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Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol
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ARTICLE IN PRESS European Journal of Radiology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol Vincent M. Levesque a,∗ , Paul B. Shyn a , Kemal Tuncali a , Servet Tatli a , Richard D. Nawfel b , Olutayo Olubiyi b , Stuart G. Silverman a a b
Division of Abdominal Imaging and Intervention, Department of Radiology, Brigham & Women’s Hospital, 75 Francis Street, Boston, MA 02115, USA Department of Radiology, Brigham & Women’s Hospital, Boston, MA 02115, USA
a r t i c l e
i n f o
Article history: Received 7 July 2015 Accepted 14 July 2015 Keywords: Cryoablation CT-guidance Radiation dose Dose reduction protocol
a b s t r a c t Purpose: To estimate and compare the radiation dose using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors. Materials and methods: An IRB-approved, HIPAA-compliant retrospective study of 97 CT-guided cryoablation procedures to treat a solitary renal tumor in each of 97 patients (64M, 33F; range 31–84 yrs) was performed. Fifty patients were treated using a standard dose protocol (kVp = 120, mean mAs = 180, monitoring scans every 3 min during freezes), and an additional 47 patients were treated using a dose reduction protocol (kVp = 100, mean mAs = 100, monitoring scans less frequently than every 3 min during freezes). Multiple Wilcoxon Mann-Whitney (rank-sum) tests were used to compare dose-length product (DLP) between the two groups. Fisher’s exact test was used to compare technique effectiveness at 12 months post ablation between the two groups. Results: Median DLP for the standard protocol group was 4833.5 mGy*cm (range, 1667–8267 mGy*cm); median DLP for the dose reduction group was 2648 mGy*cm (range, 850–7169 mGy*cm), significantly less than that of the standard protocol group (p < 0.01). The technique effectiveness for the dose reduction group was not significantly different from that of the standard protocol group at 12 month follow up (p = 0.434). Conclusion: The radiation dose during percutaneous CT-guided cryoablation of renal tumors was substantial in both the standard and the dose reduction groups; however, it was significantly lower with the protocol change that reduced dose parameters and decreased the number of CT scans. © 2015 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Image-guided percutaneous ablation of renal tumors is generally reserved for organ-confined tumors in patients for whom surgery is contraindicated or a minimally invasive option is preferred [1–3]. Thermal ablation techniques such as radiofrequency ablation (RFA) and cryoablation have been used to treat renal tumors, and are usually performed under CT-guidance [1–11]. As intermediate to long-term data are emerging, results for percutaneous CT-guided RFA and cryoablation appear promising and show potential for treating renal tumors effectively [2–11]. CT-guided percutaneous cryoablation of renal tumors has been documented as a well-tolerated procedure with a low rate of
∗ Corresponding author. E-mail address: [email protected] (V.M. Levesque).
complications [2,3,5–7,11]. In contrast to CT-guided RFA, during cryoablation, the effect of freezing (iceball formation) is clearly visualized in the tumor, renal parenchyma, and surrounding soft tissues using unenhanced CT scans. CT scanning is used during the procedure to plan the procedure, target the tumor with cryoablation probes, monitor the enlarging iceball, and survey the region after the ablation to ascertain whether the treatment is complete [2–10]. Typically, several CT scans are used to carry out these four phases of a cryoablation procedure [3,5–7]. The potential health risks associated with ionizing radiation and, in particular, the radiation dose to patients undergoing CT and fluoroscopy has attracted the attention of the medical community [12–14]. This study was conducted in order to estimate and compare the radiation doses using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors.
http://dx.doi.org/10.1016/j.ejrad.2015.07.021 0720-048X/© 2015 Elsevier Ireland Ltd. All rights reserved.
