15.4%, p < 0.001 ), 14.7% (SD=10.5%, p < 0.001), 9.8% (SD= 10.3%, p = 0.002 ), and 6.3% (SD=10.4%, p = 0.010). Conformality was also better in the AUTOplans, and spiky dose leakage away from the target was less frequent and severe, as visible in figure 2. The D2% in ring structures at 1, 2, and 3 cm distance from the PTV were 3.6, 1.9, and 1.3 Gy in AUTOplans vs. 4.7, 2.4, and 1.6 Gy in the MANplans (p< 0.001). For almost all patients, ring structures’ D2% were lowest in the AUTOplan (see figure 1, lower). Conclusion With automated Cyberknife planning, highly patient- specific parameters for optimal plan generation in Multiplan are automatically established, resulting in substantial reductions in integral dose in treatment of benign vestibular schwannoma tumors, without degrading PTV dose delivery, increasing OAR doses, or enlarging treatment time.

PO-0835 PTV margin for pelvic lymph nodes in IGRT guided prostate radiotherapy H.R. Jensen 1 , C.R. Hansen 1,2 , S.N. Agergaard 1 , E.L. Lorenzen 1,2 , L. Johnsen 1 , S. Hansen 2,3 , L. Dysager 3 , C. Brink 1,2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 University of Southern Denmark, Faculty of Health Sciences, Odense, Denmark 3 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective In recent years irradiation of the pelvic lymph nodes for high risk prostate cancer has received strong interest, as a potential way to increase locale control probability. However the prostate and the pelvic lymph nodes move independently of each other. The purpose of this study is to calculate the additional PTV margin needed for covering the pelvic lymph node region, when performing a registration and setup on the prostate with implanted gold fiducials. Material and Methods All 40 prostate patients treated at the same accelerator in 2015 were included in the study. The majority of the patients had stage T3 disease. All patients had three gold fiducials implanted into the prostate 2-3 weeks before CT simulation, which were used in the daily online IGRT. A total of 1284 cone beam CT scans were analyzed. An automatic gold seed algorithm (used as a surrogate for the prostate) and bone algorithm covering the upper pelvic and lower spine area (used as a surrogate for the lymph nodes) were performed. The deviation between the two registrations was calculated and the population based random and systematic setup error was calculated. To estimate the PTV margin needed the Van Herk margin formula was used M = 2.5 * Σ systematic + 0.7 * σ random Results The setup margin needed for the lymph node region of this patient cohort is 2.1, 6.9 and 6.6 mm for the LR, CC, AP directions, respectively (see table). This margin does not incl. any other uncertainties. The minimum deviation between prostate seed and pelvic bone match is shown as a cumulative histogram in the figure for the individual directions. More than 15.4% of the fractions have a deviation of more than 5 mm, and 5% of the fractions have a larger deviation than 7mm. The largest deviations are seen in the CC and AP direction, and a small deviation in the LR direction. The systematic and random errors are shown in the table.