For many solid tumors, grading is an integral component of routine pathology practice, providing critical information to estimate recurrence risk, guide treatment decisions, and counsel patients. In lung adenocarcinoma, the IASLC grading system is now well established and has been incorporated into the World Health Organization (WHO) Classification. However, for lung squamous cell carcinoma (LUSC), which still accounts for roughly one-quarter of lung cancers worldwide, a comparable, standardized grading system remains lacking.
Historically, LUSC has been described as well, moderately, or poorly differentiated, largely based on keratinization. More recently, the WHO has categorized LUSC into keratinizing, nonkeratinizing, and basaloid subtypes.1 However, none of these approaches has consistently provided robust, independent prognostic information across institutions and patient populations.
In a recent multi-institutional study conducted by the IASLC Pathology Committee, we sought to address this gap.2 Our goal was to develop a grading system for resected invasive LUSC that is simple, reproducible, and—most importantly—prognostically meaningful.
A Global, Multi-Cohort Effort
The study brought together large, well-annotated cohorts from multiple international institutions. Two independent training sets (n = 262 and n = 427) of patients who underwent surgical resection without neoadjuvant therapy were analyzed. All tumors were staged according to the AJCC 8th edition. We evaluated a range of histologic features previously reported to correlate with outcome, including: tumor budding, smallest tumor nest size, nuclear size and nuclear grade, mitotic count, keratinization, and spread through air spaces (STAS).
Multivariable Cox regression models were used to assess recurrence-free survival (RFS) and overall survival (OS), adjusting for age, sex, and pathologic stage. The final model was then validated in a large independent global test cohort of 827 cases from five additional institutions.
Why Tumor Budding?
Among all the histologic parameters examined, tumor budding stood out. In both training cohorts, two-tiered tumor budding was the only feature independently associated with both shorter RFS and OS.
Tumor buds were defined according to the International Tumor Budding Consensus Conference (ITBCC) 2016 recommendations: single cells or clusters of up to four tumor cells at the invasive front, counted in one hotspot at × 200 magnification and normalized to 0.785 mm².3
We explored different cutoffs. Ultimately, a threshold of ≥ 10 buds per 0.785 mm², aligned with ITBCC criteria for high budding, provided the most consistent discrimination of patient outcomes. This has led to a simple two-tier grading system:
- Low grade: 0–9 tumor buds per 0.785 mm²
- High grade: ≥ 10 tumor buds per 0.785 mm²
Importantly, intermediate-grade budding (5–9 buds) did not show independent prognostic differentiation from either low or high tumor budding, supporting the use of a two-tier system rather than a three-tier model.
Validation Across Regions and Stages
In the 827-case validation cohort, high-grade tumors were independently associated with worse outcomes. Median RFS was 4.8 years in low-grade tumors compared with 1.6 years in high-grade tumors. The system also stratified patients with stage I disease: median RFS was 7.2 years for low-grade versus 3.4 years for high-grade tumors.
High-grade tumors were also associated with larger tumor size, higher T and N stage, lymphovascular and pleural invasion, and more infiltrative growth patterns. These findings indicate that tumor budding correlates with other established markers of aggressive tumor biology.
Reproducibility in the Real World
Any grading system must be reproducible. In an interobserver study involving 10 pathologists with expertise in lung cancer who evaluated 25 cases using digital slides, overall agreement was moderate (Fleiss’ kappa = 0.524). While this finding underscores ongoing challenges in the assessment of tumor budding, it is worth noting that the methodology adheres to well-established ITBCC guidelines, and tumor budding is assessed on routine H&E slides without special stains. As digital pathology and artificial intelligence tools continue to evolve, there is substantial potential to further improve consistency.
What Does This Mean for Practice?
This proposed grading system is intentionally practical, relying on a single, clearly defined histologic parameter that can be readily incorporated into routine pathology workflows. It has been validated across geographically diverse cohorts, supporting its broad applicability.
We propose that tumor budding grade be considered for inclusion in synoptic reports for resected LUSC. Beyond prognostic stratification, the finding that high-grade tumor budding identifies higher-risk patients even within stage I disease raises important considerations for adjuvant therapy decisions and the design of future clinical trials.
Limitations
It is important to note that tumor budding-based grading should not be extrapolated to small biopsy samples. In a two-institutional study, tumor budding assessed in preoperative biopsies was neither prognostic nor predictive of tumor budding status in subsequent resections.4
Additionally, given the higher pathologic response rate to immunotherapy observed in LUSC compared to adenocarcinoma, along with the general absence of targetable EGFR or ALK alterations, the majority of patients with clinical stage IB (tumor size > 4 cm) to stage IIIA LUSC now receive neoadjuvant therapy. Accordingly, further studies are warranted to evaluate the prognostic performance of the tumor budding-based grading system in the post-neoadjuvant setting.
In Conclusion
Tumor budding emerged as the most robust and internationally validated histologic prognostic marker in resected invasive LUSC. A two-tier grading system using a cutoff of 10 tumor buds per 0.785 mm² provides a simple, biologically meaningful, and globally reproducible approach, presenting an important step toward harmonizing LUSC grading worldwide.
References
- 1. WHO Classification of Tumours Editorial Board. WHO Classification of Tumours: Thoracic Tumours. 5th ed. Lyon: IARC; 2021.
- 2. Mino-Kenudson M, Berezowska S, et al. A grading system for resected invasive squamous cell carcinoma of the lung: A multi-institutional study by the IASLC Pathology Committee. J Thorac Oncol. 2025.
- 3. Lugli A, Kirsch R, Ajioka Y, et al. Recommendations for reporting tumor budding in colorectal cancer based on the International Tumor Budding Consensus Conference (ITBCC) 2016. Mod Pathol. 2017.
- 4. Maillard M, et al. Multicenter study on tumor budding in lung squamous cell Ccrcinoma: Comparison between biopsy and resection with interobserver variability assessment. Mod Pathol. 2024.