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021
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2. Materials and methods 2.1. Subjects This retrospective study received IRB approval and complies with the HIPPA. Written informed consent was waived. The medical records of 106CT-guided renal cryoablation procedures were reviewed. The records of 57 consecutive renal cryoablation procedures performed from January 2007 through May 2008 using a standard ablation procedure scanning protocol were reviewed. Seven procedures were excluded: five, during which more than one tumor was treated, one, involving a navigation device for targeting that could have influenced the number of CT scans, and one, involving a scanner malfunction that resulted in additional scans. The remaining 50 procedures were performed in 50 patients (33 men, 17 women; mean age, 69 ± 11 yrs, range, 37–84), each with a solitary tumor (mean maximum diameter, 2.3 ± 0.9 cm, range, 0.9–5.0). No procedure was excluded because of poor image quality. There was a 23 month interval between performance of the standard protocol procedures and the dose reduction protocol procedures during which possible dose reduction strategies were considered. The records of 49 consecutive renal cryoablation procedures performed from April, 2010 through December, 2011 using a defined dose reduction scanning protocol were reviewed. Two procedures during which more than one tumor was treated were excluded. The remaining 47 procedures were performed in 47 patients (31 men, 16 women; mean age, 63 ± 12 yrs, range, 31–83) each with a solitary tumor (mean maximum diameter, 2.8 ± 1.0 cm, range, 1.2–5.2). No procedure was excluded because of poor image quality. 2.2. Procedure Cryoablation was performed using an argon gas-based system (CryoHit, Galil Medical, Arden Hills, MN). Under CT-guidance (Sensation Open, Siemens Medical Solutions, Forchheim, Germany), 17-gauge needle-like cryoablation probes were placed by an interventional radiologist. The number and positions of the probes were chosen based on the size of the tumor so that the resultant iceball would encompass the tumor plus a minimum 5 mm margin. Patients received either intravenous moderate sedation or general anesthesia. CT scans were performed during each of four phases of the procedure: planning, targeting, monitoring, and survey. An initial planning CT scan of the tumor and surrounding region was performed with a radio-opaque grid (GuideLines, Beekley Inc., Bristol, CT) placed on the patient’s skin. The location of the tumor was identified from the scan and a skin-entry site selected and marked. Following grid removal, the skin was prepared and draped in a sterile fashion. A reference needle (18-25-gauge) was inserted percutaneously and served as a guide for placing cryoablation probes using tandem technique. During this targeting phase, needle and probe placements were guided by either CT or CT fluoroscopy. CT and CT fluoroscopy were used to visualize the instruments and review adjustments. The targeting phase was complete when all cryoablation probes were in place. In the monitoring phase, cryoablation probes were activated simultaneously. Monitoring was used to confirm that the iceball encompassed the tumor plus a 5–10 mm margin and had not extended into adjacent critical structures. The cryoablation protocol consisted of a 15-minute freeze, followed by a 10-minute thaw, followed by a second 15-minute freeze. Iceball formation was monitored by CT scans obtained during each 15minute freeze. Following completion of the cryoablation protocol, the probes were removed and a final CT scan was acquired to survey the ablation coverage. Slice thickness (5 mm) and rotation time (500 ms) used for the standard group were not changed for the dose reduction group.
During each procedure in both groups, image quality and iceball visualization were deemed acceptable and comparable by the experienced interventional radiologists. Dose parameters were not altered for patient body weight in either group. In the standard protocol group (kVp = 120, mean mAs = 180), the targeting phase for 13 (26%) of 50 procedures included an ancillary component: a pre-ablation needle biopsy (n = 8) or hydrodissection via normal saline injection through additional fine needles (20–22gauge) to displace adjacent critical structures (e.g., bowel) from the treatment site (n = 5). No procedure involved both biopsy and hydrodissection. Multiple cryoablation probes were used (mean, 4, range, 2–6). CT fluoroscopy was used in 14 (28%) of the 50 procedures during the targeting phase to guide the placement of reference needles (n = 5), biopsy needles (n = 1) or cryoablation probes (n = 8). Monitoring scans were obtained every 3 min during each freeze phase. All 50 procedures were delivered according to the standard protocol plan, and were technically successful, i.e., the iceball included the entire tumor and a minimum margin of 5 mm [3]. In the dose reduction protocol group (kVp = 100, mean mAs = 100), the targeting phase for 16 (34%) of the 47 procedures included an ancillary component: a needle biopsy (n = 11) or hydrodissection (n = 5). No procedure involved both biopsy and hydrodissection. Multiple cryoablation probes were used (mean, 4 ± 1, range, 2–8). CT fluoroscopy was used in 40 (85%) of the 47 procedures in the targeting phase to guide the placement of initial reference needles, biopsy needles, or cryoablation probes. Instead of obtaining monitoring scans every 3 min as in the standard protocol, monitoring scans were obtained during each freeze only as deemed necessary by the interventional radiologists. All 47 procedures were delivered according to the dose reduction protocol plan, and were technically successful with iceball coverage of the entire tumor and a minimum margin of 5 mm [3]. At 12 month post-ablation follow up, technique effectiveness of all procedures was reviewed. Technique effectiveness was defined by “no evidence of recurrent disease” on imaging at 12 months postablation. Any case with evidence of recurrence that could require a second ablation was noted. 2.3. Data analysis Using chi-square tests, with two-sided p-values reported at a preset significance (␣) level of 0.05, the baseline distribution of gender, age, tumor size, number of cryoprobes used, and performance of an ancillary procedure were confirmed as comparable for the standard and dose reduction protocol groups (p > 0.05) (Table 1). Table 1 Radiation dose using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors: comparison of study groups.
