The following keywords work relatively well for the "co-existing" concept if proximity with the cancer term/s can be specified.
('co exist*' OR coexist* OR simultan* OR collision* OR synchron* OR concurrent OR concomitant OR contemporan* OR 'multiple primary' OR 'multiple primaries' OR 'double primary' OR 'double primaries' OR 'triple primary' OR 'triple primaries' OR multifocal OR multi-focal OR "multiple foci")
