Table 4.

Recommendations for processing and storage of FFPE samples for DNA analysis.

Factor	Recommendation
Tumour size	>5000 total nucleated cells with >10–20% neoplastic content*
Fixation method	10% buffered formalin
Fixation time	As short as possible (e.g. 3–6 h for core biopsies; maximum 24 h)
Knife blades	Replace before each block is cut to prevent cross‐contamination by tissue‐related nucleic acids
Number and thickness of sections	5–10 sections of 5–10 μm, depending on the dissected tissue size and cellularity
Decalcification procedure	Not recommended but if required for bone samples, use EDTA instead of acidic decalcification
FFPE block storage	Store in controlled environment (e.g. low humidity, 18–25 °C) to reduce oxidation and degradation of nucleic acids

EDTA, ethylenediaminetetraacetic acid.

^*5000 cells contain approximately 30 ng of DNA. At least 10–20% tumour content is required to reliably detect somatic variants at >5% allele frequency; higher tumour content may be required for detection of large somatic deletions and rearrangements.