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Year : 2012  |  Volume : 28  |  Issue : 2  |  Page : 237-238
 

Air retrograde pyelogram during fluoroscopic access for percutaneous nephrolithotomy: A measure to reduce radiation exposure


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Date of Web Publication13-Jul-2012

Correspondence Address:
Rohit Kathpalia
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How to cite this article:
Kathpalia R. Air retrograde pyelogram during fluoroscopic access for percutaneous nephrolithotomy: A measure to reduce radiation exposure. Indian J Urol 2012;28:237-8

How to cite this URL:
Kathpalia R. Air retrograde pyelogram during fluoroscopic access for percutaneous nephrolithotomy: A measure to reduce radiation exposure. Indian J Urol [serial online] 2012 [cited 2019 Oct 21];28:237-8. Available from: http://www.indianjurol.com/text.asp?2012/28/2/237/98483

Lipkin ME, Mancini JG,Zilberman DE, Raymundo ME,Yong D, Ferrandino MN, et al. Reduced radiation exposure with the use of an airretrograde pyelogram during fluoroscopic access for percutaneous nephrolithotomy.J Endourol 2011;25:563-7.



   Summary Top


One of the most critical steps during percutaneous nephrolithotomy (PNL) is obtaining access to the collecting system. Fluoroscopy is the imaging modality most often used to ai3 in access. [1] A retrograde pyelogram (RGP) using iodinated contrast is traditionally performed to delineate the collecting system but it tends to layer in the ventral calices of a prone patient and may fail to opacify dorsal calices. Air when used instead of contrast, rises dorsally allowing fluoroscopic differentiation of ventral and dorsal parts of the collecting system.

Fluoroscopy during PNL contributes to cumulative radiation exposure for patients with nephrolithiasis. The authors evaluated whether the use of air for RGP affected radiation exposure during PNL when compared with the use of a contrasted-RGP. They retrospectively reviewed 260 PNL procedures performed by a single surgeon and interventional radiologist between November 2007 and April 2009, of which 96 patients with data on their radiation dosage during PNL were included for analysis. Radiation dosage was recorded in a dose report from the intraoperative C-arm that included all radiation exposure during percutaneous access, wire placement, tract dilation, stone removal and drainage tube placement. The dose report included fluoroscopy time and a dose area product expressed in rads cm 2 that was converted to mSv using accepted conversion tables, so as to calculate the effective dose (ED). [2] The operative reports and intraoperative images were reviewed to determine if air or contrast was used to perform the retrograde pyelogram.

Multivariable linear regression was used to determine the association of ED and fluoroscopy time with the use of air pyelogram, controlling for factors thought to affect ED. It showed a trend toward decreased ED by nearly 50%, from 7.67 (CI=5.99-9.81) to 4.45 (CI=3.68-5.38) mSv in the air pyelogram group compared with the contrast pyelogram group. However, there was no difference in fluoroscopy time between the two groups.

The explanation for decreased radiation exposure with the use of air is related to the physics of image acquisition and the impact of the different densities between air and contrast. The C-arm detects different densities via the image intensifier and automatically adjusts the tube current (mAs) or tube potential (kVp) to maintain image quality. [3] This is a feature on most C-arms called "automatic brightness control." With denser materials such as iodinated contrast, the C-arm will increase tube potential or tube current to maintain the image quality; however, this also leads to increased radiation dose to the patient.

The authors concluded that the use of room air for performance of RGP during access for PNL significantly reduces radiation exposure, thus significantly decreasing the hazards of radiation including the risk of malignancy.


   Comments Top


Retrograde pyelogram is commonly used to obtain access for PNL using fluoroscopy with iodinated contrast. Using air instead of contrast is not new to urologists and has been in use over last several decades. It preferentially fills and outlines the posterior calyces when patients are lying prone, thereby allowing more rapid identification of the appropriate access site. Another advantage is that it does not obscure the view of the stone, which aides in the identification of the ideal access point to treat the targeted stone. The diminished volume of air required for identification of the posterior calyces may have a secondary benefit in reducing the extent of extravasation encountered especially when multiple access sites are required or during difficult access.

Multivariable linear regression analysis clearly demonstrated that using an air pyelogram lowered the mean adjusted ED nearly twofold, though it was not associated with decreased fluoroscopy time suggesting that it did not necessarily make access any easier or quicker. One concern when using air for retrograde pyelogram is the perceived risk of air embolism. [4] However, based upon the literature, air embolism is an extremely rare event.

There are certain limitations of the current study. It is a retrospective investigation and information on radiation dose was not saved for every PNL case. The authors have also not mentioned when they choose to do air or contrasted pyelogram. They have not mentioned what the indication is in their opinion for contrasted or air pyelogram. In our opinion, contrasted pyelography is needed in case where the PCS anatomy has not been properly deliniated pre-operatively, in patients with radiolucent stone, patients with aberrant renal anatomy (like horseshoe kidney or fused kidney). [5]

However, this is the first study to demonstrate a modifiable surgical variable that affects radiation dose to patients during PNL. Since these patients are already at increased risk, every small decrease in radiation dose is significant.

 
   References Top

1.Le Heron J. Estimation of effective dose to the patient during medical x-ray examinations from measurements of the dose area product. Phys Med Biol 1992;37:2117-26.   Back to cited text no. 1
    
2.Miller NL, Matlaga BR, Lingeman JE. Techniques for fluoroscopic percutaneous renal access. J Urol 2007;178:15-23.  Back to cited text no. 2
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3.Reilly AJ, Sutton DG. A computer model of an image intensifier system working under automatic brightness control. Br J Radiol 2001;74:938- 48.  Back to cited text no. 3
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4.Varkarakis J, Su LM, Hsu TH. Air embolism from pneumopyelography. J Urol 2003;169:267.  Back to cited text no. 4
[PUBMED]  [FULLTEXT]  
5.Goel A, Kathpalia R.Re: Reduced radiation exposure with the use of an air retrograde pyelogram during fluoroscopic access for percutaneous nephrolithotomy. (From: Lipkin ME, Mancini JG, Zilberman DE, et al. J Endourol 2011;25: 563-7). J Endourol. 2012 ;26:197  Back to cited text no. 5
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