Dosimetric study of dose de-escalation using MRI-guidance for Pd-103 low-dose-rate brachytherapy in prostate cancer

Despite the prevalence of prostate malignancies, there remains a need to improve toxicity profiles while maintaining oncologic control. Standard whole-gland treatment prescriptions have been associated with rectal, bladder, and urethral toxicity. This dosimetric study evaluated a differential dosing approach using Pd-103 low-dose-rate brachytherapy that delivers full prescription dose (125 Gy) to MRI-visible lesions while reducing dose to uninvolved prostate tissue (100 Gy).

Twenty-seven patients with unifocal, MRI-visible, biopsy-proven low- or intermediate-risk prostate cancer were identified. For each patient, two dosimetric plans were generated: a standard plan prescribing 125 Gy to the entire prostate, and a de-escalation plan prescribing 125 Gy to the MRI-visible lesion and 100 Gy to the remaining prostate. Dosimetric parameters including rectal D0.1cc and D2cc, bladder D2cc and D10cc, and urethral D10% and D30% were compared using Wilcoxon signed-rank tests.

Fifty-four plans (27 standard, 27 de-escalation) were analyzed. The de-escalation approach achieved statistically significant dose reductions to all organs at risk (p < 0.0001). Median reductions were: rectum D0.1cc -10.77 Gy (17.7%), D2cc -5.99 Gy (17.4%); bladder D2cc -13.84 Gy (20.1%), D10cc -5.88 Gy (17.4%); and urethra D10% -35.47 Gy (22.5%), D30% -30.14 Gy (21.9%). Large effect sizes were observed for urethral doses (Cohen's d = 1.52-2.01).

MRI-guided differential dosing in Pd-103 LDR brachytherapy is technically feasible and achieves substantial reductions in organ-at-risk exposure, particularly urethral doses (>20%). While these dosimetric improvements suggest potential for reduced toxicity, clinical validation through our ongoing prospective Phase II trial is needed to confirm clinical benefit.