factors influencing radiation exposure during the extracorporeal shock wave lithotripsy

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

(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 radio- graphically 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 op- erated 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 litho- tripter were included in this study. Sixty-five weighed less than 70 kg and 24 weighted 70 kg or more. Twen- ty-nine had stones that were less than 8 mrn in diame- ter, 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 intra- venous 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 mmkec of move-

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224 Wei-Chuan Chen et al.

ment during stone imaging as measured on the monitor" was followed. Operators followed this rule care- fully 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 intensi- fiers 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 thermolumines- cent dosimeter. Spot films were performed by adjust- ing 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 : ARx Xr

sz D =

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, re-

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 multi-

plied 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). Patients whose stones measured less than 8 mm in dia- meter, 8-16 mm and more than 16 mm received 2.56 rads, 2.98 rads and 3.90 rads of entrance radiation exposure respectively (pc0.05). The mean calculated entrance radiation exposures for different locations of stones, including the pelvic and calyceal areas, the upper third of the ureter and the lower third of the ureter were 2.90, 3.09 and 2.97 rads respectively (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<0.05) and the mean number of spot films was 28.4 (group A: 18.5 (6.8), range 10-35 and group B: 38.4 (11.2), range 25-50, p<0.05). The mean

Table I. Numbers of patients and mean (SO) variables

Patients Group A Group B

Male N=26 N=27 Female N = 16 N=20 Age (years) 43.5 (14.5) 48.4 (12.4) Height (cm) 162 (8) 162 (8)* Weight (kg) 62.1 (11.7) 63.7 (13.2)* Stone burden (mm) 11.3 (9.9) 8.0 (5.2) Shock wave pulses 2 269 (814) 2 498 (622)*

* p>o .o5 .

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Radiation exposure during ESWL 225

Table IV. Comparative data of radiation exposure in different reports

Table 11. Technique factors of radiation during ES WL in different reports

Fluoro- Snap- Fluoro- Snap- Fluoro- Snap- scopy shot scopy shot scopy shot Total (kVp) ( k b ) (mA) (mAs) (time) (no.) (rad)

Glaze et al. (6) 80-93 50-65 2-4 50-100 Van Swearingen

Lin & Hrejsa (8) 80-95 60-65 2.5-4.2 80-100 Carter et al. (2) 80 60 1.5 42 Chen et al. (present study) 70-80 50-55 3-4 18

et al. (7) 90-92 50-55 3.6-3.8 80

Glaze et al. (6) 1-5 min 16-30 14.8-27 Van Swearingen

et al. (1 1) 3.3 min 37 10.1 Bush et al. (1) 4.5 min 5 15.5 Lin & Hrejsa (8) 3.5 min 8 23.4 Carter et al. (2) 2.7 min 26 16.0 Daniels et al. (5) 4.1 min 6.5 - Chen et al. 25.2 sec 28.4 5.57 (3.01*) (present study)

calculated entrance radiation exposure was 3.01 rads (group A: 2.6 (l.O), range 1.0-4.5 and group B: 3.4 (0.9), range 1.1-5.8, p<0.05). If tissue: air ratio was not taken into consideration when estimating the amount of radiation exposure, total dosage would be 5.6 rads (Table IV). The stone free rate was 76% for group A and 68 Yo for group B. There was no significant difference in outcome between the groups.

DISCUSSION The amount of radiation exposure that the patient receives is determined by various factors, including the number and size of stones, the patient’s weight, the site of stones, their radiolucency, degree of balloon, selection of X-ray output and if there was water interfacing and the technique of the operation.

Radiation exposure increased with increasing number and size of stones and the patient’s weight. The higher X-ray settings (kVp and mA)

Table 111. Comparison of mean (SO) radiation exposure between group A and group B

Group A Group B

Fluoroscopy time (set) 35.14 (31.01) 15.26 (lO.ll)* No. of spot film 18.50 (6.79) 38.36 ( 1 1.16)* Radiation exposure

(rads) 2.64 (0.97) 3.38 (0.86)*

*pc0.05.

* Corrected with tissue : air ratio.

were selected for the heavier patient and the longer X-ray monitoring were required when the number and size of stones was larger. Carter et al. also found that patient exposure increased in the same circumstances (2). The location of stones also influenced the amount of radiation exposure. It seemed to be greater in the group with lower third ureteric stones, as compared with those with stones in the renal and calyceal areas, even though these stones were usually smaller. The statistical difference, however, was not significant. We thought that the ureteric stones always overlay the vertebral body and required longer fluoroscopy time for localisa- tion, and they might be overtreated and overex- posed because the impacted ureteric stones had normal patterns of disintegration after ESWL.

