IMPACT OF BEAM ENERGY AND FIELD MARGIN ON PENUMBRA AT LUNG TUMOR-LUNGPARENCHYMA INTERFACES

Purpose: To determine the characteristics of the penumbra in the regio n of the lung tumor-lung parenchyma interfaces for various radiation b eam energies and various field margins. Methods and Materials: A phant om simulating the thoracic cavity with a tumor arising within the lung parenchyma was irradiated with opposed 6-, 10-, and 18-MV photon beam s. Beam profiles were obtained at the tumor's surface and midplane usi ng radiographic film. The held edge varied from 0.0 to 3.5 cm from the gross tumor volume. The effective penumbra (distance from 80 to 20 % dose) and beam fringe (distance from 90 to 50% dose) were measured, Cl inically acceptable beam profiles were defined as those in which no po int of the planning target volume (gross tumor volume plus a 1-cm marg in) received less than 95% of the central tumor dose. Results: Mean ef fective penumbra and beam fringe were found to differ in a statistical ly significant manner with respect to energy, but not with distance fr om field edge to gross tumor volume. With the field edge less than or equal to 1.5 cm from the gross tumor volume, no energy provided an acc eptable dose distribution, as defined above. With the held edge 2 cm f rom the gross tumor volume, 6 and 10 MV provided acceptable dose distr ibutions, but 18 MV did not. With the field edge greater than or equal to 2.5 cm from the gross tumor volume, all energies provided acceptab le dose distributions. Conclusion: For irradiation of lung carcinomas in which the planning target volume includes a margin of normal lung t issue, 6- and 10-MV opposed beams yield a superior dose distribution w ith respect to penumbra at the tumor's surface and midplane, with the held edge placed 2 cm from the grass tumor volume. To achieve an equiv alent distribution with 18-MV photons, a distance of 2.5 cm from field edge to the gross tumor volume is necessary, leading to an increase i n normal lung tissue irradiated. (C) 1998 Elsevier Science Inc.