Colorectal cancer (CRC) represents a significant global health challenge, consistently ranking as the third most frequently diagnosed malignancy, with incidence and prevalence rates varying notably across different geographical regions [1]. Among the histological subtypes, adenocarcinoma is the predominant form documented in both international and regional epidemiological data [2].

Accurate pathological staging acts as the fundamental basis for guiding clinical management and predicting prognostic outcomes. The staging framework integrates clinical evaluation, radiologic imaging, surgical assessment, and comprehensive examination of resected specimens. As outlined in the AJCC Cancer Staging Manual (8th Edition), the classification system relies on three primary components: the extent of the primary tumor (pT), regional lymph node status (pN), and the presence of distant metastasis (pM) [3]. Specifically, the pT category is defined by the tumor depth of invasion, a factor that differs across stages and fundamentally dictates therapeutic decisions [4], [5].

Evaluating tumor invasiveness requires a synergistic approach that incorporates both macroscopic inspection and microscopic analysis. Meticulous gross examination is indispensable; suboptimal sampling may result in understaging, particularly in locally advanced (pT4) cases, thereby compromising patient care [4], [6], [7]. Identifying serosal (visceral peritoneal) invasion presents distinct diagnostic difficulties, as the serosal surface is prone to disruption during surgery or specimen handling. Moreover, differentiating genuine neoplastic invasion from inflammatory reactions, fibrosis, or adherence to adjacent structures remains a complex pathological task [5], [7], [8].

The prognostic significance of serosal involvement has been extensively documented in the literature. Shepherd et al. established that serosal invasion is a strong independent predictor of tumor recurrence and diminished overall survival [5]. Furthermore, investigations by Baguena et al. and Snaebjornsson et al. indicated that patients with pT4a tumors exhibit a heightened risk of peritoneal metastasis and suffer from reduced disease-free and overall survival rates compared to those with pT3 or pT4b disease [9], [10]. Variability in survival outcomes based on specific patterns of serosal invasion has also been highlighted by Wang et al. [11].

Since 2010, our institution has adopted a standardized grossing template to capture essential prognostic data, including tumor extension and lymph node metastatic status [12]. Despite this, the utility of this data for auditing laboratory quality remains underexplored. While routine reporting adheres to the Pathological Stage Classification (pTNM, AJCC 8th edition) [3], the lack of defined quality performance indicators limits the ability to systematically evaluate the efficacy of grossing practices.

International guidelines provide specific benchmarks for quality control. The Royal College of Pathologists, UK (2023), explicitly advises submitting a minimum of two tissue blocks from the tumor area nearest to the serosa to optimize diagnostic yield [13]. Regarding detection rates, Quirke et al. proposed that high-quality pathology services should identify peritoneal involvement in approximately 30% of colonic cancer resections [14]. Consistent with this, both the Colorectal Cancer Structured Reporting Protocol from Australia (2020) and the updated U.K. dataset (2023) suggest that a peritoneal involvement rate of at least 20% serves as an appropriate audit standard for pathology practice [13], [15]. Currently, however, Thailand lacks established national benchmarks for auditing colorectal cancer reporting.

The primary objective of this study is to investigate the correlation between the macroscopic characteristics of colonic adenocarcinoma, the number of blocks submitted from the serosal aspect, and the histologically confirmed depth of invasion. Additionally, we aim to establish the frequency of peritoneal invasion (pT4) within our institution. It is anticipated that these findings will provide a basis for developing standardized audit criteria for gross examination in pathology laboratories throughout Thailand.