The operator dependent factors were also an- alysed, these being no difference between group A and group B except the different technical experience of surgeon. It is not surprising that patients received lower doses of radiation when treatment is given by experienced operators. By exactly following the rule “10 mm/sec of movement during stone imaging as measured on the monitor” exactly in group-A patients, exposure time was reduced during the fluoroscopy technique. Ways to reduce radiation exposure have previously been reported but no detail sugges- tions have been given (8, 9). We believe that operative bias was minimised by familiarity with the movement system of the gantry, espe- cially the speed of that movement.

Patients undergoing ESWL in this study re-

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226 Wri-Chuan Chen et al.

ceived less radiation exposure than those in other studies (Table IV), and several factors contributed to this. Factors of technique, including kVp and mA values, for fluoroscopy and snapshot mode determined the entrance radiation exposure. The weights of our patients were not large enough to require high current (mA) while spot filming. The current we used in spot films was about one third of other series. As shown in the formulae, the dosage is propor- tional to mA, and the low current allowed one third radiation exposure compared with other series. The low current we employed in spot films did not have adverse effect on the quality of images. This is one of the reasons why less radiation was achieved, compared with other reports. To avoid overtreatment and conse- quent overexposure, the low dose spot films may be used frequently in the precise evaluation of stone disintegration. In addition, tissue:air ratio was considered in our patients when estimating the entrance radiation exposure because the inflated balloon was kept 50 mm from the patient’s body surface. The interfacing water gave the benefit of attenuating the X-ray beam without any adverse effect on the quality of the image. The 5 cm water reduced the radiation exposure to about half that in air if the tissue:air ratio was taken into account (4). The radiation exposure was calculated as being only about one sixth the dosage reported in other series with higher mA and in spite of correction.

In this study we found that tissue:air ratio should be taken into consideration, i.e. reduced the amount of radiation exposure to the patient if the inflated balloon was kept at a distance within the waterbath. Under similar circumstances, the patients had less risk of radiation exposure if the ESWL was administered by experienced operators, even though stones were larger. The aspect of technique that was the major concern in reducing radiation exposure

during ESWL was controlling the speed of the gantry movement.

ACKNOWLEDGEMENTS The authors acknowledge the technical assistance of Mr C. C. Chen and Miss J. S. Chen for critical review- ing of this manuscript.

REFERENCES 1. Bush WH, Jones D, Gibbons RP. Radiation dose

to patient and personnel during extracorporeal shock wave lithotripsy. J Urol 1987; 138: 716- 719.

2. Carter HB, Naslund EB, Riehle Jr RA. Variables influencing radiation exposure during extracorporeal shock wave lithotripsy. Urology 1987; 30:

3. Caussy C et al. Extracorporeal shock wave lithotripsy. Basel: S. Karger, 1982.

4. Chaney EL, Batchelor DA. Analytic formulae for estimation of dose along the central ray of diag- nostic X-ray beams. Am Assoc Phys Med 1981;

546-550.

8: 225-228. 5. Daniels C, MacKay J, MacGregor JH et al. Vari-

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6. Glaze S, LeBlanc AD, Bushong SC. Patient and personnel exposure during extracorporeal lithotripsy. Health Physics 1987; 53: 623-629.

7. Lee YH, Chen MT, Chang LS et al. Experience using extracorporeal shock-wave lithotripsy (ESWL) to treat upper urinary tract stones. J Surg

8. Lin PJ, Hrejsa AF. Patient exposure and radiation environment of an extracorporeal shock wave lithotriptor system. J Urol 1987; 138: 712- 115.

9. Pollack HM. Radiation exposure and extracorporeal shock wave lithotripsy. Editorial. J Urol 1987; 138: 850.

10. Riehle R, Fair W, Vaughan E. Extracorporeal shock wave lithotripsy: one year’s experience at a single center. JAMA 1986; 255: 2043.

1 1 . Van Swearingen F, McCullough DL, Dyer R et al. Radiation exposure to patients during extracorporeal shock wave lithotripsy. J Urol 1987;

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factors influencing radiation exposure during the extracorporeal shock wave lithotripsy

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