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Radiation exposure in paranasal sinus imaging
Radiation exposure in paranasal sinus imaging MICHAELJ. SILLERS,MD, FREDERICKA. KUHN,MD, and CHRISTOPHERL. VICKERY,MD, Augusta, Georgia Plain films of the paranasal sinus were the imaging technique most frequently used before the advent of computed tomography. With improved resolution, computed tomography imaging has become the standard for evaluating patients with disease of the paranasal sinuses. With the widespread availability of computed tomography scanning, patients are likely to undergo multiple examinations during the treatment of their disease. Concern has been raised over the radiation exposure to the patient, in particular, with regard to radiation-induced cataracts. The purpose of this study is to measure the radiation dose associated with a variety of techniques for imaging the paranasal sinuses. A cadaveric specimen with thermoluminescent dosimeters placed over the orbits was used to measure radiation exposure during plain-film imaging, limited computed tomography, standard axial and coronal computed tomography, thin-cut axial computed tomography, and thin-cut coronal computed tomography. The measured dose in all these techniques is less than 4% of the acute dose associated with cataract formation. [OTOLARYNGOLHEAD NECK SURG 1995;112:248-51 ,}
Plain-film imaging was used frequently for evaluating patients with diseases of the paranasal sinuses before the widespread availability of high-resolution computed tomography (CT). Plain-film imaging has been shown to have unacceptable false-positive and false-negative rates compared with CT in evaluating patients with chronic sinusitis. 1 Zinreich et al. 2 have outlined specific requirements for CT imaging of the paransal sinuses as part of the preoperative evaluation. Through the efforts of Kennedy3 and Stammberger and Posawetz 4 functional endoscopic sinus surgery for the treatment of chronic sinusitis has become the procedure of choice after medical failure. As familiarity with endoscopic techniques has increased, their application has been extended to patients with noninfectious sinonasal and orbital pathology. Successful endoscopic management of cerebrospinal fluid rhinorrhea, 5 choanal atresia, 6 Graves' ophthalmopathy, 7 and nasolacrimal stenosis8 has been reported. Patients who are treated with endoscopic techniques will frequently undergo multiple CT scans From the Medical Collegeof Georgia, Augusta. Presented at the SouthernSectionof the TriologicSociety,Marco Island, Fla., Jan. 14, 1994. Received for publication June 9, 1994; revisionreceived July 22, 1994; accepted July 29, 1994. Reprint requests: Michael J. Sillers, MD, The University of Alabama at Birmingham, 1501 5th Ave. South, Birmingham, AL 35233. Copyright© 1995by the AmericanAcademyof OtolaryngologyHead and Neck SurgeryFoundation,Inc. 0194-5998/95/$3.00 + 0 23/1/59519 248
during the course of therapy. Concern has been raised over the radiation exposure sustained from CT scan examinations of the paranasal sinuses, specifically with regard to the induction of cataracts. Investigators have found that the acute dose required to induce cataracts is 2 Gy. Over 12 weeks, 5.5 Gy may produce lens opacification. During a period of years, the threshold for cataract formation is 6 to 14 Gy. Other ocular complications, such as mucositis, keratitis, retinopathy, and optic neuropathy, require significantly higher radiation doses. 9'1° The purpose of this study is to measure the radiation exposure during plain-film imaging and a variety of CT scan techniques of the paranasal sinuses. The relative risk of cataract formation can then be determined. MATERIALS AND METHODS
Four lithium flouride thermoluminescent dosimeters (LiF TLD-100; Landaeur, Inc., Glenwood, Ill.) were placed over each orbit of a formalinfixed cadaveric specimen. Five imaging techniques were used: plain films of the paranasal sinuses, including Waters, Caldwell, lateral, and submentovertex views; standard CT (5-mm axial and 3-mm coronal cuts, no overlap); screening sinus CT (four axial and four coronal cuts); thin-cut axial CT (1.5 mm, no overlap); and thin-cut coronal CT (1.5 mm, no overlap). Plain films of the sinuses were performed on a Seimens Multix T (Siemens Medical Systems, Inc., Iselin, N.J.) machine with the standard technique.
