Journal of Cancer Research and Therapeutics Medknow Publications on behalf of the Association of Radiation Oncologists of India (AROI) ISSN: 0973-1482 EISSN: 1998-4138 Vol. 6, Num. 3, 2010, pp. 356-358

Journal of Cancer Research and Therapeutics, Vol. 6, No. 3, July-September, 2010, pp. 356-358

Case Report

Cystic metastasis versus brain abscess: Role of MR imaging in accurate diagnosis and implications on treatment

Prakash Shetty¹, Aliasgar Moiyadi¹, Gouri Pantvaidya², Supreeta Arya³

¹ Department of Surgical Oncology, Neurosurgery Services, Tata Memorial Centre, Parel, Mumbai, India
² Head and Neck Oncology Services, Tata Memorial Centre, Parel, Mumbai, India
³ Department of Radiodiagnosis, Tata Memorial Centre, Parel, Mumbai, India

Correspondence Address:Aliasgar Moiyadi, Department of Surgical Oncology, Neurosurgery Services, Room 48, Main building, Tata Memorial Hospital, E Borges Road, Parel, Mumbai - 400 012, India, aliasgar.moiyadi@gmail.com

Code Number: cr10086

PMID: 21119276

DOI: 10.4103/0973-1482.73368

Abstract

Keywords: Cerebral abscess, diffusion weighted MRI, intracranial cystic metastasis

Introduction

Detection of metastases especially intracranial metastases signals a poor prognosis in a patient with cancer. Solitary cystic metastasis is often difficult to differentiate from intra-cerebral abscess which can also occur in these patients. In the absence of histological confirmation, the intent and nature of treatment is greatly influenced by the radiological characteristics of the lesions. Differentiating an abscess from a cystic metastasis is crucial. We illustrate one such case where imaging was instrumental in guiding the therapy.

Case Report

A forty five-year-old male was referred to the neurosurgery services with sudden onset right sided hemiparesis and altered consciousness. He had been recently diagnosed with locally advanced squamous cell carcinoma of the right piriform fossa (T4, N1, M0). In view of advanced disease at presentation, he was given two cycles of neo-adjuvant chemotherapy (paclitaxel and cisplatin). The patient responded very well to the treatment and he had been planned for total laryngectomy with trachea-esophageal prosthesis. On presentation he was conscious, but irritable. He had grade 4/5 power in the right upper limb and 3/5 in the right lower limb. He also had a right sided homonymous hemianopia.

Magnetic resonance imaging(MRI) of the brain showed a large left occipito-parietal cystic swelling which was predominantly centrally hypointense on T1 weighted images, hyperintense on T2 weighted images with peripheral ring enhancement which was thicker along the posterior and inferior walls [Figure - 1]. The lesion was hyperintense on the diffusion weighted sequences and hypointense on apparent diffusion coefficient maps indicating restricted diffusion [Figure - 2]. The ADC ratio (the ratio of ADC values from the nonenhancing cystic portion of the mass to the ADC values from contralateral normal-appearing white matter) was 0.9. Spectroscopy showed raised lipid lactate in the central region without appreciable choline elevation. Additional peaks of succinate, acetate and amino acids of an abscess spectrum were not identified. Numerous small peaks at various frequencies due to poor shimming were identified probably representing noise. A differential diagnosis of necrotic metastasis versus an abscess was entertained. However, in view of its diffusion and spectroscopy features, an abscess was considered more likely and hence it was decided to operate upon the patient. A left occipital craniotomy was performed and during surgery the lesion was found to be well encapsulated with purulent contents. The lesion was completely excised. Histopathology showed the lesion to be a pyogenic abscess [Figure - 3]. There was no neoplastic tissue. Microbiological studies isolated Klebsiella pneumoniae from the pus. The patient was put on appropriate antibiotics for six weeks. He made a good neurological recovery after surgery. The post-operative MRI showed an excision cavity cyst with a thin rim of contrast enhancement and no evidence of restricted diffusion. He subsequently underwent total laryngectomy with reconstruction and tracheo-esophageal prosthesis placement. The histopathology showed complete response to induction chemotherapy at the primary site with residual disease in ipsilateral neck nodes. He was thereafter referred for concurrent chemo-radiation.

