Faculty Collaboration Database

Reducing loss in lateral charged-particle equilibrium due to air cavities present in x-ray irradiated media by using longitudinal magnetic fields. Med Phys 2001 Apr;28(4):603-11

Date

05/08/2001

Pubmed ID

11339758

DOI

10.1118/1.1357816

Scopus ID

2-s2.0-0035051983 (requires institutional sign-in at Scopus site)   20 Citations

Abstract

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 x 0.5 cm2 beamlets. Comparison is also made with 60Co 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.

Author List

Naqvi SA, Li XA, Ramahi SW, Chu JC, Ye SJ

MESH terms used to index this publication - Major topics in bold

Cobalt Radioisotopes
Electrons
Humans
Lung Neoplasms
Magnetics
Models, Statistical
Monte Carlo Method
Neoplasms
Photons
Radiometry
Radiotherapy
X-Rays