Certain types of pneumonia can appear highly suspicious for malignancy on a Computed Tomography (CT) scan. The CT scan images differences in tissue density. Both inflammatory processes (pneumonia) and uncontrolled growth (cancer) cause lung tissue to become dense, making the resulting images look remarkably similar to a radiologist. This visual overlap occurs because both conditions replace the normal air-filled spaces of the lung with material—inflammatory cells and fluid, or malignant cells.
Shared Visual Characteristics on CT Scans
The confusion on a CT scan is rooted in dense tissue replacement and the formation of distinct masses. Organizing pneumonia (OP) and chronic eosinophilic pneumonia (CEP), for example, frequently present as areas of consolidation, where the air in the lung’s air sacs is replaced by inflammatory material. This dense, airless tissue can create a mass effect that closely mimics a solid tumor, especially when the consolidation is focal and has an irregular border.
Pneumonia can also manifest as nodules and cavitation, features typically associated with early or advanced lung cancer. Infectious processes like granulomas or abscesses can form solitary or multiple round lesions (nodules) that are indistinguishable from small, early-stage tumors. Severe infections from bacteria like Klebsiella pneumoniae can cause tissue death, or necrosis, leading to a cavity within the dense lung tissue, a finding also common in aggressive squamous cell lung cancers.
The Importance of Clinical Context and Temporal Resolution
Distinguishing between infection and cancer requires integrating CT findings with the patient’s clinical picture. Acute symptoms like high fever, chills, and rapid onset of severe cough and sweating strongly suggest an infection like pneumonia. In contrast, lung cancer symptoms often develop slowly and may include unexplained weight loss, persistent cough that worsens over time, or coughing up blood (hemoptysis).
Laboratory work provides supporting evidence for inflammation. Basic blood tests often show an elevated White Blood Cell (WBC) count and a high C-Reactive Protein (CRP) level with active infection. These markers are frequently normal in early-stage lung cancer, though cancer can also cause inflammatory changes. When the initial CT scan is highly suspicious, the most practical diagnostic step is often a period of observation using temporal resolution.
In cases where infection is suspected, a follow-up CT scan is typically performed after a course of treatment, often around four to eight weeks later, to see if the suspicious lesion has resolved. If the lesion was inflammatory, it should shrink or disappear completely within this defined period, confirming a benign process. If the mass persists, grows, or shows minimal change after this time, the suspicion for malignancy increases dramatically, necessitating further, more invasive diagnostic steps.
Advanced Diagnostic Procedures for Confirmation
When clinical context and temporal resolution are inconclusive, advanced procedures determine the cellular nature of the lesion. Positron Emission Tomography (PET) scans are often the next step, measuring the metabolic activity of the tissue by tracking the uptake of a radioactive glucose tracer, Fluorodeoxyglucose (FDG). Since most cancer cells are highly active and consume glucose rapidly, they typically “light up” brightly on a PET scan.
The PET scan is not a perfect tool because inflammatory cells, such as those found in pneumonia or tuberculosis, are also highly metabolically active. They can show intense FDG uptake, leading to a false-positive result for cancer. The overlap is significant, making cellular analysis the ultimate gold standard.
Biopsy: The Definitive Diagnosis
A biopsy is the only way to achieve a definitive diagnosis, as it allows a pathologist to physically examine the cells and distinguish between inflammatory and malignant cells. The cell-level analysis resolves the ambiguity presented by initial imaging, confirming whether the patient has a treatable infection or a life-threatening malignancy.
Biopsy Techniques
This procedure can be performed using various techniques. A CT-guided needle biopsy involves passing a needle through the chest wall to sample the lesion. Alternatively, a bronchoscopy may be performed, where a flexible tube is passed down the patient’s airway to access the lesion from within the lung.