Respiratory · Oncology

Lung Cancers

Central vs peripheral, small vs non-small, and the paraneoplastic syndromes that announce them before the cough ever does.

Opening Challenge

A 65-year-old man with a 40 pack-year smoking history presents with 3 months of cough, weight loss, and confusion. He is clinically euvolemic with no edema and normal blood pressure. Serum sodium is 128 mEq/L (135-145), serum osmolality 262 mOsm/kg (275-295), urine osmolality 450 mOsm/kg (inappropriately concentrated), and urine sodium 45 mEq/L (>20 supports SIADH). Cortisol and TSH are normal. CXR shows a central hilar mass.

A) Adenocarcinoma
B) Squamous cell carcinoma
C) Small cell lung cancer (SCLC)
D) Large cell carcinoma
Start with the chemistry: low serum sodium, low serum osmolality, but urine that stays concentrated when it should be maximally dilute, plus urine sodium over 20 in a euvolemic patient with normal cortisol and thyroid. What single hormone, secreted when it should be off, retains free water and produces exactly this? Antidiuretic hormone. Now ask which cancer makes hormones it has no business making, and sits centrally at the hilum: the neuroendocrine one. Adenocarcinoma is peripheral and EGFR-driven, squamous cell makes PTHrP and causes hypercalcemia, large cell is a peripheral undifferentiated tumor. Only one tumor pairs a central mass with ectopic ADH. Central hilar mass plus euvolemic hyponatremia with concentrated urine is SIADH from small cell lung cancer until proven otherwise.
Begin
01 · The Two Worlds

Small Cell vs NSCLC

Two completely different diseases. Different location, biology, staging, and treatment.

Small Cell (SCLC)
NSCLC Overview
Location Map

Small Cell · 15% of lung cancers

Small Cell Lung Cancer

Origin: Kulchitsky cells (neuroendocrine cells) of the bronchial epithelium. This is why SCLC produces hormones ectopically.

Location: Central (hilar). Arises near the main bronchus. This is the classic central mass on CXR.

Biology: Very aggressive with rapid doubling time. Almost always metastatic at diagnosis. Nearly never surgical because of this.

Pathology stains: Chromogranin A, synaptophysin, CD56 (neuroendocrine markers), also TTF-1 positive. Small blue cells with scant cytoplasm and nuclear molding: the classic "oat cell"Oat cell = Small Cell. Tiny dark cells packed like grains of oats. Neuroendocrine origin = chromogranin + synaptophysin positive. appearance.

Treatment: Chemotherapy (cisplatin + etoposide) plus radiation for limited stage. Surgery is essentially never done.

Central hilar Neuroendocrine origin No surgery Chromogranin A Synaptophysin CD56 TTF-1 positive
Paraneoplastic syndromes from SCLC: SIADH (ectopic ADH, hyponatremia), Cushing syndrome (ectopic ACTH, hypokalemia + metabolic alkalosis + hyperglycemia), Lambert-Eaton myasthenic syndrome (anti-VGCC antibodies, proximal muscle weakness that improves with repetition). SCLC is the paraneoplastic syndrome capital of lung cancer.
From the Attending Two ectopic hormones, two tumors, and the boards rotate them forever. Ectopic ACTH and ectopic ADH both come from the neuroendocrine small cell tumor. Ectopic PTHrP, the hypercalcemia maker, comes from squamous cell. SCLC = Sodium & SteroidSCLC makes ADH (low Sodium = SIADH) and ACTH (Steroid = ectopic Cushing). Two S hormones, one Small cell. Squamous = PTHrP = high calcium. Ectopic ACTH or ADH means small cell; ectopic PTHrP with hypercalcemia means squamous cell.
Neuroendocrine cell → Secretes ADH ectopically → Free water retention → Euvolemic hyponatremia (SIADH)
02 · Ectopic Hormone Production

Paraneoplastic Syndromes

Lung cancers produce hormones the tissue was never supposed to make. Each syndrome maps to one cancer type.

