3277 - Interfractional Dosimetric Variations During MR-guided Radiotherapy as Predictors of Genitourinary Toxicity in Prostate Cancer

Purpose/Objective(s):

To assess the association of dosimetric parameters with physician and patient-reported genitourinary (GU) toxicity for prostate cancer patients undergoing MR-guided Radiotherapy (MRgRT).

Materials/Methods:

This retrospective study included patients (=18 years) with biopsy-proven prostate adenocarcinoma and no prior prostate radiation or prostatectomy who received curative intent MRgRT at a single institution. Interfractional dosimetric changes were assessed using the difference in PTV receiving 95% of prescribed dose between the 5^th and 1^st treatment fractions (? V95PTV 5-1 fx).

Results:

A total of 89 patients met inclusion criteria and were treated between 2021-2025 The mean age of the cohort was 69.9 ± 5.5 years, and mean baseline prostate size was 43.7 ± 19.5 cc. Overall, with respect to GU toxicity, 20% (n=18) had acute grade 2, 4% (n=4) had late grade 2, and 2% (n=2) had late grade 3 events. With respect to gastrointestinal (GI) toxicity, 9% (n=8) had acute grade 1 or 2 events, 1% (n=1) had acute grade 3 events, and 1% (n=1) had late grade 3 events. Higher GU toxicity grade was associated with GI toxicity (p =0.008). With regards to factors associated with GU toxicity, a significant difference was observed in the ? V95PTV 5-1 fx across GU toxicity grades (p=0.002). Grade 2 toxicity was associated with a greater mean volume change from their last to first fraction vs. those with no toxicity (Grade 0) (6.03 ± 10.65 cm³ vs. 1.35 ± 6.23 cm³, p < .001). ? V95PTV 5-1 fx was identified as a significant independent risk factor of GU toxicity grade (p < .001) when controlling for the use of ADT, prior GU condition, Gleason score, and Dmax of the bladder. Each 1 cm³ increase in V95PTV at the 5th fraction relative to the 1st fraction was associated with a 20% increase in the odds of experiencing a higher grade of GU toxicity (OR 1.20, 95% CI: 1.08–1.33).

Conclusion:

Increase of ? V95PTV 5-1 fx was associated with a 20% increase in the risk of higher GU toxicity grade, demonstrating the importance of monitoring interfractional volumetric changes throughout adaptive MRgRT treatment.

Table 1. Predictors of GU toxicity grade

*Indicates a significance of a = 0.05

Variable	Coefficient (ß)	Std. Error	Wald	Odds Ratio (eß)	95% CI for OR	p-value
? V95PTV 5th fx – 1st fx*	0.184	0.053	11.962	1.2	[1.08, 1.33]	<0.001 *
? Dmax bladder 5th fx – 1st fx	0.003	0.003	1.076	1	[0.99, 1.01]	0.3
Prior GU condition	0.661	0.522	1.605	1.94	[0.70, 5.39]	0.205
Use of ADT	-0.921	0.545	2.851	0.4	[0.14, 1.16]	0.091
Gleason Score 6.0*	3.537	1.57	5.076	34.36	[1.58, 745.5]	0.024*
Gleason Score 7.0	1.827	1.236	2.185	6.22	[0.55, 69.9]	0.139
Gleason Score 8.0	-0.801	1.762	0.207	0.45	[0.01, 14.1]	0.649