Abstract
In radiation therapy treatment planning, margins are added to the tumour volume to ensure that the correct radiation dose is delivered to the tumour in the presence of geometrical uncertainties. The van Herk margin formula (VHMF) was developed to calculate the minimum margin on the target to provide full coverage by 95% of the prescribed dose to 90% of the population. However, this formula is based on an ideal dose profile model that is not realistic for lung radiotherapy. The purpose of this study was to investigate the validity of the VHMF for lung radiotherapy with accurate dose calculation algorithms and respiratory motion modeling. Ultimately, the VHMF ensured sufficient target coverage, with the exception of small lesions in soft tissue; however, the derived PTV margins were larger than necessary. A novel planning approach using the VHMF was tested indicating the need to account for tumour motion trajectory and plan conformity.
Highlights
All of the accumulated plans demonstrated the characteristic shrinking of the 95% isodose as predicted by the van Herk model
It should be noted that the Planning Target Volume” (PTV) was the target volume of interest used to calculate the Conformation Number (CN) in the static plans as opposed to the accumulated plans where the CTV became the target volume used for the calculations
The CN of the accumulated plan is affected by both a reduction in volume from the PTV to the CTV, which should lower the CN, as well as the reduction in the volume of the 95% isodose between the static and accumulated dose distributions, which should increase the CN
Summary
Lung cancer stems from uncontrolled cell growth of lung tissue that leads to the rapid formation of harmful malignant lesions[2] It is currently the second most common cancer in Canada and continues to be the leading cause of cancer death for both men and women with a five-year relative survival ratio of only 16%1. Margin recipes are formulations that calculated the required PTV margin to provide adequate CTV dose coverage in the presence of errors for specific patient populations. They have been derived from dose-coverage probabilities, physical and biological considerations[13].
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