Correction Factors for Tumor Dose in the Chest Cavity Due to Diminished Absorption and Scatter in Lung Tissue.

Abstract

The investigation to be described here was undertaken to determine correction factors for the tumor dose of radiation in the lung and in the mediastinum, since the density of pulmonary tissue is so much less than that of muscle or water, on which the usual depth dose tables and isodose charts are based. Correction factors were determined from measurements obtained on a phantom man for 200-kv, 400-kv, and C060 radiations, for single and multiple ports, and for rotation therapy with C060. Review of Literature A number of investigators have attempted to find either a coefficient of absorption or a method of calculation to allow for the difference in absorption between muscle and lung. Failla (1) riade measurements with radium and deflated lung tissue. Weatherwax and Robb (2) determined the depth dose for 200-kv roentgen rays in a human lung placed in a water phantom and inflated to various sizes. Quimby et al. (3) measured the depth doses in the chest of a cadaver for 200-kv radiation and published correction curves. Nahon (4) investigated the transmission of 200-kv roentgen rays in the thorax and the abdomen of living subjects. He reported also (5) that depth-dose curves obtained in a plywood phantom with a density of 0.5 gm./c.c. were the same as those in the calf's thorax. Using this phantom he constructed isodose curves for the thorax for both multiple-field and rotational therapy. Kornelsen (6) developed a method for calculating the dose for a centrally located lesion by the use of an effective coefficient of linear absorption obtained by measuring the entrance dose and the exit dose with a back-scattering medium. Robbins and Meszaros (7) published curves of the transit dose rate for the chest and pelvis during rotation, based on measurements in a series of water phantoms. They also showed curves for three different patients. Introduction The calculation of dosage in the lung is not a simple matter. When radiation is directed at a pulmonary lesion, it must penetrate the thoracic cage composed of muscle, bone, and sometimes fat, then pass through normal lung to carcinomatous lung of different density, and finally make its exit through normal lung and thoracic cage, without benefit of back-scattering material behind the body. Measurements made at autopsy showed the densities of carcinomas in collapsed lungs to vary from 0.86 gm./c.c. to 1.05 gm./c.c. The rib cage does not alter the dose appreciably, but the vertebral column does. The factors which cause the greatest variation in dosage in the 200-kv region are decreased absorption in lung tissue and decreased scatter, one sometimes balancing the other. Available depth-dose tables are based on a so-called infinite phantom, at least 30 × 30 × 30 ern.