Found small lump below inside of ankle. painful to walk and hurts during rest. doc ordered mri to rule out tpd or mass. could i end up in a cast?

Depends on results: Depends on the results of mri. With tibialis posterior dysfunction, immobilization may not be enough, surgery may be necessary and then casting. Plus there will be a need to address the flat footed ness. If a mass consistent with a ganglion, have it drained as an in office service. Ask if they can remove it, as an in office service if not successful. If other mass, have removed.