International Consensus Guidelines for Radiation Target Volume Delineation for Skull Base Chordoma-A Particle Therapy Co-operative Group Initiative.

PURPOSE: To develop consensus guidelines for radiation therapy target volume delineation in skull base chordoma. METHODS AND MATERIALS: An international panel of 15 radiation oncologists with experience treating skull base chordoma received anonymized case histories and Digital Imaging and Communications in Medicine (DICOM) data of preoperative and postoperative imaging from 3 representative cases (superior, mid, and lower clival chordomas) after varied degrees of surgical resection. Panelists defined the preoperative gross tumor volume (GTV), residual GTV (if any) and an intermediate and high dose clinical target volume (CTV). A count mapping 3-dimensional histogram generated contours based on overlapping voxels included in each target to identify the approximately 40% and 75% levels of consensus in the panel. Qualitative comparison of these contours identified areas of consistent practice and clinically relevant variation. A modified Delphi consensus approach was used to formulate contouring recommendations and generate consensus contours. Similarities between individual contours and the consensus contour were assessed using Sorensen-Dice similarity coefficient and mean distance to agreement (DTA) in centimeter. RESULTS: Delineation of preoperative GTV was most consistent with a mean Dice coefficient of 0.85 and mean DTA of 0.13 cm. Residual GTV (present in 2 cases) was the least congruent with a mean Dice of 0.45 and mean DTA of 0.37 cm. Consensus modifications of residual GTV included identification of residual disease in the lateral clivus, cavernous sinus, atlantooccipital joint, and prevertebral soft tissues. The panel recommended CTV_Intermediate encompass the entire tumor bed with median 5 mm margin, the remainder of the clivus, and be modified to account for decompression of previously "pushing" tumor. CTV_High should encompass bone and mucosa of the entire primary tumor bed and any residual disease with median 5 mm margin. CONCLUSIONS: This international collaborative effort establishes contouring guidelines for skull base chordoma based on knowledge of patterns of failure, patterns of spread, surgical findings, and existing practice patterns within the particle therapy community.