RTOG 0321 is the first multi-institutional cooperative group HDR prostate brachytherapy trial with complete digital brachytherapy dosimetry data. This is a descriptive report of the data and an analysis of toxicity.
Methods and Materials
Patients are treated with EBRT 45 Gy and one HDR implant with 19 Gy in 2 fractions. Implants are done with TRUS guidance, and CT-compatible non-metallic catheters. HDR planning is done on ≤ 3 mm-thick CT slices. The “mean DVH” of the PTV, implanted volume (IP), and organs at risk are calculated. This includes the mean and standard deviation of the volume at ten-percentage-point intervals from 10%–200% of the prescribed dose. The conformal index (COIN), homogeneity index (HI), catheters/implant, and patients/institution are calculated. Multivariate analysis and Hazard Ratios calculation of all the variables against reported Grade ≥ 2 (G2+) GU adverse events (CTCAEv3) are performed.
Dosimetry data is based on 122 eligible patients from 14 institutions. The mean of PTV, IP, catheters/implant, and patients/institution are: 54 cc, 63 cc, 19 and 9. The mean of %V100PTV, V80Bladder, V80Rectum, and V120Urethra were: 94%, 0.40cc, 0.15cc, and 0.25cc. There are too few G2+ GI AE for correlative analysis, thus the analysis has been performed on the more common G2+ GU AE. There are positive correlations noted between both acute and late G2+ GU AE and urethral dose at multiple levels. Positive correlations with late AE are seen with PTV and IP at high-dose levels. A negative correlation is seen between HI and acute AE. A higher patient accrual rate is associated with a lower rate of G2+ acute and late AE.
Higher urethral dose, larger high dose volumes and lower dose homogeneity are associated with greater toxicities. A mean DVH comparison at all dose levels should be used for quality control and future research comparison.