Correction of conebeam CT values using a planning CT for derivation of the "dose of the day".

BACKGROUND AND PURPOSE Verification of the actually delivered 3D dose distribution during each treatment fraction ("dose of the day") is the most complete and clinical relevant "in-vivo" check of an IMRT treatment. To do this, during patient treatment portal dose images are routinely acquired with our electronic portal imaging device to derive the delivered fluence map for each treatment field. In addition, a conebeam CT scan is acquired just prior to treatment to derive the patient geometry at the time of treatment. However, the use of conebeam CT scans for dose calculation is hampered by inaccuracies in the conversion of CT values to electron densities due to an enlarged scatter contribution. MATERIALS AND METHODS In this work, a method is described for mapping of Hounsfield Units of the planning CT to the conebeam CT scan, while accounting for non-rigidity in the anatomy, e.g. related to weight loss, in an approximate way. The method was validated for head and neck cancer patients by comparing dose distributions calculated using adjusted Hounsfield Units with a golden standard. RESULTS AND CONCLUSIONS The observed dose differences were less than 1% in the majority of points, and in at least 96% of the points a 3D gamma analysis resulted in gamma values of less than 1 when applying a 2%/2mm criterion, showing that this straightforward approach allows for an accurate dose calculation based on conebeam CT scans.