percutaneous nephrolithotomy ( pnl ) is a common treatment option for patients with renal stones .
these emerge during initial puncture and lead to injuries in the surrounding organs ( e.g. , colon , pleural cavity , lung , spleen , liver , and duodenum ) ( 1 ) .
though very rare , they may lead to morbidity in the form of septicemia , peritonitis , abscess formation , and nephrocolic or colocutaneous fistula . hence , knowing the position of the retrorenal colon to prevent surgical complications during pnl and other surgical modalities is a priority . in the present manuscript
, we studied the number of retrorenal colon presence in the ct images taken before pnl applications .
the medical records of 804 patients who underwent pnl in our clinic from 2006 to 2012 were reviewed retrospectively .
patients ct images taken before pnl were studied for the position of both the ascending and descending colon in relation to the right and left kidneys respectively .
patients without ct images and patients with severe musculoskeletal defects , abdominal masses , previous surgery , renal abnormalities ( large renal cysts , ectopic kidney , and horseshoe kidney ) and ascites were excluded from the study .
all ct images taken in supine position were evaluated by the same radiologist for retrorenal colon presence .
ct images were as previously identified ( 2 ) ; to the horizontal plane a parallel line was drawn through the posterior edge of each kidney ( posterior renal line ) at three renal levels : upper pole , middle pole and lower pole .
the number of cases with partial colon extension posterior to the posterior renal line was determined .
these were further separated to see if any part of the colon extended directly behind the lower pole ( figures 1 - 4 ) , middle pole and upper pole of the kidney . 

figures 1 - 4 - anatomical variations of colon related to the kidney : colon extended directly behind the lower pole . 

of the 394 patients included in the present study , 178 were female and 216 were male with a mean age of 42.2 years .
27 patients ( 6.9% ) had retrorenal colon with 18 ( 4.6% ) on the left and 4 ( 1.0% ) on the right side .
the other 5 ( 1.3% ) patients had bilateral retrorenal colons ( table-1 ) . in comparison to kidney location ,
table 1distribution of retrorenal colon according to kidney level.levelleftrightbilateraltotalupper0000middle0000lower184527total184527 
  during the studied period , colonic perforation was observed in only 2 cases ( 0.3% ) that did not had ct scans taken before pnl intervention .
pnl was first described in 1976 and has become in the last 30 years almost the standard treatment modality for renal stones ( 3 ) . despite the high success rates , it is still an invasive procedure associated with significant morbidity that involves life - threatening complications .
pnl outcome depends heavily on planning and successful execution of initial kidney access as it may carry an increased risk of damage to surrounding organs . in order to gain access to the desired renal calyx with minimal complications it is necessary to evaluate renal anatomy and the surrounding organs .
intravenous urography ( ivu ) has historically been the main preoperative imaging technique for pnl .
the spread and increasing presence of multiphase ct scanners has made it the main imaging preference in many centers .
ivu however still remains valuable for the preoperative planning and evaluation of kidney stones due to its ability to demonstrate fine details in the collecting system anatomy of the kidney . but a poorly performed ivu may have limit valuable information required before pnl application . therefore , dynamic ct has recently become the gold standard in urolithiasis diagnosis and evaluation ( 4 ) .
ct used for the diagnosis of renal stones , also delineates the extent , orientation , and location of the stones within the kidney and thus enable the most suitable pnl tract selection .
moreover , by showing the relationship of the kidney and stone to the surrounding structures enables to minimize the risks of injury to the spleen , liver , or to an unsuspected retrorenal colon .
pre and post pnl related complications rate up to 83% , including extravasation , transfusion , and fever ; yet , major complications such as septicemia and colonic or pleural injury are rare ( 5 ) .
colonic perforation , a rare complication of pnl , is reported in less than 1% of cases ( 6 ) . during pnl ,
the age of the patient , existence of horse shoe kidney , previous kidney surgery , left kidney inferior location access , lateral posterior axillary line access , hypermobile kidney , and the existence of retrorenal colon are among the factors that predispose to colon perforation ( 5 , 7 ) .
a retrorenal colon is more frequently found on the left side and is most likely to be situated near the inferior kidney pole ( 8) . retrorenal colon is found in approximatively 0.6% of the general population according to the hadar - gadot and sherman et al .