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Fig. 1. Reconstructed sagittal view through c a d a v e r i c frontal recess demonstrating a separate cell within the frontal sinus. Asterisk, Frontal cell; MT, middle turbinate; IT, inferior turbinate.
All CT scans were performed on a GE 9800 Quick Scanner (GE Medical Systems, Waukesha, Wis.). Exposure time was 2 seconds at 120 kV potential and 100 mA for each scan. The radiation dose to each thermoluminescent dosimeter was measured with a Hershaw model 2000 thermoluminescence analyzer. An average dose was calculated for each orbit. RESULTS
The average radiation dose sustained by each orbit is outlined in Table 1. During the lateral view in the plain-film series, the left orbit is closer to the x-ray beam and sustains more radiation than the right orbit. The radiation exposure to the orbits is comparable with that in thin axial CT, thin coronal CT, and limited CT. The radiation dose in the standard CT is the highest of the techniques tested, but it is less than 4% of that associated with cataract formation. DISCUSSION
High-resolution CT has provided a significant advancement in the otolaryngologist's evaluation of paranasal sinus disease. The use of endoscopic in-
Table q. Radiation exposure through various
imaging techniques Average radiation [Gy] Imaging technique
Left eye
Right eye
Plain films Standard CT Thin axial CT Thin coronal CT Limited CT
0.00105 0,0727 0.0428 0,0430 0.0309
0.00065 0,0755 0.0425 0,0437 0.0247
strumentation and the adherence to the functional principles of sinus surgery has improved the ability with which medically refractory sinus disease can be treated. With the acceptance of functional endoscopic sinus surgery as the technique of choice for treating chronic sinusitis after medical failure, several problem areas have been identified. The frontal recess and frontal sinus are frequent areas for persistent or recurrent sinus disease after sinus surgery. The complexity of this anatomic region has led to the development of specific instrumentation for operating in this area. However, our ability to image this area radiographically has lagged behind
250
SILLERS et al.
Otolaryngology Head and Neck Surgery February 4995
Fig. 2. A fracture of the c a d a v e r i c skull base was purposely created with a curette in the laboratory. Sagittal reconstruction from thin coronal images shows the defect in the ethmoid cavity. Asterisk, Skull base defect; MT, middle turbinate; IT, inferior turbinate.
our ability to visualize and instrument the frontal recess and frontal sinus intraoperatively. With thin axial CT scans, high-resolution sagittal reconstruction can be performed to more clearly define the frontal recess and frontal sinus (Fig. 1). The combination of improved instrumentation and more specific imaging has enabled the performance of safe and successful surgery in this troublesome area. Patients with cerebrospinal fluid rhinorrhea after sinus surgery or trauma present an interesting challenge. Thin-cut coronal scans provide essential information in assessing the integrity of the skull base. Sagittal images can also be reconstructed from this format (Fig. 2). With the performance of limited, standard, and thin-cut CT scanning, concern has been raised over the radiation dose sustained by the patient from repeated examinations during the course of treat-
ment, in particular, with regard to radiation-induced cataracts. Radiation-induced cataracts differ initially from those that occur naturally by arising in the posterior pole of the lens. As the opacification progresses, it cannot be distinguished from cataracts arising naturally or from other causes. However, radiation-induced cataracts frequently will not progress beyond the initial stage and remain confined to the posterior subcapsular region, causing little, if any, visual impairment. Only 12% of patients who undergo low-dose radiation therapy (2 to 6.5 Gy) will have progressive opacification. The average latent period between radiation exposure and cataract formation is 8 years at low doses and 4 years at high doses (6.51 to 11.5 a y ) , 9'10This study provides evidence that the radiation dose sustained during CT imaging of the paranasal sinuses is well below that associated with cataract formation during acute
Otolaryngology Head and Neck Surgery Volume I 1 2 Number 2
and chronic exposure. This does not support obtaining C T scans without clear indications. However, the otolaryngologist m a y repeat s t a n d a r d or limited C T scans and study specific a n a t o m i c areas with thin-cut C T and sagittal reconstruction without the fear of inducing cataracts. CONCLUSIONS
CT scan administers minimal radiation to the oribits during imaging o f the paranasal sinuses. T h e radiation dose sustained by the orbit during C T scan is less than 4% of the acute dose associated with cataract formation. Thin-cut axial and coronal CT scan with sagittal reconstruction provides useful information in evaluating difficult areas within the paranasal sinuses. We thank James Cash for his efforts in performing the CT scans for this study. REFERENCES
1. McAlister WH, Lusk R, Muntz HR. Comparison of plain radiographs and CT scans in infants and children with recurrent sinusitis. Am J Roentgenol 1989;153:1259-64.
