A national dosimetry audit of intraoperative radiotherapy

Objective:

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.

Methods:

Measurements of output, isotropy and depth doses were performed using an ion chamber in solid water, thermoluminescent dosemeters and radiochromic film, respectively.

Results:

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.

Conclusion:

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.

Advances in knowledge:

The first UK interdepartmental audit of intraoperative radiotherapy builds confidence in the delivery of this treatment.Dosimetry audits have been a fundamental part of quality assurance (QA) in UK radiotherapy departments for over 20 years 1–6]. More recently, ongoing verification of established systems has been delegated to regional audit groups; however, new techniques such as intensity-modulated radiotherapy 7,8] or volumetric modulated arc therapy 9] have been assessed on a national level. This gives confidence in the accurate, safe and consistent delivery of complex treatments, regardless of location or prior experience.Intraoperative radiotherapy (IORT) has also been practised in the UK for nearly 20 years, using a compact mobile 50 kV X-ray device called PRS400 (Photoelectron Corporation, Lexington, MA) and, more recently, INTRABEAM® (PRS500; Carl Zeiss Surgical, Oberkochen, Germany). Initially, the system was used for intracranial stereotaxy 10] but found wider application in the delivery of a single fraction of radiation soon after surgical excision of breast tumours 11]. Most patients have been treated as part of the targeted intraoperative radiotherapy (TARGIT) international randomised controlled trial 12], although some other cases have also been successfully treated where external beam therapy was contraindicated 13]. Full calibration of the system is provided by the manufacturer, but recently, methods for independent dosimetry and quality assurance have been described 14,15]. The aim of this study was to audit all UK clinical centres in 2012 (