Introduction: Dermoid cysts are rare lesions of the central nervous system with a slow rate of growth. For this reason, they are rarely discovered during infancy. Although benign, they may be associated with devastating complications secondary to mass effect or meningitis. Discovery of completely asymptomatic dermoid cysts in the pediatric population is exceedingly rare. Little is known about the rate of growth for dermoid cysts and there have been no reports in the literature about the growth pattern of these lesions. We report the unique case of a posterior fossa dermoid cyst discovered in a 5-month-old girl and monitored for 2.5 years with serial imaging studies before surgical resection was performed.

Methods: Magnetic resonance studies of a posterior fossa dermoid cyst spanning the course of 2.5 years were retrospectively studied. The volume of the cyst was measured at 5 months, 11 months, 22 months and 35 months of age. These measurements enabled the analysis of the 3-dimensional rate of growth of this particular dermoid cyst.

Results: The dermoid cyst in this case report exhibited a linear growth pattern. The increase in volume followed the classic appearance of a cuboid sequence, which is also consistent with linear growth in all three dimensions.

Conclusions: Dermoid cysts present certain diagnostic challenges on magnetic resonance imaging. Unlike true tumors, which grow in an exponential pattern, dermoid cysts appear to exhibit a linear rate of growth in all three dimensions. Correct and prompt diagnosis is crucial for the early surgical treatment of these cysts in order to minimize morbidity and mortality.

Patient Care: Recognition of the limitations of magnetic resonance imaging in diagnosing dermoid cysts should prompt neurosurgeons to further investigate any questionable pediatric lesions, because correct and rapid diagnosis is crucial for early surgical treatment of these cysts to minimize morbidity and mortality.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Acknowledge the diagnostic challenges presented by dermoid cysts on magnetic resonance imaging. 2) Recognize that dermoid cysts exhibit a linear rate of growth, which is different from the growth pattern of true pediatric intracranial tumors. 3) Recognize the importance of prompt surgical resection of dermoid cysts, even in patients lacking symptomatology.

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