When lesions coalesce, forming a conglomerate, a plane between them may be maintained that would aid in obtaining maximal diameter measurements of each individual lesion. If the lesions have truly coalesced such that they are no longer separable, the vector of the longest diameter in this instance should be the maximal longest diameter for the coalesced lesion. In case of nodal lesions, the short axis of the coalesced lesion should be taken into account.
Similarly, if a target and non-target lesion coalesce, there is usually a plane of separation between the lesions which should make measuring the target lesion component possible. If it is not, then the longest diameter in this instance should be the maximal longest diameter for the coalesced lesion.
In general coalesced lesions are indicative of worsening disease and the possibility of disease progression.