3313 - Dosimetric Study of Dose De-Escalation Using MRI-Guidance in Low and Intermediate Risk Prostate Cancer Patients with Pd-103 Low Dose Rate Brachytherapy

A. Xue¹, A. Gogineni¹, L. Wang¹, L. Potters², and P. Bhupesh²; ¹Department of Radiation Medicine, Northwell, Lake Success, NY, ²Northwell, New Hyde Park, NY

Purpose/Objective(s): There exists an unmet requirement to improve toxicity profiles for prostate cancer patients with oncologic control of the disease. A prospective clinical trial is planned to evaluate mpMRI-guided dose de-escalation which is maintaining prescription dose to lesions visible on mpMRI while reducing dose to non-involved prostate. As a preliminary dosimetric analysis for this clinical trial, this study investigates whether dose de-escalation can reduce toxicity in peripheral structures (urethra, bladder, and rectum) compared to standard full dose treatment.

Materials/Methods: We reviewed 72 consecutive patients with low or intermediate risk prostate cancers treated with radiation in the last 5 years, of which 26 had a single malignant focus on biopsy and a corresponding lesion on MRI. In total, 52 dosimetric plans were generated from these patients using Pd-103 seeds, activity 2.5 U, with treatment planning software. For each patient, two plans were generated, one with a standard full prescription dose of 125 Gy to the entire prostate (standard arm) and a comparison plan with high dose to the malignant focus and 100 Gy to the rest of the prostate (de-escalation arm). For each plan, rectum 2 cc and 0.1 cc, bladder 2 cc and 10 cc, and urethra 10% doses were calculated. The calculated doses were compared between the standard and de-escalated plans using the Wilcoxon rank signed test.

Results: The prostate volume ranged from 15.9 to 78.4 cc with a mean of 47.1 cc. For the rectum, the mean dose to 0.1cc was 73.8±34 Gy on standard plans, 57.2±22.4 Gy on de-escalation plans. The mean dose to 2cc was 36.5±14.9 Gy on the standard plans, 30.9±13.3 Gy on the de-escalation plans. For the bladder, the mean dose to 2cc was 79.1±17 Gy on standard plans, 65.1±16.1 Gy on the de-escalation plans. The mean dose to 10cc was 37±9.8 Gy on standard plans, 30.9±8.9 Gy on the de-escalated plans. For the urethra, the mean D10% was 156.9±10.7 Gy on standard plans, 129±23.7 Gy on de-escalated plans. The median percent changes between the 100 Gy and 125 Gy plans were as follows: Rectum D2 cc reduced by 17.4%, Rectum D0.1 cc reduced by 17.7%, Bladder D2 cc reduced by 20.1%, Bladder D10 cc reduced by 17.4%, and Urethra D10% reduced by 22.5%. All differences were statistically significant (p < 0.0001) indicating a substantial reduction with the de-escalated arm.

Conclusion: This dosimetric study suggests that de-escalation using MR guidance for brachytherapy in prostate cancer patients will likely reduce rectal, urethral, and bladder toxicity. We plan to compare our dosimetric results with actual clinical outcomes from the prospective protocol. Abstract 3313 – Table 1

	Standard plan (125Gy)		De-escalation plan (100Gy)
	Mean Dose (Gy)	Std Dev	Mean Dose (Gy)	Std Dev
Rectum 0.1cc	73.8	34	57.2	22.4
Rectum 2cc	36.5	14.9	30.9	13.3
Bladder 2cc	79.1	17	65.1	16.1
Bladder 10cc	37	9.8	30.9	8.9
Urethra 10%	156.9	10.7	129	23.7