Small Cell Lung Cancer

Etiology

Smoking is the leading cause of lung cancer, accounting for 85% of cases. Additional risk factors for lung cancer include second-hand smoke, asbestos, radon, and other environmental factors. Smoking is associated with all types of lung cancer. However, the strongest associations are with SCLC and squamous cell lung cancer.

Cigarette smoke contains many organic and inorganic carcinogens including polycyclic aromatic hydrocarbons (PAHs), aromatic amines, N-nitrosamines, benzene, vinyl chloride, arsenic, and chromium among many others. There is a dose-dependent relationship between the degree of smoke exposure and the relative risk of lung cancer. The dosage of smoke depends on the type of cigarette, duration of inhalation, and presence of a filter.[4][5]

Epidemiology

Lung cancer is the leading cause of cancer death worldwide killing approximately 1.5 million people in 2012. In the United States, there are over 200,000 new cases per year and over 150,000 deaths per year attributed to lung cancer. SCLC comprises about 15% of cases.

Smoking is strongly associated with developing lung cancer and particularly with SCLC. The relative incidence of SCLC has decreased over the last 4 decades reflecting decreases in smoking prevalence, changes to cigarettes, and reduced occupational hazards.

There is a higher incidence of SCLC in males. However, the incidence gap between males and females has narrowed over the past 3 decades. Caucasians have had a higher incidence of SCLC than African Americans. Survival gaps between races and socioeconomic status (SES) have narrowed since 1983.

From 1983 to 2012, median survival and overall survival of SCLC patients have remained stable. Five-year survival rates have increased from 4.9% (1983 through 1993) to 6.4% (2002 through 2012). Median survival has remained stable at 7 months. Relative survival rates (RSRs) have remained stable. However, there have been greater survival improvements in younger patient groups.[6]

Pathophysiology

SCLC is divided into the 2 subtypes: oat cell carcinoma and combined-SCLC. Combined SCLC is defined as SCLC with non-small cell components such as squamous cell or adenocarcinoma.

SCLC carcinogenesis can occur by various pathways that disrupt normal DNA repair mechanisms. Common mutations in SCLC include loss of RB1 tumor suppressor gene and TP53(17p13) mutations which decrease the pro-apoptotic activity of cancer cells. Almost all SCLC tumors have a deletion in a section of the short arm of chromosome 3p which contains the tumor suppressor gene FHIT. [7][8]

Histopathology

According to the World Health Organization (WHO) Classification of Tumors, “SCLC is defined by light microscopy as a tumor with cells that have a small size, a round-to-fusiform shape, scant cytoplasm, finely granular nuclear chromatin and absent or inconspicuous nucleoli.” Necrosis may be extensive, and cells manifest a high mitotic rate. SCLCs originate from epithelial cells, and up to 90% will express thyroid transcription factor-1 (TTF1). Epithelial cell markers including cytokeratin can be used to distinguish SCLC from lymphoma and other neuroendocrine tumors.

History and Physical

A presenting patient is usually a male, older than 70, and a current or former smoker. Patients often have multiple cardiovascular or pulmonary comorbidities.

SCLC typically occurs in the central airways and is often aggressive with a short doubling time and high mitotic rate. Symptoms are usually rapid-onset and begin 8 to 12 weeks before presentation. Signs and symptoms depend on the location and bulk of the primary tumor. They may include a cough, wheezing, and hemoptysis. Local intrathoracic tumor growth can affect the superior vena cava (resulting in superior vena cava syndrome), chest wall, or esophagus. Extrapulmonary distant spread can present with neurological problems, recurrent nerve pain, fatigue, and anorexia. Nearly 60% of patients present with metastatic disease. Most common sites of metastases include the brain, liver, adrenal glands, bone, and bone marrow. If left untreated, SCLC is characterized by rapid tumor progression with a median survival of 2 to 4 months.

SCLC is the most common solid tumor to cause paraneoplastic syndromes. Paraneoplastic syndromes are caused by ectopic hormone production or immune-mediated tissue destruction.

