Correlation of Prostate-specific Antigen Kinetics with Acute Toxicity Following Stereotactic Body Radiotherapy: A Single Institution Review

Summary: Stereotactic body radiotherapy (SBRT) has been demonstrated to be an effective treatment modality for localized prostate cancer. This study aimed to investigate prostate-specific antigen (PSA) kinetics in relation to acute toxicity. After obtaining IRB approval, a retrospective review was conducted on patients with pathologically confirmed, localized prostate cancer who underwent SBRT alone from 2018 to 2024 at a single institution. Patients who were placed on androgen deprivation therapy (ADT) were excluded from the analysis. Per institutional standards, patients received 36.25 – 40 Gy in 5 fractions. Linear multivariable analysis was performed to compare PSA kinetics between groups. NCI CTCAE version 5.0 was used to assess genitourinary toxicity. A total of 41 patients were included in this study. The median age of the patients was 70 years. Median PSA at diagnosis was 7.58 ng/mL (interquartile range [IQR], 5.2 – 12.05). The median follow-up was 11 months (IQR, 7.75-13.25). Hydrogel insertion was performed in 85% of patients prior to receiving SBRT. Among the patients examined, 24% developed Grade 2 genitourinary toxicity. The pattern of PSA change was not significantly different (p=0.579) based on whether the patients experienced acute toxicity, with a slope of -0.359 ng/mL/month in patients with no toxicity and -0.297 ng/mL/month in those with toxicity. The pattern of PSA change was also not significantly different (p=0.157) across grade groups with slopes of -0.704, -0.310, and -0.379 ng/mL/month in grade group 1, 2, and 3, respectively. This institutional retrospective review found that PSA kinetics in patients with localized prostate cancer treated with SBRT were not significantly affected by the development of acute toxicity or by stratification of grade groups. Similar rates of PSA decline across the groups suggest that acute toxicity may not be a major determinant of PSA response following SBRT. Our findings align with previously published data, demonstrating that SBRT is an effective treatment for localized prostate cancer, achieving consistent PSA reduction across all grade groups. Further research with a larger patient population and longer follow-up may be warranted to confirm these results. Cody R. Kilar, DO, PhD (Presenting Author) - Department of Radiation Oncology, West Virginia University Cancer Institute; Matthew Trotta, MD (Co-Author) - Department of Radiation Oncology, West Virginia University Cancer Institute; John Knoth, MD (Co-Author) - Department of Radiation Oncology, West Virginia University Cancer Institute; Ramon A. Siochi, PhD, DABR (Co-Author) - Department of Radiation Oncology, West Virginia University Cancer Institute; David A. Clump, MD, PhD (Co-Author) - Department of Radiation Oncology, West Virginia University Cancer Institute; Muhammad M. Fareed, MD (Co-Author) - Department of Radiation Oncology, West Virginia University Cancer Institute