The effect of altering the input parameters into a Pharmacokinetic (PK) model on output parameter values generated was investigated. This included determining the variations induced by using individualized versus population based Arterial Input Function and haematocrit values in the model. This was completed for multiple DCE-MRI scans acquired along the course of radiotherapy treatment for 5 head and neck cancer patients. Qualitatively, most patients had similar trendlines when comparing between each combination of parameter inputs. However quantitatively, the %difference between baseline and subsequent weeks highlighted significant impacts caused by input parameter selection; having implications for applications in treatment response monitoring.