What is the most likely diagnosis for a well‑circumscribed, lobular mass in the left frontal lobe appearing three years after cranial irradiation?

Radiation-Induced Meningioma

The most likely diagnosis is radiation-induced meningioma. A well-circumscribed, lobular mass appearing in the left frontal lobe three years after cranial irradiation strongly suggests this diagnosis, particularly given the temporal relationship and characteristic imaging features 1, 2.

Key Diagnostic Features Supporting This Diagnosis

Temporal Relationship

• Radiation-induced meningiomas typically develop within the previously irradiated field with a mean latency period of 18.7-26.5 years after high-dose cranial radiation 2, 3
• However, shorter latency periods are well-documented, particularly in younger patients who received radiation during childhood 1, 4
• Cases with latency periods as short as 14 months have been reported, though 3 years falls within the recognized spectrum 4
• Younger age at time of irradiation correlates with shorter latency periods and higher risk of developing secondary tumors 5, 3

Radiographic Characteristics

• Well-circumscribed, lobular masses are classic imaging features of meningiomas 1
• The location within the frontal lobe is consistent with radiation-induced meningiomas, which develop within the radiation field 2, 6
• Meningiomas typically appear as homogeneously enhancing lesions on contrast-enhanced imaging 1

Critical Clinical Considerations

Aggressive Behavior Pattern

Radiation-induced meningiomas behave more aggressively than spontaneous meningiomas and require heightened vigilance 3:

• 100% recurrence rate after initial resection has been documented 3
• 38% demonstrate atypical or malignant histopathological features compared to only 8.5% of spontaneous meningiomas 6, 3
• Multiple meningiomas occur in 31% of radiation-induced cases 3
• Mean age at presentation is significantly younger (38.5-38.8 years) compared to spontaneous meningiomas (60.1 years) 6, 3

Histopathological Distinctions

• Complex cytogenetic aberrations are characteristic, particularly involving chromosomes 1p (89% of cases) and 6q (67% of cases) 3
• Progesterone receptor positivity and low proliferation indices do not correlate with benign behavior in radiation-induced meningiomas, unlike spontaneous cases 3
• WHO grade distribution shows 86.5% grade I and 11.5% grade II (atypical) in radiation-induced cases 6

Management Approach

Diagnostic Confirmation

• Gadolinium-enhanced MRI is the preferred imaging modality to characterize the lesion 1
• Histological confirmation through surgical resection is essential, as neuroradiological investigations alone are insufficient for definitive diagnosis 1
• Biopsy should be avoided in favor of complete resection when feasible, as it provides both diagnostic tissue and therapeutic benefit 1

Treatment Strategy

Optimal surgical resection with margins as wide as possible while avoiding functional deficits is the standard of care 1:

• Complete resection should be the goal given the 100% recurrence rate with incomplete resection 3
• Use of technical aids including neuronavigation, surgical microscopy, and intraoperative brain mapping optimizes surgical outcomes 1
• Postoperative MRI within 72 hours establishes baseline for surveillance 1

Adjuvant Therapy Considerations

• For atypical (WHO grade 2) or incompletely resected meningiomas, adjuvant radiotherapy should be considered 1
• However, additional radiation carries risk of inducing further secondary neoplasms in this already at-risk population 1
• Stereotactic radiosurgery may be appropriate for small, surgically inaccessible lesions 1

Surveillance Requirements

Lifelong clinical and radiological surveillance is mandatory for all patients with prior cranial irradiation 1, 2:

• Serial MRI imaging at regular intervals to detect new lesions early 1
• Enhanced vigilance for multiple meningiomas, which occur more frequently in radiation-induced cases 2, 3
• Monitor for signs of intracranial hypertension, focal neurological deficits, or seizures 7

Common Pitfalls to Avoid

• Do not assume benign behavior based on WHO grade I histology or favorable molecular markers - radiation-induced meningiomas are inherently more aggressive regardless of grade 3
• Do not perform stereotactic biopsy alone - complete resection provides both diagnosis and optimal treatment 1
• Do not delay intervention once diagnosis is suspected - early complete resection offers the best chance of disease control given the certain recurrence pattern 3
• Do not discontinue surveillance after successful resection - lifelong monitoring is required due to high recurrence rates and risk of additional radiation-induced tumors 1, 2