Discussion topic:
When using inhomogeneity corrections on treatment plans, explain how the dose is effected by the presence of bone and air. Which way do the isodose lines shift? Why? How accurately do the algorithms display these isodose lines?
Discussion post:
Inhomogeneity corrections are vital to accurate dosimetry. Tissue densities vary widely in treatment areas such as the lungs and bone, as such beam attenuation or lack thereof must be considered when producing an accurate treatment plan. Heterogeneity correction is a computation method utilized by treatment planning systems that accounts for differences in tissue density. When a photon beam traverses an air cavity the result is an increase in dose at the air cavity-tissue interface. Conversely, when a photon beam interacts with bone the area behind the bone, distal to the treatment head will experience an area of lower dose. In a lung treatment plan the low attenuation coefficient of the airfilled lung tissue compared to the high attenuation coefficient of bone produces a great deal of inhomogeneity in dose distribution.
Departmental physics guidelines typically seek to deliver a treatment plan within 3% accuracy. The 2004 TG-65 AAPM study published in Med Phys sought to ensure treatment accuracy by determining the effect of inhomogeneity on radiation treatment planning.1 This study found that isodose distributions designed to emulate a lung cavity showed areas with as high as an 11% difference when compared to a homogeneous cube measurement. The study predominantly examined the current (2004) heterogeneity correction methods in use such as convolution, superposition, Monte Carlo, and found some small variations. However, TG-65 also pointed out how greatly heterogeneity can affect a plan that does not account for differences in tissue density.1
References
Papanikalou N, Battista J, Boyer A, et al. Tissue inhomogeneity corrections for megavoltage photon beams. Med Phys. AAPM report 85. 2004. https://www.aapm.org/pubs/reports/rpt_85.pdf