Medical negligence by doctor

A year ago my mother underwent gall bladder surgery at a reputed clinic at cuttack (Odisha). We also had paid the amount for biopsy of the gall bladder after its removal. The reports do not indicate anything out of the ordinary. However, a year later i.e. now my mother has been diagnosed with cancer and PET scan suggests that the point of origin was the gall bladder. And now my mother can't undergo surgery as the nodes have thickened. The doctors at Tata Memorial, where my mother is undergoing treatment presently, have confirmed that the cancerous tissues could have been easily removed via surgery a year back. It has come as a complete shock to our family that in spite of having the gall bladder from which they could have removed multiple samples for biopsy, the clinic removed just 1 sample. Whereas medically, the correct practice is to remove 5 different samples from different areas of the organ for biopsy. We do have the biopsy report and the PET scan report as well. And I want to take severe action against the clinic. Can you please suggest the right course of action. Thank you.