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Objectives: To report feasibility, early toxicity, and PSA kinetics following gantry-based, stereotactic radiotherapy (SBRT) boost within a prospective, phase 2, multicenter study (PROMETHEUS: ACTRN12615000223538). Methods: Patients were treated with gantry-based SBRT, 19–20 Gy in two fractions delivered 1 week apart, followed by conventionally fractionated IMRT (46 Gy in 23 fractions). The study mandated MRI fusion for RT planning, rectal displacement, and intrafraction image guidance. Toxicity was prospectively graded using the Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v4). Results: Between March 2014 and July 2018, 135 patients (76% intermediate, 24% high-risk) with a median age of 70 years (range 53–81) were treated across five centers. Short course (≤6 months) androgen deprivation therapy (ADT) was used in 36% and long course in 18%. Rectal displacement method was SpaceOAR in 59% and Rectafix in 41%. Forty-two and ninety-three patients were treated at the 19 Gy and 20 Gy dose levels, respectively. Median follow-up was 24 months. Acute grade 2 gastrointestinal (GI) and urinary toxicity occurred in 4.4 and 26.6% with no acute grade 3 toxicity. At 6, 12, 18, 24, and 36 months post-treatment the prevalence of late grade ≥2 gastrointestinal toxicity was 1.6, 3.7, 2.2, 0, and 0%, respectively, and the prevalence of late grade ≥2 urinary toxicity was 0.8, 11, 12, 7.1, and 6.3%, respectively. Three patients experienced grade 3 late toxicity at 12 to 18 months which subsequently resolved to grade 2 or less. For patients not receiving ADT the median PSA value pre-treatment was 7.6 ug/L (1.1–20) and at 12, 24, and 36 months post-treatment was 0.86, 0.36, and 0.20 ug/L. Conclusions: Delivery of a gantry-based SBRT boost is feasible in a multicenter setting, is well-tolerated with low rates of early toxicity and is associated with promising PSA responses. A second transient peak in urinary toxicity was observed at 18 months which subsequently resolved. Follow-up is ongoing to document late toxicity, long-term patient reported outcomes, and tumor control with this approach.
The purpose of this paper is to describe a practical approach to commissioning and quality assurance (QA) of a dedicated wide-bore 3 Tesla (3T) magnetic resonance imaging (MRI) scanner for radiotherapy planning. Methods: A comprehensive commissioning protocol focusing on radiotherapy (RT) specific requirements was developed and performed. RT specific tests included: uniformity characteristics of radio-frequency (RF) coil, couch top attenuation, geometric distortion, laser and couch movement and an end-to-end radiotherapy treatment planning test. General tests for overall system performance and safety measurements were also performed. Results: The use of pre-scan based intensity correction increased the uniformity from 61.7% to 97% (body flexible coil), from 50% to 90% (large flexible coil) and from 51% to 98% (small flexible coil). RT flat top couch decreased signal-to-noise ratio (SNR) by an average of 42%. The mean and maximum geometric distortion was found to be 1.25 mm and 4.08 mm for three dimensional (3D) corrected image acquisition, 2.07 mm and 7.88 mm for two dimensional (2D) corrected image acquisition over 500 mm × 375 mm × 252 mm field of view (FOV). The accuracy of the laser and couch movement was less than ±1 mm. The standard deviation of registration parameters for the end-to-end test was less than 0.41 mm. An on-going QA program was developed to monitor the system's performance. Conclusion: A number of RT specific tests have been described for commissioning and subsequent performance monitoring of a dedicated MRI simulator (MRI-Sim). These tests have been important in establishing and maintaining its operation for RT planning.
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