Quadrant dosimetry as a predictor of biochemical relapse in 125I prostate brachytherapy

Spadinger, Ingrid; Morris, W. James; Keyes, Mira; Liu, Mitchell; Shaffer, Richard; Thompson, Anna; Chng, Nicholas; Woods, Ryan

doi:10.1016/j.brachy.2010.06.009

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Volume 10, Issue 2 , Pages 87-97, March 2011

Quadrant dosimetry as a predictor of biochemical relapse in ¹²⁵I prostate brachytherapy

Ingrid Spadinger
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
- Corresponding author. Department of Medical Physics, BC Cancer Agency, 600 West 10th Avenue, Vancouver, BC V5Z 4E6, Canada. Tel.: +1-604-877-6000; fax: +1-604-877-6059.
W. James Morris
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Mira Keyes
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Mitchell Liu
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Richard Shaffer
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Anna Thompson
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Nicholas Chng
- Provincial Prostate Brachytherapy Program, BC Cancer Agency, Vancouver, British Columbia, Canada
Ryan Woods
- Population Oncology, BC Cancer Agency, Vancouver, British Columbia, Canada

Received 19 January 2010; received in revised form 28 May 2010; accepted 7 June 2010. published online 09 August 2010.

Abstract

Purpose

To evaluate region-specific dose metrics as predictors of biochemical relapse in prostate brachytherapy patients.

Methods and Materials

In a cohort of 1006 low-risk and “low-tier” intermediate-risk prostate brachytherapy patients treated to a planned dose of 144Gy mPD (minimal peripheral dose), 30 of 32 with biochemical relapse (nadir+2ng/mL definition) had postimplant CT scans available for retrospective analysis. These were matched to nonrelapsing controls from the same era. Three copies of each CT were created and, after randomization and deletion of identifiers and original contours, were re-contoured by three radiation oncologists. Prostate contours were then divided into quadrants: Anterior-Superior (ASQ), Posterior-Superior (PSQ), Anterior-Inferior (AIQ), and Posterior-Inferior (PIQ), and dosimetric parameters calculated. Results were analyzed using mixed-effects linear regression and multivariate logistic regression.

Results

Whole prostate volume of the prostate receiving at least 100% of the prescribed dose (V₁₀₀) and minimum dose, as a percentage of the prescribed dose, received by 90% of the prostate volume (D₉₀) were similar for relapses and controls (p=0.40 and 0.48, respectively). Among the quadrants, the largest differences between relapses and controls were seen for the AIQ. Mean AIQ V₁₀₀s were 91.2% (relapses) and 95.5% (controls) (p=0.096), and D₉₀s were 112.8% (relapses) and 119.3% (controls) (p=0.145). Overall, the lowest doses were in the ASQ, but were not very different for relapses and controls (V₁₀₀=76.5% and 78.5%, respectively) (p=0.54). On multivariate analysis along with various clinical parameters, AIQ metrics approached significance at the p ≤ 0.05 level in models that also included initial prostate-specific antigen, androgen suppression, and risk group.

Conclusions

Although whole prostate dose metrics did not predict for biochemical relapse in our data set, dose to the AIQ was predictive in multivariate analysis.

Keywords: Prostate cancer, Prostate brachytherapy, PSA failure, Sector analysis, Dose response