Introduction Neuroendocrine tumours (NETs) are a relatively rare, but deadly group of cancers. Since the cells of origin are neuroendocrine, diverse tumours can arise throughout the body. Often, patients are presented with metastases, making resection as a treatment strategy alone insufficient. In the clinic, localization of NETs has been assessed by targeting the somatostatin receptor 2 (SSTR2) which is overexpressed on tumour cells, with radiolabeled somatostatin analogues. Radiolabeling of these somatostatin analogues (e.g. DOTA-(Tyr3)-octreotate (DOTA-TATE)) with the DNA damaging nuclide (Lutetium-177) is currently utilised to treat the disease: peptide receptor radionuclide therapy (PRRT). PRRT with the use of 177Lu-DOTA-TATE proves effective in improving progression-free survival and life quality. However, mortality rates are still high. To increase the efficacy of PRRT we set out to dissect the cellular response of tumour cells to radionuclide therapy, the effect on dose-limiting organs (bone marrow and kidney) and mechanisms of therapy induced DNA damage in vitro and in vivo. Material and methods The SSTR2-expressing small cell lung cancer cell line NCI-H69 was treated with 177Lu-DOTA-TATE and fixed for assessment of DNA damage by means of 53 BP1 and γH2AX stainings on multiple time points. Also, BALB/c-nude mice were engrafted subcutaneously with NCI-H69 cells and were treated with 177Lu-DOTA-TATE after tumorigenesis. Mice were euthanized at different time points until 14 days post-treatment, and tumours, bone marrow and kidneys were excised for downstream analyses. Results and discussions After PRRT we observed a time-dependent intensity in DNA damage signalling and subsequent repair both in vitro and in vivo. We observed the peak of DNA double-strand break repair after two days. Although NCI-H69 tumours have a homogeneous nature, we observed intra-tumoral heterogeneity in cell death and activation of the DNA damage response. Furthermore, acute, but transient damage was observed in the kidneys and bone marrow. Conclusion With this investigation, we studied in detail the radiobiology of PRRT in a preclinical setting. Our results will contribute to a better understanding of PRRT effects. This might help to improve future patient treatment by finding the optimal therapeutic window for radiosensitization of the tumour without increase of side effects.