Computed tomography surveillance scanning after lung cancer surgery: mathematical optimization of scanning interval based on tumour biology†

To determine the optimal computed tomography (CT) scanning interval for the detection of a new primary lung cancer and recurrent disease, utilizing the known mathematical formula for tumour doubling. Doubling time = T-i x [log2/3 x log(D-i/D-o)]; where: T-i = interval time, D-i = initial diameter and D-o = final diameter. Three doubling times were utilized for demonstration of the principle, 30, 80 and 100 days. A worst-case scenario for a doubling time of 30 days indicates that a 2-mm tumour will need 210 days (7 months) to reach 10 mm in diameter and 300 days (10 months) to reach 20 mm in diameter. Over a 5-year (60 months) follow-up period, this indicates that eight CT scans will be required if a threshold of 10 mm is desired or six if a threshold of 20 mm is desired. For an 80-day doubling time over a 5-year (60 months) follow-up period, three CT scans will be required if a threshold of 10 mm is desired or two if a threshold of 20 mm is desired and for a 100-day doubling time. Assuming complete histological clearance of the primary lung cancer and that recurrence occurs from a microscopic focus, a time period of 1700 days (56 months) is required to reach 10 mm in diameter. The exact timing of interval CT scanning to detect recurrence and new primary tumour depends on philosophy; however, three monthly CT scanning is probably inappropriate, and scanning every 7 months is probably the shortest interval that is clinically useful, particularly for small-cell lung cancer in the first year after treatment. We recommend, based on mathematical modelling, a scanning interval post-potentially curative resection surgery for primary lung cancer of 18 months, which is different from the current guidelines on surveillance, for non-small-cell lung cancer.