Dosimetric considerations in radiation therapy of coin lesions of the lung

The dose distribution in small lung lesions (coin lesions) is determined by the combined effects of reduced attenuation and electronic disequilibrium. The magnitude of the dose delivered also depends on the algorithm used to correct for reduced lung density. These effects are investigated experimentally and computationally for 10 MV photons. Using a polystyrene miniphantom embedded in cork or cedar, thermoluminescent dosimetry and film dosimetry was performed to investigate interface effects and the central dose per monitor unit (MU). Three frequently applied calculation techniques--no density correction, ratio of tissue maximum ratios (TMRs), and the Batho correction--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. The measurements at the entrance surface and center of the miniphantom agreed well 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 lung density, the lesion dose may be larger or smaller than the prescribed dose by as much as 20% in extreme cases. A clinical example is presented. In comparing clinical results of treatments of small lung lesions, it is important to be aware of the density correction used.