ESTRO meets Asia 2024 - Abstract Book

S286

Interdisciplinary – Urology

ESTRO meets Asia 2024

3. Lawton CA, Glisch C, Byhardt RW, Sehring S, Hartz A, Cox JD. Extended-field radiation therapy for prostatic carcinoma with para-aortic lymph node metastasis. Am J Clin Oncol. 1986; 9[4]:302-6.

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148

Proffered Paper

MRI-guided radiotherapy for focal boosting of T3b prostate cancer

Uulke A. van der Heide 1 , Matthijs G. Dassen 1 , Anja Betgen 1 , Peter de Ruiter 1 , Joeke van der Linden 1 , Lisa Wiersema 1 , Ben Neijndorff 1 , Floris J. Pos 1 , Tomas Janssen 1 , Leontien Abbenhuis 2 , Peter van Kollenburg 2 , Casper Reijnen 2 , Robert Jan Smeenk 2 , Ellen Brunenberg 2 1 Radiation Oncology, the Netherlands Cancer Institute, Amsterdam, Netherlands. 2 Radiation Oncology, Radboud University Medical Center, Nijmegen, Netherlands

Purpose/Objective:

Focal boosting of prostate cancer, escalating the radiation dose to the GTV inside the prostate, benefits biochemical failure-free survival in patients with high-risk disease. This was also observed in patients with T3b stage disease, where the tumor invades into the seminal vesicles. In combination with ultra-hypofractionation, focal boosting is however challenging for T3b patients due to the proximity of rectum and bladder as well as the day-to-day and intra-fraction mobility of the seminal vesicles. Therefore, T3b patients are generally excluded from ultra-hypofractionated treatments with focal boosting. Here we investigate the potential for focal boosting of T3b tumors using MRI-guided on-line adaptive radiotherapy on a Unity MR-linac (Elekta AB) and determine the focal boost dose that can be achieved without violating organ-at-risk (OAR) constraints in T3b tumors as compared to tumors that are confined to the prostate gland (T2-T3a).

Material/Methods:

Data were used from 23 patients with T2-T3a prostate cancer, who received focal boosting radiotherapy in 5 fractions on the Unity MR-linac within a prospective clinical trial. For the purpose of the current study, the clinical GTV delineations were replaced by artificial GTVs, representative of tumors with seminal vesicle invasion, delineated by a radiation oncologist. Subsequently, the CTV was defined as the entire prostate including the seminal vesicles plus a margin of 3 mm around the GTV, excluding OARs. A PTV margin of 5 mm was applied. For each MR scan a treatment plan was generated by an automated treatment planning application, simulating an online adaptive MRI-guided workflow. Patients were planned to receive at least 35 Gy to the CTV, with an isotoxic focal boost to the GTV up to 50 Gy. Plans were generated for the artificial GTVs with seminal vesicle invasion and compared to the clinical plans for the real GTVs without seminal vesicle invasion. Dose distributions were optimized each fraction based on adapted contours (D planned,i ). Towards the end of a treatment fraction an MRI scan was acquired to assess intra-fraction motion (MRI during ). The planned dose was projected on that