SII.LERS el al.
25'I
2. Zinreich SJ, Kennedy DW, Stammberger H, et al. Paranasal sinuses: CT imaging requirements for endoscopic surgery. Radiology 1987;163:769-75. 3. Kennedy DW. Functional endoscopic sinus surgery: technique. Arch Otolaryngol 1985;111:643-9. 4. Stammberger H, Posawetz W. Functional endoscopic sinus surgery: concepts, indications, and results of the Messerklinger technique. Eur Arch Otorhinolaryngol 1990;247: 63-76. 5. Mattox DE, Kennedy DW. Endoscopic management of ce~ rebral spinal fluid leaks and encephaloceles. Laryngoscope 1990;100:857-62. 6. Stankiewicz JA. The endoscopic repair of choanal atresia. OTOIARVNGOLI-I~aDNECk:SuRo 1990;103:931-7. 7. Kennedy DW, Goodstein ML, Miller NR, et al. Endoscopic transnasal orbital decompression. Arch Otolaryngol Head and Neck Surg 1990;116:275-82. 8. Metson R. The endoscopic approach for revision dacryocystorhinostomy. Laryngoscope 1990;100:1344-7. 9. Hall EJ. Radiobiology for the radiologist. 2rid ed. Philadelphia: Harper and Row, 1978:350-6. 10. Mettler FA, Moseley RD. Medical effects of ionizing radiation. Orlando, Fla2 Grune and Stratton, Inc., 1985: 136-8.
Plain-film imaging was used frequently for evaluating patients with diseases of the paranasal sinuses before the widespread availability of high-resolution computed tomography (CT). Plain-film imaging has been shown to have unacceptable false-positive and false-negative rates compared with CT in evaluating patients with chronic sinusitis. 1 Zinreich et al. 2 have outlined specific requirements for CT imaging of the paransal sinuses as part of the preoperative evaluation. Through the efforts of Kennedy3 and Stammberger and Posawetz 4 functional endoscopic sinus surgery for the treatment of chronic sinusitis has become the procedure of choice after medical failure. As familiarity with endoscopic techniques has increased, their application has been extended to patients with noninfectious sinonasal and orbital pathology. Successful endoscopic management of cerebrospinal fluid rhinorrhea, 5 choanal atresia, 6 Graves' ophthalmopathy, 7 and nasolacrimal stenosis8 has been reported. Patients who are treated with endoscopic techniques will frequently undergo multiple CT scans From the Medical Collegeof Georgia, Augusta. Presented at the SouthernSectionof the TriologicSociety,Marco Island, Fla., Jan. 14, 1994. Received for publication June 9, 1994; revisionreceived July 22, 1994; accepted July 29, 1994. Reprint requests: Michael J. Sillers, MD, The University of Alabama at Birmingham, 1501 5th Ave. South, Birmingham, AL 35233. Copyright© 1995by the AmericanAcademyof OtolaryngologyHead and Neck SurgeryFoundation,Inc. 0194-5998/95/$3.00 + 0 23/1/59519 248
during the course of therapy. Concern has been raised over the radiation exposure sustained from CT scan examinations of the paranasal sinuses, specifically with regard to the induction of cataracts. Investigators have found that the acute dose required to induce cataracts is 2 Gy. Over 12 weeks, 5.5 Gy may produce lens opacification. During a period of years, the threshold for cataract formation is 6 to 14 Gy. Other ocular complications, such as mucositis, keratitis, retinopathy, and optic neuropathy, require significantly higher radiation doses. 9'1° The purpose of this study is to measure the radiation exposure during plain-film imaging and a variety of CT scan techniques of the paranasal sinuses. The relative risk of cataract formation can then be determined. MATERIALS AND METHODS
