ESTRO 2022 - Abstract Book

S1162

Abstract book

ESTRO 2022

All patients were treated with Cyberknife, that consist of a lightweight linear accelerator mounted on a robotic system, giving the ability to deliver highly non-coplanar dose distributions. It differs from conventional SBRT modalities with its real-time image-guidance system which allows for more accurate radiation delivery. Results The study population of 82 patients was between 48 and 80 years old (median 64 years) and 193 metastases were treated. The mean follow-up time was 7.47 months. The prostate-specific antigen level was before radiosurgery between 0.27 and 109 ng/ml (Average 14.45 ng/ml). The average planning target volume (PTV) was 43.94 (range: 0.19–695.34) cm3. Multiple fractions were applied in 46.6% of the lesions with a median prescription dose of 24 Gy (range: 5-15). The Gleason score was between 6 and 9 (median 7). The diagnostic imaging used were PSMA-PET-CT (95.3%), CT (86.5%) and MRI (56.9%). The group with the highest overall survival (OS) was Group B (97.92 months), followed by Group C and A (86.16 and 82.56 months respectively). The average OS resulted in 88.8 months. Also, the ADT- Free Survival measured in Group B, was of 3.95 years. There were no major adverse effects during or after radiosurgery. Conclusion For oligometastatic prostate cancer, the metastasis Directed therapy (MDT) delivered using Cyberknife- stereotactic radiosurgery is a safe and efficient option with excellent local control rates and Overall-survival. We recommend testing MDT in larger prospective, multicenter, randomized trials. L. Kerkmeijer 1 , M. Kunze-Busch 2 , R.J. Smeenk 1 , P. Van Kollenburg 1 , L. Abbenhuis 1 , T. Scheenstra 1 , L. Engels 1 , C. Van der Sande 1 , N. Verheyden-Beckers 1 , L. Spee 1 , L. Hinke 1 , E. Brunenberg 1 , E. Van der Bijl 1 1 Radboud UMC, Radiation Oncology, Nijmegen, The Netherlands; 2 Radboud UMC, Radiation Oncology , Nijmegen, The Netherlands Purpose or Objective The addition of a focal boost to the intraprostatic lesion(s) improves outcome for patients with intermediate and high risk prostate cancer. Furthermore, this can be achieved with no additional toxicity or impact on quality of life, as was demonstrated in the multicenter phase III randomized FLAME trial. The purpose of the present analysis was to evaluate the technical and clinical feasibility of a novel dose-adaptive workflow on an 1.5T MR-linac for an ultra-hypofractionated focal boost scheme. Materials and Methods Until October 2021, 20 patients with intermediate and high risk localized prostate cancer were treated at the Radboudumc on the MR-linac in 5 fractions of 7 Gy with an iso-toxic focal boost up to 50Gy within a multicenter phase II study. The focal boost dose was reduced if necessary to adhere to the constraints of the organs at risk. In a fully adaptive Adapt-To-Shape (ATS) workflow the contours of the reference MRI were propagated to the daily MRI and manually adapted when required within a ring of 1cm around the target volume. Based on these contours the reference plan was adapted using a full re- optimisation. The optimisation goals (iso-constraints) of the GTV, CTV and OARs for the daily online plan were manually adjusted to acquire an optimal balance of a GTV dose as high as possible, within the OAR constraints. A maximum of three optimisation rounds with iso-constraints adaptation was allowed to prevent prolonged intrafraction time. After completion of plan optimisation, the ATS workflow was followed by a position verification scan. When intrafraction motion was ≥ 2 mm, an additional translation (Adapt-To-Position) was performed. During beam-on, a 3D T2 weighted image was acquired. After each fraction, all contours were adapted by an experienced RTT on the beam-on scan and the delivered dose per fraction for all structures was accumulated. The actual delivered doses of all previous fractions were taken into account for adjustment of the iso-constraints in the remaining fractions when deemed necessary or the creation of a new reference plan, see Figure 1 for a visual of the workflow. PO-1371 Dose-adaptive MR-linac workflow for hypofractionated focal boost radiotherapy in prostate cancer