SU‐E‐T‐165: Characterization of Dose Distributions in High‐Dose‐Rate Surface Brachytherapy

Abstract

Purpose:To characterize dose distributions in high‐dose‐rate(HDR) surface brachytherapy using an Ir‐125 source for different geometries, field sizes and topology of the clinical targets(CT). To investigate the depth doses at the central axis(CAX), edges of the treatment fields(E), and lateral dose distributions(L) present when using flap applicators in skin cancer treatments.Methods:When malignancies diagnosed on the skin are treated, various geometries of the CT require proper adaptation of the flap or custom‐made applicators to the treatment site. Consequently, the dose at the depth on CAX and field edges changes with variation of the curvatures and size of the applicators. To assess the dose distributions, we created a total of 10 treatment plans(TP) for 10×10 and 20×20 field sizes(FS) with a step size of 1cm. The geometry of the applicators was: planar(PA), curved to 30(CA30) and 60(CA60) degrees with respect to the CAX, half‐cylinder(HC), and cylindrical shape(CS). One additional TP was created in which the applicators were positioned to form a dome shape(DS) with a diameter of 16cm. This TP was used to emulate treatment of the average sized scalp. All TPs were optimized to deliver a prescription dose at 8mm equidistantly from the planes containing the dwell positions. This optimization is equivalent to the clinical arrangement since the SSD for the flap applicators is 5mm and the prescription depth is 3mm in the majority of clinical cases.Results:The depths (in mm) of the isodose lines were: FS(10×10):PA[90%(9.1CAX,8.0E,7.6L),50%(28.3CAX,20E,17.3L), 25%(51.1CAX,40E,27L)],CA30[90%(10.3CAX,8.2E,7.9L),50%(32.1CAX, 16.2E,15.8L),25%(61.3CAX,36.7E,31.8L)],CA60[90%(12.2CAX,6.1E,6.3L ),50%(47CAX,14E,16.6L),25%(70.8CAX,31.9E,35.4L)],HC[90%(11.1CA X,6.3E,7.3L),50%(38.3CAX,14.6E,16.1L),25%(66.2CAX,33.8E,34.2L)];FS (20×20):PA[90%(11.1CAX,9.0E,7.0L),50%(34.4CAX,21.9E,15.3L),25%(7 0.4CAX,50.9E,34.8L)],CA30[90%(10.9CAX,7.5E,7.1L),50%(38.8CAX,16. 7E,15.4L),25%(80.7CAX,39.9E,35.7L)],HC[90%(11.1CAX,10.9E,10.6L),5 0%(34.4CAX,38.8E,47.2L),25%(70.4CAX,80.7E,96.6L);DS[90%(12.1CA X,8.5E,6.8L),50%(60.6CAX,17.7E,15.8L),25%(109.6CAX,40.3E,34.3L)].Conclusion:These results can be used for the approximate calculation and quick assessment of the radiation dose to the organs‐at‐risk at depth (heart,brain,thyroid, optical nerve) or at the surface distance from the CT (breast,eyes, gonads), and only the geometry of the CT needs to be measured. Understanding of the dose distributions in HDR surface brachytherapy may improve clinical decision making process.