Stereotactic radiosurgery-radiotherapy: Should Monte Carlo treatment planning be used for all sites?

To evaluate a Monte Carlo (MC) treatment planning system for CyberKnife treatments of cranial and extracranial lesions and determine whether it is necessary for all treatment sites. Dose distributions are compared to those calculated with a ray-tracing algorithm. Maximum doses and dose-volume histograms for the target and selected critical structures are analyzed.

The CyberKnife is used for stereotactic radiosurgery-radiotherapy of intracranial lesions (91) as well as stereotactic body radiotherapy for lesions in the spine (24), lung (58), and pelvis (36). The Multiplan system is an inverse treatment planning system which uses an effective path length (EPL) algorithm (sometimes referred to as ray-trace) for dose calculations. In addition, an MC algorithm became clinically available in late 2007.

The maximum doses calculated by the EPL to targets in the lung were uniformly larger than the doses calculated by MC by up to a factor of 1.32. In addition, large differences in target and critical organs' dose coverage were observed. In general, more beams traversing larger distances through low density lung are associated with larger differences. For other sites such as brain and pelvis targets the differences in maximum doses and tumor coverage were generally less than 5% between the 2 calculation methods.

The MC algorithm should be consistently used for treatment plans of lung lesions and lesions near large air cavities, but the faster EPL algorithm is adequate for treatment sites with less tissue heterogeneity.