Distinct Histological Patterns in Anal Canal Cancer: Insights from the Japanese National Registry

Highlights

• In contrast to Western populations where squamous cell carcinoma (SCC) predominates, adenocarcinoma accounts for the majority (66.8-75.5%) of anal canal cancer cases in Japan.
• Japanese SCC cases show a significant female predominance (71.5%), which is higher than figures reported in Western literature.
• Human papillomavirus (HPV) remains a critical driver for SCC in Japan, with an 85% positivity rate and HPV-16 as the dominant genotype.
• Treatment paradigms in Japan have shifted dramatically from primary surgical resection to chemoradiotherapy (CRT), yielding survival outcomes comparable to surgery.

The Clinical Challenge of Anal Canal Carcinoma

Anal canal carcinoma (ACC) represents a rare but clinically significant subset of gastrointestinal malignancies. Historically, its management has been complicated by the anatomical complexity of the anal canal, which serves as a transition zone between the glandular epithelium of the rectum and the squamous epithelium of the perianal skin. While global oncology guidelines often group ACC as a single entity dominated by squamous cell carcinoma (SCC), emerging data from East Asia, particularly Japan, reveal a starkly different epidemiological and histological landscape. Understanding these regional variations is paramount for clinicians and researchers aiming to standardize care and improve outcomes in rare cancer populations.

Methodological Framework: The JSCCR Registry Analysis

To clarify these epidemiological nuances, Yamada et al. conducted a comprehensive analysis utilizing data from the Japanese Society for Cancer of the Colon and Rectum (JSCCR) registry alongside a nationwide multi-institutional study. This research framework allowed for the evaluation of large-scale clinicopathological features, human papillomavirus (HPV) status, and long-term survival trends. By comparing these findings with established Western registry data (such as SEER or European databases), the study provides a high-resolution view of how ACC manifests differently across diverse geographic and ethnic cohorts.

Results: A Striking Divergence in Histological Distribution

The most significant finding of this investigation is the reversal of histological prevalence between Japan and the West. In Western nations, SCC accounts for approximately 70% to 85% of all anal canal malignancies, largely driven by the high prevalence of HPV infections. Conversely, the Japanese data reveals that adenocarcinoma is the most common histological type, comprising between 66.8% and 75.5% of cases. SCC, the Western hallmark of the disease, represents only 16.2% to 24.4% of cases in Japan.

This discrepancy raises important questions regarding the biological and environmental drivers of ACC in Japan. It suggests that many cases classified as anal canal cancer in Japan may originate from the glandular tissue above the dentate line or potentially from anal glands and fistulae, which are more likely to result in adenocarcinoma. This highlights a critical need for international standardization in classification and staging, particularly regarding the anatomical boundaries that define the anal canal versus the low rectum.

Demographics and the Role of HPV

Among the Japanese SCC cohort, there is a notable demographic shift. Women represent 71.5% of SCC cases, a higher proportion than typically observed in Western studies. This gender distribution suggests that screening and prevention strategies for anal SCC in Japan must be uniquely tailored to female populations. Despite the lower overall incidence of SCC, the role of HPV is consistent with global patterns. The study found that 85% of SCC cases were HPV-positive, with HPV-16 being the most prevalent genotype. This confirms that while SCC is rarer in Japan, its viral etiology remains identical to that of Western cases, underscoring the potential benefit of expanded HPV vaccination programs in the region.

Evolution of Treatment Modalities: From Surgery to CRT

Management strategies for ACC in Japan have undergone a significant transformation over the last three decades. In the 1990s, primary surgical intervention (often involving abdominoperineal resection) was the mainstay of treatment, with chemoradiotherapy (CRT) utilized in only about 14% of cases. However, following the global success of the Nigro protocol and subsequent clinical trials, the adoption of CRT in Japan surged to over 80% after 2010. The analysis demonstrates that this shift has not compromised patient outcomes; survival rates for those receiving CRT are now comparable to those who underwent radical surgery, with the added benefit of sphincter preservation and improved quality of life.

Clinical Implications and Expert Commentary

From a clinical perspective, the predominance of adenocarcinoma in Japanese ACC cases necessitates a diagnostic approach that prioritizes differentiation from low rectal cancer. Since the treatment for anal adenocarcinoma often mirrors that of rectal cancer (e.g., total mesorectal excision) while SCC is primarily managed with CRT, a correct histological diagnosis is the cornerstone of effective therapy.

Experts suggest that the high prevalence of adenocarcinoma in Japan may be partly attributed to the historical definitions used in the Japanese Classification of Colorectal, Appendiceal, and Anal Carcinoma, which may differ slightly from the AJCC/UICC staging systems. To facilitate better international collaboration and comparison, Japanese institutions are increasingly moving toward harmonized staging criteria. Furthermore, the high HPV positivity rate in the SCC subgroup serves as a strong call to action for public health officials to promote the HPV vaccine as a primary prevention tool for both cervical and anal malignancies.

Conclusion: Harmonizing Global Management Strategies

In summary, the epidemiology of anal canal cancer in Japan presents a unique challenge that diverges from the Western paradigm. While adenocarcinoma is the dominant subtype, the characteristics of HPV-associated SCC remain globally consistent. The successful transition to CRT as a standard of care for SCC in Japan marks a significant milestone in oncological practice. Moving forward, the expansion of HPV vaccination and the adoption of unified international classification systems will be essential to optimize clinical management and bridge the gap between regional epidemiological trends.

References

Yamada K, Saiki Y, Takano S, Tanaka M, Fukunaga M, Nakamura Y, Yonemura K, Sugimoto K, Iwasaki Y, Tsuji Y, Takano M. Cancer epidemiology in rare and hereditary colorectal diseases 2) anal canal cancer (cancer statistics). Int J Clin Oncol. 2025 Dec 26. doi: 10.1007/s10147-025-02947-3. Epub ahead of print. PMID: 41452529.