Scand J Urol Nephrol25:223-226, 1991
FACTORS INFLUENCING RADIATION EXPOSURE DURING THE EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY Wei-Chuan Chen, Ying-Huei Lee, Ming-Tsun Chen, Jong-Khing Huang and Luke S. Chang From the Division of Urology, Department of Surgery, National Yang-Ming Medical College, and Veterans General Hospital-Taipei, Taipei, Taiwan, Republic of China Scand J Urol Nephrol Downloaded from informahealthcare.com by Freie Universitaet Berlin on 10/31/14 For personal use only.
(Submitted April 25, 1990. Accepted for publication October 1 1 , 1990)
Abstract. A prospective evaluation of 89 consecutive sessions of extracorporeal shock wave lithotripsy (ESWL) was undertaken to try and find the best way of minimising the amount of exposure to radiation. Forty-two patients were randomly allocated to undergo ESWL treatment by experienced surgeons (group A), and 47 to undergo the treatment by inexperienced surgeons (group B). The mean calculated entrance radiation exposure was 3.01 rads (group A: 2.64 (0.97) rads, range 1.00-4.48, group B: 3.38 (0.86) rads, range I . 1 1-5.75). Among factors that influenced radiation exposure, the tissue: air ratio should be borne in mind and the level of skill in controlling movement ofgantry was the most important in reducing the exposure to radiation. Key words. extracorporeal shock wave lithotripsy, radiation exposure, tissue: air ratio.
Extracorporeal shock wave lithotripsy (ESWL) has been used successfully to disintegrate urinary tract stones (3, 7, 10). Biplanar fluoroscopy and video spot filming technique are used to localise and monitor the targeted stones radiographically during treatment. The radiation exposure to patients has been estimated and may be determined by various factors including the size of patient, number and size of stones, location of stones, radiolucency of stones, selection of radiation output, degree of balloon inflation, and operative skill (2, 5, 6, 8 , 9 , 11). As the first four factors were beyond the operator’s control, this study was designed to evaluate the degree of exposure to radiation depending on who operated during ESWL, and the identification of the operator-dependent factors that would reduce radiation exposure.
PATIENTS AND METHODS A total of 89 patients undergoing ESWL during July 1989 using the modified Dornier Model HM3 lithotripter were included in this study. Sixty-five weighed less than 70 kg and 24 weighted 70 kg or more. Twenty-nine had stones that were less than 8 mrn in diameter, 42 were 8-1 6 mm and 18 were more than I6 mm. The stones of 54 patients were in the pelvic and calyceal areas, I7 were in the upper third of the ureter and 16 in the lower third ureter. The patients were randomly divided into two groups (Table I): in group A (n=42) the ESWL was caused by experienced surgeons and in group B (n=47) by inexperienced ones. Bowel preparation was given the night before ESWL to obtain a clear image. A stent was used to help locate small or transparent stones (1 1). Under intravenous anesthesia with tramadol hydrochloride, the patients were placed on a chair-like support system and positioned in a large immersion tank filled with degassed and demineralised water. The side of body to be treated was tilted 20” for renal stones and 30-45” for ureteric stones. We attempted t o locate the stones on F2 position before using the radiographic video monitor system. Fluoroscopy and spot filming with two Philips generator tubes positioned at an angle of 90” were used to monitor location and disintegration of stone during the procedure. The initial radiographic setting (kVp and mA) was selected according to patient’s size and body mass. As the patients in this study hardly varied in size a setting of 70 kVp, 3 mA t o 80 kVp, 4 mA was selected for fluoroscopy. Spot filming factors were set to 50-55 kVp and 18 mAs. Because the mA values were low and the tissue: air ratio was taken into consideration in measurements of radiation exposure when there was an interface of 5 cm water or more, the spot films were taken to evaluate disintegration more often than any other (Table 11). Gentle respiration was achieved with the patient’s cooperation during ESWL. This also allowed us to use fluoroscopy less often to determine the respiratory movements of the stones, though this mode is usually used whenever possible to diminish radiation exposure. The rule of “10 m m k e c of moveSrand J Urol Nephrol25
Scand J Urol Nephrol Downloaded from informahealthcare.com by Freie Universitaet Berlin on 10/31/14 For personal use only.
Wei-Chuan Chen et al.
ment during stone imaging as measured on the monitor" was followed. Operators followed this rule carefully to reduce the time taken in moving the gantry. The patients were moved without continuous fluoroscopy and the cones were reduced to the smallest possible size to reduce radiation exposure. Image intensifiers were equipped with inflatable balloons so that they could be immersed in water. The balloon of tube was filled with sufficient air to sustain its natural shape-that is, it was filled with air but not inflated to the extent that the rubber of the balloon was fully expanded. The inflated balloons were placed around the patient along the path of the X-ray beams. The length between the balloon tip and the surface of the patient that was immersed in water was 50 mm. The target stone must be placed at the second focus of the ellipsoid, which was 68 cm from the focal spot of the X-ray, but the radiation were likely to enter the body surface only 60 cm from the focal spot. Measurements of the patient's entrance exposure at 60 cm from the Philips X-ray tubes were made with a thermoluminescent dosimeter. Spot films were performed by adjusting the mA to 56 mA with an averaged 50 kVp for 320 msec, resulting in an approximate X-ray output of 5.14 mWmA at 60 cm distance or exposure rate (Xr) of 18.5 R cm2/mAs.For fluoroscopy, the mean output at 70 kVpl3.5 mA was 15.5 mWmA or exposure rate of 55.8 R cm2/mAs. Entrance radiation exposure was estimated by the following formula (10):
mAsX F x T : A R x Xr
where D = dose in rads (one rad = Gy) at the point of interest along the central ray; T : AR = tissue :air ratio in rad/R for a standard field size at the depth of calculation; S = distance in cm from the X-ray target to the point of calculation F = dimensionless factor to correct the T:AR for non-standard field sizes, and R cm2 Xr = exuosure rate in air in mAs The tissue: air ratio at a depth of 5 cm was given by equation (1) and the factor to correct the TAR for non-standard filed sizes was shown in equation (2) (10).
When considering interfacing 5 cm water with a small field size (10 c m x 10 cm) in our series, the tissue : air ratio and F value was 0.60 and 0.91, reScand J Urol Nephrol25
spectively. The product of tissue : air ratio and F would be used in correction of radiation. The fluoroscopy time and number of spot films of the 87 patients during ESWL were recorded. The amount of entrance radiation exposure was calculated according to the following equation: 1) Exposure rate at the maximum observed multiplied by fluoroscopy "in" time for the procedure = fluoroscopic exposure 2) Exposure/spot filming multiplied by total number of spot films = video filming exposure The entrance exposure then resulted in 0.04 rad/ spot film and 1.8 rads/min for fluoroscopy.
RESULTS The mean (SD) weight, number and size of stones, then site, the fluoroscopy time, number of spot films, and the calculated entrance radiation exposure are shown in Tables I, I1 and 111. The averaged calculated entrance radiation exposure was 0.03 Gy for patients who weighed less than 70 kg and 0.04 Gy for those who weighed 70 kg or more (p 0.05). Mean fluoroscopy time was 25.2 sec (group A: 35.1 (31.1), range 14-48 and group B 15.3 (lO.l), range 8-31, p