# Liquid Biopsy and Molecular Biomarkers in Mucinous Appendiceal and Colorectal Tumors: Current Evidence and Unmet Challenges in Precision Oncology

## TL;DR

This review discusses the challenges and current evidence of using liquid biopsy in mucinous appendiceal and colorectal tumors, emphasizing the need for tailored approaches due to their unique biology.

## Contribution

The paper provides a context-dependent framework for interpreting liquid biopsy results in mucinous tumors, highlighting the limitations and research needs specific to this subgroup.

## Key findings

- Mucinous tumors have low ctDNA detectability due to peritoneal compartmentalization and limited vascular access.
- Current liquid biopsy evidence is fragmented and not extrapolated from non-mucinous colorectal cancer paradigms.
- Compartment-specific sampling and non-mutational platforms may improve sensitivity in low-shedding mucinous malignancies.

## Abstract

Liquid biopsy tests tumor-related signals in body fluids, most often blood, to support cancer diagnosis and follow-up. In mucinous and peritoneal-dominant cancers (such as appendiceal tumors and pseudomyxoma peritonei), disease may remain largely confined to the abdominal cavity and release very little tumor material into the bloodstream. Therefore, a negative blood-based liquid biopsy does not necessarily mean that disease is absent. Mucinous appendiceal and colorectal tumors represent a biologically distinct and clinically underrepresented subgroup in which the performance and interpretation of liquid biopsy differ substantially from non-mucinous colorectal cancer, yet disease-specific guidance remains limited. In this review, we explain why detection can be low, summarize which body fluids and test approaches may be more informative in this setting (including blood and peritoneal fluid), and discuss how results should be interpreted for common clinical contexts such as genetic testing when tissue is limited, monitoring after surgery, and assessment of treatment response. Importantly, we propose a clinically oriented, context-dependent interpretation framework that distinguishes between informative positive findings and potentially misleading negative results in low-shedding disease. We also highlight current limitations and research priorities needed to improve clinical use and outline practical considerations for integrating liquid biopsy into multidisciplinary decision-making and patient counseling in mucinous malignancies.

Mucinous appendiceal and colorectal tumors represent a biologically and clinically distinct subset of gastrointestinal malignancies, defined by abundant extracellular mucin, characteristic molecular alterations, and a strong predilection for peritoneal dissemination. Despite these distinctive features, they remain relatively underrepresented in biomarker-driven clinical frameworks, and evidence specific to liquid biopsy performance in this subgroup is fragmented. These features complicate clinical assessment and limit the applicability of tissue-based and circulating biomarkers that perform well in non-mucinous colorectal cancer. Although liquid biopsy is now integral to precision oncology across multiple solid tumors, its role in mucinous disease remains incompletely defined. This review synthesizes evidence on liquid biopsy and molecular biomarkers in mucinous appendiceal and colorectal tumors, with emphasis on circulating tumor DNA (ctDNA) and emerging multi-analyte approaches. Rather than extrapolating from non-mucinous colorectal cancer paradigms, we examine liquid biopsy performance through the lens of mucinous tumor biology and peritoneal compartmentalization. We summarize how peritoneal-dominant spread, limited vascular access, low tumor cellularity, and sequestration of malignant cells within mucin pools constrain biomarker shedding into peripheral blood and contribute to low ctDNA detectability. We discuss the clinical implications for postoperative surveillance after cytoreductive surgery and hyperthermic intraperitoneal chemotherapy, molecular profiling when tissue is limited, and longitudinal monitoring in selected patient subsets. A central focus is the context-dependent interpretation of positive versus negative plasma findings, particularly in low-shedding disease, where false reassurance may occur. We also review strategies beyond plasma ctDNA, including compartment-specific sampling and non-mutational platforms that may improve sensitivity in low-shedding settings. By integrating biological rationale with comparative findings across studies, we identify key evidence gaps and priorities for prospective, biologically stratified validation. We further outline practical considerations for integrating liquid biopsy results into multidisciplinary decision-making in mucinous malignancies. Aligning biomarker class and sampling compartment with mucinous tumor biology may improve interpretability and support more precise clinical management.