Modelling ordinal response to treatment in a real-world cohort study

The Italian Thyroid Cancer Observatory foundation maintains a database including all records on patients with confirmed diagnosis of thyroid cancer reported by several special- ized centers in the country. One of the objectives of this project is to monitor the evolution over time of the response to treatment, which is a synthesis of serum values and ultrasound imaging, measured over an ordinal 4-point scale. The response to treatment is measured at 12 months, 3 and 5 years since the initial treatment; patients may underwent additional treatments, between measurement occasions, which may alter the initial risk composition. Our analysis focuses on modelling the response at 12 months as a function of baseline risk classification, clinical and surgical information. Further, we aim at exploring the transition between response categories when we consider the 12 months and 3 years measurements, since these may be affected by additional treatments occurred in the meanwhile.