Teaching Case

This patient presented with a 2-month history of unrelenting headaches associated with nausea and vomiting. There were no focal neurological deficits. He had a background of advanced prostate cancer initially diagnosed 4 years before. He developed bony pelvic and lumbar vertebral metastases 2 years later but had been clinically stable lately.

Unenhanced selected axial CT image (Fig. 1 a) shows a right frontal calvaria-based lesion with a large (3 cm) intracranial component extending internally. This is associated with considerable regional mass effect and adjacent parenchymal distortion. A small extra-cranial component is also seen extending out of the calvarium (this was retrospectively palpable clinically).

Following administration of contrast (Fig. 1 b), there is homogenous enhancement throughout the lesion. The images obtained slightly more inferiorly (Fig. 2 a, b) show evidence of invasive enhancing tissue into the adjacent right frontal sinus (arrowed on Fig. 2 a). These images also show a smaller but similarly enhancing left posterior parieto-occipital subcalvarial lesion (arrowed on Fig. 2 b). Axial bone windowed CT images (Fig. 3 a, b) show multiple areas of ill-defined permeative calvarial destruction interspersed by more distinct lytic lesions. No purely intracerebral lesions were seen.

A biopsy of the large right frontal lesion confirmed metastatic prostatic carcinoma.

Background
Metastatic prostate cancer most commonly occurs in lymph nodes and bones [1]. While sclerotic metastases are the dominant type of bony metastases in prostate cancer, lytic metastases can occur in approximately 5% of cases [2-3]. Bony metastases from prostate cancer can widely affect the skeleton (including the calvarium).

Calvarial metastases in general tend to occur during more advanced stages of metastatic malignancy [4]. Prostate cancer metastases account for around 6% of all calvarial metastatic lesions [5]. Intracerebral metastases from prostate cancer are, however, extremely rare and are seen only in around 0.6% of all prostate cancer cases. When they do occur, they often represent a terminal event of the disease [6-8].

Clinical perspective
Most patients with calvarial metastases are asymptomatic [5] and this may well be attributed to small sizes of most lesions. Some lesions (especially if large) have a potential to produce a variety of symptoms which in turn are often dependent on the location where they occur in the skull. The variation in symptoms tends to be particularly pronounced if the metastases occur around the skull base and close to cranial nerves. In isolated cases, cranial vault metastases can be large, produce significant mass effect and can show symptoms very similar to those of intracerebral metastases [9].

Imaging perspective
Given the known background prostatic malignancy with established skeletal metastases elsewhere, a key challenge for the radiologist in this case is to decide whether the calvaria-based enhancing lesions represent prostate cancer metastases or some alternative pathology. The presence of the second left parietal enhancing subcalvarial lesion suggests multifocal pathology while the absence of purely intracerebral lesions and the concurrent presence of numerous lytic calvarial lesions suggest that the metastatic pathology is essentially skeleton-centred. The lack of a periosteal reaction implies an aggressive process.

In situations such as this case, it is vital to review imaging features of the other skeletal lesions outside the skull on concurrent and/or prior imaging. Some of the vertebral lesions in this case had been reported as being lytic on prior imaging (actual images were not available).

Take home message
This case highlights some unusual features where calvarial metastases from prostate cancer can be lytic, be large in size, show considerable intracranial extension and sinus extension. It broadens the radiologist's perspective of the imaging spectrum of metastatic calvarial lesions from prostate cancer and makes a radiologist ponder upon a meaningful differential diagnosis when faced with such a presentation.