Abstract
INTRODUCTION: Prostate cancer is the leading cause of cancer treatment worldwide, reflecting its high incidence, high cure rate. Most patients with prostate cancer will present with localized disease, and the majority will have low-risk or intermediate-risk disease. Management options include definitive external beam radiotherapy, radical prostatectomy, brachytherapy, active surveillance and more recently Radiosurgery SBRT.
Hypofractionation using higher doses per fraction allows for similar results to conventional RT in shorter time in selected patients. Stereotactic body radiotherapy SBRT has shown in a multi-institutional consortium report of 1100 patients 5-year biochemical relapse–free survival rates of 95% for patients with low-risk disease and 84% for patients with intermediate-risk disease. Based on the overall favorable outcomes of these studies, in 2014 the NCCN guidelines added radiosurgery as another treatment option. Our objective is to present biochemical response and long-term outcomes associated with Linac SBRT in 40 men with prostate cancer treated with VMAT SBRT
MATERIALS & METHODS: Between September 2015 and December 2020, 40 patients with prostate cancer were treated at our institution with Linac-based SBRT without fiducials, spacers, or rectal balloons. Ages ranged from 51 to 92 with a median of 70 years. Thirty-five patients had low and favorable intermediate risk disease and five had unfavorable or high-risk disease including oligo-metastases. Fourteen patients received initial androgen deprivation therapy (ADT) and three patients had previous focal ablative therapies including HIFU and cryoablation. All patients were treated with VMAT using two to four 6 MV FFF arcs. First and confirmatory CBCT was performed prior to every fraction. Total dose ranged from 30 Gy to 40 Gy delivered in 5 fractions. The most frequent dose scheme was 36.25 Gy in 5 fractions of 7.25 Gy. This regimen was used in 34 (85%) of the patients. The seminal vesicles were included in the target volume in 37 patients (92.5%). Thirty-four patients (85%) received 25 Gy using 5 Gy/fx with a simultaneous integrated boost (SIB). Three patients were treated only to the prostate gland. Our first patient was treated with a rectal balloon. Follow-up ranged from 3 to 58 months with a median of 24 months.
PSA was collected from pre-treatment to the most-recent follow-up time for each patient. The patients were divided in 2 groups: Group 1 included 35 patients with low and favorable intermediate risk prostate cancer. Group 2 consisted of 5 patients with unfavorable intermediate and high-risk prostate cancer and/or presence of metastatic disease. The Gastrointestinal (GI) and Genitourinary (GU) toxicities were evaluated according to the RTOG criteria.
RESULTS: A decrease in the PSA levels was observed in all patients of group 1 with the caveat that in one patient with prostatomegaly the response took longer. Biochemical responses of more than 70% and 85% at 12 and 24 months respectively were observed. PSA levels continued to decrease thereafter or remained stable. Patients that were under short-term androgen deprivation therapy (ADT) showed a sharper initial decline in the first follow-up PSA. Once ADT was discontinued the response followed the same pattern previously described. No significant toxicity was observed. Regarding daily localization, CBCT images, prior to every fraction showed consistently less than 2 mm shift in the position of the target.
Two patients from Group 2 showed the same response as Group 1 with PSA of 0.1 after 9 and 54 months. One of them was treated with salvage radiosurgery due to previous radiation therapy to the pelvis for colorectal cancer. The other patient received radiosurgery to the prostate and to the adjacent oligometastatic lesion in the ischium. PSA for the other three patients increased due to the development of distant metastases with primary under control.
Fifty percent of the patients developed grade 1 acute GU toxicity and 20% experienced grade 1 late toxicity. No acute or subacute GI toxicity was encountered.
CONCLUSIONS: All patients responded to SBRT. The biochemical response over time was consistent with previously described patterns in patients with low and favorable intermediate risk prostate cancer treated with SBRT. Our initial results suggest that Linac-based SBRT using CBCT is safe and effective, without the need for rectal balloon or fiducials.
Our review demonstrates that SBRT can be also used in patients with high and unfavorable risk. Local control was obtained in patients previously treated with radiation therapy, HIFU and in patients with oligometastatic disease. This unfavorable group although small suggests that SBRT can be used as an alternative to a protracted course of radiation therapy.
