Remote afterloading patient-specific brachytherapy with liquid radioisotope for irradiation of extensive scalp lesions: A Monte Carlo study.

Abstract

The aim of this study is to propose a remote afterloading patient-specific brachytherapy technique for total scalp irradiation by utilizing liquid radioisotope as well as a three-dimensional (3D) printer and to find an optimal radioisotope for the suggested technique. We designed a brachytherapy device composed of liquid radioisotope tank, tube, patient-specific applicator, and a thin flexible pouch. The liquid radioisotope tank, tube, and the flexible pouch are interconnected one another to constitute a closed loop system. The pouch is located inside the solid patient-specific applicator; therefore, when the liquid radioisotope is injected into the pouch, the pouch is inflated and fills the space inside the applicator. The 3D-printed patient-specific applicator keeps the uniform thickness of the liquid radioisotope conforming patient's contour. To investigate an optimum condition for the suggested system, we performed Monte Carlo simulation with the GEANT4 simulation toolkit. To find the optimal radioisotope, percent depth doses (PDDs) of P-32, Sr-89, Y-90, and I-125 solutions were acquired in a rectangular parallelepiped phantom. For the selected radiation source, PDDs as well as dose rates in spherical phantoms with radii of 7.7cm (infant head size) and 9.1cm (adult head size) were acquired. To deliver prescription doses at 4-mm depth regions (scalp region), 1-mm-thick Y-90 and 5-mm-thick I-125 in liquid form were found to be feasible for the suggested technique. For both spherical phantoms with radii of 7.7 and 9.1cm, when delivering 2Gy at the 4-mm depth region with the 1-mm-thick Y-90 and 5-mm-thick I-125 sources, 53.3 and 3.8Gy were delivered at the surface regions, respectively (delivery time=111.1 and 3.5min with 1GBq/ml solutions). The PDDs of Y-90 and I-125 became less than 1% at depths greater than 8 and 50mm, respectively. The remote afterloading patient-patient specific brachytherapy with I-125 or Y-90 in liquid form seems feasible for total scalp irradiation.