High-dose-rate brachytherapy boost for prostate cancer: Analysis of dose-volume histogram parameters for predicting late rectal toxicity.

PURPOSE: To determine the significance of dose-volume histogram parameters for predicting late rectal toxicity (LRT) after single-fraction high-dose-rate brachytherapy (HDRBT) boost and external beam radiotherapy (EBRT) in prostate cancer. MATERIALS AND METHODS: Three hundred patients with intermediate- or high-risk prostate cancer were included between August 2010 and March 2015. Treatment comprised a single-fraction HDRBT boost of 15.0 Gy plus EBRT (46.0 Gy delivered in 23 fractions) or an HDRBT boost of 9.5 Gy plus EBRT (60.0 Gy delivered in 30 fractions) if the seminal vesicles were infiltrated using real-time transrectal ultrasound-based planning. LRT was evaluated every 3 months after the end of the combined treatment using the Common Terminology Criteria for Adverse Events, version 4.0. The minimum dose received by the most exposed 0.1 and 2.0 cm3 volume of the rectum (D0.1 cc/D2cc) was analyzed by estimating the biologically equivalent rectal dose according to the recommendations of the Groupe Européen de Curiethérapie/European Society for Radiotherapy and Oncology and an ordinal regression analysis was performed. RESULTS: LRT was observed in 62 patients (20.7%) at a median followup of 33 (range, 2-68) months. Twenty patients (6.7%) developed grade 2 and 3 patients (1%) developed grade 3 LRT. A significant association was observed between D2cc and the probability of developing grade 1-3 LRT (p = 0.04). CONCLUSIONS: D2cc is associated with the occurrence of LRT in HDRBT-treated prostate cancer patients. The dose constraints proposed and recommended by experienced HDRBT centers must be investigated to determine the threshold dose through long-term and prospective studies.

An evaluation of the robustness of organ-at-risk recommendations made by GEC/ESTRO according to interobserver variability: a single-center experience.

PURPOSE: Groupe Européen de Curiethérapie (GEC) and European Society for Radiotherapy & Oncology (ESTRO) has proposed a rectal dose constraint of the most exposed 2-cc volume (D2cc of ≤ 75 Gy EQD2α/ß = 3) during external-beam plus high-dose-rate brachytherapy (HDR-BT) in localized prostate cancer patients. This study aimed to evaluate D2cc for rectal contouring via interobserver variability. MATERIAL AND METHODS: Four blinded observers contoured rectums of 5 patients. Rectal contouring anatomical limits were determined through previous consensus. Dose-volume histogram (DVH) dosimetric parameters (D0.1cc, D1cc, and D2cc) were analyzed according to GEC/ESTRO recommendations and subjected to intra- and interobserver comparisons. Latter comparisons involved coefficients of variation. For each parameter, the mean, standard deviation (SD), and range were evaluated. The effect of interobserver variation on total dose was analyzed by estimating the biologically equivalent rectal dose (EQD2α/ß = 3). RESULTS: Interobserver coefficients of variation for D0.1cc, D1cc, and D2cc were 5.7%, 4.5%, and 4%, respectively. The highest interobserver rectal delineation variation yielded a rectal dose difference up to 5.8 Gy EQD2. Estimated intraobserver variation for the reported D2cc was 5.5% in the worst-case scenario (non-significant). CONCLUSIONS: We observed acceptable interobserver variability in EQD2 for D2cc, with strong impacts on clinical threshold levels (D2cc ≤ 75 Gy EQD2) in some cases. This small, single-center analysis will be extended in a multicenter study.