ESTRO 2024 - Abstract Book

S5570

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

1 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada. 2 University of Toronto, Radiation Oncology, Toronto, Canada

Purpose/Objective:

Magnetic resonance imaging (MRI) offers the ability to visualize gross disease in prostate cancer. The purpose of this study was to assess the feasibility and describe the initial experience delivering a focal boost to intraprostatic gross tumour volume (GTV) using MR-guided external beam radiation therapy on a 1.5 T MR-Linac.

Material/Methods:

Between March and October of 2023, 10 patients with prostate cancer were enrolled onto a prospective study and treated with a focal boost of 15 Gy in one fraction to their GTV(s), followed by 30 Gy in 5 fractions to the whole gland. GTV volume(s) were required to be less than 1/3 of the total prostate volume. Patients were treated entirely using the 1.5T MR-Linac (Unity, Elekta). Patients were asked to completely void their bladder prior to entering the treatment room for their GTV boost treatment and were asked to fill their bladder for their whole gland treatment. Patients were positioned supine with minimal immobilization devices. The clinical target volume (CTV) for the focal boost was equal to the GTV, and the margin expansion used to generate the planning target volume (PTV) was 3mm in the left right direction, and 4mm in all other directions from the contoured GTV volume. The PTV margin used to treat the whole gland prostate was a 5mm symmetrical expansion from the prostate. For both phases of treatment, a 9-beam intensity modulated radiation therapy (IMRT) reference plan was generated ensuring acceptable target coverage and sparing of the contoured organs at risk (OARs) including the urethra, bladder, penile bulb, rectum, and other bowel structures that were within 2cm of the PTV. The daily online adaptive procedure reflected an adapt to shape (ATS) workflow performed on the first 3D T2-weighted localization image. Additional ATS or adapt to position (ATP) procedures, along with additional verification imaging were done in scenarios where large intrafraction motion was exhibited by the patient. On the additional verification image, if large intrafraction motion was observed, the session would be aborted on the initial treatment day (day 0), and the patient treated on a subsequent day. Reference treatment plans were generated for all 10 patients, three of whom had two GTVs for the focal boost phase. One patient had a rectal spacer insertion prior to simulation. Of 10 patients, 7 received treatment on day 0 for their focal boost. For this cohort, 30% of patients were unable to receive treatment on day 0 of their focal boost phase, which was greater than the 20% who were unable to receive treatment on their first fraction of the whole gland phase. The most common reason for the inability to proceed with treatment on day 0 was due to large intrafraction motion resulting in the CTV excursion beyond the PTV even with additional ATS or ATP procedures. For the 4 instances in which an additional ATS or ATP procedure successfully corrected the prostate motion for the focal boost (2 on day 0, and 2 on the subsequent appointment), the average translational shifts observed were -0.08mm (range = -0.2mm 0mm) in the lateral direction, -0.20mm (range = -0.38mm-0.03mm) in the superior-inferior direction, and -0.18mm (range = -0.33mm-0.13mm) in the anterior-posterior direction. Of 5 patients with under-coverage of the target volumes on the daily adaptive plan to meet organ at risk dose constraints, 4 had similar deviations on their reference plans. The median step-and-shoot IMRT beam-on time was 13 minutes (range = 8-15minutes). The median total treatment time was 53 minutes (range = 42-68 minutes). Results: