Anterior Visual Pathway(AVP) Meningiomas

"Introduction: Intensity Modulated Radiation Therapy (IMRT) is therapy is the standard technique to minimize dose to nearby organs at risk (OAR) when treating tumors involving or abutting the Optic Nerve. Proton therapy may further decrease the dose to OAR while maintaining primary target coverage. A dosimetry study compared IMRT photons to pencil-beam protons to evaluate the utility of protons in these tumors.Methods: Four representative tumors involving the AVP, treated with IMRT were planned with pencil-beam proton planning. Equivalent target coverage and OAR dose- volume restrictions were applied to both modalities, and dosimetric outcomes compared. Patient 1: large orbital schwannoma. Patients 2,3,4: orbital optic nerve sheath, clinoid, and cavernous sinus meningiomas.Results: Patient 1: ipsilateral retina V20, and mean dose were significantly less with protons. Chiasm dose was substantially less with protons. Pituitary mean dose was substantially less with protons Patient 2 and 3: Ipsilateral retina, pituitary doses, and brain V5Gy, all received significantly less dose with protons. Chiasm dose was no different in patient 2 (due to proximity to target volume), but markedly reduced in patient 3. Pt four: Chiasm V40Gy, Pituitary mean and maximum dose, and ON V40 are all less with proton therapy. Brain V5 was less for protons (50%-70%) in all patients. Conclusions: Pencil beam protons decrease dose to OAR in all but small apical tumors. OAR 1cm to 5cm distant to target volumes receive substantially less dose with protons but clinically relevant differences with IMRT is variable with cavernous sinus and para-clinoid tumors benefiting for more OAR. Brain dose is much less with protons (V5Gy). Protons should be considered for large benign tumors involving the AVP where OAR dose differences impact long term toxicity risk. Small apical tumors can be treated with IMRT photons. Reduced brain dose theoretically may reduce second malignancies."