Samples
In total, 116 primary tumor blocks from 116 patients (74 relapsers and 42 non-relapsers) were sampled for DNA extraction and molecular analysis. The median DNA concentration was 28 ng/μl (range 4–493 ng/μl) with elution volumes of 25 μl resulting in a median total yield of 0.7 μg/tumor. The median tumor block age was 11 years (range 5-36 years). From the 116 tumor blocks sampled, 50 produced DNA of sufficient quality for both MLPA and BRAF/NRAS analysis, 25 produced results for MLPA only, 21 produced results for BRAF/NRAS only and 20 did not produce any results. Therefore, the success rate for MLPA was 65% (75/116) and that for BRAF/NRAS was 61% (71/116). There was no significant difference in the distribution of block age between samples with a MLPA result (median 10 years, range 5–36) and those that failed (median 12 years, range 5–24), P = 0.3 (Wilcoxon rank test). The strongest predictor of assay success was DNA quantity. Failure was much more likely to occur in assays with <100 ng DNA input (44% fail rate at <100 ng DNA compared to 17% fail rate at >100 ng DNA), according to the DNA quantity control peaks within each MLPA assay.