Gender Female Male Age (years) ≤60 >60 Tumor size (cm) ≤2 >2, <4 ≥4 Number of cryoprobes used ≤3 ≥4 Ancillary procedure performed Yes No
Standard protocol
Dose reduction protocol
17 33
16 31
9 41
14 33
23 24 3
13 30 4
15 35
18 29
13 37
16 31
P 0.996
0.173
0.174
0.389
0.387
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Table 2 Median dose-length product (DLP) for the standard and dose reduction protocols in patients undergoing CT-guided percutaneous cryoablation of renal tumors. Procedure phase
Median standard protocol DLP
Median dose reduction protocol DLP
P
Total Targeting Monitoring
4833.5 mGy*cm 2087 mGy*cm 1733 mGy*cm
2648 mGy*cm 1092 mGy*cm 866 mGy*cm
<0.01 <0.01 <0.01
The total dose-length product (DLP) for each group was calculated using the total radiation dose of each procedure: the sum of the scanner’s calculated DLP of all CT and CT fluoroscopy acquisitions. The DLP was also calculated separately for the individual CT and CT fluoroscopy acquisitions during the targeting and monitoring phases for each group. The median DLPs for the targeting and monitoring phases of the two groups were compared for subanalysis as these were the only phases in which the protocols differed. Multiple Wilcoxon (rank-sum) Mann–Whitney tests were used to compare total DLP, targeting phase DLP, and monitoring phase DLP, between the two groups after confirming that the dose distribution patterns differed from a normal distribution pattern. Fisher’s exact test was used to compare technique effectiveness at 12 months post ablation between the two groups. Only 2-sided p-values at preset significant level (␣) were reported. All statistical analyses were conducted using STATA Version 11.2 (StataCorp LP, College Station, Texas). 3. Results The median overall DLP for the standard protocol group was 4833.5 mGy*cm (range, 1667–8267 mGy*cm). The median overall DLP for the dose reduction protocol group was 2648 mGy*cm (range, 850–7169 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). In the standard protocol group, the median DLP in the targeting phase was 2087 mGy*cm (range 380–4951 mGy*cm). The median DLP in the monitoring phase was 1733 mGy*cm (range 667–3200 mGy*cm) (Table 2). In the dose reduction group, the median DLP in the targeting phase was 1092 mGy*cm (range, 220–4998 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). The median DLP in the monitoring phase was 866 mGy*cm (range, 158–2479 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). At 12 month follow up, the technique effectiveness for the standard protocol group (n = 50) was 100% (43/43 procedures; five patients were lost to follow up and two patients had no recurrence with follow up of less than 12 months). At 12 month follow up, the technique effectiveness for the dose reduction group (n = 47) was 97% (32/33 procedures; seven patients were lost to follow up and seven patients had no recurrence with follow up of less than 12 months). The technique effectiveness for the dose reduction group was not significantly different from that of the standard protocol group at 12 month follow up (p = 0.434). 4. Discussion Reports describing radiation dose from CT-guided liver tumor ablation procedures suggest an almost two-fold greater radiation exposure during cryoablation compared to RFA procedures [15–16]. One of these reports also found almost three times the radiation exposure from CT-guided renal cryoablation procedures compared to CT-guided liver RFA procedures [16]. In our study, using DLP as an estimate, the radiation dose of CT-guided renal cryoablation was substantial (reaching as high as 8267 mGy*cm) and varied over a wide range. Cryoablation typically requires placement of a greater number of probes than RFA procedures, thereby requiring an increased number of scans. Some
procedures required only two or three probes to treat smaller tumors with no adjacent critical structures. Others required five or six probes to treat larger tumors that were close to critical structures such as the bowel and ureter. The number of probes was determined by the need to achieve an iceball of sufficient size to cover the tumor plus a minimum margin of 5 mm [3]. Some small tumors were difficult to target requiring additional scans because of their small size or particular location. Other small tumors were not sufficiently well defined on unenhanced CT scans and therefore, warranted more probes and larger iceballs. Others were located centrally near vasculature, and more probes were used to counteract heat sink effects. We analyzed both the total DLP values incurred during the entire procedure, and separately, the component DLP values incurred during the targeting and monitoring phases in both groups. The targeting and monitoring phases were the portions of the procedures primarily impacted by the dose reduction protocol. We did not compare the DLP values in the planning and surveying phases of the two groups because the protocol for these phases was not changed and these phases contributed a substantially lower radiation dose than the targeting and monitoring phases. Radiation during the targeting phase was an important contributor to total radiation dose. Using CT fluoroscopy during the targeting phase helped minimize the number of helical CT scans required. Indeed, the increased use of CT fluoroscopy during targeting in the dose reduction protocol was accompanied by a significant decrease in radiation dose compared to that of the standard protocol group. CT fluoroscopy was used in 85% of cases in the dose reduction protocol group, but only in 28% of cases in the standard protocol group, suggesting that the judicious use of CT fluoroscopy during the targeting phase can reduce both the number of helical CT scans required and the overall radiation dose to the patient. A key advantage of cryoablation relative to other ablation technologies is the ability to visualize the entire extent of the ablation throughout the procedure, allowing interventional radiologists to treat the tumor completely while minimizing damage to adjacent critical structures. However, this advantage carries with it the added radiation dose of monitoring CT scans. The radiation dose reduction protocol employed in this study provides a markedly reduced radiation dose to patients. The dose reduction was attributable to lower CT tube potential and fewer CT scan acquisitions in the dose reduction protocol group. During the monitoring phase of procedures in the dose reduction protocol group, the iceballs remained well-depicted using the lower tube potential, and did not compromise the ability to achieve a technically successful ablation as the interventional radiologist always had the option of scanning more frequently when necessary. Technique effectiveness for both groups was high and demonstrated that our dose reduction protocol reduced radiation dose to the patient without sacrificing long term desirable outcomes. Although not used in this study, ultrasound can be used adjunctively to expedite the real-time targeting of a tumor without ionizing radiation exposure to the patient. Some tumors are not visible with ultrasound, and ultrasound is not ideal for monitoring cryoablation because the iceball is largely obscured by acoustic shadowing. Other strategies to reduce radiation dose during CTguided cryoablations may include the use of a computer-based navigational device to reduce the number of targeting scans [17,18].
