CHEST RADIOLOGY

The patient is a known case of acute myelogenous leukemia on chemotherapy, with prolonged aplasia. He presents with acute onset fever of 10 days duration. A CT scan is asked for following a chest radiograph. 
What is the diagnosis?

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Imaging Findings reveal a nodular lesion in the right upper lobe. A faint haze with irregularity is seen at the periphery of this lesion. In view of the history of immunocompromised status, fungal etiology was suggested.

CT guided FNAC revealed scant cellularity on smear. Sheets of classic fungal filaments were noted (hyphae).

Diagnosis
Pulmonary Aspergillosis

Discussion
The CT halo sign (Fig 4) is a specific sign that can be used to diagnose invasive pulmonary aspergillosis in patients with acute leukemia, undergoing aplasia-producing chemotherapy. This sign consists of a round pulmonary mass or nodule with a surrounding halo of intermediate CT attenuation; this halo represents hemorrhage around a focal area of lung infarction caused by the Aspergillus organism that invades pulmonary vessels. Other angio-invasive infections (mucormycosis, Candida torupolis, and angio-invasive Pseudomonas) may produce similar CT findings, but invasive pulmonary aspergillosis is the commonest species to produce this finding in such a clinical setting.

The lung infection then follows a typical pattern of progression and resolution. As the bone marrow recovers from aplasia and as white cell count returns to normal, the pulmonary lesion begins to cavitate, producing an air-crescent sign. With further healing, this lesion shrinks and fades away, leaving behind a thin-walled cyst or linear scar.

Contribution
Dr. Narayani, Dr Sanjeev Mani, Mumbai