Volumetric‐based image guidance is superior to marker‐based alignments for stereotactic body radiotherapy of prostate cancer

PURPOSES: The aim of this study was to evaluate a dual marker-based and soft-tissue based image guidance for inter-fractional corrections in stereotactic body radiotherapy (SBRT) of prostate cancer. METHODS/MATERIALS: We reviewed 18 patients treated with SBRT for prostate cancer. An endorectal balloon was inserted at simulation and each treatment. Planning margins were 3 mm/0 mm posteriorly. Prior to each treatment, a dual image guidance protocol was applied to align three makers using stereoscopic x ray images and then to the soft tissue using kilo-voltage cone beam CT (kV-CBCT). After treatment, prostate (CTV), rectal wall, and bladder were delineated on each kV-CBCT, and delivered dose was recalculated. Dosimetric endpoints were analyzed, including V36.25 Gy for prostate, and D0.03 cc for bladder and rectal wall. RESULTS: Following initial marker alignment, additional translational shifts were applied to 22 of 84 fractions after kV-CBCT. Among the 22 fractions, ten fractions exceeded 3 mm shifts in any direction, including one in the left-right direction, four in the superior-inferior direction, and five in the anterior-posterior direction. With and without the additional kV-CBCT shifts, the average V36.25 Gy of the prostate for the 22 fractions was 97.6 ± 2.6% with the kV x ray image alone, and was 98.1 ± 2.4% after applying the additional kV-CBCT shifts. The improvement was borderline statistical significance using Wilcoxon signed-rank test (P = 0.007). D0.03 cc was 45.8 ± 6.3 Gy vs. 45.1 ± 4.9 Gy for the rectal wall; and 49.5 ± 8.6 Gy vs. 49.3 ± 7.9 Gy for the bladder before and after applying kV-CBCT shifts. CONCLUSIONS: Marker-based alignment alone is not sufficient. Additional adjustments are needed for some patients based kV-CBCT.