Risk of local recurrence
Studies and clinical experience have shown that even if the primary bulk of tumour is removed there is still a risk of local recurrence in the surrounding tissue. This risk is dependent on the tumour site, subtype and histological grade. The use of very wide margins in surgery can eliminate this risk (eg. mastectomy for breast cancer).
Use of radiotherapy for subclinical disease
Radiotherapy is a useful method of treating these local sites as it preserves normal tissue while killing tumour stem cells. The relatively small number of tumour cells allows smaller doses than would be required for a bulky mass - see the section on random cell kill and Poisson statistics.
Some large tumours are unable to be resected completely by surgery - for instance, some lung tumours be unresectable despite lobectomy or pneumonectomy. The removal of a large number of tumour stem cells is still beneficial as it leaves fewer potential targets for radiation.