In clinical oncology, confirmatory trials may track disease response endpoints using clinical response criteria such as RECIST. This algorithm reflects the clinical practice of surveilling the disease status with periodic follow-up scans, and sum of longest diameters drawn on lesions in 3D space that grow by a certain percentage are declared "progressive disease" and are considered an event in time to event analyses.
Standard statistical analysis plans specify censoring criteria in these cases that roughly require that subjects
- Attend scans at a desired frequency.
- Are censored at the last time point of an evaluable scan
- Are considered an "event" at the moment it's known that event criteria are met (i.e. death, or radiographic progression)
Example of an accepted, publicly available protocol and statistical analysis plan for these cases can be found on the US government clinical trials database: https://clinicaltrials.gov/ct2/show/NCT04516746
Typically scans occur every 2 months, and if 2 scans are missed, the subject is censored at the last evaluable scan. Even if PD is known to occur at 4 months and a few days, the patient is censored due to the long period with unknown clinical status.
The idea of finding the exact time point at which a disease meets criteria for PD is a theoretical issue that has no bearing on clinical practice. In practice, patients monitor the disease status in exactly the same way: attending periodic scans, and (more or less) presenting with symptoms of unequivocal disease progression. As such there's no misleading information or bias in using the actual date of radiologic progression as the event time, even though the subject may have met those criteria days or weeks prior to the scan.