Imaging algorithm to differentiate intracranial infection from malignancy

A 57-year-old man with no stated past medical history presented to the ED with complaints of headache and blurry vision that were getting progressively more severe. These symptoms began about 6 months before presentation. He also noted worsening bloody bowel movements.

A CT scan of the brain without contrast was performed on admission. It revealed an enhancing right parietal lobe lesion. A chest/abdomen/pelvis CT with IV contrast was subsequently performed that showed multiple large lymph node masses in the mediastinum and right hilum, as well as a mass in the right upper lobe. Two lesions were noted in the liver, as well as possible rectosigmoid malignancy with possible extension into the perirectal space and the rectosigmoid colon.

Further imaging was performed with contrast enhanced MRI to characterize the right parietal lobe lesion. The findings were most suggestive of a right parietal lobe abscess. The patient underwent right craniotomy and evacuation of the right parietal lobe lesion. Pathological evaluation confirmed that the lesion was an abscess.

The patient was treated with IV antibiotics. During hospitalization, he underwent biopsy of the right upper lobe lung mass. Pathology revealed metastatic adenocarcinoma consistent with a rectal primary. Colonoscopy and flexible sigmoidoscopy showed a large rectal mass that was nearly obstructing the rectum. The patient is currently undergoing systemic chemotherapy with concurrent radiation treatment.

Figure 1. A noncontrast head CT was performed in a 57-year- old man with new onset left facial paresis, leftward tongue deviation and left upper extremity twitching. An intra-axial mass was present in the right parietal lobe, with surrounding vasogenic edema. Courtesy of M Ghesani, MD

Discussion

In the United States, about 145,000 new cases of large bowel cancer are diagnosed each year. Of these, about 105,000 are colon, and the others are rectal cancers.

The mainstay of treatment for colorectal cancers is surgical resection. Screening methods, in practice, include fecal occult blood testing, endoscopy or radiology based on clinical presentation. Based on the timing of presentation and appropriate workup, diagnosis at an earlier stage of disease can be made and can reduce cause-specific mortality.

The most common symptoms at presentation are hematochezia or melena, abdominal pain caused by obstruction or perforation, peritoneal spread, a change in bowel habits or unexplained microcytic iron deficiency anemia. Other symptoms include weakness/fatigue and unintentional weight loss. Specifically for rectal cancer, tenesmus can be a common presentation because the cancer itself may involve the pelvic floor muscles. Also, neuropathic pain may be caused by a locally advanced lesion involving the sciatic or obturator nerve.

Figure 2. Given the likelihood for intracranial metastasis, a subsequent CT scan of the chest, abdomen and pelvis was obtained. Examination of the chest revealed mediastinal lymphadenopathy with a speculated mass in the right upper lobe (a). There were two liver nodules suspicious for metastases (not shown). Examination of the pelvis revealed asymmetric rectal wall thickening with perirectal infiltration and enlarged perirectal vasculature, suggesting primary rectal carcinoma (b).

It is estimated that about one-fifth of patients present with distant metastatic disease. Spread is by either lymphatic or hematogenous dissemination; however, disease may also spread contiguously. The most common sites of metastasis are the regional lymph nodes, liver and lungs, as well as locally in the peritoneum. Specifically, distal rectal malignancies metastasize primarily to the lungs. It is hypothesized that this is due to hematogenous dissemination into the venous system, namely the inferior vena cava.

There are also a variety of unusual presentations of colorectal cancer. These include fever of unknown origin and intra-abdominal, retroperitoneal or abdominal wall abscesses. Streptococcus bovis bacteremia and Clostridium septicum sepsis are due to underlying colonic malignancies in a subset of patients as well.

In this case, the patient presented to the ED with neurologic complaints and was found to have a mass in the right parietal lobe. In adults, metastases account for as many as 40% of intracranial masses seen on CT scans, most commonly lung and breast. The additional chest and abdominal CT findings supported the likelihood of intracranial metastasis. However, the brain MRI findings in this patient were more characteristic of an abscess rather than metastasis.

Figure 3. Contrast-enhanced MRI of the brain revealed a thin-walled intra-axial lesion situated at the gray-white matter junction in the right frontal lobe. It was hyperintense on T2-weighted images, with peripheral intrinsic T1 hyperintensity, which is hypointense on T2 weighted sequence (b). Axial, coronal and sagittal Gadolinium-enhanced images demonstrated strong peripheral enhancement with no internal enhancement (c). There was uniform restricted diffusion with corresponding decreased signal on ADC map associated with the lesion, characteristic of an abscess rather than metastasis. All photos courtesy of M Ghesani, MD

Intracranial abscesses and metastases often show a high degree of vasogenic edema, as was seen in this case. Abscesses and metastases are associated with internal low T1 weighted signal, with high T2 signal seen in abscesses and variable T2 signal seen in metastases. The ring-enhancement pattern in this patient is also nonspecific; the differential for intracranial ring-enhancing lesions includes metastatic or primary tumors, radiation necrosis, multiple sclerosis, lymphoma, resolving infarct and subacute hematoma.

Metastases enhance in variable patterns, including the ring-like enhancement classically seen in abscesses. Abscesses are classically associated with restricted water diffusion, showing increased signal on the diffusion-weighted imaging series and corresponding signal drop-out on the apparent diffusion coefficient maps. Metastases usually do not demonstrate restricted diffusion, although some necrotic tumors (for example, squamous cell carcinoma) may show restricted diffusion. Finally, the thin rim of low T2 and high T1 weighted signal characterizes the wall of the abscess and would be unusual for necrotic tumors. Thickness, irregularity and nodularity are associated with tumors.

In summary, the MRI findings of restricted diffusion, ring-like enhancement and the presence of a thin wall with intrinsic T1 signal are all features that suggest abscess rather than metastasis. The patient underwent surgical excision of the abscess, and subsequent histological analysis confirmed the diagnosis.

Amit Patel, MD, is a fellow in oncology at St. Luke’s-Roosevelt Hospital Center.

Iwao Tanaka, MD, is a resident in radiology at St. Luke’s-Roosevelt Hospital Center.

Munir Ghesani, MD, is an attending radiologist at St. Luke’s-Roosevelt Hospital Center and associate clinical professor of radiology at Columbia University College of Physicians and Surgeons.

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