Dosimetry of small x-ray beams for stereotactic radiotherapy

Abstract

The dosimetry of small [formula omitted] width) x-ray beams, such as those used in stereotactic radiotherapy, is much more complex than that of those used in routine clinical treatments. A thorough understanding of the properties of both small beams and small detectors is necessary to determine the optimum detector to use in each measurement situation. Accurate and reproducible experimental methods must also be developed to measure absolute and relative doses, obtain precise beam data and subsequently verify the delivered treatment dose. This work is an investigation of the above aspects of small beam dosimetry, with particular reference to the types of small fields used for stereotactic radiotherapy in the Edinburgh Cancer Centre. These are fields formed by circular stereotactic collimators (12.5 to 40 mm diameter), used in conjunction with arc therapy for the treatment of small brain lesions. Several detectors were compared in the measurement of percentage depth doses, tissue maximum ratios, off axis ratios, head scatter and relative output factors, on a 6 MV linac. This included the testing of three new, commercially available detectors. Clinical beam data were obtained via detector comparison and recommendations made as to the best methodology for each measurement parameter. The most accurate and reproducible technique for head scatter factors was extended to smaller stereotactic collimators (5 to 10 mm diameter) and square fields with widths of 10 to 20 mm. These were shaped with both the movable linac collimators and the multileaf collimator and the results will be applied to the measurement of the small subfields used in intensity modulated radiotherapy (IMRT). A verification phantom was designed to be compatible with the stereotactic head frame and the properties of various small detectors were investigated for use in the phantom to measure point doses in both single arcs and multiple non-coplanar plans. The recommendations on beam data acquisition and dose verification were applied to two additional linacs. On all machines, the dose to the isocenter was verified in several typical treatment plans to within 2% of the calculated dose, for all clinical collimators. The results confirm the accuracy of the measurement processes used. The verification technique also provides the basis for a proposed audit of dosimetry in all stereotactic centers in the UK. © 2004, American Association of Physicists in Medicine. All rights reserved.