Increased 18F-FDG Uptake in the Obturator Muscles Due to Denervation-Driven Atrophy

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Abstract
Case Report A 52-year-old female patient with a history of rectal carcinoma underwent targeted radiotherapy on right-sided pelvic lymph node metastases (42 Gy in 21 fractions). She later developed local post-radiation retroperitoneal fibrosis with resulting ureteric stenosis. Serial oncological follow ups with 18 F-FDG PET-CT showed a right-sided increased uptake in the obturator internus (Figures 1 and 2, X) and obturator externus muscles, at a significant distance inferiorly to the irradiated zone. This uptake first became visible 22 months after the last radiotherapy session, reached a peak after 25 months (Figure 1), and all but disappeared after 32 months without any further oncologic treatment. It was associated with gradual and permanent muscular volume loss compared to the contralateral side. This volume loss reached a maximum after 37 months (Figure 2). The previous pelvic irradiation suggests that post-irradiation scarring led to injury of the