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021
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Since the entire iceball is visible also with MRI, ionizing radiation exposure can be totally eliminated by performing the entire procedure using MRI-guidance [6,11,19]. Finally, it should be noted that the radiation dose incurred during an ablation procedure may constitute only a fraction of the total cumulative radiation dose a patient receives in the course of care. It is likely that many patients treated with CT-guided cryoablation also undergo a diagnostic CT scan, a pre-ablation CT-guided biopsy, and several follow-up CT scans [20,21]. Furthermore, some patients may require additional cryoablation procedures, either to retreat a recurrent tumor or to treat other renal tumors. In view of our findings, we suggest that radiologists performing CT-guided percutaneous cryoablations assess their techniques and implement methods to limit radiation exposures to levels that are as low as reasonably achievable. A limitation of this study was that radiation dose was estimated retrospectively using DLP. We acknowledge that DLP does not accurately quantify dose to an individual patient. However, we believe DLP can serve as a reference value against which other studies of radiation dose can be compared, particularly those that evaluate the effects of dose reduction strategies. DLP has been used in a recent study to estimate radiation dose to patients undergoing CT-guided interventional procedures [22]. 5. Conclusion The findings of this study can be expected to contribute to the existing knowledge in the field of CT-guided percutaneous cryoablation of renal tumors by demonstrating that (a) potentially substantial radiation doses are received by patients during CTguided percutaneous cryoablation of renal tumors; (b) radiation dose reductions are achievable by decreasing tube current and kilovoltage of targeting and monitoring scans and reducing the number of monitoring scans; and, (c) further dose reductions are possible through the use of CT fluoroscopy during the targeting phase so as to limit the number of helical CT scans acquired. The findings of this study have an important implication for patient care: the potential radiation doses received by patients during CT-guided percutaneous renal cryoablation procedures should alert interventional radiologists of the need for adopting procedural techniques and dose reduction protocols that limit radiation dose. Conflict of interest None
References [1] M. Beland, P.R. Mueller, D.A. Gervais, Thermal ablation in interventional oncology, Semin. Roentgenol. 42 (2007) 175–190. [2] M. Saksena, D.A. Gervais, Percutaneous renal tumor ablation, Abdom. Imaging 34 (2009) 582–587. [3] R.N. Uppot, S.G. Silverman, R.J. Zagoria, K. Tuncali, D.D. Childs, D.A. Gervais, Imaging-guided percutaneous ablation of renal cell carcinoma: a primer of how we do it, AJR Am. J. Roentgenol. 192 (2009) 1558–1570. [4] K. Krehbiel, A. Ahmad, J. Leyendecker, R. Zagoria, Thermal ablation: update and technique at a high-volume institution, Abdom. Imaging 33 (2008) 695–706. [5] C.S. Georgiades, K. Hong, C. Bizzell, J.F. Geschwind, R. Rodriguez, Safety and efficacy of CT-guided percutaneous cryoablation for renal cell carcinoma, J. Vasc. Intervention Radiol. 