Session Item

Daily image guided radiotherapy does not necessarily handle anatomical changes concerning organs at risk (OAR). Thus, dose to the OAR may be a limiting factor for stereotactic body radiation therapy (SBRT) as the dose may exceed tolerance levels. The aim is to justify a patient specific dose plan Quality Assurance (QA) for individual treatment sessions. 

Patient material consist of 24 patients referred for lung SBRT (3 x 22.5 Gy). A CBCT acquired prior at each treatment fraction, are matched trough a workflow starting with a bone match follow by a soft tissue (tumor) match (Online-match). As an offline strategy a solitary bony match is performed, leading to strategies with one starting from a bone match and the other from the tumor match. the three CBCTs are deformable registered to the planning CT (Velocity 4.1, Varian Medical Systems) and a ‘plan-of-the-day’ is calculated on the generated artificial-CT (aCT). The calculated dose distribution is deformed back to the original CT, then subtracted from the planed dose distribution. The dose difference for the OARs are examined. Due to OAR dose constrains defined by maximum values, only the 200 voxels (0.4 cm³) in an OAR (OAR₂₀₀) receiving the larges additional doses are included.

A total of 60 structure sets (10 patients, three CBCT and two deformations) are examined for correlation between the deform-registered and radiographer contoured OAR on each aCT. Dice-Similarity coefficient (DSC) and Hausdorff distance (HD) are calculated, in order to ensure the quality of the deformable registrations.

The association between the deform-registered and radiographer contoured structures (fig. 1) shows mean DSC ranging [0.92; 0.98] and the mean HD ranging [0.06; 0.10] cm. The 95% percentile HD ranging from [0.12; 0.38] cm, with deviations primarily from the esophagus and heart due to broad distributions.

The added mean dose to OAR₂₀₀ (fig. 2) ranging from 0.5 Gy for the trachea and up to 5 Gy for the aorta. The maximum dose difference for a single OARs is 18 Gy for the aorta. When comparing OAR₂₀₀ dose difference for the two match strategies it is see that on average OAR₂₀₀ receive more [0.1-1.1 Gy]   with Online-match then with Bone-match.

Patient specific dose plan QA shows in general good agreement within the deform-registered structures and radiographer contoured structures. The rather large HD for some regions of structures may be explained by variation in inter radiographer variations. However, large dose difference in some OAR (aorta and bronchial tree), indicates that inter fractional patient specific QA may be required to avoid exceeding dose constrains.