Avid 18F-FDG uptake of pectoralis major muscle: an equivocal sequela of strenuous physical exercise
1 Department of Radiology, Heart, Stroke and
Cancer Centre, Kuala Lumpur, Malaysia
2 Centre for Molecular Imaging, The Peter
Mac-Callum Cancer Centre, Melbourne, Australia
3 University of Melbourne, Melbourne, Australia
Avid functional 18F-FDG uptake of skeletal
muscle is a known false positive finding of PET-CT study especially after
involuntary muscle exercise just prior to the study. We describe the case of a
50-year-old man in whom the finding of avid 18F-FDG uptake of
pectoralis major muscle was encountered during investigation of metastatic
melanoma. � 2009 Biomedical Imaging and Intervention Journal. All rights
Keywords: 18F-FDG uptake; pectoralis major muscle;
The purpose of this case report is to highlight an
unexpected pitfall of 18F-FDG uptake of pectoralis major muscle,
which is potentially caused by exercise of the upper limb muscles.
18F-FDG is a common PET radiotracer used in
oncology and its property as a glucose analogue is known to play a vital role
in the detection of most malignant tissue. Nevertheless, 18F-FDG is
not a tumour-specific substance as it shows non-specific affinity in other
normal tissue especially in the skeletal muscle .
Active skeletal muscle is a common area for interpretative
pitfall which is related to physiological FDG uptake . In addition, other
reasons for 18F-FDG uptake in muscle are also observed in diabetic
patient following administration of insulin, surgical intervention or following
Muscle uptake can be attributed to voluntary or
involuntary muscle activity. Voluntary muscle activity consists of
activities such as talking, chewing, and exercising-induced rhabdomyolysis.
Involuntary muscle activity would include laboured breathing or
stress-induced muscle spasms.
Normal muscles accumulates little FDG, uptake of grade 1
or less. Muscles exercised just prior to or around the time of FDG injection
can exhibit uptake of grade 3 to 4 . Notwithstanding the known normal
physiological FDG uptake in muscle, grade 3 to 4 muscle exercised within more
than 24 hours remains a poorly defined issue.
A 50-year-old gentleman with a small right ankle melanoma
had been successfully managed with surgical removal of the melanoma in 2001. He
had right inguinal nodal recurrence after 6 years disease-free progression. A
PET-CT study performed in June 2007 revealed unexpected intense symmetrical
uptake of the pectoralis major muscle (Figure 1). The patient was noted to have
undergone strenuous upper limbs exercise 24 hours prior to the imaging study.
Clinically, the disease showed a favourable response following several courses
of high dose adjuvant interferon alfa- 2b therapy. Biochemical blood analysis
was generally unremarkable aside from minimal derangement of the liver
In our case, an avid 18F-FDG uptake in
pectoralis major muscle is conceptually attributed to some form of strenuous
eccentric exercise 24 hours prior to PET-CT study.
Exercise is a well known mechanism that is capable of
inducing various components of the immune system . IL-6 is a lymphokine that
would substantiate immune-protection against muscle damage by maintaining
glucose uptake in the muscle following prolonged exercise . Glucose uptake
by skeletal tissue is insulin-dependent via recruitment of the GLUT4 (glucose
transporter) from the interior of the cell to the plasma membrane. In human
skeletal muscle, exercise increases GLUT-4 and hexokinase II and glycogenin
gene expression. Nevertheless, in-vivo study reveals that one hour of moderate
intensity exercise increases Hexokinase II transcription mRNA and protein
levels up to 3 hours after the end of exercise .
The fact is that a substantiated FDG accumulation in
skeletal muscle as shown in our case cannot be entirely explained by the
influence of intramuscular glucose uptake for more than 24 hours following
A possible explanation of what appears to augment FDG
avidity in this case might have been attributed to ongoing rhabdomyolysis
following strenuous exercise. Rhabdomyolysis appears to be a relatively common
sequela of strenuous exercise . Of note, numerous case reports have linked
rhabdomyolysis to strenuous activities such as military basic training and
weight lifting . This is further substantiated by a large screening to date
involving 337 military recruits whose blood were sampled during their first six
days of conditioning which revealed that an approximately 40% of the subjects
have some degree of rhabdomyolysis .
It is noteworthy that the cause of prolonged avid FDG
muscle uptake in our case remains unclear as there were neither symptoms
related to muscle damage i.e aching nor available biochemical analysis
pertaining to the level of creatinine kinase and urine myoglobin levels.
Unexpected FDG muscle uptake may occur more than 24 hours
following exercise as illustrated in our case. The exact explanation is unclear
but underlies the importance of adequate patient preparation and instructions
before a FDG PET scan.
Figure 1 FDG PET scan (coronal view. Grade 3 to 4 symmetrical F-18 FDG uptake of the pectoralis major muscle) [Image courtesy of Centre for Molecular Imaging, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia].
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Kemppainen J, Fujimoto T, Kalliokoski KK et al. Myocardial and skeletal muscle glucose uptake during exercise in humans. J Physiol 2002; 542(Pt 2):403-12.
Kraniou Y, Cameron-Smith D, Misso M et al. Effects of exercise on GLUT-4 and glycogenin gene expression in human skeletal muscle. J Appl Physiol 2000; 88(2):794-6.
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|Received 15 February 2009; accepted 16 April 2009
Correspondence: Clinical Associate Professor, Department of Radiology, the University of Melbourne, Director of Clinical PET, Centre for Molecular Imaging, The Peter MacCallum, Cancer Centre, 12 Cathedral Place, East Melbourne VIC 3002, Australia. Tel.: +61-3-9656-1852; Fax: +61-3-9656-1826; E-mail: email@example.com (Eddie Lau).
Please cite as: Fathinul F, Lau WFE,
Avid 18F-FDG uptake of pectoralis major muscle: an equivocal sequela of strenuous physical exercise, Biomed Imaging Interv J 2009; 5(2):e7