Excessive applicator radiation leakage for a common therapeutic kilovoltage system.

Abstract

The objective of this work is to characterise out- of- field leakage radiation emanating from clinical applicators of the WOmed T-200 kilovoltage therapy machine. To identify points of leakage, radiosensitive film was affixed to the walls and base plate of each applicator. Quantitative assessment of leakage radiation was conducted with a 0.23 cm3 ionisation chamber following the International Electrotechnical Commission standard. Film was also used to illustrate leakage distribution in the patient plane. Angular and energy dependences of the leakage radiation were quantified as well as a two-dimensional leakage profile in the plane parallel to one applicator. Leakage was found when the diameter of primary collimator of the kV tube exceeded the external dimension of the applicator wall. In the patient plane all applicators showed similar leakage rates with the leakage distribution dependent upon applicator design. Mean patient plane leakage was 1.37% of central axis air kerma rate, exceeding the 0.5% limit specified in the standard. Leakage was shown to be profoundly energy dependent with maximum leakage of 11.8% for the 200 kV beam, 1.3% for 150 kV and 0.2% for 100 kV. Angular dependence measurements showed a 10.3% change in leakage between the minimum and maximum positions. The combination of shielding thickness, primary collimator design and applicator dimensions permits unwanted radiation to contribute dose outside the treatment field when energies ≥150 kV are used. Even carefully designed modern kv therapy systems can exhibit leakage in some areas. Thorough assessment of leakage is needed prior to release for clinical use.