simon focus simon focus a simon focus is a tuberculosis (tb) nodule that can form in the apex of the lung when a primary tb infection elsewhere in the body spreads to the lung apex via the bloodstream. simon focus nodules are often calcified. the initial lesion is usually a small focus of consolidation, less than 2cm in diameter and located within 1 to 2 cm of the apical pleura. in adolescence, simon foci may become reactivated and develop into assmann foci. such foci are sharply circumscribed, firm, gray-white to yellow areas that have a variable amount of central caseation and peripheral fibrosis.