iformis muscle were observed from the pelvis MRI that had been performed after the disease had progressed and de- creased muscle strength was found (Fig. 2B). 　　Based on the abnormal findings from the MRI, a local anesthetic and steroid were injected into the piriformis muscle so that the pain could be considerably reduced, but the patient's pain relapsed after several months. For a more definite diagnosis and treatment, surgical treatment was recommended, but it was not conducted because of the patient's refusal. 　　Thickening of the piriformis muscle is usually found in the piriformis syndrome. When there is a fatty atrophy finding in the piriformis muscle, there may be no symp- toms at all [10], but it can compress the sciatic nerve by causing a muscular shortening. In addition, it is known that pain relief by injection in the piriformis muscle is not easily achieved [5]. It is assumed that relief of the symp- tom after the injection in the piriformis muscle in this case report might be the result of the anti-inflammatory action of the steroid. We have decided that the patient's symp- toms were due to the proximal sciatic neuropathy caused by post-radiotherapy fatty atrophy of the piriformis mus- cle, based on the series of changes in the piriformis muscle as shown by the MRI, clinical symptoms, and electro- myography findings. 　　When treating cancer pain, the pain due to the dam- age of muscles or nerves that may be followed by radio- therapy and chemotherapy must be considered. Especially when radiating pain occurs in buttocks and legs after ra- diotherapy of a tumor existing in the pelvis, evaluation of not only the spinal lesion, but also proximal sciatic neuro- pathy, must be conducted. SY Jeon, et al / Post-radiation Piriformis Syndrome 91 REFERENCES 1. Kirschner JS, Foye PM, Cole JL. Piriformis syndrome, diagno- sis and treatment. Muscle Nerve 2009; 40: 10-8. 2. Rossi P, Cardinali P, Serrao M, Parisi L, Bianco F, De Bac S. Magnetic resonance imaging findings in piriformis syn- drome: a case report. Arch Phys Med Rehabil 2001; 82: 519-21. 3. Gillette EL, Mahler PA, Powers BE, Gillette SM, Vujaskovic Z. Late radiation injury to muscle and peripheral nerves. Int J Radiat Oncol Biol Phys 1995; 31: 1309-18. 4. Lewis AM, Layzer R, Engstrom JW, Barbaro NM, Chin CT. Magnetic resonance neurography in extraspinal sciatica. Arch Neurol 2006; 63: 1469-72. 5. Filler AG, Haynes J, Jordan SE, Prager J, Villablanca JP, Farahani K, et al. Sciatica of nondisc origin and piriformis syndrome: diagnosis by magnetic resonance neurography and interventional magnetic resonance imaging with outcome study of resulting treatment. J Neurosurg Spine 2005; 2: 99-115. 6. Chang CW, Shieh SF, Li CM, Wu WT, Chang KF. Measurement of motor nerve conduction velocity of the sciatic nerve in patients with piriformis syndrome: a magnetic stimulation study. Arch Phys Med Rehabil 2006; 87: 1371-5. 7. Saphner T, Gallion HH, Van Nagell JR, Kryscio R, Patchell RA. Neurologic complications of cervical cancer. A review of 2261 cases. Cancer 1989; 64: 1147-51. 8. Kim JW, Lee PB, Park CD, Choi SJ, Choi JB, Moon JY. Ultrasound-guided sciatic nerve block for the treatment of radiation therapy induced sciatic neuritis: a case report. Korean J Pain 2009; 22: 186-90. 9. Abu-Rustum NR, Rajbhandari D, Glusman S, Massad LS. Acute lower extremity paralysis following radiation therapy for cervical cancer. Gynecol Oncol 1999; 75: 152-4. 10. Russell JM, Kransdorf MJ, Bancroft LW, Peterson JJ, Berquist TH, Bridges MD. Magnetic resonance imaging of the sacral plexus and piriformis muscles. Skeletal Radiol 2008; 37: 709-13.