Microboost dose escalation for localized prostate cancer within a statewide radiation oncology quality consortium.

Abstract

298 Background: The phase III FLAME trial demonstrated that in localized prostate cancer, an external beam radiotherapy (EBRT) simultaneous integrated microboost to an MRI-defined dominant intraprostatic lesion improves biochemical control without affecting toxicity and quality of life. Given the complexity required to deliver high microboost doses, we hypothesized that practice patterns are variable in clinical practice. We aimed to characterize microboost utilization and evaluate dosimetric parameters within the statewide Michigan Radiation Oncology Quality Consortium (MROQC). Methods: Men with intermediate or high-risk prostate adenocarcinoma treated with curative intent radiotherapy for intact disease were included. Data was prospectively collected, including T/N-category, Gleason score, prostate-specific antigen, and percent positive biopsy cores. Full DICOM files were available for dosimetric data. Multivariable analyses (MVA) were used to evaluate associations between receipt of microboost, known prognostic factors, and fiducial marker and rectal spacer placement (advanced image guided radiation therapy (IGRT)). Results: From 10/26/20 to 06/26/23, 741 patients across 26 centers were enrolled, 71% (n=528) with intermediate-risk and 29% (n=213) with high-risk disease. Androgen deprivation therapy was planned in 61%. EBRT + whole-gland brachytherapy boost was utilized in 29% (n=217/741) of patients. Of those treated with EBRT (71%, n=524/741), 10% received a microboost (n = 53/524). Brachytherapy boost and microboost were used in 9 and 7 centers, respectively. Most patients received either conventional fractionation or moderate hypofractionation- 11% and 66% in EBRT without microboost and 21% and 60% with microboost, respectively. Microboost treatment was associated with use of a planning MRI (91% vs. 62%, p <0.0001) as well as fiducial marker and rectal spacer placement (76% vs. 45%, p <0.0001). Median prostate planning target volume (PTV) was smaller in microboost patients (127 cc vs. 91 cc, p = 0.002); median boost volume was 21 cc. Median boost dose (D0.1cc) was 117% (IQR 115% – 119%) of the PTV prescription. Microboost patients had significantly higher mean bladder D1cc[%] (105.7% vs. 103.4%, p = 0.002) and numerically lower rectal D1cc[%] (90.7% vs. 94.1%, p = 0.065). On MVA, receipt of microboost was significantly associated with grade group 4 or 5 disease, planning MRI use, and fiducial and rectal spacer use. Conclusions: Within a large, diverse prospective cohort of men with prostate cancer treated in both academic and community settings, a microboost was utilized in 10% of EBRT cases. Microboost use was associated with higher grade group, MRI planning, and advanced IGRT. EBRT microboost is an emerging dose escalation strategy and further studies confirming safety and improved clinically meaningful outcomes may increase uptake in routine practice.