A comparison of high-energy oblique lung irradiation techniques

The lung cancer death rate in the U.S. rose 440% between 1957-59 and 1987-89, from 5.4 to 29.4 per 100,000. While surgical resection of small, localized carcinomas offers the best prognosis, only 15-20% of lung cancers fall into this category. The remaining 80-85% of patients are generally candidates for radiation therapy. Typically, the tumor volume (plus a 2 cm margin) and the mediastinum will be irradiated, using parallel opposed anterior and posterior ports, until the spinal cord has reached tolerance at 45 Gy. At this point, an off-cord lateral or oblique treatment technique will be used to complete the prescribed dose to the tumor. The depth to isocenter for oblique ports may easily be 15 cm. With this depth, a high-energy x-ray beam seems to be required; however, the beam may pass through a significant portion of lung tissue, reducing the equivalent depth. Another factor to consider is the build-up region beyond the lower density lung tissue. Two different energy beams, 6 MV and 18 MV, were compared for the oblique treatment ports. Plans were run using a thorax CT slice of an anthropomorphic phantom for parallel opposed oblique fields at these two energies, each with and without CT correction. Further data were collected for comparison by thermoluminescent dosimetry measurements. This paper describes the process and results obtained.