TU-AB-201-08: Rotating Shield High Dose Rate Brachytherapy with 153Gd and 75Se Isotopes

Abstract

Purpose: To introduce rotating shield brachytherapy (RSBT) for different cancer sites with 1⁵3Gd and ⁷⁵Se isotopes. RSBT is a form of intensity modulated brachytherapy, using shielded rotating catheters to provide a better dose distribution in the tumour while protecting healthy tissue. Methods: BrachySource, a Geant4-based Monte Carlo dose planning system was developed for investigation of RSBT with 1⁵3Gd and ⁷⁵Se for different cancer sites. Dose distributions from 1⁵3Gd, ⁷⁵Se and 192Ir isotopes were calculated in a 40 cm radius water phantom by using the microSelectron-v2 source model. The source was placed inside a cylindrical platinum shield with 1.3 mm diameter. An emission window coinciding with the active core of the source was created by removing half (180°) of the wall of the shield. Relative dose rate distributions of the three isotopes were simulated. As a proof of concept, a breast cancer patient originally treated with Mammosite was re-simulated with unshielded 192Ir and shielded 1⁵3Gd. Results: The source with the lowest energy, 1⁵3Gd, decreased the dose on the shielded side by 91%, followed by ⁷⁵Se and 192Ir with 36% and 16% reduction at 1 cm from the source. The breast cancer patient simulation showed the ability of shielded 1⁵3Gd to spare the chest wall by a 90% dose reduction when only one emission window angle is considered. In this case, fully covering the PTV would require more delivery angles and the chest wall dose reduction would be less, however, the simulation demonstrates the potential of shielded 1⁵3Gd to selectively isolate organs at risk. Conclusion: Introducing 1⁵3Gd and ⁷⁵Se sources combined with RSBT will allow escalation of dose in the target volume while maintaining low doses in radiation sensitive healthy tissue. Tailoring treatments to each individual patient by treating all parts of the tumour without over-irradiation of normal tissues will be possible. The author acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290), and the Quebec Fonds de recherche Nature et Technologies.