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Meningiomas are the most common primary intracranial tumor. They are developed at the expense of the arachnoid, with an overall incidence which has increased over the past decade. To date, there is no published specific guideline about meningiomas target volume. No prospective study has defined a consensus for delineation in meningiomas’ radiotherapy. Therefore, target volume definition is mainly based on retrospective studies, with a heterogeneous population of patients. The aim of this paper is to describe delineation guidelines for meningiomas’ radiotherapy as an adjuvant or definitive treatment with Intensity Modulated Radiation Therapy (IMRT) and stereotactic radiation therapy (SRT) techniques.

This guideline is based on a consensus endorsed by a global multidisciplinary group of brain tumor experts’ member of the ANOCEF (French neuro-oncology association). A two round modified Delphi consensus was achieved, and the consensus was adopted by the RAND/UCLA method. The third round was carried out in videoconference, in order to allow experts to debate and argue on remaining uncertain proposals.

Twenty experts from 17 radiotherapy center participated. After 3 rounds, all the proposals resulted in a consensus.

The ANOCEF guideline committee proposed to perform an unenhanced planning CT scan, merged with a post-contrast MRI obtained at the time of radiotherapy and preoperative MRI in case of adjuvant treatment.

GTV is defined by T1 contrast-enhancing lesion, thickened meninges, and directly invaded bone.

For IMRT, the CTV include:
- Grade I: No margin around the GTV.
- Grade II: Margin of 5 mm to expand GTV in normal brain tissue, hyperostosis, along the unthickened meninges and venous sinuses if the GTV is coming into contact.
- Grade III: Margin of 10 mm to expand GTV in normal brain tissue, hyperostosis, along the unthickened meninges, and optic or cranial nerves in contact with GTV.

In case of bone invasion, a margin of 5 or 10 mm in the healthy bone around the GTV is recommended, for grade II or III respectively. Otherwise, it is considered as an anatomical barrier and does not need to be included in the target volume.

In case of post-operative radiotherapy, no additional margin is required for CTV for grade I around tumor bed. A 5 and 10 mm margin is required for grade II and III. The cranial flap should only be included in the CTV only over 5 or 10 mm for grades II or III, in case of initially invaded bone. The drill holes and osteotomy areas should be included if they come into contact with target volume.

SRT is not recommended for grades II and III, excluding relapse situation. CTV corresponds to GTV without additional margin.

The current consensus provides a detailed delineation guideline for meningioma, suggesting smaller margins than the major studies published to date.