Aims: Delays between surgery and the delivery of radiotherapy may allow brain tumours to grow beyond the planned radiotherapy fields and therefore reduce the effectiveness of radiotherapy. This pilot study aimed to ascertain whether significant growth of brain tumours occurs between post-biopsy imaging and the start of radiotherapy. Materials and methods: Two estimates of tumour volume were obtained from contrast-enhanced magnetic resonance images obtained within 3 days of surgical debulking/biopsy (postoperative), and shortly before starting radiotherapy (pre-radiotherapy). The postoperative and pre-radiotherapy volumes were compared to assess tumour growth and expansion of the tumour margin. The enhancing tumour volume was measured on a workstation using two methods: tracing the area of the lesion on each slice on which it appeared and summing the volume in each slice, and by measuring the largest diameters in three planes. The ease of use and intra-operator variability of the two methods were compared. Results: The median time between postoperative and pre-radiotherapy scans was 31.5 days (range 15-53 days). Both methods found that statistically significant tumour growth occurred between postoperative and pre-radiotherapy imaging. The tumour area method found median postoperative volume of 42 849 mm(3) (range 4843-148 047 mm(3)), and median pre-radiotherapy volume of 49 382 mm(3) (range 9327-150 850 mm(3)) and a median growth of 35.07% (range 0-105%). The enhancing tumour margin on the pre-radiotherapy scan overlapped the margin of the postoperative scan by a maximum of 20 mm. Our study found that the diameter method gave lower estimates of tumour growth than the area method. The diameter method was inaccurate when tumours were small or irregularly shaped. Conclusion: As a 2-3 cm margin is usually included around the tumour when planning radiotherapy, it seems unlikely that the visible tumour actually grew outwith the planned radiotherapy fields. However, cells beyond the tumour margin visible on imaging could be outside the planned radiotherapy field. This paper highlights difficulties in determining the most appropriate time for baseline radiotherapy planning imaging to be carried out.