Lung Tumor Grading & Histologic Classification
Invasion assessment, AIS/MIA/lepidic-predominant classification, IASLC grade, neuroendocrine tumor grade, and AJCC 4-tier grading.
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This tool is built from a supplied CAP protocol excerpt covering lung histologic type and histopathologic grade only; it does not cover TNM staging, margins, lymph nodes, or other reporting elements. Non-small cell lung carcinoma is not an acceptable final diagnosis in resection specimens (small biopsies excepted). This tool is an educational and documentation-support aid only and does not replace professional pathologist judgment, the current WHO Classification of Tumours, or the current AJCC Cancer Staging Manual.
Reference
Histologic type & sampling
The WHO histologic classification for tumors of the lung, including carcinoids, is recommended for consistency in reporting. A designation of non-small cell lung carcinoma is acceptable in small biopsies but not in resection specimens. Lung carcinomas should be adequately sampled to ensure defining features are represented and invasion can be thoroughly assessed.
WHO invasion criteria (adenocarcinoma)
Invasion is defined as any one of: (1) any histologic subtype other than a lepidic pattern (i.e., acinar, papillary, micropapillary, and/or solid); (2) tumor cells infiltrating myofibroblastic stroma; (3) vascular or pleural invasion; (4) spread through air spaces (STAS).
STAS is defined as micropapillary clusters, solid nests, or single cells of tumor extending beyond the tumor edge into the air spaces of the surrounding lung parenchyma. STAS is an independent prognostic factor in major lung cancer histologic types and is associated with increased recurrence after limited resection (segmentectomy, wedge resection). STAS should not be incorporated into the measurement of tumor size.
AIS / MIA / lepidic-predominant adenocarcinoma
A diagnosis of AIS or MIA requires the lesion to be entirely submitted for histopathologic examination, and applies only to a solitary lesion ≤3 cm in diameter.
MIA = lepidic-predominant tumor ≤3 cm with an invasive component ≤0.5 cm, provided none of: (1) lymphatic, blood vessel, or pleural invasion, (2) tumor necrosis, (3) STAS.
Tumors ≤3 cm with ≤0.5 cm invasion but exhibiting 1 or more exclusionary features are classified as lepidic-predominant adenocarcinoma (not AIS/MIA).
AIS is categorized as Tis (AIS); MIA is categorized as T1mi.
A lepidic-predominant tumor >3.0 cm with no invasion, or ≤0.5 cm of invasion, is classified as lepidic-predominant adenocarcinoma and categorized as pT1a — there is insufficient data to conclude these behave the same as ≤3.0 cm tumors meeting AIS/MIA criteria.
IASLC histopathologic grading — non-mucinous adenocarcinoma
Predominant pattern and proportion of non-predominant subtypes (specified semi-quantitatively in 5% increments) together improve prediction of outcome. A complex glandular pattern is defined by fused glands or single cells infiltrating a desmoplastic stroma. Prognosis: lepidic-predominant (best) > acinar/papillary-predominant (intermediate) > solid/micropapillary-predominant (worst). Immunohistochemistry can aid classification in poorly differentiated cases, particularly for solid-type adenocarcinoma vs. nonkeratinizing squamous cell carcinoma.
Scope: use of this grading scheme is required for non-mucinous adenocarcinomas. There is currently no established grading scheme for invasive mucinous adenocarcinoma or squamous cell carcinoma of the lung.
| Grade | Definition |
|---|---|
| G1 | Well-differentiated — lepidic-predominant with no or <20% high-grade pattern |
| G2 | Moderately differentiated — acinar or papillary-predominant with no or <20% high-grade pattern |
| G3 | Poorly differentiated — any tumor with ≥20% high-grade pattern (solid, micropapillary, cribriform, or complex glandular pattern) |
Neuroendocrine tumor grade
Use of this scheme is required for neuroendocrine tumors; classification continues to rest on diagnostic criteria rather than a separate quantitative grading formula.
| Grade | Diagnosis |
|---|---|
| Low-grade | Typical carcinoid / neuroendocrine tumor, grade 1 |
| Intermediate-grade | Atypical carcinoid / neuroendocrine tumor, grade 2 |
| High-grade | Neuroendocrine carcinoma — large cell carcinoma or small cell carcinoma |
AJCC 4-tier histopathologic grading scheme
May be used, per the AJCC, for tumors without an established tumor-specific grading scheme.
| Grade | Definition |
|---|---|
| GX | Cannot be assessed |
| G1 | Well-differentiated |
| G2 | Moderately differentiated |
| G3 | Poorly differentiated |
| G4 | Undifferentiated |
Source
Built directly from a supplied CAP cancer protocol excerpt (Lung_5.1.0.0.REL_CAPCP, Histologic Type and Histopathologic Grade sections), which cites: (1) WHO Classification of Tumours Editorial Board, Thoracic Tumours, 5th ed., IARC, 2021; (2) Travis WD, Eisele M, Nishimura KK, et al., J Thorac Oncol. 2024;19(7):1028-1051 (STAS as a 9th-edition TNM descriptor); (3) Van Schil PE, Asamura H, Nishimura KK, et al., J Thorac Oncol. 2024;19(5):749-765 (T-descriptor revisions). Consult the current CAP protocol and AJCC Cancer Staging Manual directly for full reporting requirements.