192Ir low dose rate brachytherapy for boosting locally advanced prostate cancers after external beam radiotherapy: A phase II trial

Background and purpose: To evaluate on 201 locally advanced prostatic cancers prospectively treated in a phase II trial, the efficacy of a combination of external beam radiotherapy (39.6 Gy) and 192Ir low dose rate brachytherapy (Bt) (40-45 Gy). Patients and methods: Sixty-four patients were included in the intermediate prognosis group with only one of the following adverse factors (PSA>10 ng/ml, Gleason score≥7 or clinical stage≥T2b) and 137 in the unfavourable group when at least two of these factors were present. Results: The actuarial 4 years biochemical no evidence of disease is 82.8% for the entire population. It is, respectively, 97 and 76% in the intermediate and unfavourable prognosis groups (P<0.0001). Grade≥3 late urinary complications occurred in 13 patients (6.5%). Eight patients (4%) presented late grade 2 rectal complications but no grades 3-5 was observed. Conclusions: Even if an α/β of 1.5-3 Gy theoretically favours the use of a high dose rate mode of irradiation, the early results presented here are as good as those reported for similar groups of patients with high dose rate treatments. Late toxicity is identical but our urinary toxicity is within the less favourable and rectal toxicity within the most favourable results. We can postulate that while inducing very high hyperdosage regions (V150) mainly focused on the peripheral zone, most of the Bt techniques consist of a more ablative treatment. Many of the radiobiological studies on Bt did not in fact take into account the heterogeneity of irradiation inside the CTV. This study highlights the need to explore pulsed dose rate therapies, permanent implant and new available radioisotopes such as 169Ytterbium that will offer the safety of low and lower dose rates. The actual late toxicity of the different Bt techniques is not yet inexistent indeed