CT semiotics of lung adenocarcinoma
In modern diagnostic radiology, the evaluation of the solid component is crucial for tumor staging and prognosis.
- Adenocarcinoma in situ manifests as a pure ground-glass opacity (pure ground-glass lesion / GGO) — a focal opacity through which normal lung structures remain clearly visible. The prognosis for such findings is excellent, with survival rates approaching 100%.
- Minimally invasive adenocarcinoma (Minimally invasive adenocarcinoma) is characterized predominantly by lepidic (lepidic) growth. On CT, it appears as a subsolid nodule with a total size of up to 3 cm, wherein its solid component does not exceed 5 mm.
Due to the extremely slow growth of part-solid nodules (part-solid nodules), international guidelines (Fleischner Society, BTS) require prolonged follow-up—up to 5 years. If a new solid element appears within the nodule or the existing one begins to increase in size, it is considered a marker of aggressive growth and warrants active workup or treatment.
Pericystic tumors
Particular diagnostic challenges are presented by pericystic tumors (Pericystic Tumours). These are malignant nodules (most commonly adenocarcinomas) that develop directly within the wall of a cystic area of lung destruction—for example, within an emphysematous bulla, fibrous cavity, bronchiectasis, or subpleural bleb (bleb).
In the early stages of development, the solid soft-tissue element in the cyst wall is so thin that it is extremely difficult to recognize. This frequently leads to delayed diagnosis. Furthermore, the small size of the neoplastic component makes percutaneous or bronchoscopic biopsy technically challenging and associated with a high risk of complications.
Additional evaluation methods: contrast and PET/CT
Specialized imaging techniques are used to characterize indeterminate lung nodules (indeterminate lung nodule).
Contrast-enhanced nodule evaluation Basic principle: malignant lesions actively take up contrast material. If there is no enhancement, the risk of malignancy is effectively excluded. This method works exceptionally well for soft-tissue density nodules, but its diagnostic value drops sharply in the presence of calcification, cavitation, or ground-glass attenuation.
Positron emission tomography (PET/CT) Fluorodeoxyglucose (FDG) is used as the radiopharmaceutical. The method demonstrates high sensitivity (90%) and specificity (83%) for detecting malignant lesions.
Limitations of PET/CT:
- Size < 1 cm: high risk of false-negative results, especially in poorly differentiated adenocarcinoma subtypes and carcinoids.
- Size < 6 mm: the procedure is currently not indicated.
- False-positive results: increased tracer uptake can occur in sarcoidosis, inflammatory conditions, and granulomatous infections.
Evaluation of solid renal masses
The evaluation of solid masses is not limited to lung tissue. Computed tomography is the imaging modality of choice for characterizing solid renal masses as well. Quality diagnostics here are impossible without intravenous contrast administration and precise adherence to scanning phases:
- Non-contrast phase: essential for detecting calcifications.
- Corticomedullary phase (25–40 sec): important for assessing tumor vascularity and the status of the renal vein.
- Nephrographic phase (90–100 sec): optimal for visualizing central masses and small medullary lesions.
If only a single phase must be used to reduce radiation dose, the nephrographic phase is always chosen. Dynamic contrast-enhanced CT (DCE-CT) is used to assess microcirculation. However, it is important to consider the risks: the effective radiation dose for such a study ranges from 13.7 to 28.7 mSv, creating a lifetime risk of radiation-induced cancer of approximately 1 in 1,000.
How to remember the CT contrast phases for the kidneys: "NCP" — Non-contrast (baseline, calcifications), Corticomedullary (vasculature, blood), Nephrographic (neoplasms, parenchymal lesions).
Class and exam questions
How does ground-glass opacity visually differ from a solid nodule?
With pure ground-glass opacity, normal anatomical structures of the lung can be clearly traced through the area of consolidation. A solid nodule has a higher X-ray attenuation and completely obscures the underlying structures.
What is the danger of subsolid nodules?
Occult invasive growth may begin inside such nodules. The appearance of new solid components or enlargement of existing ones (especially those measuring more than 5 mm) is a red flag requiring urgent treatment or an expanded workup.
Can lung cancer be completely ruled out if a nodule on CT does not enhance?
For soft-tissue density nodules, the absence of contrast enhancement effectively rules out malignancy. However, this rule does not apply to nodules with calcifications, necrotic cavities, or ground-glass architecture.
What else to study on this topic
- CT semiotics and differential diagnosis of pleural effusion
- The split-pleura sign in malignant lesions
- Application of dual-energy CT (DECT) for generating virtual non-contrast images
- Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and evaluation of contrast kinetics
- Assessing the efficacy of anti-angiogenic therapy using perfusion CT
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