Tumor burden monitoring using cell-free tumor DNA could be limited by tumor heterogeneity in advanced breast cancer and should be evaluated together with radiographic imaging

Abstract

Background: Accurate measurement of tumor burden in breast cancer disease is essential to improve the clinical management of patients. In this study, we evaluate whether the fluctuations in the fraction of PIK3CA mutant allele correlates with tumor response according to RECIST criteria and tumor markers quantification.

Methods: Eighty six plasma samples were analyzed by digital PCR using Rare Mutation Assays for E542K, E545K and H1047R. Mutant cfDNA and tumor markers CA15-3 and CEA were compared with radiographic imaging.

Results: The agreement between PIK3CA mutation status in FFPE samples and circulating tumor DNA (ctDNA) was moderate (K = 0.591; 95% IC = 0.371-0.811). Restricting the analysis to the metastatic patients, we found a good agreement between PIK3CA mutation status assessed in liquid and solid biopsy (K = 0.798 95%; IC = 0.586-1). ctDNA showed serial changes with fluctuations correlating with tumor markers 15.3 and CEA in 7 out of 8 cases with Pearson correlation coefficients ranging from 0.99 to 0.46 and from 0.99 to 0.38 respectively. Similarly, fluctuations in the fraction of PIK3CA mutant allele always correlated with changes in lesion size seen on images, although in two cases it did not correlate with treatment responses as defined by RECIST criteria.

Conclusion: oncogenic mutation quantification in plasma samples can be useful to monitor treatment outcome. However, it might be limited by tumor heterogeneity in advanced disease and it should be evaluated together with radiographic imaging.

Keywords: Breast cancer; PIK3CA; cfDNA; dPCR.

Figures

Fig. 1

Panel (A) shows plasma levels of H1047R PIK3CA mutation (%) on cfDNA quantified by dPCR and CEA (ng/ml) tumor marker across four time points (months) (Pearson correlation coefficient =0.92) from patient 2, PD = progressive disease. Panel (B) shows plasma levels of H1047R PIK3CA mutation (%) on cfDNA across four time points (months). Disease status as ascertained on a CT-scan at two time points is marked with a discontinuous line. The use of adryamicine based chemotherapy is indicated by gray shading. As shown, PIK3CA mutation quantification increased until October when the patient 44 was diagnosed as having a progressive disease (PD). Subsequently, the patient received a different line of palliative treatment. PIK3CA mutation significantly decreased from 2.08% (CI 1.768% – 2.429%) to 0.81% (0.487% – 1.342%) correlating with the partial response (PR) assessed by CT-scan. Panel (C) shows the plasma levels of tumor markers 15–3 (U/ml) and CEA (ng/ml) and CT-scans showing the bone metastasis, lung metastases and the axillary adenopathy from patient 31. In this case the H1047R mutation quantification went down to 0% over the course of treatment (from 0.19 to 0%) correlating with tumor markers, the visceral metastases (white arrow) and the adenopathies (circle) although the bone metastasis increased significantly in size (white arrow)

Fig. 2

E545K assay scatter plots from patient of Patient 3 plasma samples and the corresponding CT scan images showing the hilar and mediastinal adenophaties and the lung metastases (arrows and circles). The asterisk indicates the lung hilum vessels