Tissue sampling
Our pages about biopsies describe in some detail the standard work-up of biopsy samples and resected solid tumours in the pathology laboratory. In normal clinical practice, these tissues are seriously under-sampled, with typically only a tiny fraction of the material being examined in detail (this can be as little as 0.0005 % of the material). It would be impractical to individually examine hundreds or even thousands of samples of a resected tissue material. This limitation creates a challenge: how can the pathologist ensure that they choose a meaningful selection of specimen for examination, in particular how can it be ensured that the selection is unbiased and representative of the tissue volume. These limitations are well documented and understood.
There is extensive statistical analysis and theory about sampling in homogeneous and heterogeneous materials – a generally important analytical aspect, not just in the pathology laboratory. As long as the material to be examined is known to be homogeneous, the task is fairly straightforward (a common situation, for example, in quality control in a range of production lines in various industries). In such a scenario, a fairly small number of samples taken randomly from the material will be sufficient to characterise the bulk of the material with sufficient accuracy and reproducibility; the number of necessary samples is predominantly determined by the uncertainty associated with the experimental determination of the chosen property / parameter of the bulk material.
Unfortunately, the situation with tissue samples in the pathology laboratory is considerably more complicated as all tissue materials are heterogeneous, to varying degrees. This is particularly relevant for head & neck malignancies which tend to be highly heterogeneous solid tumours (this high heterogeneity also impacts / limits a number of methods recently suggested to assess surgical margins during surgery (mass spectrometry and Raman spectroscopy) both rely on tissue-data bases for comparison with actual readings, a difficult problem when confronted with heterogeneous tumour tissues). Being able to characterise the overall properties of solid tumour tissues is becoming increasingly important. For example, different types and levels of mutation burden of tumours are important to know in order to select the most appropriate treatment options for a particular type of tumour.
Any improvements with regard to under-sampling issues have to be practical so that they can be implemented in clinical routines (examining very large numbers of sample specimen would be prohibitively work-intensive). It has been suggested to exploit tissue material more completely than is current practice. Currently, once the selected few samples are taken from the tissue, prepared and examined, the remainder of the tissue material is considered clinical waste and is, typically, incinerated.
Instead, after standard selective sample examination, the residual tissue material is kept and homogenised (literally by blending it). The homogenisation of the tissue material does avoid selection bias in traditional selective (under)sampling approaches. This is followed by purification steps and analysis for relevant biomarkers (such as sequencing for certain mutations) in the homogenised material. Many of these steps and examinations rely on established methods, sometimes referred to as ‘liquid biopsies’. This analysis gives a comprehensive average measure of the important parameters and characteristics for the bulk of the tissue material, with potential to reduce uncertainties in decisions about the most appropriate treatment options. Furthermore, analysis of biomarkers of the homogenised tissue material can provide an information baseline to monitor metastasis alongside its cell type of origin, and to identify cell types (variants) evading treatment. In the context of research, such representative tissue sampling and more complete tissue characterisation are vital in further investigating the biochemistry and biology of tumour emergence, formation and evolution, including the development of metastases, the recurrence of primary tumours, and understanding poor response to immunotherapies.