Abstract: Background and purpose: Radiotherapy (RT) is one of the most important therapeutic tools for esophageal cancer. Because tumors are heterogeneous, including for ¹⁸F-FDG uptake and, most likely, for radioresistance, selective boosting of high FDG uptake zones within the tumor has been suggested. Therefore, it is critical to know whether the location of these high FDG uptake patterns within the tumor remains stable during RT. Methods: Twenty-two patients with esophageal squamous cell carcinoma treated with concurrent chemo-radiation underwent repeated ¹⁸F-FDG PET-CT scans before RT and after 20 fractions of RT. On all scans, the high and low FDG uptake regions were auto-delineated using several standard uptake value (SUV) thresholds, varying from 40% to 70% of SUV_max on the pretreatment scan [gross tumor volume (GTV)_40%pre, GTV_50%pre, GTV_60%pre, GTV_70%pre] and from 70% to 90% of SUVmax on the dur-treatment scan (GTV_70%dur, GTV_80%dur, GTV_90%dur) and fixed thresholds of 2.5 and 5 (GTV_2.5pre, GTV_5pre). The volumes and overlap fractions (OF) of these delineations were calculated to demonstrate the stability of the high FDG uptake regions during RT. Results: The high uptake regions within the tumor during RT largely corresponded (OF>70%) with the 50% SUV_max high FDG uptake area (GTV_50%pre) of the pretreatment scan. The hotspot within the residual area (GTV_90%dur) was completely within the GTV and pre-radiotherapy high uptake regions (OF=100%). Although the location of the high FDG uptake patterns within the tumor during RT remained stable, the delineated volumes varied markedly. Conclusion: The location of the high FDG uptake areas within the tumor remained stable during RT. This knowledge may enable selective boosting of high FDG uptake areas within the tumor.

Key words: Esophageal squamous cell carcinoma, Radiotherapy, PET-CT