Biparietal foramen are not an uncommon finding on a routine skull radiograph, but occasionally they can become "giant," simulating a lytic lesion.
According to available sources, these foramina are heritable in an autosomal dominant pattern, and represent part of a spectrum of bifid cranium (bifid cranium at one extreme and parietal foramen at the other).
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1. Fein JM, Brinker RA. "Evolution and significance of giant parietal foramina" J. Neurosurg. Vol:37. October, 1972.
2. Little BB, Knoll KA, Klein VR, et al. "Hereditary Cranium Bifidum and Symmetric Parietal Foramina Are the Same Entity." American Journal of Medical Genetics 35:453-458 (1990)