Four lithium flouride thermoluminescent dosimeters (LiF TLD-100; Landaeur, Inc., Glenwood, Ill.) were placed over each orbit of a formalinfixed cadaveric specimen. Five imaging techniques were used: plain films of the paranasal sinuses, including Waters, Caldwell, lateral, and submentovertex views; standard CT (5-mm axial and 3-mm coronal cuts, no overlap); screening sinus CT (four axial and four coronal cuts); thin-cut axial CT (1.5 mm, no overlap); and thin-cut coronal CT (1.5 mm, no overlap). Plain films of the sinuses were performed on a Seimens Multix T (Siemens Medical Systems, Inc., Iselin, N.J.) machine with the standard technique.
Otolaryngology Head and Neck Surgery Volume 112 Number 2
SILLERS et al.
249
Fig. 1. Reconstructed sagittal view through c a d a v e r i c frontal recess demonstrating a separate cell within the frontal sinus. Asterisk, Frontal cell; MT, middle turbinate; IT, inferior turbinate.
All CT scans were performed on a GE 9800 Quick Scanner (GE Medical Systems, Waukesha, Wis.). Exposure time was 2 seconds at 120 kV potential and 100 mA for each scan. The radiation dose to each thermoluminescent dosimeter was measured with a Hershaw model 2000 thermoluminescence analyzer. An average dose was calculated for each orbit. RESULTS
The average radiation dose sustained by each orbit is outlined in Table 1. During the lateral view in the plain-film series, the left orbit is closer to the x-ray beam and sustains more radiation than the right orbit. The radiation exposure to the orbits is comparable with that in thin axial CT, thin coronal CT, and limited CT. The radiation dose in the standard CT is the highest of the techniques tested, but it is less than 4% of that associated with cataract formation. DISCUSSION
High-resolution CT has provided a significant advancement in the otolaryngologist's evaluation of paranasal sinus disease. The use of endoscopic in-
Table q. Radiation exposure through various
imaging techniques Average radiation [Gy] Imaging technique
Left eye
Right eye
Plain films Standard CT Thin axial CT Thin coronal CT Limited CT
0.00105 0,0727 0.0428 0,0430 0.0309
0.00065 0,0755 0.0425 0,0437 0.0247
strumentation and the adherence to the functional principles of sinus surgery has improved the ability with which medically refractory sinus disease can be treated. With the acceptance of functional endoscopic sinus surgery as the technique of choice for treating chronic sinusitis after medical failure, several problem areas have been identified. The frontal recess and frontal sinus are frequent areas for persistent or recurrent sinus disease after sinus surgery. The complexity of this anatomic region has led to the development of specific instrumentation for operating in this area. However, our ability to image this area radiographically has lagged behind
250
SILLERS et al.
