Conformal radiotherapy planning of cervix carcinoma: differences in the delineation of the clinical target volume. A comparison between gynaecologic and radiation oncologists.

Purpose: To assess uncertainties in the definition of the clinical target volume (CTV) for patients scheduled for primary radiotherapy of cervix carcinoma. Methods and materials: Seven physicians (five radiation oncologists and two gynaecologists) independently contoured the CTVs for three patients. All observers were provided with the same clinical information. CTVs were entered directly in the treatment planning system. Differences were analysed qualitatively and quantitatively. Results: The qualitative analysis revealed a good agreement by all observers on anatomical structures identified to be at risk for tumour spread. Quantitatively, however, a large interobserver variability was found. The ratio between largest and smallest volumes ranged between 3.6 and 4.9 for all observers (3.6 ‐4.9 for the radiation oncologists, 1.3 ‐ 2.8 for the gynaecologists). The median three-dimensional difference in gravity centres ranged between 10.9 and 26.3 mm for the respective patients. The ratio of common volumes to encompassing volumes ranged between 0.11 and 0.13 for the radiation oncologists, and between 0.30 and 0.57 for the gynaecologists. Conclusions: Although there was a good consistency in outlined anatomical structures, for the radiation therapy of carcinomas of the uterine cervix a large interobserver variability in CTV delineation concerning the magnitude and relative location of volumes was observed. Compared to other factors, e.g. set-up and organ motion, interobserver variability in CTV definition seems to have the highest impact on the geometrical accuracy in the radiotherapy of this tumour entity. q 2002 Elsevier Science Ireland Ltd. All rights reserved.