The most common paraneoplastic syndromes include:

• SIADH (syndrome of inappropriate antidiuresis) in 15% to 40% of patients. This may present as weakness, dysgeusia, and clinical euvolemia.
• Ectopic Cushing syndrome in 2% to 5% of patients. Production of ectopic corticotropin (ACTH) resulting in elevated free cortisol levels. Can present with edema, proximal myopathy, and hypokalemic alkalosis.
• Lambert-Eaton Myasthenic syndrome in 3% of patients. This presents as a weakness of proximal arms and legs, relieved temporarily after exercise. Can involve autonomic nervous system.

Evaluation

Lung cancer should always be considered in a smoker or former smoker who presents with new onset cough or hemoptysis. It is important to compare new imaging with prior imaging, noting any changes. Patient’s suspected of having lung cancer will undergo an extensive imaging workup and biopsy for diagnosis with most patients being diagnosed based on small biopsy and cytology specimens.[9][10][11]

Initial imaging workup includes plain film radiography and contrast-enhanced computed tomography (CT) scans of the chest and upper abdomen, brain MRI and PET/CT.

Laboratory studies ordered to evaluate for the presence of neoplastic syndromes include complete blood count (CBC), electrolytes, calcium, alkaline phosphatase, alanine aminotransferase (ALT) and aspartate aminotransferase (AST), total bilirubin, and creatinine.

The method of attaining tissue depends on tumor location. A biopsy is typically obtained via CT-guided biopsy or transbronchial.

Treatment / Management

Treatment is highly dependent on stage. Patients with limited-stage SCLC are candidates for curative-intent radiation therapy and chemotherapy. Patients with extensive-stage disease are treated with chemotherapy with radiation (RT) reserved for select candidates and palliation.[12][13](B3)

Limited Stage

While the majority of patients present with locally advanced disease, a small minority may present with T1-2N0 disease. These patients may be candidates for lobectomy with mediastinal sampling or dissection. If surgery demonstrates mediastinal nodal involvement, adjuvant chemoradiation is indicated. If they are not surgical candidates, stereotactic ablative body therapy (SABR) may be employed. Adjuvant cisplatin-based systemic therapy typically follows both local approaches.

The remainder of limited stage patients is treated with concurrent chemotherapy and radiation. Cisplatin and etoposide is the current standard of care. The addition of radiation to chemotherapy significantly increases local control and overall survival. The early incorporation of RT with CT has been shown to further increase overall survival compared to late incorporation. Prophylactic cranial irradiation (PCI) is indicated for patients with a complete or partial response to therapy. PCI significantly increases overall survival and decreases the incidence of brain metastases.[14][15](B2)

Extensive Stage

The mainstay of therapy for patients with the extensive stage is systemic chemotherapy. Patients who have a complete or partial response to therapy can be evaluated for consolidative thoracic RT. This therapy has been associated with an increase in overall survival and is typically well tolerated. PCI is also indicated for ES and has been associated with an increase in survival with a decrease in brain metastases incidence. There is currently no role for the addition of consolidation radiation to limited volume metastatic sites.

Novel agents for SCLC are currently under investigation and include immunotherapies and targeted therapies previously approved for NSCLC and other malignancies. Immunotherapies include immune checkpoint inhibitors Nivolumab (PD-1 inhibitor antibody), Pembrolizumab (anti-PD1 antibody), Ipilimumab (CTLA-4 inhibitor antibody). Targeted therapies in clinical trials include Rovalpituzumab tesirine (Delta-like protein three antibody-drug conjugate).[16]

Differential Diagnosis

• Hamartoma
• Granuloma
• Lymphoma
• Non small cell lung cancer
• Carcinoid

Staging

SCLC is distinguished according to the Veterans Administration Lung Cancer Study Group staging system into Limited Disease (LD) and Extensive Disease (ED). The main criteria for determining LD and ED are whether a reasonable radiation plan can safely encompass disease extent. Two-thirds of patients present with extensive stage or metastatic disease.

TNM staging is useful for patients who are candidates for surgical resection, clinical research, and cancer registries.

Complications

The majority of complications are from the lung cancer itself or chemotherapy and radiation. 

Patients may also develop paraneoplastic syndromes like hypercalcemia, myasthenia gravis like symptoms and SIADH.