, ct images showed that the left colon was posterior in 16.1% of cases , and the right colon was posterior in 9% of cases at the level of the lower pole ( 11 ) .
total ratio of retrorenal colon was 1.7% , with 1.2% on the left side , 0.3% on the right side and 0.2% bilateral in the atar et al .
hopper et al . reported in their study based on 500 abdomen ct scans that the overall frequency of retrorenal colon was 1.9% if the images were taken in supine position .
when 90 patients were studied in the prone position , retrorenal colon was found in 10% ( 12 ) . in the present study , retrorenal colon in pnl patients
was found in 27 cases , ( 6.9% ) of which 18 ( 4.6% ) were on the left side , 4 ( 1.0% ) on the right side , and 5 ( 1.3% ) bilateral .
all ct scans in the present study were performed in supine position . however , in the literature the prevalence of retrorenal colon varies according to the patients position .
al . study , analyzing a series of 500 supine and 90 prone abdominal ct scans , determined the colon positioned posterior to the kidney in 1.9% and 10% of cases , respectively .
they found that there is a high risk of colon perforation during pnl in the prone position ( 13 ) .
if supine and prone positioned patient images are to be compared , the prone position seems to be associated with a significantly shorter nephrostomy tract length and a greater number of potential puncture sites .
a shorter tract length may ease percutaneous access and nephroscope mobility within the collecting system and thus improve stone - free rate and decrease hematuria risk .
a greater range of potential access angles may result in decreased risk of visceral injury ( 14 ) . in the croes pnl study group
1079 pnl applications were conducted in prone and 232 in supine positions without revealing a difference in complication rates ( 15 ) .
pnl was applied a short time ago mainly in endourology clinics ; yet , pnl related complications were generally documented by authors and departments whose main interest is pnl .
however , nowadays pnl has become a routine practice in all urology clinics and the reported complications may differ from the actual ones . in the present study , the retrorenal colon rate of 6.9% of patients who underwent pnl shows actually how close the urologists are to intervene to a retrorenal colon perforation .
unfortunately , in the two retrorenal colon perforation cases , no ct images were taken before pnl intervention and thus our ability to evaluate colon perforation risk proactively was nullified . on the other hand
, no colon perforations occurred in any of the cases who had ct images obtained before pnl intervention . in order to prevent colon perforation during pnl ,
alternative imaging techniques are available . however , a precise and reliable standardization of imaging techniques seems currently not possible .
thus , ultrasound or ct guided puncture to the kidney calyx , in cases with previously reported anatomic variations without inflicting harm to other organs , might be used .
moreover , 3d ct can provide excellent representation of kidney anatomy and be used while planning a pnl intervention ; however , it does not show the relationship between the calices and different organs , such as the colon in particular and kidney motion ( 16 ) .
retrorenal colon is more frequently found on the left side and on the lower pole of the kidney .
therefore , when accessing the lower pole of the kidney , especially on the left side , the risk of colonic injuries should be considered during pnl .
 objective the aim of this study was to investigate retrorenal colon incidence in percutaneous nephrolithotomy ( pnl ) interventions made in our clinic.materials and methods clinical data of 804 pnl patients , accumulated over a 7 year period ( 2006 - 2012 ) , was surveyed . 
 the patient files were reviewed retrospectively , and only those who had abdominal computed tomography ( ct ) images before pnl intervention were included in the study . in the ct images , the position of both the ascending and descending colon in relation to the right and left kidneys were evaluated.results according to our hospital reports , 394 patients with ct images were included in the present study 27 patients ( 6.9% ) had retrorenal colon , of which 18 ( 4.6% ) were on the left side , 4 ( 1.0% ) on the right side and 5 ( 1.3% ) had bilateral retrorenal colons . 
 colonic perforation complication was seen only in two patients and the colonic perforation rate was 0.3% . 
 these two cases had no ct images.conclusions pnl , in the process of becoming the standard treatment modality , is a safe and reliable technique for renal stone treatment . 
 colonic injury should be taken into consideration during pnl interventions of the lower pole of the kidney ( especially on the left side ) due to the location of retrorenal colon . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS