To estimate the biochemical cure rate of a recently closed cesium-131 prostate brachytherapy trial using the BED method described by Stock, et al. (IJROBP 64(2):527).
Materials/Methods
Stock, et al., used a simplified biologically effective dose (BED) model to explain their iodine and palladium prostate brachytherapy results. They determined that a BED of 150 Gy calculated from the prostate D90 was the biological dose required to achieve biochemical cure. They used an alpha-beta ratio of 2.0 in their calculations. It can be shown that the similarity of their palladium and iodine results means that the alpha-beta ratio for this disease should be very close to 1.5. Using this assumption and the simplified model, we have calculated the BED for the 100-patient multi-institutional trial which has recently closed. This trial used Cs-131 as monotherapy for early stage prostate cancer. Dosimetric analysis was performed at a single facility following electronic submission of the implant records and the two-week post implant CT scan. Correction for the recently changed dose constant was applied prior to conversion to BED using the evaluated D90. PSA levels were obtained 1, 4, 6, 12, 18 and 24 months post implant.
Results
D90 (corrected) for the implants ranged from 97.2 to 156.7 Gy with an average of 122 ± 11.4 Gy. The converted BED ranged from 124 to 227 Gy with an average of 165 + 19.5 Gy. Based upon the palladium and iodine experience and the above analysis, the implant series is expected to achieve a biochemical control rate of 80%. Patient followup ranges from 2 to 28 months with a median of 11.9 months. The average PSA levels measured at the 0, 1, 2, 4, 6, 12 and 18 month marks were 6.8, 3.2, 2.1, 1.4, 1.1, 0.9 and 0.6 ng ml−1. No patients have failed according to either the ASTRO or Houston definitions of biochemical failure.
Conclusions
It is too early for failures to have occurred in the data set, but the success of the clinical trial, and the dosimetric targets established, suggest that Cesium-131 is an acceptable alternative to iodine and palladium for prostate brachytherapy. It is possible to perform implants with the isotope and achieve high cure rates while minimizing complications from the procedure.
1International Medical Physics Service, Helotes, TX
FULL TEXT