Discussion

Intracranial metastases usually imply disseminated disease and forebode a poor prognosis in patients with cancer. Detection of an intracranial mass on imaging in patients with a known primary raises the possibility of metastasis. Often the diagnosis of an intracranial metastasis is made radiologically without histological confirmation. However, radiological evaluation has its own limitation. Though the MRI has increased the specificity of diagnosis, routine MRI sequences may still not be able to differentiate between different intracranial lesions. The classical appearance of a metastasis is a solid enhancing mass with well-defined margins and extensive edema. Occasionally, central necrosis produces a ring enhancing mass. Such a lesion is difficult to differentiate from an abscess. Recently, diffusion and spectroscopy sequences have proven useful to differentiate between the two entities.

Diffusion weighted imaging has been widely reported to help in differentiating between an abscess in capsular stage and a necrotic neoplasm. ^{[1],[2],[3],[4],[5],[6],[7],[8]} Diffusion weighted imaging characterizes a lesion based on the free diffusing property of water molecules in that lesion. The diffusion data can be presented as signal intensity or as an image map of the apparent diffusion coefficient (ADC). Calculation of the ADC requires two or more acquisitions with different diffusion weightings. A low ADC corresponds to high signal intensity (restricted diffusion), and a high ADC corresponds to low signal intensity on diffusion-weighted images. The ADC ratios for all pyogenic abscesses in a report varied from 0.45 to 0.8 and cystic necrotic tumor typically had higher values. ^[5]

Pus in an abscess cavity is a viscous fluid of inflammatory cells, necrotic tissue, bacteria and proteins. This viscous fluid grossly restricts the motion of water molecules, accounting for restricted diffusion. In contrast, the necrotic portion of a tumor contains less viscous fluid with necrotic tumor tissue and few inflammatory cells as compared to an abscess, resulting in free diffusion of water molecules. ^[1] However, there have been exceptions to the rule. Park and Holtas in two different case reports have reported necrotic brain metastases with low ADC values. ^[9],[10] The reason for restricted diffusion was probably early necrosis with intracellular edema. The presence of a complete T2 weighted hypointense ring, though more common in an abscess, is not always very helpful in excluding a neoplasm. However, the use of ADC ratios along with T2 weighted rim characteristics of the lesion have been seen to increase the accuracy (up to 94.3%) in differentiating brain abscesses from cystic or necrotic neoplasms. ^[8]

Magnetic resonance spectroscopy also has been used to differentiate between cystic neoplasms and abscesses. The spectral pattern of an abscess shows elevation of acetate, succinate, lactate as well as amino acids such as valine, leucine and isoleucine. These amino acids are not seen in the in vivo proton MR spectra of brain tumors. ^[11] It is also seen that with a TE of 135, phase inversion occurs as a result of J-coupling between lactate and amino acid, but not in lipid, a feature which is helpful along with presence or absence of acetate or succinate in differentiating between brain abscesses from tumor. ^[12],[13] In our case, only a lipid lactate peak was seen, which may exist in both brain tumors and abscess. However, MRS is more time consuming and less specific as compared to diffusion weighted imaging.

The significance of correctly diagnosing a mass lesion in the brain in patients with a primary cancer cannot be overemphasized. A careful decision based on clinical findings (site of primary, propensity for intracranial metastases, systemic spread) and radiological features is essential. Though a tissue diagnosis is ideal, it may not be possible always. In patients with a known primary, labeling such a mass as a metastasis would upstage the disease, altering the intent of treatment and often relegating the patient to palliative therapy. As in our case, the presence of a left occipital ring enhancing lesion in a known case of locally advanced squamous cell carcinoma of the larynx would have raised the possibility of brain metastasis. However, squamous cell carcinomas of the head and neck rarely metastasize to the brain. In addition, he had received chemotherapy which itself may predispose individuals to infective complications such as abscess. ^[14] As the imaging findings were strongly in favor of a cerebral abscess, it was decided to aggressively treat the patient and resect the cerebral lesion. Surgery proved the lesion to be an abscess confirming the radiological diagnosis. Not only it improved the neurological outcome of the patient, but it also allowed definitive radical treatment of the primary which may otherwise not have been offered to the patient.

References

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