Application of radiosurgery principles to a target in the breast: A dosimetric study

Purpose: To investigate the technical and physical feasibility of using a r adiosurgery-like technique to irradiate a small target within the breast wi th a single fraction. Material and Methods: During diagnostic biopsy, a tan talum surgical clip is placed in the lesion identified at mammography. Tran sverse CT scans over the entire breast are obtained, as the patient lies pr one on a special table that allows the breast to hang down. The clip is use d as a reference point to define the isocenter of the radiation treatment. Results: The clip is visible on port films taken with a 4 MV beam, allowing the isocenter to be set to its planned location. No movement of the hangin g breast is visually detected. The possible beam directions are enclosed by a 220 degrees horizontal x 180 degrees vertical angular interval. Dosimetr y of two "radiosurgical" examples, (A) seven fixed horizontal beams and (B) six 45 degrees arcs and a 90 degrees sagittal are using a 4 MV x-ray beam with a 32 mm diameter collimator, are discussed. Both field arrangements pr oduce adequate tumor coverage: the minimum target dose is 83% of the dose m aximum in the fixed beam arrangement and 86% in the multiarc setup. In arra ngement A the lung and other tissues external to the breast receive dose on ly from scattered radiation. In arrangement B the maximum lung dose is less than 5% of the dose to isocenter. Conclusion: From a dosimetric point of v iew both described techniques are feasible, and the radiosurgery-like treat ment is executable. (C) 2000 American Association of Physicists in Medicine . [S0094-2405 (00)04205-X].