Abstract
External beam radiotherapy remains the primary treatment modality for localized prostate cancer. The radiobiology of prostate carcinoma lends itself to hypofractionation, with recent studies showing good outcomes with shorter treatment schedules. However, the ability to accurately deliver hypofractionated treatment is limited by current image-guided techniques. Magnetic resonance imaging is the main diagnostic tool for localized prostate cancer and its use in the therapeutic setting offers anatomical information to improve organ delineation. MR-guided radiotherapy, with daily re-planning, has shown early promise in the accurate delivery of radiotherapy. In this article, we discuss the shortcomings of current image-guidance strategies and the potential benefits and limitations of MR-guided treatment for prostate cancer. We also recount present experiences of MR-linac workflow and the opportunities afforded by this technology.
Highlights
Prostate cancer has accounted for 23.2% of all male cancer diagnoses in Europe in 2020 so far [1], a large proportion of whom will be treated with external beam radiotherapy (EBRT) for localized disease
Inter- and intra-fractional variability of target organ morphology and position as well as organ-at-risk (OAR) deformation limit the safety of dose escalation and hypofractionation with current image-guided radiotherapy (IGRT) techniques
The HYPO-RT-PC trial, comparing UHF for localized prostate cancer to conventional fractionation, reported significantly higher levels of patient-reported acute bowel and urinary toxicity with UHF [11], though late-term toxicity appeared equivalent regardless of treatment arm. These findings were not correlated in acute toxicity findings from the PACE-B trial in which the SBRT cohort reported similar levels of acute toxicity to the standard fractionation cohort [10]
Summary
Prostate cancer has accounted for 23.2% of all male cancer diagnoses in Europe in 2020 so far [1], a large proportion of whom will be treated with external beam radiotherapy (EBRT) for localized disease. MRguided radiotherapy (MRgRT) brings IGRT to a higher level with improved soft tissue contrast and online adaptive planning allowing for greater accuracy of fraction delivery. The acquisition of a verification image subsequent to contouring and planning allows for there to be a shift of the new plan immediately prior to beam on (called ‘Adapt-toPosition’ workflow) to account for any prostate motion, which occurs during the workflow This is due to rectal or bladder filling. The ongoing Magnetic Resonance Imaging-Guided Stereotactic Body Radiotherapy for Prostate Cancer trial (MIRAGE trial, NCT04384770) is a phase III randomized study comparing standard CT-guided SBRT versus MRIguided SBRT, with the primary endpoint of acute grade ≥2 genitourinary (GU) toxicity It is designed as a superiority study, and secondary endpoints include patient-reported outcomes and late toxicity. It is too early to form robust toxicity comparisons between MRgRT and non-MRgRT SBRT trials, but outcomes encourage further prospective and long-term trials to interrogate this important point
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