Background: Larger maximum tumor diameter (MTD) has been associated with worse prostate cancer outcomes for those undergoing surgery as well as salvage radiation. MTD is also an important consideration for patients weighing active surveillance. However, the impact of MTD in intermediate and high-risk prostate cancer treated with external beam radiotherapy (EBRT) and brachytherapy boost is unknown. We set out to evaluate MTD of the dominant nodule on MRI as a prognostic factor in patients treated with EBRT and brachytherapy boost for localized prostate cancer. Methods: Patients with prostate cancer treated with EBRT and brachytherapy boost were identified from an institutional database. In patients with a pretreatment MRI, data on MTD were retrospectively collected. Clinical data including age, ADT use, pretreatment PSA, International Society of Urologic Pathology (ISUP) group, clinical T-stage, and presence of adverse pathology on imaging (either seminal vesicle invasion or extraprostatic extension) were also collected. Multivariable and univariable cox proportional hazards models for biochemical failure (BF) and distant metastasis (DM) were produced in patients with MTD grouped by receiver operating characteristic (ROC) cutpoint. Cumulative hazard functions for BF and DM were compared with log-rank test and stratified by ISUP group. Results: Of 191 patients treated with EBRT and brachytherapy boost, 113 had pretreatment MRI and available MTD measurement. Median follow up was 40 months (interquartile range 23-66 months) and median MTD was 17 mm (interquartile range 13-22mm). Increasing MTD was associated with higher T stage and increased ADT use. ROC cutpoint optimization identified MTD of 24mm to be the optimal cut-point for both BF and DM. On univariate log-rank analysis, patients with MTD > 24mm had higher 5-year BF (31% vs 4%, p = 0.004) and DM (21% vs 4%, p = 0.002) than those with MTD≤24. Stepwise multivariable cox model for BF (P = 0.130, HR 1.08, 95% CI 0.98-1.21) and DM (P = 0.115, HR 1.09, 95% CI 0.98-1.23), MTD did not demonstrate statistical significance when controlling for clinical t-stage, adverse pathologic features on imaging, ISUP group, and ADT use. However, in patients with ISUP group 4-5 disease, MTD > 24 was independently associated with increased risk of DM (P = 0.032, HR 1.18, 95% CI 1.01-1.37). Conclusions: This is the first study to evaluate MTD on MRI as a prognostic factor in the setting of brachytherapy boost. These results demonstrate that for patients treated with EBRT and brachytherapy boost, MTD is independently associated with risk for metastasis in patients with ISUP grade 4 and 5 disease. Although these results require further validation, this suggests a possible role for MTD as a factor in risk assessment models and clinical decision-making.