An evaluation of the variability of tumor-shape definition derived by experienced observers from CT images of supraglottic carcinomas (ACRIN protocol 6658)

Purpose: Accurate target definition is considered essential for sophisticated, image-guided radiation therapy; however, relatively little information has been reported that measures our ability to identify the precise shape of targets accurately. We decided to assess the manner in which eight "experts" interpreted the size and shape of tumors based on "real-life" contrast-enhanced computed tomographic (CT) scans. Methods and Materials: Four neuroradiologists and four radiation oncologists (the authors) with considerable experience and presumed expertise in treating head-and-neck tumors independently contoured, slice-by-slice, his/her interpretation of the precise gross tumor volume (GTV) on each of 20 sets of CT scans taken from 20 patients who previously were enrolled in Radiation Therapy Oncology Group protocol 91-11. Results: The average proportion of overlap (i.e., the degree of agreement) was 0.532 (95% confidence interval 0.457 to 0.606). There was a slight tendency for the proportion of overlap to increase with increasing average GTV. Conclusions: Our work suggests that estimation of tumor shape currently is imprecise, even for experienced physicians. In consequence, there appears to be a practical limit to the current trend of smaller fields and tighter margins.