A systematic evaluation of air cavity dose perturbation in megavoltage x‐ray beams

Abstract

The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.

Evaluation and Commissioning of Commercial Monte Carlo Dose Algorithm for Air Cavity

“…A re-build up region occurs as electrons are once again generated in water-equivalent material. Allen et al reported that the dose reduction near an air cavity was greater for smaller field size, higher energy, larger air cavity size, and shallower depth in water where the air cavity was situated [10]. Klein et al results showed that following a 2.0 cm wide air channel for a 4 MV X-ray with 4 × 4 cm 2 field, there was an 11% under dose at the distal interface, while a 2.0 cm cubic cavity yielded only a 3% loss [4].…”

Dosimetric accuracy of the Acuros XB and Anisotropic analytical algorithm near interface of the different density media for the small fields of a 6- MV flattening-filter-free beam

“…On the other hand, the under dosing that gives rise to the skin-sparing effect could reduce the dose delivered to the rectal wall during treatment of prostate cancer, as reported by Holmes et al (1) .…”

Dosimetric evaluation of IMRT plan for homogenous and inhomogeneous medium using AAPM TG-119 protocol

“…In addition, about the measurement in phantom C, the dose reduction is higher than the measurement in phantom B, because the location of measurement is in the low-density area [4] [15]. The phenomena might be caused by the target mass attenuation which might involve in the measurements.…”

“…In fact, human body consists of materials having different densities such as lungs, soft tissues and bones which may represent inhomogeneity condition. Hence, the procedures of dose verification in the protocol might not fully represent the dose verification for human body (patients) and might cause a dose perturbation due to the inhomogeneity issues [4]. Moreover, the dose perturbation due to inhomogeneity issues might be more complex due to the use of small field technique for treating small volume targets.…”