ESTRO 36 Abstract Book

S449 ESTRO 36 _______________________________________________________________________________________________

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.

Results Both IMPT or target tailoring by excluding the proximal uterus resulted in significant reductions of V 15Gy , V 30Gy , V 45Gy and D mean for bladder and small bowel. Compared to conventional volumes, target tailoring by excluding the non-invaded uterus resulted in an average reduction of the primary ITV and PTV of 37% and 8%, respectively. IMPT would have reduced the estimated NTCP for small bowel toxicity (≥grade 2) from 25% to 18%, and would be additionally reduced to 9% when IMPT were combined with MRI-based target tailoring. Major NTCP reductions of >10% were predicted in four patients (36%) by IMPT, and in six patients (55%) when IMPT were combined with MRI-based target tailoring. Patients benefitted most (NTCP reduction >10%) from one of the investigated approaches if the V 45Gy for bowel cavity was >275 cm 3 during standard IGART alone; a similar reduction in NTCP from the combined approached would have been obtained in patients with a V 45Gy for bowel cavity >200 cm 3 . Conclusion In patients with cervical cancer, both 1) proton therapy and 2) target tailoring by excluding the radiologically uninvolved part of the uterine corpus led to a significant dose reduction to surrounding OARs, which separately would already yield a clinically important decrease in small bowel toxicity, which is cumulative if both approaches would be combined. Reference [1] de Boer P, Bleeker MCG, Spijkerboer AM, et al. Eur J Radiol Open. 2015;2:111–7. PO-0834 Automated planning to reduce integral dose in robotic radiosurgery for benign tumors L. Rossi 1 , A. Méndez Romero 1 , M. Milder 1 , E. De Klerck 1 , S. Breedveld 1 , B. Heijmen 1 1 Erasmus Medical Center, Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective Highly conformal dose distributions and minimizing integral dose are essential in radiosurgery of benign vestibular schwannoma (VS) tumors to avoid long term side effects. This includes avoidance of secondary tumor induction in these long surviving patients. High delivery accuracy can be obtained with the robotic CyberKnife (CK, Accuray Inc, Sunnyvale, USA) due to real time image- guided tracking, allowing small PTV margins. However, optimal plan quality may be hampered by the current trial-and-error planning approach, as it strongly depends on the planner’s experience and available planning time. We have developed a system for fully automated CyberKnife treatment planning. In this study, we have used this system to automatically generate plans for vestibular schwannoma patients (AUTOplan) and we have compared them with plans that were manually generated in clinical routine (MANplan), both with the IRIS collimator. Material and Methods Both MANplans and AUTOplans were genereated with the Multiplan TPS (Accuray Inc). For AUTOplanning, a fully automatic pre-optimization was performed with our in-