RECIST 1.1 principles should be followed. In general, when a lesion cannot be assessed the entire timepoint assessment should be considered to be not evaluable (NE). RECIST 1.1 describes how to manage lesions that have become so small they cannot be measured.
iRECIST adds an additional element, as progression is only confirmed at the “next assessment”, and so the question arises of whether iCPD can be assigned If there is an intervening NE between iUPD and what would be iCPD. iRECIST recommends that the NE TP assessments be disregarded, and the next evaluable assessment be considered the ‘next assessment’. Clearly, this does not apply to scenarios where lesions are NE because of massive increases in size, the development of large effusions, are an increase in size leading to lobar collapse (for lung lesions).