4D planning over the full course of fractionation: assessment of the benefit of tumor trailing.

Tumor trailing techniques have been proposed as a method of reducing the problem of intrafraction motion in radiotherapy. However the dosimetric assessment of trailing strategies is complicated by the requirement to study dose deposition over a full fraction delivery. Common 4D planning strategies allowing assessment of dosimetric motion effects study a single cycle acquired with 4DCT. In this paper, a methodology to assess dose deposited over an entire treatment course is advanced and used to assess the potential benefit of tumor trailing strategies for lung cancer patients. Two digital phantoms mimicking patient anatomy were each programmed to follow the tumor respiratory trajectory observed from 33 lung cancer patients. The two phantoms were designed to represent the cases of a small (volume = 13.6 cm3) and large (volume = 181.7 cm3) lung lesion. Motion margins required to obtain CTV coverage by 95% of the prescription dose to 90% of the available cases were computed for a standard treatment strategy and a trailing treatment strategy. The trailing strategy facilitated a margin reduction of over 30% relative to the conventional delivery. When the dose was computed across the entire delivery for the 33 cases, the trailing strategy was found to significantly reduce the underdosage to the outlier cases and the reduced trailing margin facilitated a 15% (small lesion) and 4% (large lesion) reduction for the mean lung dose and 7% (small lesion) and 10% (large lesion) for the mean esophagus dose. Finally, for comparison an ideal continuous tracking strategy was assessed and found to further reduce the mean lung and esophagus dose. However, this improvement comes at the price of increased delivery complexity and increased reliance on tumor localization accuracy.