Reducing loss in lateral charged-particle equilibrium due to air cavities present in x-ray irradiated media by using longitudinal magnetic fields

The underdosing of lesions distal to air cavities, such as those found in u pper respiratory passages, occurs due to the loss in lateral charged-partic le equilibrium (CPE). The degree of underdosing worsens for smaller field s izes, resulting in more frequent recurrence of the cancer treated. Higher p hoton energies further aggravate the outcome by producing longer second bui ld-up regions beyond the cavity. Besides underdosing, the larger lateral sp read of secondary electron fluence in the air cavity produces diffuse dose distributions at the tissue-air interface for shaped or intensity modulated fields. These disequilibrium effects create undesirable deviations from th e intended treatment. The clinical concern is further intensified by the fa ilure of traditional treatment planning systems to even account for such de fects. In this work, the use of longitudinal magnetic fields on the order o f 0.5 T is proposed for alleviating lateral electronic disequilibrium due t o the presence of air cavities in the irradiated volume. The magnetic held enforces lateral CPE by restricting the lateral range of electrons in the a ir cavity. The problem is studied in a simple water-air-water slab geometry using EGS4 Monte Carlo simulations for 6 MV photons. Electronic disequilib rium is evaluated for beams of various sizes, shapes and intensity distribu tions constructed by linear superposition of the dose distributions for 0.5 x 0.5 cm(2) beamlets. Comparison is also made with Co-60 irradiation. The results indicate that the lateral confinement of secondary electrons in the air cavity by sub-MRI strength longitudinal fields is effective in reducin g deterioration of dose distributions near tissue-air interfaces. This can potentially reduce recurrence rates of cancers such as the larynx carcinoma . (C) 2001 American Association of Physicists in Medicine.