Magnetic resonance linear accelerator boost for para-urethral cancer: a new treatment paradigm replacement for brachytherapy.

This study evaluates the use of magnetic resonance-guided radiation therapy (MRgRT) as an alternative to brachytherapy in treating para-urethral gynecological cancers, particularly for patients who are not candidates for brachytherapy. Five female patients with advanced para-urethral gynecological cancers underwent MRgRT using a custom 3-dimensional-printed intravaginal cylinder for image registration and treatment alignment. MRgRT was administered as a five-fraction adaptive boost following standard chemoradiation, with each fraction utilizing the cylinder to achieve precise positioning and improve organ sparing. A 1.5T magnetic resonance linear accelerator was used to deliver adapt-to-shape treatment, allowing real-time adjustments to compensate for anatomical variations. The cylinder served not only as a surrogate for accurate image registration but also as a spacer to displace the rectum from high-dose regions. The median follow-up period was 14.4 months, during which all patients completed treatment with no grade >3 genitourinary toxicities. Acute toxicities included dysuria and vaginal pain, while chronic toxicities, such as urinary incontinence and mild cystitis, were recorded in a subset of patients. Treatment achieved an overall survival rate of 100% and a recurrence-free survival rate of 80%. Dosimetric analysis demonstrated effective target coverage with minimal exposure to surrounding organs, particularly sparing the urethra from hotspots, unlike traditional brachytherapy. These results suggest that MRgRT with a vaginal cylinder offers a promising approach for managing para-urethral gynecological cancers in patients ineligible for brachytherapy. Further studies are warranted to validate these findings and refine treatment protocols.