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Some children with optic pathway gliomas present with proptosis related to intraorbital tumor extension. The radiological assessment of chemotherapeutic response in these patients can be complicated by irregular tumor shape and lack of relation between tumor volume and cosmetic effect.We propose that proptosis measurements and derivation of a proptosis index can be a useful adjunct to the measurement of tumor volume in the radiological assessment of chemotherapeutic response. The proptosis index was derived as the ratio of the difference in proptosis between eyes postchemotherapy to that prechemotherapy. A series of six patients with proptosis and the diagnosis of an optic nerve tumor from an optic pathway glioma registry demonstrate by case example the correlation between the proptosis index and the clinical and radiographic response to chemotherapy.We have found that a proptosis index <1 correlates with a chemotherapeutic maintained response and an index >1 correlates with progressive disease.