Variability in target volume delineation on CT scans of the breast

Purpose: To determine the intra- and interobserver variation in delineation of the target volume of breast tumors on computed tomography (CT) scans in order to perform conformal radiotherapy. Materials and Methods: The clinical target volume (CTV) of the breast was d elineated in CT slices by four ra ration once og sts on our clinically used delineation system. The palpable glandular breast tissue was marked with a lead wire on 6 patients before CT scanning, whereas 4 patients were scanne d without a lead wire. The CTV was drawn by each observer on three separate occasions. Planning target volumes (PTVs) were constructed by expanding th e CTV by 7 mm in each direction, except toward the skin. The deviation in t he PTV extent from the average extent was quantified in each orthogonal dir ection for each patient to find a possible directional dependence in the ob server variations. In addition, the standard deviation of the intra- and in terobserver variation in the PTV volume was quantified. For each patient, t he common volumes delineated by all observers and the smallest volume encom passing all PTVs were also calculated. Results: The patient-averaged deviations in PTV extent were larger in the p osterior (42 mm), cranial (28 mm), a dial (24 mm) directions than in the an terior (6 mm), caudal (15 mm), and lateral (8 mm) directions. The mean intr aobserver variation in volume percentage (5.5%, 1 SD) was much smaller than the interobserver variation (17.5%, 1 SD). The average ratio between the c ommon and encompassing volume for the four observers separately was 0.82, 0 .74, 0.82, and 0.80. A much lower combined average ratio of 0.43 was found because of the large interobserver variations. For the observer who placed the lead wire, the intraobserver variation in volume was decreased by a fac tor of 4 on scans made with a lead wire in comparison to scans made without a lead wire. For the other observers, no improvement was seen. Based on th ese results, an improved delineation protocol was designed. Conclusions: Intra- and especially interobserver variation in the delineati on of breast target volume on CT scans can be rattier large. A detailed del ineation protocol making use of CT scans with lead wires placed on the skin around the palpable breast by the delineating observer reduces the intraob server variation. To reduce the interobserver variation, better imaging tec hniques and pathology studies relating glandular breast tissue to imaging m ay be needed to provide more information on the extent of the clinical targ et volume. (C) 2001 Elsevier Science Inc.