Chest wall irradiation with MLC-shaped photon and electron fields.

Abstract

To improve the treatment technique for chest wall irradiation, using the multileaf collimator (MLC) of the MM50 Racetrack Microtron to shape both photon and electron beams, and to check the dose delivery in the match-line region of these fields for the routine and improved technique. Using diode and film phantom measurements, the optimal number of photon beam segments and their positions relative to the electron beam were determined. On phantoms, and during actual patient treatment using in vivo dosimetry, the dose homogeneity in the match-line region was determined for both the routine and improved techniques. Three photon beam segments (9-mm gap, perfect match, and 9-mm overlap) were used to match the electron beam, resulting in minimum-maximum dose values in the match-line region of 88-109%, compared to 80-115% for the routine technique (2 photon beam segments). During patient treatment, the average minimum and maximum dose values were 95% and 115%, respectively, compared to 78% and 127%, respectively, for the routine technique. The interfraction variation in dose delivery was reduced from 11.0% (1 SD) to 4.6% (1 SD). The actual treatment time was reduced from 10 to 4.5 min. Using the MLC of the MM50 to shape both photon and electron beams, an improved treatment technique for chest wall irradiation was developed, which is less labor intensive, faster, and yields a more homogeneous, and better reproducible dose delivery.