the purpose of bone scintigraphy is to portray areas of new bone formation within the skeleton .
this is useful in imaging reaction of bone to tumor , fracture , and infection .
since approximately half of the administered radioisotope is excreted through renal filtration , abnormalities of the urinary system are also frequently noted during bone scintigraphy . in such cases , to reach an accurate diagnosis , the interpreting physician must first recognize which structures are involved in the uptake and the significance of the uptake .
we describe a rare case in which extraosseous bone scan tracer accumulation was noted in a renal calculus .
a 60-year - old man , who had difficulty in passing urine for the past 6 months and a complaint of lower back pain presented to our department . on ultrasound imaging , he was found to have an enlarged prostate .
transrectal ultrasound - guided biopsy revealed adenocarcinoma of prostate , gleason 's score 3 + 3=6 .
his psa was normal ( 1.3 ng / ml ) and serum alkaline phosphatase was elevated 58 u / l ( normal range 30 - 50
u / l ) . a 99 m technitium methylene diphosphonate ( 99 m tc - mdp ) bone scan was performed .
the scan [ figures 1 and 2 ] showed mildly increased tracer uptake in lumbar vertebrae and focal accumulation of tracer in lower pole of the left kidney .
fusion imaging [ figure 3 ] , using single photon emission computed tomography along with x - ray computed tomography ( spect - ct ) of the lumbar spine was performed to characterize the vertebral tracer uptake .
lasix 40 mg was given 45 minutes prior to spect - ct to monitor the renal tracer accumulation .
high tracer uptake in a lower pole of the left kidney was found localized in a calculus in the lower calyx of kidney .
the ct attenuation factor was 1060 hounsfield unit , compatible with that of a renal calculus .
( a ) anterior and ( b ) posterior views , showing mildly increased tracer uptake in the lumbar vertebrae ( black arrow ) and intense , focal localization of the tracer in the lower pole of left kidney ( red arrows ) .
( a - b ) lateral and ( c - d ) oblique views clearly demonstrate the focal tracer uptake outside the skeletal structures in left kidney ( arrows ) .
( a ) coronal , ( b ) sagittal and ( c ) transaxial ct images ; ( d ) coronal ( e ) sagittal and ( f ) transaxial spect images ; ( g ) coronal , ( h ) sagittal and ( i ) transaxial spect - ct images .
the ct attenuation factor was 1060 hounsfield unit , compatible with that of a renal calculus .
we concluded this as extra - skeletal bone scintigraphy tracer uptake in a renal calculus .
bone scintigraphy is a valuable diagnostic tool in the evaluation of patients with a variety of osseous abnormalities
. however , accumulation of bone scan tracer outside the skeleton can pose a difficulty in reporting for a nuclear medicine physician , especially if only planar imaging is performed and the tracer uptake is overlapping or is in close vicinity of the skeleton .
localization of bone scan tracer in a renal calculus has been reported in the past and its use in preoperative in vivo localization of the renal calculus has been explored .
retention of bone scan tracer on ureteric calculi and bladder calculus has been reported in the past.[35 ] in one of the studies , autoradiography of ureteric calculus demonstrated peripheral tracer distribution within the calculus .
accumulation of the radionuclide due to sluggish flow and its absorption onto the crystal surface , within the calculus , were suggested as possible mechanisms of tracer uptake .
the other common causes of urinary system localization of the bone scan tracer are dilation of urinary - collecting system , bladder diverticulum , and presence of an ureterostomy bag.[69 ]
conditions causing extraskeletal accumulation of bone scan tracer must be kept in mind while reporting a bone scan .
our case demonstrates a rare occurrence of extraosseous bone tracer accumulation in a renal calculus .
it also highlights important role played by spect - ct in localizing the extraskeletal tracer uptake .
spect - ct can be used effectively when an extraskeletal uptake is encountered on planar bone imaging .
 extraosseous localization of radioisotope , used in bone scan , in a variety of physiological and pathological conditions is a well - known phenomenon . 
 the causes of extraosseous accumulation of bone - seeking radiotracers should be kept in mind when bone - imaging studies are reviewed to avoid incorrect interpretations . 
 we report an extremely rare occurrence of extraosseous accumulation of bone scintigraphy tracer in a renal calculus , in a patient with adenocarcinoma of prostate , that was demonstrated by single photon emission computed tomography and computed tomography ( spect - ct ) fusion imaging . 