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

OBJECTIVES: 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.
METHODS: where: Ti interval time, Di initial diameter and Do final diameter. Three doubling times were utilized for demonstration of the principle, 30, 80 and 100 days.
RESULTS: 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.
CONCLUSIONS: 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.