A national dosimetry audit of intraoperative radiotherapy.

Abstract

National dosimetry audits are a fundamental part of quality assurance in radiotherapy, especially for new techniques. Intraoperative radiotherapy with a compact mobile kilovoltage X-ray source is a novel approach for the treatment of breast and other cancers. All seven current clinical sites in the UK were audited by a single visiting group and set of measurement equipment. Measurements of output, isotropy and depth doses were performed using an ion chamber in solid water, thermoluminescent dosemeters and radiochromic film, respectively. The mean difference between measured and planned dose across all centres was -3.2±2.7%. Measured isotropy was within ±3% around the lateral plane of the X-ray source and +11±4% in the forward direction compared with the lateral plane. Measured depth doses were agreed within 5±2% of manufacturer-provided calibration values or a mean gamma index of 97% at a tolerance of 7%/0.5 mm. Agreement within measurement uncertainties was found for all three parameters except forward anisotropy, which is unlikely to be clinically significant. Steep dose gradients increase the sensitivity to small variations in positioning, but these tests are practical for use in interdepartmental audits and local baseline comparison. The first UK interdepartmental audit of intraoperative radiotherapy builds confidence in the delivery of this treatment.