Dosimetry measurements with an intra-operative x-ray device

The INTRABEAM 50 kV x-ray device can be used for intra-operative partial breast irradiation. Spherical applicators are added to the source probe to deliver a radially symmetric radiation dose. Dosimetric data for calculation of absorbed dose were measured for this unit and a superficial unit with a similar beam quality, as defined by half value layer (HVL). Chamber calibration factors, N-K, and chamber correction factors, k(ch), were determined based on HVL, according to the IPEMB code of practice and addendum. Depth doses were also measured using an ionization chamber and GafChromic EBT film. HVL was measured as 0.85-1.30 mm Al across the range of applicator sizes. Values for NK and k(ch) were found to be similar for the two units and all INTRABEAM applicator sizes. Therefore, calibration of ionization chambers, radiochromic film and other relative dosimeters could be performed on the superficial unit. This has the advantage of higher dose rates and lower dependence on small variations in detector positioning. Depth dose measurements performed using film also agreed with chamber values, published and manufacturer data, giving a simple and robust method for commissioning and regular quality assurance.