288 LETTERS

dates for SWL be first screened with a 99mTc-MDP scan in thehope of predicting the likelihood of successful stone disinte-gration. Assuming that such a test exists, it seems doubtful thatthe savings produced by its routine use would outweigh thecosts of its universal application.

We do not mean this communication to cast an irrevocableshadow over the possibility that some day, we will be able toimage kidney stones by means of scintigraphy. In fact, we

would like nothing better than to learn that the authors had man-

aged to accomplish such a scientific tour de force. Our pointis that, as of now, they have not.

Sincerely,Howard M. Pollack, M.D.

Abass Alavi, M.D.Department of Radiology

University of Pennsylvania Medical Center3400 Spruce Street

Philadelphia, PA 19104

Response by the Authors:Our initial studies were performed in vitro to evaluate the

binding capabilities of 99mTc-MDP. Fifty-two urinary stoneswere extracted via antegrade percutaneous or retrogradeureteroscopic access, all with more than 70% of one composi-tion, as determined by Calculab. These stones were measuredin three dimensions and weighed. They were incubated in99mTc-MDP for 10 minutes, rinsed three times with 50 mL ofnormal saline, and placed in a scintillation counter. The percent uptake of the isotope was then calculated as the counts perunit of surface area and per unit of volume.

The relative activities of the stones separated them into threedistinct groups. The first group (N = 15; 29%) had uptake ofless than 1%. The second group (N = 19; 37%) had more than1% uptake but less than 5%. The third group (N = 18; 35%)had more than 5% uptake and included 11 stones that boundmore than 10%. The results according to stone composition are

summarized as follows:

Mean PerCent Uptake

±SDRange

Uric acidCystineAB

StruviteBrushiteCa phosphateCa oxalate dihydrateCa oxalate

monohydrateCa oxalate, <5% uptakeCa oxalate, >5% uptake

1.89 ± 0.31

0.21 + 0.22

13.18 ± 1.222.06 ± 0.987.77 ± 1.491.93 ± 1.56

3.23 ± 0.961.57 ± 0.39

10.45 ± 1.21

1.07-2.96

0.18-0.255.90

10.04-15.201.08-3.040.48-15.20.17-5.03

0.19-12.50.19-4.498.29-12.50

All the calcium oxalate dihydrate stones bound less than 5%,whereas the calcium oxalate monohydrate stones bound with a

wide range throughout the entire spectrum. This may reflectthe variability among these stones found in clinical practice, inthat some are recalcitrant to most forms of fragmentation,whereas others can be fragmented easily with routine SWL.These data suggest to us that 99mTc-MDP actually bound to theurinary stones in our in vivo study. The exact site of binding isunknown. However, it appears that stones without apatite crys-tals can bind 99mTc-MDP.

The concern that the images may represent stagnant urinesurrounding these calculi is greatly diminished by selecting forthe analysis only those patients without evidence of hy-dronephrosis as documented by ultrasonography. Thus, only 15of our 20 patients were included in the data analysis.

We agree that theoretically, a sphere with a uniform surfacethat binds isotope in a uniform fashion on its surface shouldappear as a radioactive halo around a less active or photopeniccenter. However, a two-dimensional projection of a three-di-mensional sphere in the presence of an absorber, as performedin this study, would yield a uniform image, as we have found.Additionally, human urinary calculi are not smooth, and manysmall crevices could allow binding.

We agree that Figure 1A is mislabeled: it is an early filmfrom an intravenous urogram. Figures 1C and 2B, however, areafter furosemide washout in a non-hydronephrotic kidney.Although there is no foolproof way to ensure that this imagerepresents a calculus, there is no evidence of uptake in the con-

tralateral kidney, nor of substantial amounts of 99mTc-MDP re-

maining in the bladder, suggesting to us that these images rep-resent renal calculi. Figure 3B shows radioactivity in the lowerpole of the right kidney. A preoperative intravenous urogramconfirms that there is no hydronephrosis in this kidney.

We do not mean to imply that all patients with urinary stonesshould be imaged with this modality. However, a test that couldpotentially discriminate between those stones that would frag-ment easily with SWL and those that would require percuta-neous extraction would help the clinician decide on the mosteffective treatment approach. Further research will help us de-termine if this imaging modality is such a test.

Marshall L. Stoller, M.D.Department of Urology

University of California-San Francisco