SU‐E‐T‐889: Target Definition in TomoTherapy Lung SBRT Treatment Plans

Abstract

Purpose: An unresolved question in lung stereotactic body radiotherapy (SBRT) treatment planning is whether a free breathing computed tomography (CT) image set, an image set with the electron density of the internal target volume (ITV) set to water, or an averaged 4DCT scan provides the most realistic representation of the physical situation. Methods: TomoTherapy Hi Art treatment plans created on free breathing scans for three patients receiving lung SBRT at our institution were compared to plans created on 1) a scan with the ITV electron density set to water, 2) a scan with the planning target volume (PTV) electron density set to water, 3) a phase averaged 4DCT, and 4) a time averaged 4DCT. The comparison plans were made by creating TomoTherapy delivery quality assurance (DQA) phantoms from the additional image sets, then recalculating the free breathing plan on these phantoms. All doses were exported to MIMVista for analysis. Results: The maximum isodose line covering the entire PTV and ITV showed little change among the different plans for each patient. The maximum point dose to the ITV and the PTV was similar for the free breathing and average scans. The maximum point doses showed a slight increase for both the ITV and PTV override plans for all patients. The PTV override plan pushed the maximum point dose out of the ITV into the PTV. Conclusions: Plans calculated on the scans with density overridden target volumes show a marked change in dose distribution and target coverage from plans calculated on free breathing and average scans. The different treatment planning techniques are clearly not equivalent. The use of a poor model could lead to over or under‐dosing of the target. Further study is needed to determine which plan most accurately models the true physical situation.