As described here, SimBiology conserves amounts when changing compartment volumes. If I were building the model, I would not use SimBiology's compartment volume to track the tumor volume. Instead, I would track tumor volume in a "non-constant SimBiology parameter" and set me SimBiology compartment volume to some standard reference volume (e.g., 1 milliliter).
I already logged an enhancement request in our database to allow conservation of concentrations instead of amounts. I'll make sure to add this use case to the request.
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