Radiotherapy is widely used to treat breast cancer, but recurrence occur in some patients. Radiotherapy has been shown to enhance neural signalling in normal tissues, and signalling from the sympathetic nervous system (SNS) is implicated in the development and progression of cancer. This raises the possibility that neural signalling might contribute to recurrence after radiotherapy. To investigate the effect of radiotherapy on SNS signalling in tumours, we treated mice bearing 4T1.2 and 66cl4 orthotopic mammary tumours with a single dose of 8 Gy image-guided superficial radiotherapy. Radiotherapy reduced primary tumour volume by 3.5-fold (p < 0.0001) but not metastasis (p > 0.05) in both models of breast cancer. Using immunofluorescence staining to map SNS nerves in mammary tumours, we found that radiotherapy increased neural density by 3-fold (p = 0.02). Furthermore, histofluorescence analysis showed that radiotherapy increased the levels of SNS neurotransmitter in tumours. Using qRT-PCR to quantify neurotrophin gene expression, we found that radiotherapy increased key SNS growth factor NGF, by 3.4-fold (p = 0.03) in tumours at 10 days after irradiation. Preliminary experiments targeting radiotherapy-induced nerve growth indicate that blocking SNS signalling with the beta-blocker propranolol during radiotherapy can delay the onset of metastasis. These findings suggest that radiotherapy may upregulate SNS signalling in tumours, with adverse effects on cancer recurrence. Ongoing experiments are evaluating how to optimally combine beta blockers and radiotherapy to delay metastasis and improve cancer outcomes.