Purpose: To determine the characteristics of the penumbra in the region of the lung tumor-lung parenchyma interfaces for various radiation beam energies and various field margins. Methods and Materials: A phantom simulating the thoracic cavity with a tumor arising within the lung parenchyma was irradiated with opposed 6-, 10-, and 18-MV photon beams. Beam profiles were obtained at the tumor's surface and midplane using radiographic film. The field 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. Clinically acceptable beam profiles were defined as those in which no point of the planning target Volume (gross tumor volume plus a 1-cm margin) received less than 95% of the central tumor dose. Results: Mean effective penumbra and beam fringe were found to differ in a statistically significant manner with respect to energy, but not with distance from field edge to gross tumor volume. With the field edge ≤1.5 cm from the gross tumor volume, no energy provided an acceptable dose distribution, as defined above. With the field edge 2 cm from the gross tumor volume, 6 and 10 MV provided acceptable dose distributions, but 18 MV did not. With the field edge ≤22.5 cm from the gross tumor volume, all energies provided acceptable dose distributions. Conclusion: For irradiation of lung carcinomas in which the planning target volume includes a margin of normal lung tissue, 6- and 10-MV opposed beams yield a superior dose distribution with respect to penumbra at the tumor's surface and midplane, with the field edge placed 2 cm from the gross tumor volume. To achieve an equivalent 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 in normal lung tissue irradiated.