19 (2008) 1302–1310. [6] T.D. Atwell, M.A. Farrell, B.C. Leibovich, et al., Percutaneous renal cryoablation: experience treating 115 tumors, J. Urol. 179 (2008) 2136–2140. [7] P.J. Littrup, A. Ahmed, H.D. Aoun, et al., CT-guided percutaneous cryotherapy of renal masses, J. Vasc. Intervention Radiol. 18 (2007) 383–392. [8] R.J. Zagoria, M.A. Traver, D.M. Werle, M. Perini, S. Hayasaka, P.E. Clark, Oncologic efficacy of CT-guided percutaneous radiofrequency ablation of renal cell carcinomas, AJR Am. J. Roentgenol. 189 (2007) 429–436. [9] D.A. Gervais, F.J. McGovern, R.S. Arellano, W.S. McDougal, P.R. Mueller, Radiofrequency ablation of renal cell carcinoma: part 1, Indications, results, and role in patient management over a 6-year period and ablation of 100 tumors, AJR Am. J. Roentgenol. 185 (2005) 64–71. [10] A.H. Mahnken, D. Rohde, D. Brkovic, R.W. Günther, J.A. Tacke, Percutaneous radiofrequency ablation of renal cell carcinoma: preliminary results, Acta Radiol. 46 (2005) 208–214. [11] S.G. Silverman, K. Tuncali, E. vanSonnenberg, et al., MRI-guided percutaneous cryotherapy of renal tumors: initial experience in 23 patients, Radiology 236 (2005) 716–724. [12] D. Brenner, E.J. Hall, Computed tomography – an increasing source of radiation exposure, N. Engl. J. Med. 357 (2007) 2277–2284. [13] Computed Tomography and Radiation Exposure Letters to the Editor. N Engl J Med 2008; 358:850-853. [14] E.S. Amis Jr, P.F. Butler, E. Kimberly, et al., American College of Radiology white paper on radiation dose in medicine, J. Am. Coll. Radiol. 4 (2007) 272–284. [15] B.K. Park, P.R. Morrison, S. Tatli, et al., Estimated effective dose of CT-guided percutaneous cryoablation of liver tumors, Eur. J. Radiol. 81 (2012) 1702–1706. [16] S. Leng, J.A. Christner, S.K. Carlson, et al., Radiation dose levels for interventional CT procedures, AJR Am. J. Roentgenol. 197 (2011) W97–103. [17] L. Crocetti, R. Lencioni, S. Debeni, T.C. See, C.D. Pina, C. Bartolozzi, Targeting liver lesions for radiofrequency ablation: an experimental feasibility study using a CT-US fusion imaging system, Invest. Radiol. 43 (2008) 33–39. [18] S.G. Silverman, K. Tuncali, P.R. Morrison, MR-imaging guided percutaneous tumor ablation, Acad. Radiol. 12 (2005) 1100–1109. [19] P.R. Morrison, S.G. Silverman, K.T. Tuncali, S. Tatli, MRI-guided cryotherapy, J. Magn. Reson. Imaging 27 (2008) 410–420. [20] A. Sodickson, P.F. Baeyens, K.P. Andriole, C.T. Recurrent, et al., cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults, Radiology 251 (2009) 175–184. [21] S. Javadi, J.U. Ahrar, E. Ninan, S. Gupta, S.F. Matin, K. Ahrar, Characterization of contrast enhancement in the ablation zone immediately after radiofrequency ablation of renal tumors, J. Vasc. Intervention Radiol. 21 (2010) 690–695. [22] N.C. Paik, Radiation dose reduction in CT fluoroscopy-guided lumbar interlaminar epidural steroid injection by minimizing preliminary planning imaging, Eur. Radiol. 24 (2014) 2109–2117.
