ESTRO 2024 - Abstract Book

S2622

Clinical - Urology

ESTRO 2024

patients treated with an empty bladder, and evaluate its impact on prostate motion. We also aim to report acute genitourinary (GU) toxicity outcomes and the impact of bladder filling and prostate motion on this.

Material/Methods:

Seventy-two patients consecutively treated on the Elekta Unity MR-Linac system with prostate ultra hypofractionation were retrospectively reviewed. Patients were simulated with an empty bladder using an institutional MR-simulation workflow, and initial baseline and daily online adaptive SBRT treatment plans were created on the Monaco Treatment planning System, which has been previously described(1). Clinical target volume (CTV) encompassed the prostate and base of the seminal vesicles. The planning target volume (PTV) was created from a 0.3cm isotropic margin on the CTV, which was treated to 40Gy over 5 fractions. Gross tumor volume (GTV) was delineated based on MR-defined biopsy proven PIRADS 4/5 lesions with a 0.2-0.3cm PTV margin and boosted to 45Gy over 5 fractions. Prior to treating with the daily adapted plan, a repeat verification scan was acquired and rigidly registered with the initial daily scan to evaluate for prostate motion during the planning phase. A virtual couch shift was performed to match the prostate contours to the new prostate position using the adapt-to-position (ATP) method. Shifts in the x, y and z direction were recorded (cm). A net shift was calculated using the Euclidean distance in the 3-dimensional space for each fraction. Bladder volume was recorded in cubic centimeters (cc) on the initial scan and on the verification scan for every fraction for every patient. Relative bladder volume changes were defined as the difference in bladder volume between the initial bladder volume and the verification bladder volume as a percentage of the initial bladder volume. Acute toxicity (less than six months post treatment initiation) was evaluated using international prostate symptom score (IPSS) questionnaires, and Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0. Correlation of the percentage bladder change with the Euclidean distance was evaluated using Pearson’s correlation. The impact of relative bladder filling and the magnitude of the net shift adopted on acute toxicity was evaluated using the Wilcoxon rank sum test. The mean relative bladder filling across all fractions for 72 patients was 42.4% (SD 30). Table 1 outlines the mean relative bladder filling for each fraction numbered 1-5. Fraction 1 demonstrated increased bladder filling compared with the remaining fractions 2-5 in the treatment course. Table 1 also outlines the average shift in the superior inferior, anterior-posterior, and left- right direction) for each fraction, as well as the average “net shift”. Seven of 59 patients (11.9%) had a moderate increase in IPSS score following radiation (defined ≥10 points above baseline). Just two of 64 patients (3%) had Grade2 or greater urinary toxicity (one with grade 2 retention and one with grade 3 incontinence. Correlations between relative bladder filling and prostate shift were mostly insignificant (p values 0.05-0.46) (Table 1). There was no association between relative bladder filling and ≥G2 acute toxicity or change in IPSS (p values 0.4, 0.3 respectively). Similarly, the net shift was not associated with either outcome (p values 0.4, 0.3 respectively). Results: