PSA Kinetics and Outcomes after Magnetic Resonance Image-Guided Stereotactic Body Radiotherapy for Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Using moderate or ultra-hypofractionation, which is also known as stereotactic body radiotherapy (SBRT) for treatment of localized prostate cancer patients has been increased. Assessment of the efficacy of treatment response after radiotherapy relies heavily on the monitoring of prostate specific antigen (PSA) levels and PSA kinetics. The aim of our study was to evaluate tumor control outcomes with PSA response and toxicity after magnetic resonance image-guided adaptive radiotherapy (MRgRT) for prostate cancer patients. <h3>Materials/Methods</h3> A total of 95 prostate cancer patients treated with MRgRT were included in the study. Treatment was performed with IMRT (step and shoot) technique and daily plan adaptation using MRgRT. Patients received 36.25 Gy in 5 fractions, per institutional standards. Outcome measurements included tumor control evaluation with PSA levels, acute and late toxicities. Nadir PSA was defined as the lowest serum PSA value from the time of SBRT completion to date of last follow-up. Biochemical failure (BCF) was defined using the Phoenix definition (nadir + 2 ng/mL). <h3>Results</h3> The median follow-up for our cohort was 23.7 months (range; 3-41.4 months). The median age was 74 years (range; 50-87 years). The median initial PSA (iPSA) was 7.6 ng/mL (range; 2.7-44 ng/mL) for all patients. According to the NCCN guidelines, 13 (13.7%), 53 (55.8%) and 29 (30.5%) patients were in low-, intermediate- and high-risk group, respectively. Twenty-nine patients (30.5%) received androgen deprivation therapy (ADT). The median nadir PSA (nPSA) for all patients was 0.72 ng/mL (range, 0.01-4.96 ng/mL). For the entire cohort, 52.6% patients had an nPSA <1 ng/mL, with 35.8% of patients reaching an nPSA 0.5 ng/mL and 20% reaching an nPSA 0.2 ng/mL. For the patients who didn't receive ADT, median nPSA was 1.26 ng/mL (range; 0.16-4.96 ng/mL); 45.0% patients had an nPSA <1 ng/mL, with 21.3% of patients reaching an nPSA 0.5 ng/mL and 3.3% reaching an nPSA 0.2 ng/mL. For the patients who received ADT, median nPSA was 0.06 ng/mL (range; 0.01-1.55 ng/mL); 95.8% patients had an nPSA <1 ng/mL, with 87.5% of patients reaching an nPSA 0.5 ng/mL and 70.8% reaching an nPSA 0.2 ng/mL. A patient who did not yet meet the Phoenix criteria for biochemical recurrence but a PSA rising was diagnosed with two bone metastases with imaging evaluation and one patient with a nPSA and positive PSMA-PET had radical prostatectomy. MRgRT was well tolerated by all patients. There was no Grade ≥3 GU or GI toxicity. <h3>Conclusion</h3> In conclusion, MRgRT represents a new method for delivering SBRT with markerless soft tissue visualization, online adaptive planning and real-time tracking. Our study suggests that ultra-hypofractionation with MRgRT has an acceptable toxicity profile with favorable efficacy and PSA response. Further studies with longer follow-up time and larger cohorts are warranted.