Purpose: To investigate the potential of FDG‐PET imaging for delineating the surgical cavity in post‐operative partial breast irradiation patients. Method and Materials: A DCIS breast cancer patient was imaged with a GE Discovery ST PET‐CT scanner approximately 2 weeks post lumpectomy. Following the treatment planning CT, a single‐bed (15 cm) FDG‐PET scan was dynamically acquired in 5‐sec intervals over 15 mins. The raw PET data was combined to form bins ranging from 30 sec to 15 min. These data were reconstructed by the GE scanner through an iterative OSEM algorithm, and hardware fused to the treatment planning CT. The value of PET in visualizing the lumpectomy cavity border was investigated through visual comparisons of fused PET‐CT images, the evolution of PET intensity for various breast points, and signal‐to‐noise measurements across the lesion. Results: The PET image showed clear signal enhancement near the lumpectomy cavity. This enhancement formed a ring in each axial slice, matching the locations of surgical clips. Enhancement was also apparent where the cavity border was difficult to evaluate by CT density or clips alone. The ring presented a significantly higher SUV than other breast tissue (2.2 vs. <1.4), while the region inside the ring had a lower SUV than the (presumably benign) glandular tissue in the same breast (1.2 vs. 1.4). The SUV values were transient below 5 min, but remained stable thereafter. Conclusion: FDG‐PET may provide useful information for delineating lumpectomy cavity borders by elucidating regions of enhanced radiotracer uptake due to inflammation. The hypointense PET volume enclosed by the high activity ring may represent fluid and non‐viable tissue stemming from post‐surgical changes, explaining its lower FDG uptake and further supporting the suggestion that the ring corresponds to the cavity border itself. On current hardware, five‐minute scans were required to achieve stable SUVs.