ctDNA-Response evaluation criteria in solid tumors – a new measure in medical oncology

In the metastatic setting, most decisions during systemic palliative therapies are based on the imaging-based Response Evaluation Criteria in Solid Tumors (RECIST), which is, however, known to be a suboptimal surrogate marker for the clinical outcome overall survival. Over the past decade, research has brought focus to the potential of circulating tumour DNA in cancer. However, at present, there is no generally accepted classification of quantitative changes during the treatment course, and prospective investigations can therefore not be validated. We here propose, for the first time, a response classification based on circulating tumour DNA measurements and its confidence intervals, a "ctDNA-RECIST" that has proven valuable in retrospective studies and goes along with the conventional RECIST classification. We aim to raise the topic for discussion and to encourage analyses of ctDNA data along this line.