EFFECT OF LOW-DENSITY LATERAL INTERFACES ON SOFT-TISSUE DOSES

Purpose: Doses at the interface between tissue and low-density inhomog eneities with the interface positioned perpendicular to the beam direc tion have been well studied. When the inhomogeneity lies parallel to t he beam direction (i.e., a lateral interface), the resulting dose dist ribution 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 lun g and esophageal cancer. The purpose of this study was to evaluate the dose distribution along lateral interfaces and to determine the impli cations for treatment. Methods and Materials: A polystyrene and cork s lab 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 betwee n the field edge and the target volume. Measurements were made using a parallel plate ionization chamber. With the chamber position held con stant, polystyrene slabs were added between the cork and the chamber t o 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 interfac e region were lower by as much as 10% compared to doses in a homogeneo us phantom. For a given cork width and field size, the magnitude of th e 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, un derdoses 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. Howeve r, doses right at the interface were 1% greater for 6 MV and 3% less f or 18 MV than doses in a homogeneous phantom. For a given cork width, the interface doses were not significantly dependent on field width bu t decreased by an additional 2-3% as the length decreased to 4 cm. Add itional decreases were also observed when the measurement depth decrea sed to 3 cm. With a l-cm width of cork in the field, a lateral distanc e of 3-4 mm from the interface was necessary to ensure doses of at lea st 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 inhomogeneitie s. The magnitude depends primarily on the width of the inhomogeneity s een in the treatment field, but also on field size, depth, and beam en ergy. 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 highe r-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 l east 96% of those calculated with no inhomogeneity corrections. High-e nergy beams are often used to treat sites in the thorax or breast to i mprove dose homogeneity throughout the treatment volume. Potential und erdosing due to the presence of lung should be considered and may requ ire a decrease in beam energy or an increase in the margin between the target volume and the field edge to ensure adequate treatment. (C) 19 97 Elsevier Science Inc.