Acknowledgments The authors thank Nina Geller, PhD, and the late Paul R. Morrison, MS, for their assistance in preparations of portions of the manuscript.
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021
ARTICLE IN PRESS European Journal of Radiology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol Vincent M. Levesque a,∗ , Paul B. Shyn a , Kemal Tuncali a , Servet Tatli a , Richard D. Nawfel b , Olutayo Olubiyi b , Stuart G. Silverman a a b
Division of Abdominal Imaging and Intervention, Department of Radiology, Brigham & Women’s Hospital, 75 Francis Street, Boston, MA 02115, USA Department of Radiology, Brigham & Women’s Hospital, Boston, MA 02115, USA
a r t i c l e
i n f o
Article history: Received 7 July 2015 Accepted 14 July 2015 Keywords: Cryoablation CT-guidance Radiation dose Dose reduction protocol
a b s t r a c t Purpose: To estimate and compare the radiation dose using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors. Materials and methods: An IRB-approved, HIPAA-compliant retrospective study of 97 CT-guided cryoablation procedures to treat a solitary renal tumor in each of 97 patients (64M, 33F; range 31–84 yrs) was performed. Fifty patients were treated using a standard dose protocol (kVp = 120, mean mAs = 180, monitoring scans every 3 min during freezes), and an additional 47 patients were treated using a dose reduction protocol (kVp = 100, mean mAs = 100, monitoring scans less frequently than every 3 min during freezes). Multiple Wilcoxon Mann-Whitney (rank-sum) tests were used to compare dose-length product (DLP) between the two groups. Fisher’s exact test was used to compare technique effectiveness at 12 months post ablation between the two groups. Results: Median DLP for the standard protocol group was 4833.5 mGy*cm (range, 1667–8267 mGy*cm); median DLP for the dose reduction group was 2648 mGy*cm (range, 850–7169 mGy*cm), significantly less than that of the standard protocol group (p < 0.01). The technique effectiveness for the dose reduction group was not significantly different from that of the standard protocol group at 12 month follow up (p = 0.434). Conclusion: The radiation dose during percutaneous CT-guided cryoablation of renal tumors was substantial in both the standard and the dose reduction groups; however, it was significantly lower with the protocol change that reduced dose parameters and decreased the number of CT scans. © 2015 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Image-guided percutaneous ablation of renal tumors is generally reserved for organ-confined tumors in patients for whom surgery is contraindicated or a minimally invasive option is preferred [1–3]. Thermal ablation techniques such as radiofrequency ablation (RFA) and cryoablation have been used to treat renal tumors, and are usually performed under CT-guidance [1–11]. As intermediate to long-term data are emerging, results for percutaneous CT-guided RFA and cryoablation appear promising and show potential for treating renal tumors effectively [2–11]. CT-guided percutaneous cryoablation of renal tumors has been documented as a well-tolerated procedure with a low rate of
∗ Corresponding author. E-mail address: [email protected] (V.M. Levesque).
