PA25.1-7 | Respiratory System — Case Study

CLINICAL SCENARIO

A clinico-pathological case of a chronic heavy smoker presenting with productive cough, haemoptysis, and weight loss, in whom imaging reveals a central lung mass. Students reason through the differential diagnosis, interpret histopathological findings, classify the tumour type, recognise a paraneoplastic syndrome, apply TNM staging principles, and trace the smoking-attributable pathological background (COPD). The case integrates PA25.1–PA25.7 (respiratory system pathology) at Year-2 MBBS clinico-pathological correlation depth.

Section 1 — Differential Diagnosis

Based on the clinical features (central mass, heavy smoking, haemoptysis, weight loss, superior vena cava obstruction signs) construct a prioritised differential diagnosis. Include at least three diagnoses. For each, list the supporting features present in this case and the key feature(s) that would most help distinguish it from the others.

Guidance: Your top differential should be lung carcinoma. Rank tuberculosis second (endemic setting, cough, haemoptysis, weight loss) and consider lung abscess third. For each diagnosis, map at least two specific features from the scenario — do not list generic differentials without case-specific reasoning. Explicitly note why the central location, pack-year history, and the sputum cytology morphology shift the probability toward carcinoma.

Section 2 — Histopathology Interpretation

The bronchoscopic biopsy report reads: 'sheets of small, round-to-oval cells with scant cytoplasm, nuclear moulding, finely stippled 'salt-and-pepper' chromatin, absent nucleoli, and brisk mitotic activity; immunohistochemistry positive for synaptophysin, chromogranin A, CD56, and TTF-1; Ki-67 index 85%.'

Interpret these findings. Name the tumour type, explain the significance of each immunohistochemical marker, and state the cell of origin. How does the histological appearance of this tumour differ from that of squamous cell carcinoma and adenocarcinoma?

Guidance: Identify this as Small Cell Lung Carcinoma (SCLC). Explain that synaptophysin and chromogranin A confirm neuroendocrine differentiation; CD56 marks neural cell adhesion; TTF-1 supports pulmonary origin (also expressed in SCLC). The cell of origin is the Kulchitsky (neuroendocrine) cell of the bronchial mucosa. Contrast: squamous cell carcinoma shows intercellular bridges, keratin pearls, and positive p40/CK5/6 on IHC; adenocarcinoma shows glandular architecture, mucin, and is TTF-1+/Napsin-A+ but NETs markers negative.

Section 3 — Tumour Typing and Central vs Peripheral Classification

Classify lung carcinomas into SCLC and NSCLC. For NSCLC, list the major subtypes and their typical anatomical locations (central vs peripheral). Explain why SCLC and squamous cell carcinoma tend to be central tumours, while adenocarcinoma tends to be peripheral. Explain the clinical significance of the SCLC vs NSCLC distinction for management.

Guidance: SCLC (~15%): central, arising from neuroendocrine cells of large bronchi; highly aggressive, early metastasis, almost always disseminated at diagnosis — treated primarily with chemotherapy+immunotherapy, not surgery. NSCLC (~85%): squamous (central, proximal bronchi, related to squamous metaplasia in smokers), adenocarcinoma (peripheral, subpleural, can arise in non-smokers, associated with EGFR/ALK mutations), large-cell carcinoma (peripheral, diagnosis of exclusion). Central location in squamous/SCLC explains haemoptysis and obstructive pneumonia.

Section 4 — Paraneoplastic Syndromes

Mr. Kumar has hyponatraemia (Na 118 mEq/L) and markedly elevated serum ACTH. Identify the paraneoplastic syndrome(s) these biochemical findings suggest. Explain the mechanism for each, name the ectopic hormone or antibody responsible, and describe the expected clinical manifestations if left untreated. Which lung carcinoma subtype is most strongly associated with paraneoplastic syndromes, and why?

Guidance: Hyponatraemia in SCLC = SIADH (Syndrome of Inappropriate ADH secretion) — ectopic ADH produced by tumour cells leads to dilutional hyponatraemia; manifestations: nausea, confusion, seizures. Elevated ACTH = ectopic ACTH syndrome (Cushing's syndrome variant) — tumour-derived ACTH drives bilateral adrenal hyperplasia; clinical features: hypertension, hypokalaemia, muscle weakness, hyperglycaemia. SCLC is most commonly associated because tumour cells derive from amine-precursor uptake and decarboxylation (APUD / neuroendocrine) cells with intrinsic peptide-secretion machinery. Other SCLC paraneoplastics: Lambert-Eaton syndrome (anti-VGCC antibodies), limbic encephalitis (anti-Hu antibodies).

Section 5 — Staging and Metastatic Pathways

Using TNM principles, assign a provisional stage to Mr. Kumar's tumour based on the available clinical and imaging data. Identify all T, N, and M descriptors you can determine. Then describe the typical routes of metastatic spread for SCLC — which organs are most commonly involved and through which pathways (lymphatic, haematogenous, or direct)? What is the most common site of SCLC distant metastasis, and what symptoms might it produce?

Guidance: T: T3 or T4 depending on mediastinal invasion (central mass abutting main bronchus, right UL collapse — likely T3 at minimum; mediastinal invasion would be T4). N: N2 (ipsilateral mediastinal/hilar nodes 2.1 cm). M: no confirmed distant metastasis in the data provided — M0 clinically, but SCLC staging in practice uses limited vs extensive disease (LD = one hemithorax ± nodes; ED = beyond). This case = Limited Disease. Metastatic routes: early haematogenous spread (to liver, adrenals, bone, brain — brain most common distant site, can cause headache, seizures, focal deficit, personality change); lymphatic to mediastinal nodes → SVC obstruction (already seen).

Section 6 — Smoking-Attributable Pathology and COPD Background

Mr. Kumar's spirometry showed an FEV1/FVC ratio of 0.62 with FEV1 55% predicted. Interpret this spirometric pattern and classify the severity of COPD by GOLD criteria. Describe the underlying pathological changes in the airways and lung parenchyma that produce this obstructive pattern in a chronic smoker. Explain how the COPD background may complicate the presentation and management of his lung carcinoma. What fraction of lung carcinomas are attributable to tobacco smoke, and through what molecular mechanisms does smoke carcinogenesis occur?

Guidance: FEV1/FVC <0.70 = obstructive pattern; FEV1 55% = GOLD Grade 2 (moderate). COPD pathology: chronic bronchitis (mucous gland hypertrophy → Reid index >0.4, goblet-cell metaplasia, luminal mucus plugs) + emphysema (centriacinar in smokers — destruction of respiratory bronchioles and alveolar walls, loss of elastic recoil). COPD complicates carcinoma: masks worsening dyspnoea, poor surgical candidacy (reduced respiratory reserve), overlapping symptoms (cough, sputum). ~85–90% of lung carcinomas attributable to tobacco. Molecular mechanisms: polycyclic aromatic hydrocarbons (PAHs) and nitrosamines form DNA adducts → TP53, KRAS, RB1 mutations; oxidative stress; chronic inflammation → NF-κB pathway activation → pro-survival signalling. SCLC specifically: near-universal RB1 and TP53 loss.

PEER REVIEW

You are reviewing a peer's case-study response on lung carcinoma pathology. Your role is to give constructive, evidence-based feedback that helps your peer improve their understanding.

Review checklist:
1. Differential diagnosis: Is carcinoma ranked first? Are the supporting features case-specific (not generic)?
2. Histopathology: Is SCLC correctly identified? Are IHC markers explained with mechanisms, not just listed?
3. Tumour classification: Is the SCLC/NSCLC distinction clear? Is the location rationale (central vs peripheral) linked to cell of origin?
4. Paraneoplastic syndromes: Are both SIADH and ectopic ACTH named with their ectopic hormones? Is the APUD/neuroendocrine link made?
5. Staging: Are T, N, M descriptors assigned with case-specific evidence? Is the LD/ED framework mentioned?
6. COPD and carcinogenesis: Is the spirometry correctly graded? Is at least one molecular mechanism of tobacco carcinogenesis named (not just 'smoking causes cancer')?

Feedback format: Write 150–250 words. Start with one strength (specific, not just 'good work'). Then list 2–3 specific points for improvement. End with one question to prompt deeper thinking. Do not assign a score — that is done separately.