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Reducing loss in lateral charged‐particle equilibrium due to air cavities present in x‐ray irradiated media by using longitudinal magnetic fields
Author(s) -
Naqvi Shahid A.,
Li X. Allen,
Ramahi Shada W.,
Chu James C.,
Ye SungJoon
Publication year - 2001
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1357816
Subject(s) - electron , irradiation , photon , materials science , range (aeronautics) , fluence , dosimetry , monte carlo method , magnetic field , secondary electrons , computational physics , physics , atomic physics , optics , nuclear medicine , nuclear physics , composite material , medicine , statistics , mathematics , quantum mechanics
The underdosing of lesions distal to air cavities, such as those found in upper respiratory passages, occurs due to the loss in lateral charged‐particle equilibrium (CPE). The degree of underdosing worsens for smaller field sizes, resulting in more frequent recurrence of the cancer treated. Higher photon energies further aggravate the outcome by producing longer second build‐up regions beyond the cavity. Besides underdosing, the larger lateral spread 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 the intended treatment. The clinical concern is further intensified by the failure of traditional treatment planning systems to even account for such defects. In this work, the use of longitudinal magnetic fields on the order of 0.5 T is proposed for alleviating lateral electronic disequilibrium due to the presence of air cavities in the irradiated volume. The magnetic field enforces lateral CPE by restricting the lateral range of electrons in the air cavity. The problem is studied in a simple water‐air‐water slab geometry using EGS4 Monte Carlo simulations for 6 MV photons. Electronic disequilibrium is evaluated for beams of various sizes, shapes and intensity distributions constructed by linear superposition of the dose distributions for 0.5 × 0.5 cm 2beamlets. Comparison is also made with60 Co irradiation. The results indicate that the lateral confinement of secondary electrons in the air cavity by sub‐MRI strength longitudinal fields is effective in reducing deterioration of dose distributions near tissue‐air interfaces. This can potentially reduce recurrence rates of cancers such as the larynx carcinoma.