Consistency of Performance of Intra-Operative Radiotherapy Equipment – Photon Radiosurgery System
K. S. Armoogum, J. M. Parry, C. D. Mackay, S. Souliman
Department of Radiotherapy and Oncology, Ninewells Hospital and Medical School, Tayside University Hospitals NHS Trust, Dundee DD1 9SY, UK.
The past 20 years have seen increasing use of more conservative methods to treat early stage breast cancer 1. Intra-Operative RadioTherapy (IORT) avoids unnecessary treatment of the whole breast and delivers a critical dose to the tumour bed only. It is currently being investigated whether a single high dose of radiation will impart the same clinical benefit as a standard course of external beam therapy (typically 6 weeks).
Ninewells Hospital has four Photon Radiosurgery Systems (Carl Zeiss Surgical GmbH) currently used to treat breast and neurological tumour sites. The PRS comprises a portable x-ray generator, control console, bespoke QA tools and a mobile gantry. To comply with the Ionising Radiations Regulations 2 we investigated the dosimetric characteristics of each source and its performance stability during individual treatments as well as over a period of time.
Characteristics investigated were half value layer, output decay factor, internal rate monitor (IRM) reproducibility and depth-doses in water.
The half value layer was determined by the broad beam method, in which the x-ray probe was placed 20 cm away from the ionisation chamber, with high purity aluminium attenuators placed at the midpoint 3. To ascertain the effects of beam hardening at clinical depths, a solid water attenuators of 5 and 10 mm was placed between the x-ray probe and the aluminium attenuators, approximately 2 cm away from the probe. To determine the constancy of output from the x-ray sources, the ion chamber current was monitored over a typical clinical treatment time of 30 minutes to deduce an output decay factor.
IRM reproducibility was investigated under various exposures 4. Exposures were controlled using a pre-set number of IRM counts at a count rate of approximately 7 x 104 counts per second. Beam parameters of 50kV and 40µA were used. Measurements were made for exposures corresponding to counts of 4, 20, 40 and 60 x 106, equivalent to exposures of 1, 5, 10 and 15 minutes. Calculated and actual treatment times terminated on IRM limit were recorded for 16 patients. Depth-dose curves in water were obtained by measuring the chamber output at distances of between 10 and 35 mm away from the probe.
The equivalent energies for the beam attenuated by 5 and 10 mm of solid water were derived from data tables in ICRU Report 17 and found to be approximately 12 and 24 keV respectively. The average output level over a period of 30 minutes was found to be 98.9%. The average difference between the preset IRM limit and the IRM count on beam termination was found to be less than 0.5%. For breast IORT, the average difference between the calculated and actual treatment times was found to be 0.30% (0.47% for neurological treatments). It was found that beam attenuation in water varied by approximately 1/r3 (Fig.1).
Figure 1: Dose in water versus distance over the range 10-35 mm at 50kV and 40µA.
The x-ray sources have proven to be stable over time. Most measurements were found to lie within the manufacturer’s tolerances and an intercomparison of these checks suggests that the four x-ray sources have similar performance characteristics.
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 Ionising Radiations Regulations (1999). SI No. 3232. HSE, London.
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