Lung dose in electron beam therapy of chest wall

The dose to lung tissue in patients whose chest walls are irradiated with 6 and 9 MeV electron beams was estimated. Thermoluminescent dosimetry measurements in thoracic Section 18 of Rando phantom were compared with calculations by two different methods, the effective depth method and the average absorption equivalent thickness (AET) method. The calculations were based on a density of 0.45 (relative to water) for lung tissue in the Rando phantom which was determined from CT scan. The measurements agreed with calculations by the effective depth method within 7 per cent for 9 MeV electron beam at locations where lung tissue thickness was less than 4.5 cm. Larger discrepancy between measurement and calculations was found at greater depths. The effective depth method gave better agreement with measurements (within 14 per cent) compared with the average AET method. The effective depth method was used, to calculate the lung tissue dose at different depths beneath the chest wall. A lung density of 0.25 was assumed to conform to real irradiation situation. The calculations show that for a 9 MeV electron beam dose to lung at 5 cm depth beyond a 2 cm thick chest wall could be as high as 72 per cent of the dose at the maximum buildup depth. It is recommended that post-mastectomy patients with chest wall thickness less than 2 cm should be irradiated with an electron energy of less than 9 MeV or that the chest wall thickness be artificially increased with tissue equivalent bolus.