The Heavy Cavalry of Colorectal Cancer

Personalized healthcare requires solid, reproducible biomarkers to stratify cancer patients into prognostic subgroups. One such factor emerging in colorectal cancer is tumor budding, a novel and promising histopathological biomarker now classified as an additional prognostic factor in the eighth edition of the TNM Classification of Malignant Tumors 2017 (1).

Tumor budding describes the presence of single tumor cells or small tumor clusters of up to four cells. These “buds” are detached from the main mass and are usually located in the stroma at the invasive front of the tumor. Studies carried out on patients with colorectal cancer show that a higher number of tumor buds is associated with an increase in vessel invasion, lymph node metastasis, and distant metastatic disease. These associations indicate that tumor buds – or at least a subset of the budding cells – can invade the extracellular matrix and disseminate through blood vessels.

Located within the tumor microenvironment, tumor buds have been shown, at least in part, to display features of epithelial-to-mesenchymal transition (EMT). They also overexpress markers of cell invasion, migration, and survival, as well as deregulating proteins involved in the Wnt signaling pathway. Tumor buds frequently disrupt E-cadherin expression and prevent β‑catenin expression at the cell membrane, often along with nuclear translocation of β‑catenin (2). These morphologic and molecular characteristics make tumor buds the attacking “heavy cavalry” at the invasive tumor front, counteracted by immune cells as the defenders (2) (3).

A budding practice

Given that colorectal cancer patients within the same disease stage have heterogeneous outcomes, tumor budding could be an important biomarker to better resolve these differences. Put simply, the more tumor buds identified in the histological evaluation of colorectal cancer, the worse the patient’s prognosis. Tumor buds are classified as either peritumoral or intratumoral and are visible on standard hematoxylin and eosin (H&E) staining.

In daily practice, tumor budding may impact at least three potential clinical scenarios. For early colorectal cancer that begins in polyps (pT1), peritumoral budding is associated with the presence of lymph node metastasis, which provides an indication that an oncologic resection should be considered to establish the lymph node status (4). In stage II colorectal cancer that doesn’t include lymph node metastasis, tumor budding is an independent prognostic factor associated with worse disease-free and overall survival. In stage II colorectal cancer patients with high-grade budding, adjuvant therapy should potentially be considered (5). Finally, another promising clinical scenario has recently emerged: preoperatively treated rectal cancers. Why is tumor budding a useful biomarker in this setting? Because intratumoral budding can be seen in colorectal cancer biopsies and, therefore, identifying this feature in a colon or rectal cancer patient may prove useful in their preoperative management (6).

Although tumor budding is still a marker of tumor progression in more advanced and metastatic colorectal cancer (stage III and IV), we are still investigating its role in clinical practice. Patients with stage III colorectal cancer normally benefit from treatment with adjuvant therapy because clinically occult micrometastases may still be present after surgery, leading to disease recurrence. Tumor budding may not play an imminent role in decisions about therapeutic management, but high-grade budding is still an indicator of the potential progression of a local into a distant metastatic disease. It could therefore be used to optimize the clinical management of stage III colorectal cancer patients.

In stage IV colorectal cancer, the liver is the main site for distant metastases. For patients with isolated colorectal cancer, the regional treatment of liver metastases – including surgery alone or in combination with systemic chemotherapy – may be considered. In this clinical scenario, tumor budding could potentially be assessed in biopsies of colorectal cancer liver metastases (intrametastastic budding), or in resected colorectal cancer liver metastases (perimetastatic budding). Nevertheless, there is not enough data in the literature to make any conclusions on the prognostic or predictive role of tumor budding in colorectal cancer liver metastases.

Introducing new guidelines

Given the amount of evidence that supports the clinical value of tumor budding in colorectal cancer, it is somewhat surprising that this feature has not yet been universally accepted into diagnostic practice. The main reason for the absence of tumor budding in previous colorectal cancer guidelines and protocols has been the lack of an international standardized scoring system. For this reason, the International Tumor Budding Consensus Conference (ITBCC) was held in Bern, Switzerland, in April 2016. Its goal? For 23 experts in GI pathology from all over the world to meet and propose a reproducible, cost-effective scoring system based on the available data. The group reached a consensus based on the following statements (7):

• Tumor budding is defined as a single tumor cell or a cell cluster of up to four tumor cells.

• Tumor budding is an independent predictor of lymph node metastasis in pT1 colorectal cancer.

• Tumor budding is an independent predictor of survival in stage II colorectal cancer.

• Tumor budding should be taken into account along with other clinicopathological factors in a multidisciplinary setting.

• Tumor budding is counted on H&E.

• Intratumoral budding in colorectal cancer has been shown to be related to lymph node metastasis.

• Tumor budding is assessed in one hotspot (in a field measuring 0.785 mm²) at the invasive front.

• For tumor budding assessment in colorectal cancer, the hotspot method is recommended.

• A three-tier system should be used along with the budding count to facilitate risk stratification in colorectal cancer.

• Tumor budding should be included in guidelines and protocols for colorectal cancer reporting.

• Tumor budding and tumor grade are not the same.

The results of the ITBCC were published in 2017 in Modern Pathology (7). Five practical steps are proposed for scoring tumor budding in daily practice:

1. Determine the field (specimen) area for the 20X objective lens of the microscope based on the eyepiece field number diameter.

2. Select the H&E slide with greatest degree of budding at the invasive front.

3. Scan 10 individual fields at medium power (10X objective) to identify the “hotspot” at the invasive front.

4. Count tumor buds in the selected “hotspot” (20X objective).

5. Divide the bud count by the normalization factor to determine the tumor bud count per 0.785 mm².

These five steps allow the pathologist to select the budding category (BD1 – low, BD2 – intermediate, or BD3 – high) based on bud count and indicate the absolute count per 0.785 mm².

The ITBCC guidelines should not be regarded as the endpoint of tumor budding, but as a basis for large retrospective and prospective clinical trials. Indeed, the ITBCC guidelines have already been applied in several studies and are also included in guidelines and protocols (8) (9) (10). Many pathologists working with tumor budding in daily practice are asking themselves why it was not defined based on immunohistochemistry, as this better visualizes tumor buds – especially in a highly inflamed peritumoral environment. This was discussed in depth at the ITBCC in 2016; the main argument for using H&E was based on the data available in the literature, which clearly favors the use of H&E stains to assess tumor budding in colorectal cancer. Additionally, educational sessions on the interpretation of tumor budding, similar to those organized for PD-L1 scoring, may not only further develop digital image analysis, but also minimize the inter-observer variability for tumor budding in colorectal cancer.

An increase in the number of biopsy studies that focus on tumor budding will support the implementation of intratumoral budding in preoperative biopsies of colon and rectal cancer. From the tumor microenvironment perspective, the inclusion of tumor budding and immune cells into a prognostic score is an interesting approach (3) (11). Indeed, the inclusion of tumor- and host-related biomarkers better reflects the “attacker–defender” approach and the role of cancer and immune cells in the tumor microenvironment of colorectal cancer. From a molecular perspective, the detection of potentially predictive and prognostic target molecules is a crucial next step (2). Specifically, the discovery of predictive tumor budding molecules would be a promising therapeutic approach, directly targeting the heavy cavalry of colorectal cancer and potentially defining an anti-budding therapy in the future.

Tumor budding seems to be a simple, reproducible, and robust histopathological biomarker. Its use is applicable not only to colorectal cancers, but also to other solid tumors, such as cancer of the oral cavity, lung, pancreas, esophagus, breast, and urinary bladder. In my opinion, there is a promising outlook for this new approach!