Effect of low-density lateral interfaces on soft-tissue doses

Purpose: Doses at the interface between tissue and low-density inhomogeneities with the interface positioned perpendicular to the beam direction have been well studied. When the inhomogeneity lies parallel to the beam direction (i.e., a lateral interface), the resulting dose distribution is not as well known. Lateral lung-soft-tissue interfaces are common in many fields used to treat malignancies in the thorax region including tangential breast fields and anteroposterior fields for lung and esophageal cancer. The purpose of this study was to evaluate the dose distribution along lateral interfaces and to determine the implications for treatment. Methods and Materials: A polystyrene and cork slab phantom was irradiated from the side to simulate treatment fields with lateral lung-soft-tissue interfaces. The beam was positioned with the isocenter in polystyrene and the field edge in cork. Cork slabs (0.6-2.5 cm) were used to simulate different thicknesses of lung between the field edge and the target volume. Measurements were made using a parallel plate ionization chamber. With the chamber position held constant, polystyrene slabs were added between the cork and the chamber to study the dose distribution in the interface region. Interface doses were studied as a function of the amount of cork in the field, field size, beam energy (6-18 MV), and depth. Results: Doses in the interface region were lower by as much as 10% compared to doses in a homogeneous phantom. For a given cork width and field size, the magnitude of the underdose increased by several percent as the x-ray energy increased from 6 to 18 MV. The underdose at the interface was 5% for 6 MV and 8% for 18 MV X-rays with a l-cm cork width. For a 2.5-cm cork width, underdoses of 2.5% and 3% at distances up to 2.5 and 4 mm lateral to the interface were observed for 6- and 18-MV X-rays, respectively. However, doses right at the interface were 1% greater for 6 MV and 3% less for 18 MV than doses in a homogeneous phantom. For a given cork width, the interface doses were not significantly dependent on field width but decreased by an additional 2-3% as the length decreased to 4 cm. Additional decreases were also observed when the measurement depth decreased to 3 cm. With a l-cm width of cork in the field, a lateral distance of 3-4 mm from the interface was necessary to ensure doses of at least 98% of the homogenous dose with 6-MV X-rays. A lateral distance of 6-7 mm was necessary for 10- and 18-MV X rays. Conclusion: Underdosing will occur in the soft tissues adjacent to low-density inhomogeneities. The magnitude depends primarily on the width of the inhomogeneity seen in the treatment field, but also on field size, depth, and beam energy. For treatment fields with a lateral lung interface, a segment of tissue approximately 3-4 mm thick for 6 MV and 6-7 mm thick for higher-energy beams may be underdosed. Lung widths of greater than or equal to 1.75 cm as observed on film will generally guarantee doses of at least 96% of those calculated with no inhomogeneity corrections. High-energy beams are often used to treat sites in the thorax or breast to improve dose homogeneity throughout the treatment volume. Potential underdosing due to the presence of lung should be considered and may require a decrease in beam energy or an increase in the margin between the target volume and the field edge to ensure adequate treatment. (C) 1997 Elsevier Science Inc.