Dosimetric impact of PTV margin reduction in CT-guided compared with MRI-guided stereotactic body radiotherapy for localized prostate cancer
Faculty and Abstracts
Purpose: The MIRAGE trial (NCT 04384770) reported an acute toxicity benefit for patients with localized prostate cancer randomized to MRI-guided stereotactic body radiotherapy (MRgRT) compared with CT-guided stereotactic body radiotherapy (CTgRT). This toxicity improvement was attributed to PTV margin reduction from 4mm to 2mm, enabled by improved soft tissue resolution during planning and real-time target motion tracking with MRgRT. However, there are clinical and radiotherapy planning conformality limitations with current MRgRT technology, and smaller PTV margins with CTgRT may be feasible with triggered imaging and automatic beam hold. We evaluated the dosimetric impact of reducing PTV margins in CTgRT-arm compared with MRgRT-arm patients in the MIRAGE trial.
Methodology: Ten CTgRT-arm patients from the MIRAGE trial were retrospectively re-planned with a 2mm PTV margin from the 4mm PTV margin per MIRAGE protocol. We measured PTV, rectum, and bladder dosimetric parameters, and compared these parameters within the same patients between their 2mm and 4mm PTV margin plans and with 10 matched MRgRT-arm patients.
Results: In both the CTgRT and MRgRT groups, all patients received 40Gy in 5 fractions to the prostate and proximal seminal vesicles, and 5 received a simultaneous integrated boost to 42Gy to the GTV. Dosimetric parameter analysis results are reported in the accompanying table. Overall, PTV coverage was significantly improved and rectal and bladder dosimetric parameters were similar or numerically but non-significantly reduced in 2mm CTgRT plans compared with MRgRT plans.
Conclusions: Reducing PTV margins from 4mm to 2mm in CTgRT resulted in similar to superior dosimetry compared with 2mm MRgRT PTV margins. Future studies will evaluate additional dosimetric parameters, including rectum and bladder maximum radiation dose, extend the findings to a larger group of the CTgRT patients, and analyze CTgRT with 3mm PTV margins in a similar manner. Prospective evaluation is required to validate the safety and efficacy of PTV margin reductions for patients undergoing CTgRT for localized prostate cancer.
