How to Access Tumor-Infiltrating Lymphocytes (TILs) in Cancer Patients
TILs are accessed through standard histopathological evaluation of hematoxylin and eosin (H&E)-stained tissue sections obtained from tumor specimens, either via core needle biopsy or surgical resection, requiring no special tissue processing beyond routine pathology procedures. 1
Tissue Acquisition Methods
Primary Diagnostic Setting
- Core needle biopsies are sufficient for TIL assessment in the pre-treatment setting, particularly before neoadjuvant therapy 1
- Surgical specimens (lumpectomy or mastectomy) provide optimal tissue for TIL evaluation in the adjuvant setting 1
- One formalin-fixed paraffin-embedded (FFPE) block per patient is adequate for assessment 1
Post-Treatment Setting
- Core biopsies after short-term treatment (targeted therapy or chemotherapy) can be used for TIL assessment, but only when tumor cells are present 1
- Cores with only scarred and inflammatory stroma without tumor cells should not be assessed 1
- Surgical specimens from residual disease after neoadjuvant chemotherapy require assessment of the entire residual tumor bed 1
Technical Specifications for Assessment
Slide Preparation
- Standard 4-5 μm FFPE sections are optimal and sufficient 1
- H&E staining only - no immunohistochemistry or special stains required for routine assessment 1
- Frozen sections are not validated outside research settings and cannot be recommended 1
Microscopic Evaluation Parameters
- Magnification: ×200-400 (ocular ×10 with ×20-40 objective) for detailed assessment 1
- Initial scanning at ×50-100 to survey the tumor bed, followed by detailed evaluation at higher magnification 1
Standardized Assessment Methodology
Areas to Include
- Evaluate only within invasive tumor borders 1
- Stromal compartment is the primary focus - report as percentage of stromal area occupied by TILs 1
- For residual disease post-neoadjuvant therapy, assess within the residual tumor bed as defined by Residual Cancer Burden (RCB) criteria 1
Areas to Exclude (Critical Pitfalls)
- Exclude TILs outside tumor borders 1
- Exclude areas around ductal carcinoma in situ (DCIS) and normal lobules 1
- Exclude zones with necrosis, crush artifacts, and regressive hyalinization 1
- Exclude previous core biopsy sites 1
- Do not focus on hotspots - use average assessment across entire tumor area 1
Cellular Components to Score
- Include all mononuclear cells (lymphocytes and plasma cells) 1
- Exclude polymorphonuclear leukocytes 1
Quantification Approach
Reporting Method
- Report as continuous percentage variable for stromal TILs (e.g., 15%, 30%, 60%) 1
- The denominator is the area of stromal tissue, not the number of stromal cells 1
- Pathologists may round to nearest 5-10% in daily practice, though more precise reporting is preferable for research 1
Sample Size Requirements
- One section per patient is generally sufficient for tumors ≤2 cm 1
- For residual tumor beds >2 cm: assess one slide per cm of tumor bed (e.g., 5 cm tumor requires minimum 5 slides) 1
Special Considerations
Computational Assessment
- Machine learning algorithms are emerging but require manual annotation of tumor regions by experienced pathologists to exclude non-invasive epithelium and artifacts 1
- The FDA provides online CME-accredited TIL training for pathologists at https://ceportal.fda.gov/ 1
Clinical Context
- TIL assessment on H&E is the preferred method for daily practice over immunohistochemical characterization, as it is technically feasible, requires less tissue, and correlates well with immune gene expression 1
- This approach provides the quickest route to obtaining evidence without requiring block recollection from existing trials 1
Therapeutic TIL Harvesting
- For TIL immunotherapy (distinct from TIL assessment), surgical resection of sufficient tumor tissue is required for ex vivo TIL expansion and manufacturing 2
- This therapeutic application requires specialized infrastructure and is currently approved for metastatic melanoma refractory to checkpoint inhibitors 2