BACKGROUND: and Purpose: Image-guided radiotherapy (IGRT) is widely used but data directly relating setup errors to patient outcome is scarce. This study investigates the relationship between residual IGRT shifts and overall patient survival, and uses the observed relations to identify structures sensitive to radiation dose.
MATERIALS AND METHODS: Residual shift data for 780 NSCLC patients was summarised for each patient over the course of treatment by determining the mean shifts, standard deviations and the vector shift in the direction of the heart. These variables were related to overall survival, and significant variables were used to produce Kaplan-Meier plots of survival. The effect of shift directionality was studied by splitting the cohort into left, right, anterior, posterior, superior and inferior groups, and by analysing the vector shift in the direction of the heart. The observed relationship was independently validated in an oesophageal cancer cohort (n = 177).
RESULTS: The shift data showed strong associations with survival. Left and right cohorts showed opposite directional shift effects, suggesting shifts towards the mediastinum have a negative effect on survival. Projection of the vector shift in the direction of the heart showed that patients with a residual shift towards the heart have significantly worse overall survival (p = 0.007, hazard ratio 1.091). The same effect was observed in the oesophageal cancer cohort (p = 0.041, hazard ratio 1.164).
CONCLUSIONS: Residual shift metrics derived from IGRT data can categorise both NSCLC and oesophageal cancer patients into populations with significantly different survival times based on the size of the residual shift in the direction of the heart, thus providing evidence of the importance of using strict IGRT protocols to spare organs at risk and at the same time highlighting the heart as a dose sensitive organ.