Dosimetry of Continuous Random Motion in High Dose Rate Strontium-90 Plesiotherapy

Strontium-90 plesiotherapy delivers high doses of radiation to superficial lesions (<3 mm depth) with excellent sparing of deeper tissues. The sealed-source applicator tip is circular and 8-10 mm in diameter. Larger treatment fields are treated with multiple overlapping fields. The applicator cannot conform to angular extensions of irregularly shaped fields, resulting in unnecessary treatment of normal tissue, and overlapping fields can overdose in some areas. With strontium-90 sources that have high activity and thus a high dose rate, small variations in duration and position of placement can be even more consequential. This prospective investigation used radiochromic film to evaluate the dosimetry of adjacent, static, overlapping fields compared with that of the same target area treated with the applicator in continuous motion intended to achieve the prescribed exposure to all points within the target. The intent was to minimize over- and underdosed areas. Three shapes of target field were used: single 2 cm circle, curvilinear field, and cloverleaf. Application times were calculated to deliver the dose to the surface, and the procedure was repeated by four operators for qualitative evaluation (1 Gy/100 cGy to target) and four operators to generate quantitative data (5 Gy/500 cGy to target). We found significant differences between methods among operators, dependent on the number of fields, and in dose drop-off at the margin of the treated field. In almost every comparison, continuous motion was more homogenous (p ≤ .01 for standard deviation from expected dose). The continuous motion technique can be used for strontium plesiotherapy when the applicator must be held.