Emulation of gynecological brachytherapy doses with external beam

Purpose: To evaluate the viability of external beam methods in substitution or replacement of brachytherapy techniques, while maintaining similar dosimetric performance. Methods: Three original tandem and ring brachytherapy plans were re-planned using external beam techniques. A physician recreated the target volumes on CT scans without instruments present, and volumetric photon and intensity modulated proton plans were generated to deliver either a uniform dose or to emulate the brachytherapy distribution with the escalated dose core. Proton plans were additionally generated using several different planning parameters. Resultant plans were compared dosimetrically by evaluating the dose to 2 cm³ for critical organs, the dose coverage to 90% of the target volume, the low-dose normal tissue exposure at 20% and 50% of the prescription, and the target volume receiving 200% and 300% of the prescription for the escalated-dose plans. Results: External beam plans performed well, achieving comparable D90 coverage and critical structure dose to 2 cm³. Higher robustness distance settings increased the D100 for proton plans, along with increases to critical structure doses. Photon plans achieved good escalated dose patterns when emulating the brachytherapy distribution, with better result for the 200% volume than the 300% volume. Photon plans did not meet the 300% coverage in any plans. Escalated proton plans did not meet the brachytherapy dose volumes in any plan, and doses along the entry paths rose to over 150% of the prescription. Conclusion: Brachytherapy is still the preferred method for delivery of the local boost for cervical cancer, despite continued advancement of external beam technologies. Brachytherapy is able to achieve the highest target dose heterogeneity while still protecting organs at risk. External beam photon plans are superior to proton plans when attempting to emulate the brachytherapy dose heterogeneity.