KEY POINTS
- An international PTCOG panel of 15 radiation oncologists from the United States, Europe and Asia/Australasia independently contoured three representative skull base chordoma cases with superior, middle and inferior clival disease. Consensus recommendations were developed using a modified Delphi process.
- Preoperative GTV delineation showed relatively strong agreement, with a mean Dice coefficient of 0.85 and mean distance to agreement of 1.3 mm. Residual postoperative GTV was much less consistent, with mean Dice only 0.45 and mean distance to agreement 3.7 mm.
- Areas particularly vulnerable to missed residual disease included the lateral clivus, cavernous sinus, atlanto-occipital joint and prevertebral soft tissues. The panel stressed reviewing the preoperative GTV, operative findings and postoperative MRI together rather than interpreting postoperative imaging in isolation.
- For CTV_Intermediate, intended for approximately 50-54 Gy, the panel recommends including the entire tumor bed with a median 5-mm mucosal margin, plus the remainder of the clivus. The volume should be edited after surgical decompression rather than continuing into displaced temporal lobe tissue or above the restored optic apparatus.
- For CTV_High, conventionally treated to approximately 74 Gy, the consensus is to cover the bone and mucosa of the entire preoperative tumor bed even after gross total resection, generally using a median 5-mm margin. This differs importantly from approaches that irradiate only visible postoperative residual disease.
- Previously involved dura should be included in CTV_High when tumor extended into the prepontine cistern, but the CSF itself should not be included. Previously involved adjacent bone such as the petrous apex or occipital condyle receives a median 5-mm margin, while previously involved prevertebral soft tissue receives a median 4-mm margin.
- The postoperative GTV should not automatically include nasoseptal flap or fat-graft tissue within the surgical defect, although the bone and mucosa of the original tumor bed remain part of the target. Air within the sinonasal surgical cavity is excluded from both consensus CTVs.
- The panel reached 100% agreement that all gross residual disease should remain within the target even when it abuts the brainstem or optic apparatus, while accepting that actual high-dose coverage may need to be compromised to respect critical OAR constraints.
- Although developed primarily by particle-therapy specialists, the authors explicitly state that the target-volume principles are applicable to fractionated photon as well as particle therapy. The recommendations remain expert consensus derived from three representative cases rather than prospective outcome validation.
CLINICAL TAKEAWAY
These guidelines provide a much-needed common contouring framework for skull base chordoma and may be particularly useful for postoperative cases where anatomy has changed substantially after decompression. The key principle is to treat the full preoperative high-risk tumor bed rather than visible residual disease alone, while anatomically editing CTVs around restored normal structures.
SOURCE
International Journal of Radiation Oncology, Biology, Physics