To help pathologists avoid the different types of pitfalls in the assessment of TILs we designed an educational tool.

This tool uses both conventional pictures of microscopic slides and digitized whole microscopic slides (also called whole slide images (WSIs)) of biopsies and resection specimens of breast and lung cancer to illustrate these pitfalls. Despite the technical challenges of the use of WSIs -with the introduction of whole slide scanners- WSIs have become the standard for education in pathology. WSIs have a better resolution then conventional pictures. As is currently the case, we use annotations to make it possible to give detailed instructions and explanations directly on the images so to use WSIs  as pathologist read slides in their daily practice. He/she can navigate through the image and to zoom in and out. This creates a learning environment which is closer to the environment of our daily practice.

The use of WSIs has further advantages :

1. To avoid some of the pitfalls, such as misinterpretation of invasive cancer cells from a lobular breast carcinoma or apoptotic cells as TILs, the tissue should be examined at higher magnification, which WSIs can offer.
2. WSIs can better demonstrate possible problems due to the heterogeneity of the infiltrate or differences between different areas of the tumour.
3. WSIs make it also easier to demonstrate the relation between TILs in the tumour and the infiltrate in the surrounding (normal) tissue.

At this point in time, we included several examples of each of the pitfalls. We invite pathologists worldwide to help us extend this library of images to make this collection a ‘living’ library and continuously evolving learning tool for the pathology community.

• Click here to see more reference images and pitfall exercises