Otolaryngology Head and Neck Surgery February 4995
Fig. 2. A fracture of the c a d a v e r i c skull base was purposely created with a curette in the laboratory. Sagittal reconstruction from thin coronal images shows the defect in the ethmoid cavity. Asterisk, Skull base defect; MT, middle turbinate; IT, inferior turbinate.
our ability to visualize and instrument the frontal recess and frontal sinus intraoperatively. With thin axial CT scans, high-resolution sagittal reconstruction can be performed to more clearly define the frontal recess and frontal sinus (Fig. 1). The combination of improved instrumentation and more specific imaging has enabled the performance of safe and successful surgery in this troublesome area. Patients with cerebrospinal fluid rhinorrhea after sinus surgery or trauma present an interesting challenge. Thin-cut coronal scans provide essential information in assessing the integrity of the skull base. Sagittal images can also be reconstructed from this format (Fig. 2). With the performance of limited, standard, and thin-cut CT scanning, concern has been raised over the radiation dose sustained by the patient from repeated examinations during the course of treat-
ment, in particular, with regard to radiation-induced cataracts. Radiation-induced cataracts differ initially from those that occur naturally by arising in the posterior pole of the lens. As the opacification progresses, it cannot be distinguished from cataracts arising naturally or from other causes. However, radiation-induced cataracts frequently will not progress beyond the initial stage and remain confined to the posterior subcapsular region, causing little, if any, visual impairment. Only 12% of patients who undergo low-dose radiation therapy (2 to 6.5 Gy) will have progressive opacification. The average latent period between radiation exposure and cataract formation is 8 years at low doses and 4 years at high doses (6.51 to 11.5 a y ) , 9'10This study provides evidence that the radiation dose sustained during CT imaging of the paranasal sinuses is well below that associated with cataract formation during acute
Otolaryngology Head and Neck Surgery Volume I 1 2 Number 2
and chronic exposure. This does not support obtaining C T scans without clear indications. However, the otolaryngologist m a y repeat s t a n d a r d or limited C T scans and study specific a n a t o m i c areas with thin-cut C T and sagittal reconstruction without the fear of inducing cataracts. CONCLUSIONS
CT scan administers minimal radiation to the oribits during imaging o f the paranasal sinuses. T h e radiation dose sustained by the orbit during C T scan is less than 4% of the acute dose associated with cataract formation. Thin-cut axial and coronal CT scan with sagittal reconstruction provides useful information in evaluating difficult areas within the paranasal sinuses. We thank James Cash for his efforts in performing the CT scans for this study. REFERENCES
1. McAlister WH, Lusk R, Muntz HR. Comparison of plain radiographs and CT scans in infants and children with recurrent sinusitis. Am J Roentgenol 1989;153:1259-64.
SII.LERS el al.
25'I
2. Zinreich SJ, Kennedy DW, Stammberger H, et al. Paranasal sinuses: CT imaging requirements for endoscopic surgery. Radiology 1987;163:769-75. 3. Kennedy DW. Functional endoscopic sinus surgery: technique. Arch Otolaryngol 1985;111:643-9. 4. Stammberger H, Posawetz W. Functional endoscopic sinus surgery: concepts, indications, and results of the Messerklinger technique. Eur Arch Otorhinolaryngol 1990;247: 63-76. 5. Mattox DE, Kennedy DW. Endoscopic management of ce~ rebral spinal fluid leaks and encephaloceles. Laryngoscope 1990;100:857-62. 6. Stankiewicz JA. The endoscopic repair of choanal atresia. OTOIARVNGOLI-I~aDNECk:SuRo 1990;103:931-7. 7. Kennedy DW, Goodstein ML, Miller NR, et al. Endoscopic transnasal orbital decompression. Arch Otolaryngol Head and Neck Surg 1990;116:275-82. 8. Metson R. The endoscopic approach for revision dacryocystorhinostomy. Laryngoscope 1990;100:1344-7. 9. Hall EJ. Radiobiology for the radiologist. 2rid ed. Philadelphia: Harper and Row, 1978:350-6. 10. Mettler FA, Moseley RD. Medical effects of ionizing radiation. Orlando, Fla2 Grune and Stratton, Inc., 1985: 136-8.