By: Philip Bryant
The application of large format histology in routine clinical diagnosis is reasonably well established. However, large specimens such as breast, bowel, kidney and prostate are frequently sliced in the dissecting room and processed in multiple smaller, standard size tissue cassettes. Logically, the more samples that are obtained from large tumor specimens, the more reliable the pathology, yet protocols for the routine sampling of these tumors often recommend taking fewer samples. Taking a one centimeter block for each centimeter of tumor diameter is often the general consensus with the provision for extra samples to be taken from areas of tumor that may have a different color, texture or area of suspicion. Since cancers are the culmination of many complex changes in cell metabolism, tumors are often dissimilar at different sites. These changes termed intra-tumor heterogeneity (ITH) have an uncertain behavior with often unique and unpredictable tumor patterns. As a result, detection of these changes is often dependent upon the accuracy of tumor sampling by the pathologist.
One of the most common methods of dissecting large tumor samples is the ‘bread-loafing’ technique which is able to determine tumor extent and assessment of surgical resection margins (Figure 1a). However, this method has a high false negative rate if only selected slices are processed (Figure 1b). In contrast, complete circumferential (peripheral and deep margin) assessment allows for complete examination of tumor and the surgical margins (Figure 2a+b). Consequently, this method is superior to ‘bread-loafing’ due to its low false negative error rates.
An alternative to the more traditional practices that are used in tissue dissection is the procedure known as multi site tumor sampling, a simple method that clearly improves routine detection of complex changes in tumors. The technique is based on a divide-and-conquer (DAC) algorithm which involves selecting certain areas of the tumor for analysis with the goal that those areas are representative of the entire tumor. This strategy has been used to solve complex problems by selecting the most appropriate cells for examination. This is especially important for large tumors which have a propensity for incomplete sampling (Figure 3a+b). The practice of multi site tumor sampling is carried out by the application of a cutting grid to the slice of tumor in order to obtain multiple tissue cubes, several of which are placed into each standard size tissue cassette. This procedure is cost effective by limiting the quantity of standard cassettes used whilst increasing the number of tumor areas sampled (Figure 3c+d).
Irrespective of which of these methods are employed for sampling large tumors, the use of standard size processing cassettes and multiple tissue blocks still remains commonplace. Consequently, at microscopy, the specimens often need to be pieced together like jigsaws since the stained sections are frequently presented over many slides. The use of large format cassettes alleviates this problem by aiding diagnosis and turnaround times through improvement in visibility as the entire tissue sample can often be viewed on a single slide. Because these large format (supa mega) cassettes are capable of holding 10 times the volume of conventional standard size cassettes (Figure 4), they are a more efficient way of assessing large areas of tissue, thereby avoiding the problems of under sampling that often occurs with tumor specimens. This is particularly important in cancer diagnostics since in the early stages, tumor development is often so involved that conventional histology sampling is seldom sufficient to document its complexity.
Easily adapted to the needs of the routine diagnostic laboratory, large format histology has become the established method for not only assessing characteristics such as intra-tumor heterogeneity, tumor size, extent and distribution, but also for identifying the involvement of surgical margins. The large format technique can be adopted for all tissue types and tumors and the production of digitalized whole slide images is also possible using available scanners. This is especially significant when tumor evaluation of virtual slides is able to offer diagnostic clues to the classification, prognosis and prediction of response of the disease to treatment. Large format histology has proven to be a cost effective method that is able to meet the needs of the modern histology laboratory, particularly in the multidisciplinary approach to cancer diagnosis.
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