Prostate cancer skull metastasis invading the brain

Clinical History

The patient underwent a re-staging bone scan for advanced metastatic prostate carcinoma under hormone suppression therapy. He complained of low back pain and headaches. Prostate-specific antigen was 250 ng/ml.

Imaging Findings

Bone scintigraphy showed disseminated osteoblastic activity with punctum maximum in the lower lumbar spine and pelvis. There was also a large circumscribed activity zone in the right posterior calvarium (Fig. 1). Cranial CT including bone algorithm reconstructions was performed. A large osteoblastic metastasis of the right posterior calvarium was confirmed. Pre- and post-contrast soft window CT images of the brain showed a hypervascular mass extending from the skull bone into the brain. The borders towards the brain were irregular, suggesting direct invasion rather than displacement (Fig. 2). There were also a few liquid inclusions at the tumour periphery, indicating previous haemorrhage. Radiation therapy of the lesion was initiated.

Discussion

Bone metastases to the skull are increasingly found on imaging studies, due to more effective treatment and higher life expectancy of patients with advanced neoplastic diseases [1]. Skull metastases are far more common in breast cancer than in other neoplasms. Prostate cancer, in spite of its distant location, is the second most common primary tumour metastasising to the skull [2]. Most skull metastases are purely intraosseous lesions confined to the calvarium or skull base [1]. Dural involvement, either by continuous or by haematogenous spread is extremely uncommon [3]. Both calvarial and epidural metastases typically do not transgress the dura, which forms a barrier against tumour spread. Bone metastases extending intracranially therefore show a characteristic biconvex or nodular shape, usually associated with brain displacement away from the inner table [4]. In the present case, the brain parenchyma was not displaced, but infiltrated by a mass with a broad base at the inner table, irregular borders with the brain, and avid contrast enhancement. At the interface of tumour and brain tissue, hypoattenuating, non-enhancing foci were present, probably indicating previous haemorrhage.
Direct invasion of the brain parenchyma from a metastasis of the calvarium, to our knowledge, has been described only once, in a patient with large cell lung cancer, in whom the neoplastic brain invasion finally resulted in a fatal intracerebral haemorrhage [5]. The present description is probably the first case of a gross brain invasion from a purely osteoblastic skull metastasis. Compared with metastases of the skull base, which may occasionally lead to cranial nerve palsies, the diagnosis is often delayed in metastases of the calvarium. Radionuclide bone scans are still extremely valuable to detect those lesions. Activity and distribution of metastases on a whole-body skeletal scintigram may also suggest the nature and location of the primary tumour. Although CT is better in characterising bony lesions of the skull, MR is more sensitive and provides more detailed information about dural and brain involvement [4]. However, in the absence of a bony skull lesion, the differentiation between intra- and extraaxial tumours at the brain periphery may be difficult [6].

Differential Diagnosis List

Final Diagnosis

Skull metastasis with intracerebral extension

URL:	https://www.eurorad.org/case/9416
DOI:	10.1594/EURORAD/CASE.9416
ISSN:	1563-4086