Air Cavity Effects on the Absorbed Dose for 4-, 6- and 10-MV X-ray Beams: Larynx Model

Kim, Chang Seon; Yang, Dae Sik; Kim, Chul Yong; Choi, Myung Sun

Abstract

PURPOSE
When an x-ray beam of small field size is irradiated to target area containing an air cavity, such as larynx, the underdosing effect is observed in the region near the interfaces of air and soft tissue. With a larynx model, air cavity embedded in tissue-equivalent material, this study is intended for examining parameters, such as beam quality, field size, and cavity size, to affect the dose distribution near the air cavity.
MATERIALS AND METHODS
Three x-ray beams, 4-, 6- and 10-MV, were employed to perform a measurement using a 2cm (width)xL (length in cm, one side of x-ray field used)x2cm (height) air cavity in the simulated larynx. A thin window parallel-plate chamber connected to an electrometer was used for a dosimetry system. A ratio of the dose at various distances from the cavity-tissue interface to the dose at the same points in a homogeneous phantom (observed/expected ratio, O/E), normalized buildup curves, and ratio of distal surface dose to dose at the maximum buildup depth were examined for various field sizes. Measurement for cavity size effect was performed by varying the height (Z) of the air cavity with the width kept constant for several field sizes.
RESULTS
No underdosing effect for 4-MV beam for fields larger than 5cmx5cm was found. For both 6- and 10-MV beams, the underdosing portion of the larynx at the distal surface was seen to occur for small fields, 4cmx4cm and 5cmx5cm. The underdosed tissue was increased in its volume with beam energy even for similar surface doses. The relative distal surface dose to maximum dose was changed to 0.99 from 0.95, 0.92, and 0.91 for 4-, 6-, and 10-MV, respectively, with increasing field size, 4cmx4cm to 8cmx8cm. For 6- and 10-MV beams, the dose at the surface of the cavity ismeasured less than the predicted by about two and three percent, respectively, but decrease was found for 4-MV beam for 5cmx5cm field. For the 4cm x L x Z (height in cm), varying depth from 0.6 to 4.8cm, cavity, O/E > 1.0 was observed regardless of the cavity size for any field larger than about 8cmx8cm.
CONCLUSION
The magnitude of underdosing depends on beam energy, field size, and cavity size for the larynx model. Based on the result of the study, caution must be used when a small field of a high quality x-ray beam is irradiated to regions including air cavities, and especially the region where the tumor extends to the surface. Low quality beam, such as, 4-MV x-ray, and larger fields can be used preferably to reduce the risk of underdosing, local failure. In the case of high quality beams such as 6- and 10-MV x-rays, however, an additional boost field is recommended to add for the compensation of the underdosing region when a typically used treatment field, 5cmx5cm, is employed.