Monday, July 2, 2012

Out of Control: CT Scans in Renal Colic

The Gist:   Patient's don't keep track of their imaging and often underestimate the amount of radiation they've been exposed to through medical imaging.  Renal colic patients are at a high risk of cumulative radiation exposure so consider beginning the diagnostic evaluation other modalities, such as renal ultrasound (US), in relatively high risk populations.  CT scans aren't bad, but should probably used in series after US and with consideration.

Who should I really worry about?  If the imaging is absolutely necessary, then a theorized risk doesn't outweigh the potential harm from not performing the scan.  Period.  Also, younger people are more radiosensitive so imaging the geriatric population is less of an issue from the radiation standpoint.  For a review of radiation in medical imaging, check this out.

Patients with multiple medical problems.  Many patients, if not most patients, have no idea how much imaging they've had.  A study by Baumann et al in Annals of EM surveyed patients regarding their perception of radiation in medical imaging.  
  • In this study (n=1,168), 365 of participants reported no previous CT scan, of these 39% had at least one documented CT scan in that medical system (2). As radiation is additive, how can one estimate a patient's radiation risk with no idea of exposure?  
Patients with renal colic.  Presently, the "gold standard" diagnostic modality for renal colic exists as the CT scan.  A retrospective chart review published by Broder, et al provides insight into the magnitude of the problem of radiation exposure in evaluation of 356 patient encounters (n=306 patients) for suspected renal colic over 10 month period.   
  • 74% got a CT (n=262)
  • Many got more than one CT scan in that period:
    • Nearly half of the patients (49.3%, n=151) had 2 CT scans during the time period
    • 15% had 3 CT scans  (n=46) 
    • 4.9% had 4 CT scans  (n=15) 
    • 1% (n=3) had 7 CT scans
    • 1.6% (n=5) had >9 CT scans
  • Three patients had excessive scans:  28 y/o female had 14 CTs, a 53 y/o male had 25 CT scans, and a 42 y/o female had 22 CT scans
    • 38% of CTs were normal and only 2% (6 scans) showed an urgent or emergent cause of symptoms
    • 21% (n=56) of CT scans showed complicated urolithiasis
    • Only 6% of those who had a CT scan underwent a procedure
    A typical single detector unenhanced CT in suspected renal colic exposes a patient to approximately 6.5 mSv (8.5 mSv for multidetector unenhanced CT) (3).  For simplicity sake, suppose scans were single detector CT limited protocol (not complete abdomen/pelvis scans) then the cumulative dose for some of the patients reached near 50 mSv.  If there's a 1 in 1000 risk of cancer caused by radiation per 10 mSv, then at least half of the patients in this cohort tip that balance.  Some of the patients received over 100 mSv of radiation within this short time frame.  Additionally, the mean age in this study was 38.9 years, placing these patients in a more radiosensitive category than their geriatric peers.

    Also, keep in mind that the above Broder, et al study was conducted in 2003.  Another review, published in Academic Emergency Medicine demonstrated that between 1996-2007, CT usage for flank pain 4% to 42.5%, a 10-fold increase.  Thus, the Broder study may actually underestimate the radiation exposure for this population.  Unfortunately, this increase didn't result in an increase in the diagnosis of the badness that emergency physicians look for (therefore, likely not imparting a significant patient benefit) (1).

    There is a solution...ultrasound and common sense.  For the former, Mike and Matt come through on Ultrasound Podcasta podcast filled with great visual examples of hydronephrosis for practice.  SonoSpot, a relatively new blog, also has a great entry on renal ultrasound.
    • Curvilinear or phased array low frequency probe 
    • Placing the probe in the right and left mid-axillary line at the lower rib cage
    • Slowly fan up and down to evaluate the longitudinal view of the kidney for dilation of the collecting system and renal pelvis
    • Rotate 90 degrees to see the transverse view of the kidneys and slowly fan
    • Look at the bladder
    • Consider administering a 500 mL bolus of fluid before US
    • Add an aorta US in older patients with higher suspicion for AAA
    • Do a U/A
    • Get a CT if needed
    I'm do I interpret what I see?  The name of the game is hydronephrosis, which correlates to nephrolithiasis as stones are poorly visualized in US.
    • Sensitivity 73-97%; Specificity 73-83%
    • Study with highest sensitivity administered 500 mL fluid bolus prior to US
    The statistics listed below are from an excellent review article (abstract here). 
    Mild: renal pelvis dilation.
    • 88% stones < 5 mm  (95% CI 79.2-92.3).  Almost all of these stones pass spontaneously (76.7%)
    • Treat clinically and D/C with follow up if improved.
    • If fail medical management, get CT scan
    Moderate: renal pelvis and calyces dilated
    • 69.8%  stones <5 mm (95% CI 54.9-81.4)
    • Consider treating clinically and D/C with follow up if improved.
    • If fail medical management, get CT scan.
    Severe hydronephrosis: effacement of entire collecting system
    • 80%  stones > 5mm (95% CI 37.6-96.4).  These stones are less likely to pass spontaneously. 
    • CT scan/urology consult  
    1.  Westphalen A,
    2.  Hsia R, 
    3. Maselli J, 
    4. Wang R, 
    5. Gonzales R.  
    6. Radiological Imaging of Patients With Suspected Urinary Tract Stones: National Trends, Diagnoses, and Predictors Academic Emergency Medicine Volume 18Issue 7pages 699–707, July 2011
    1. Howard P. Forman.  
    2. Radiation Dose Associated with Unenhanced CT for Suspected Renal Colic: Impact of Repetitive Studies AJR Am J Roentgenol. 2006 Apr;186(4):1120-4.

    No comments:

    Post a Comment