Syndrome Trace It Lung Cancer Type Key Clue
SIADH Ectopic ADH secretion SCLC Euvolemic hyponatremia Na 120s (135-145), urine Na >20 (mEq/L), urine osm inappropriately high, no edema
Cushing syndrome Ectopic ACTH secretion SCLC Rapid onset, no moon face (ACTH surge too fast). K 2.8 (3.5-5.0), hyperglycemia, metabolic alkalosis (HCO3 high)
Lambert-Eaton syndrome Anti-P/Q voltage-gated calcium channel antibodies SCLC Proximal weakness that IMPROVES with repetition. Opposite of MG. Hyporeflexia, autonomic features.
Hypercalcemia Ectopic PTHrP (mimics PTH at receptor) Squamous cell carcinoma Ca 12-14 (8.5-10.5), PTH suppressed 8 (15-65) because PTHrP is invisible to the PTH assay, no bone mets needed
Carcinoid syndrome Serotonin (needs liver mets) Bronchial carcinoid Flushing, diarrhea, right-heart valve disease, urine 5-HIAA elevated
Clubbing / HOA Unknown (likely VEGF, platelet factor) Any type (especially adenocarcinoma) Periosteal new bone formation, painful joints, clubbing of digits
From the Attending Lambert-Eaton and myasthenia gravis are mirror images. LEMS antibodies hit the presynaptic P/Q calcium channel, so the first few contractions are weak, but with repeated use calcium builds up at the terminal and strength climbs. MG antibodies block the postsynaptic ACh receptor, so repeated use exhausts it and weakness worsens. LEMS gives hyporeflexia that improves after exercise; MG keeps normal reflexes and hits the eyes first. Weakness that gets better with use is Lambert-Eaton from small cell; weakness that fatigues with use is myasthenia from thymoma.
From the Attending Ectopic ACTH from small cell does not look like the textbook Cushing face. The tumor moves too fast for moon facies and striae to form. What dominates is the biochemistry: profound hypokalemia, hyperglycemia, and metabolic alkalosis, with very high cortisol and ACTH that high-dose dexamethasone fails to suppress. A rapid, biochemistry-heavy Cushing with no cushingoid body habitus and non-suppressible ACTH is ectopic ACTH from small cell lung cancer.
03 · Local Invasion Syndromes

Pancoast, SVC, and RLN

Lung cancer invading structures around it. Each structure tells you the story.

Pancoast Tumor ▶
Location: Apex of the lung (superior sulcus). Invades the thoracic inlet.

Cancer type: Usually squamous cell carcinoma (or adenocarcinoma).

Structures invaded:
1. Sympathetic chain (T1): Horner syndrome: ptosis (drooping eyelid), miosis (small pupil), anhidrosis (no sweating on that side of face). All ipsilateral.
2. Brachial plexus (C8-T2): Shoulder and arm pain radiating down the inner forearm and ulnar fingers. Weakness of intrinsic hand muscles.
3. Ribs and vertebrae: Bone erosion visible on imaging.

Treatment: Concurrent chemoradiation first, then surgical resection if downstaged.
Apex mass Horner (ptosis+miosis+anhidrosis) Brachial plexus pain Rib erosion
SVC Syndrome ▶
Mechanism: Obstruction or compression of the superior vena cava. The SVC drains the head, neck, and upper extremities. When blocked, drainage backs up.

Symptoms: Facial and neck swelling (plethora), arm swelling, JVD, cyanosis, headache that worsens when bending forward or lying flat (increases venous pressure).

Most common cause: Lung cancer (SCLC or right-sided masses, because the SVC runs on the right). Second cause: mediastinal lymphoma.

Diagnosis: CT chest with contrast to locate obstruction and identify the mass.

Treatment: Radiation to shrink the mass. Stenting for rapid relief of obstruction. Treat the underlying cancer.
Facial + neck swelling JVD Headache when bending SCLC or lymphoma
Recurrent Laryngeal Nerve Palsy ▶
Which nerve: Left recurrent laryngeal nerve (RLN). The left RLN loops under the aortic arch before ascending to the larynx. This long thoracic course makes it vulnerable.

The right RLN hooks under the right subclavian artery and does not enter the mediastinum, so it is rarely affected by lung cancer.

Cause: Left-sided lung cancer or enlarged mediastinal lymph nodes compressing the left RLN.

Symptom: Hoarseness. Unilateral vocal cord paralysis. Voice is weak and breathy.

Why this matters: New hoarseness in a smoker with lung cancer = left RLN involvement = mediastinal disease. Changes staging and resectability.
Hoarseness Left-sided mass Left RLN under aortic arch Unilateral vocal cord palsy
Horner syndrome anatomy for boards: Ptosis + miosis + anhidrosis, all ipsilateral. Caused by disruption of the sympathetic chain. The three-neuron arc: hypothalamus to T1 (first order), T1 to superior cervical ganglion (second order = Pancoast hits here), superior cervical ganglion to eye and face (third order). Second-order lesions classically from Pancoast or aortic dissection.
04 · Treatment Logic

Staging & Treatment

SCLC uses its own 2-stage system. NSCLC uses TNM. Treatment depends entirely on stage plus mutation status.

SCLC Staging
NSCLC Staging
SCLC uses a 2-stage system, not TNM. Limited vs extensive. The question is: does it fit in one radiation field?

SCLC Limited Stage

Fits in One Radiation Field

Definition: One hemithorax plus ipsilateral mediastinal and supraclavicular nodes. All disease fits within a tolerable radiation port.

Treatment: Cisplatin + etoposide chemotherapy PLUS concurrent thoracic radiation. Concurrent is better than sequential.

After response: Prophylactic cranial irradiation (PCI). Brain is the most common site of SCLC relapse. PCI reduces risk of brain mets and improves survival.

Prognosis: Median survival 15-20 months. Small fraction achieve long-term remission.

One hemithorax Chemo + radiation PCI after response

SCLC Extensive Stage

Beyond One Radiation Field

Definition: Disease beyond one hemithorax. Contralateral nodes, malignant pleural effusion, distant mets (brain, liver, adrenals, bone). Most SCLC presents here.

Treatment: Cisplatin or carboplatin + etoposide chemotherapy. Now add atezolizumab (PD-L1 inhibitor) to chemotherapy for extensive stage (current standard of care).

No PCI in extensive stage if brain MRI is negative (prior practice was PCI for all, now controversial).

Prognosis: Median survival 8-13 months. High response rate to first-line chemo but almost universal relapse.

Distant mets Chemo only Add atezolizumab No PCI if brain MRI negative
SCLC board trap: The question will give you limited-stage SCLC and ask the best treatment. The answer is concurrent chemoradiation (NOT sequential, NOT surgery, NOT chemo alone). If PCI is offered as a choice after a complete response to chemoradiation for limited-stage, that is also correct.
05 · Clue-by-Clue Reasoning

Elimination Game

One vignette. Four suspects. Use the clues to eliminate until one remains.

A 55-year-old woman who has never smoked presents for evaluation of an incidentally found lung nodule. CT chest shows a peripheral lung nodule with ground-glass opacity and a lepidic growth pattern. Molecular testing returns an EGFR exon 19 deletion.
Small Cell Lung Cancer
Squamous Cell Carcinoma
Adenocarcinoma
Large Cell Carcinoma
Clue 1: Never-smoker + peripheral location + ground-glass opacity + lepidic growth pattern.

SCLC is central (hilar) and occurs almost exclusively in heavy smokers. Squamous cell is also central and strongly smoking-related. Both are eliminated by peripheral location plus never-smoker demographics.
Clue 2: EGFR exon 19 deletion.

EGFR mutations are the defining molecular feature of adenocarcinoma, especially in never-smoking women. Large cell carcinoma is undifferentiated with no defined targetable driver mutation. The EGFR finding is so specific to adenocarcinoma that it eliminates large cell.
Adenocarcinoma. Never-smoker + peripheral + lepidic growth + EGFR mutation is the archetype adenocarcinoma presentation. Osimertinib is the targeted therapy.
06 · See It

Clinical Atlas

The images the board pairs with each tumor. Tap any to expand.

CT chest showing a lung mass
Lung Mass · CT Chest
Squamous cell carcinoma histology
Keratinizing · Squamous
Small cell lung carcinoma oat cell histology
Oat Cells · Small Cell
Lung adenocarcinoma acinar histology
Glandular · Adenocarcinoma
Digital clubbing of the fingers
Digital Clubbing · HOA
CT chest with adenocarcinoma
Peripheral Nodule · Adeno CT
07 · Watch It Happen

The SIADH Engine

Release ectopic ADH from the small cell tumor and watch free water back up into the blood. Sodium falls in real time.

Central SCLC (neuroendocrine) tumor Collecting duct aquaporins open Blood ectopic ADH released
Serum sodium: 140 mEq/L (135-145)
The payoff: ADH that should be off keeps the collecting duct permeable. Free water is reabsorbed into blood, diluting sodium while the patient stays euvolemic. No edema, low serum osmolality, but urine stays concentrated. That is SIADH, and from a central lung mass it is small cell until proven otherwise.
08 · Lock The Pattern

Five Tumors, Five Reflexes

Tap each beat. Say your answer first, then reveal. These are the one-line reflexes the board rewards.

Central mass + euvolemic hyponatremia + concentrated urine?
Small cell lung cancer (SIADH). Neuroendocrine tumor making ectopic ADH. Also the source of ectopic ACTH and Lambert-Eaton.
Central cavitating mass + hypercalcemia + suppressed PTH?
Squamous cell carcinoma (PTHrP). Keratin pearls, intercellular bridges, p40/p63 positive. PTHrP is invisible to the PTH assay.
Peripheral nodule + never-smoker + ground-glass + EGFR?
Adenocarcinoma. Most common lung cancer overall and the non-smoker tumor. TTF-1 positive, lepidic growth, osimertinib for EGFR.
Peripheral, undifferentiated, all stains negative, poor prognosis?
Large cell carcinoma. The diagnosis of exclusion: no glandular, squamous, or neuroendocrine markers. No targeted therapy.
Young non-smoker + flushing, diarrhea, right-heart valve disease?
Bronchial carcinoid. Low-grade neuroendocrine, serotonin driven, urine 5-HIAA elevated. Resect for cure; octreotide for symptoms.
09 · Board-Style Walkthrough

Board-Style Walkthrough

Original third-order vignettes. Shuffled, never-repeat, full reasoning for every choice. Right-click or long-press to cross out; double-tap to highlight. Answer first, then tap each wrong option to walk the chain.