Background and purpose: Radiotherapy (RT) is one of the most important therapeutic tools for esophageal cancer. Because tumors are heterogeneous

including for

18

F-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

18

F-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 (OF70%) 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.