Relevant risk of carboplatin underdosing in cancer patients with normal renal function using estimated GFR: lessons from a stage I seminoma cohort

Background: Seminoma stage I is the most frequent testis cancer and single-dose carboplatin (AUC7) is an effective and widely used adjuvant treatment. Underdosing of carboplatin by 10% has been shown to almost double the rate of relapse and hence correct dosing based on accurate GFR measurement is crucial. The gold standard of GFR measurement with a radiolabelled isotope is expensive and not readily available. In many institutions, it is replaced by GFR estimation with the Cockcroft-Gault formula, which might lead to significant carboplatin underdosing and potentially inferior clinical outcome. Methods: Retrospective analysis of all patients with stage I seminoma treated with adjuvant carboplatin between 1999 and 2012. All patients had serum creatinine measured and underwent GFR measurement with a radioisotope (Cr-51 EDTA or Tc-99m DTPA), which was compared with seven standard GFR estimation formulae (Cockcroft-Gault, CKD-EPI, Jelliffe, Martin, Mayo, MDRD, Wright) and a flat dosing strategy. Bias, precision, rates of under-and overdosing of GFR estimates were compared with measured GFR. Bland-Altman plots were done. Results: A total of 426 consecutive Caucasian male patients were included: median age 39 years (range 19-60 years), median measured GFR 118 ml/min (51-209), median administered carboplatin dose 1000 mg (532-1638). In comparison to isotopic GFR measurement, a relevant proportion of patients would have received <= 90% of carboplatin dose through the use of GFR estimation formulae: 4% using Mayo, 9% Martin, 18% Cockcroft-Gault, 24% Wright, 63% Jelliffe, 49% MDRD and 41% using CKD-EPI. The flat dosing strategy, Wright and Cockcroft-Gault formulae, showed the smallest bias with mean percentage error of +1.9, +0.4 and +2.1, respectively. Conclusions: Using Cockcroft-Gault or any other formula for GFR estimation leads to underdosing of adjuvant carboplatin in a relevant number of patients with Seminoma stage I and should not be regarded as standard of care.