ESTRO 2021 Abstract Book

S1126

ESTRO 2021

The highest grade of a given toxicity that occurred in a time period (3 months FU=0 to 3 months after treatment; 6 months FU=3 to 6 months after treatment). Data available: baseline=68/156 patients; 3 months FU=39/147 patients; 6 months FU=20/124 patients Conclusion Ultrahypofractionated 1.5T MR-Linac treatment of localized PCa is effective and safe (no grade ≥3 GU or GI toxicity). In the first 6 months following treatment, patients reported stable erectile function. No significant deterioration of PROs at 3- and 6-months FU was observed. PO-1375 Feasibility of neurovascular sparing MR-guided adaptive radiotherapy for prostate cancer F. Teunissen 1 , R. Wortel 2 , J. Hes 1 , T. Willigenburg 1 , J. de Boer 1 , R. Meijer 2 , H. van Melick 3 , H. Verkooijen 4 , J. van der Voort van Zyp 1 1 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands; 2 University Medical Center Utrecht, Urology, Utrecht, The Netherlands; 3 St. Antonius Hospital, Urology, Nieuwegein, Utrecht, The Netherlands; 4 University Medical Center Utrecht, Imaging and Oncology Division, Utrecht, The Netherlands Purpose or Objective Erectile dysfunction is a common adverse effect of external beam radiation therapy for localized prostate cancer (PCa), probably due to damage to surrounding neural and vascular tissue. MR-guided on-line adaptive radiotherapy (MRgRT) enables high-resolution MR imaging during dose delivery and facilitates correction for both inter- and intra-fraction movement and tissue deformations, paving the way for a neurovascular sparing approach to reduce erectile dysfunction after radiotherapy for PCa. Materials and Methods Monaco 5.40 (Elekta AB) was used to generate 5x7.25 Gy neurovascular sparing MR-Linac treatment plans for an unselected consecutive series of 10 localized PCa patients, previously treated with conventional 5x7.25 Gy MRgRT. Seven-field intensity modulated radiation therapy technique was used. The gross tumor volume (GTV) included the MR visible tumor with a 4mm isotropic margin excluding OAR and the planning target volume (PTV) included the GTV and prostate body with a 5mm isotropic margin. In addition to the organs at risk (OAR) such as rectum and bladder, dose constraints for the neurovascular bundles (NVBs), the internal pudendal arteries (IPAs), the corpora cavernosa (CCs), and the penile bulb (PB) were established and dose prescriptions for GTV and PTV were adapted (figure 1). For the treatment planning the primary goal was to achieve clinically acceptable dose coverage for both GTV and PTV, secondary sparing of OAR, and tertiary sparing of neurovascular structures. When constraints of the NVB could not be met, a dose as low as reasonably achievable was pursued. Dose to regions of interest were compared between the neurovascular sparing plans and conventional plans. Figure 1: Neurovascular sparing 5x7.25Gy MRgRT dose constraints and an example of a treatment plan delivered dose and dose distribution (patient 3)

Results Constraints for the IPAs, CCs, and PB were met in all 10 cases. Constraints for the NVBs were met in 5 cases bilaterally, in 3 cases unilaterally, and were not met in 2 cases (figure 2). In the cases where the NVB constraint was not met, the mean dose was still significantly lower compared to the conventional plans (mean Dmean 27.75 Gy vs. 34.90 Gy). Unfavorable cases for neurovascular sparing were those with a GTV in the dorsolateral position, a wider spread of the NVBs around the prostate, and IPAs running in closer proximity along the prostate. Figure 2: Relative location of GTV, CTV, NVBs and IPAs per patient to scale at level of largest GTV surface on