DOSIMETRIC CONSIDERATIONS IN RADIATION-THERAPY OF COIN LESIONS OF THELUNG

Purpose: The dose distribution in small lung lesions (coin lesions) is determined by the combined effects of reduced attenuation and electro nic disequilibrium. The magnitude of the dose delivered also depends o n the algorithm used to correct for reduced lung density. These effect s are investigated experimentally and computationally for 10 MV photon s. Methods and Materials: Using a polystyrene miniphantom embedded in cork or cedar, thermoluminescent dosimetry and film dosimetry was perf ormed to investigate interface effects and the central dose per monito r unit (MU), Three frequently applied calculation techniques-no densit y correction, ratio of tissue maximum ratios (TMRs), and the Bathe cor rection-were also used to calculate the dose per MU. The measurements and calculations were compared with a one-dimensional phenomenological theory with parameters taken from the literature. Results: The measur ements at the entrance surface and center of the miniphantom agreed we ll with the predictions of the phenomenological theory.; The interface regions are usually thin enough (2-3 mm) to be clinically unimportant for 10 MV. Depending on the algorithm used to correct for decreased l ung density, the lesion dose may be larger or smaller than the prescri bed dose by as much as 20% in extreme cases. A clinical example is pre sented. Conclusions: In comparing clinical results of treatments of sm all Lung lesions, it is important to be aware of the density correctio n used.