Table 3.

Difficulties in tumour growth and treatment response modelling relating to suitable input biological data.

Parameters/information that are difficult to obtain (quantitatively)	Reason for the difficulty	Overcoming the difficulty
Aggressiveness and time of onset of accelerated repopulation	Intertumour variability that is unknown cannot measure/estimate for individual patients without cell biopsy sample used in in vitro tests which may alter results	Grouping patients into tumours that are likely to have slow or fast repopulation by some means of genetic/pathologic testing—however methods currently unknown
The extent of various mechanisms responsible for tumour repopulation during treatment	The interplay between recruitment, accelerated stem division, abortive division and loss of asymmetrical division, in stem cells makes it difficult to evaluate their individual effect	Research stem cell properties for rapidly proliferating tumours.Sensitivity study on each individual and combined parameter when modelling
Input of individualised tumour data, for example, intrinsic radiosensitivity, differences in stem/transit or quiescent cell radiosensitivity	Currently no pretreatment testing due to logistics and time of testing	Research cell type/proliferative capacity-dependent radiosensitivities for different tumour cell lines, individualised radiosensitivity pre-treatment testing (requires staff/money/time)
Tumour oxygenation/reoxygenation	Different in every tumour, changes in time, access to equipment, for example, daily/weekly PET, invasive nature of in vivo quantitative data gathering, for example, Eppendorf/Oxy Lab probe	Access and research into to the feasibility and drug development for daily/weekly PET scans, with tracers that can image hypoxic regions with various thresholds, for example, 2.5, 5.0, 10 mm Hg
Drug pharmacokinetics	Lack of quantitative in vivo assays	Using in vitro data if existent, parameter estimation and sensitivity study. Molecular pharmacological modelling is required
Cell survival data for chemotherapy	Lack of mathematical formalism equivalent to the LQ model used in radiotherapy	Using in vitro data if existent for that particular agent