Is It Necessary to Perform Post-Implant Dosimetric Analysis in Low-Dose-Rate Brachytherapy for Prostate Cancer? A Quality Assurance Assessment

To determine if computerized tomography (CT)/magnetic resonance imaging (MRI)-based day 0 (d0) dosimetry is a meaningful predictor of d21 dosimetry in low-dose-rate brachytherapy (LDR-BT) for localized prostate cancer (PCa). We performed a retrospective analysis of men with localized (T1-2 N0 M0/X), low/intermediate-risk prostate cancer treated with LDR-BT for PCa from 2003-2012 at a National Cancer Institute-designated cancer center. All men underwent an intra-operative transrectal ultrasound to generate a real-time LDR-BT plan with customized needle positions and seed-spacer sequences. The primary dose (D) constraint was > 145 Gy prescribed to 90% of the prostate volume (D90 > 145 Gy); a secondary constraint was that the volume (V) of the prostate receive > 90% of the prescription dose (V100 > 90%). CT/MRI-fusion was used for post-implant dosimetry at d0 and d21. Paired t-tests were used to compare change in D or V at the time points. Logistic regression was used to construct receiver operating characteristic (ROC) curves for achieving each constraint at d21, based on d0 D90 and V100 as continuous variables. Youden's index was used to determine D and V cut-points which maximized equally weighted sensitivity and specificity. Two hundred seventy eight men were evaluated. In assessment of dose prescriptions, from d0 to d21, mean D90 increased from 133 to 150 Gy, and mean V100 increased from 85% to 91%. For patients who met the D90 > 145 Gy constraint at d0, 90% maintained this constraint at d21; of those below the D90 constraint at d0, 46% had an increase to D90 > 145 Gy at d21. Similarly, for patients who met the V100 > 90% at d0, 83% maintained this level at d21; of those below the V100 constraint at d0, 47% had an increase to V100 > 90% at d21. The optimal cut-point for d0 D90 was 135 Gy with 84% of these patients maintaining a d21 D90 > 145 Gy. For d0 V100, the optimal cut-point was 87%, with 83% of these patients maintaining a d21 V100 > 90%. Meeting dosimetric constraints on d0 does not obviate d21 dosimetric analysis in men receiving LDR-BT for PCa. Constraints used for dose prescriptions on d0 are not ideal predictors of d21 dosimetry.