complications [2,3,5–7,11]. In contrast to CT-guided RFA, during cryoablation, the effect of freezing (iceball formation) is clearly visualized in the tumor, renal parenchyma, and surrounding soft tissues using unenhanced CT scans. CT scanning is used during the procedure to plan the procedure, target the tumor with cryoablation probes, monitor the enlarging iceball, and survey the region after the ablation to ascertain whether the treatment is complete [2–10]. Typically, several CT scans are used to carry out these four phases of a cryoablation procedure [3,5–7]. The potential health risks associated with ionizing radiation and, in particular, the radiation dose to patients undergoing CT and fluoroscopy has attracted the attention of the medical community [12–14]. This study was conducted in order to estimate and compare the radiation doses using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors.
http://dx.doi.org/10.1016/j.ejrad.2015.07.021 0720-048X/© 2015 Elsevier Ireland Ltd. All rights reserved.
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021
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2. Materials and methods 2.1. Subjects This retrospective study received IRB approval and complies with the HIPPA. Written informed consent was waived. The medical records of 106CT-guided renal cryoablation procedures were reviewed. The records of 57 consecutive renal cryoablation procedures performed from January 2007 through May 2008 using a standard ablation procedure scanning protocol were reviewed. Seven procedures were excluded: five, during which more than one tumor was treated, one, involving a navigation device for targeting that could have influenced the number of CT scans, and one, involving a scanner malfunction that resulted in additional scans. The remaining 50 procedures were performed in 50 patients (33 men, 17 women; mean age, 69 ± 11 yrs, range, 37–84), each with a solitary tumor (mean maximum diameter, 2.3 ± 0.9 cm, range, 0.9–5.0). No procedure was excluded because of poor image quality. There was a 23 month interval between performance of the standard protocol procedures and the dose reduction protocol procedures during which possible dose reduction strategies were considered. The records of 49 consecutive renal cryoablation procedures performed from April, 2010 through December, 2011 using a defined dose reduction scanning protocol were reviewed. Two procedures during which more than one tumor was treated were excluded. The remaining 47 procedures were performed in 47 patients (31 men, 16 women; mean age, 63 ± 12 yrs, range, 31–83) each with a solitary tumor (mean maximum diameter, 2.8 ± 1.0 cm, range, 1.2–5.2). No procedure was excluded because of poor image quality. 2.2. Procedure Cryoablation was performed using an argon gas-based system (CryoHit, Galil Medical, Arden Hills, MN). Under CT-guidance (Sensation Open, Siemens Medical Solutions, Forchheim, Germany), 17-gauge needle-like cryoablation probes were placed by an interventional radiologist. The number and positions of the probes were chosen based on the size of the tumor so that the resultant iceball would encompass the tumor plus a minimum 5 mm margin. Patients received either intravenous moderate sedation or general anesthesia. CT scans were performed during each of four phases of the procedure: planning, targeting, monitoring, and survey. An initial planning CT scan of the tumor and surrounding region was performed with a radio-opaque grid (GuideLines, Beekley Inc., Bristol, CT) placed on the patient’s skin. The location of the tumor was identified from the scan and a skin-entry site selected and marked. Following grid removal, the skin was prepared and draped in a sterile fashion. A reference needle (18-25-gauge) was inserted percutaneously and served as a guide for placing cryoablation probes using tandem technique. During this targeting phase, needle and probe placements were guided by either CT or CT fluoroscopy. CT and CT fluoroscopy were used to visualize the instruments and review adjustments. The targeting phase was complete when all cryoablation probes were in place. In the monitoring phase, cryoablation probes were activated simultaneously. Monitoring was used to confirm that the iceball encompassed the tumor plus a 5–10 mm margin and had not extended into adjacent critical structures. The cryoablation protocol consisted of a 15-minute freeze, followed by a 10-minute thaw, followed by a second 15-minute freeze. Iceball formation was monitored by CT scans obtained during each 15minute freeze. Following completion of the cryoablation protocol, the probes were removed and a final CT scan was acquired to survey the ablation coverage. Slice thickness (5 mm) and rotation time (500 ms) used for the standard group were not changed for the dose reduction group.
During each procedure in both groups, image quality and iceball visualization were deemed acceptable and comparable by the experienced interventional radiologists. Dose parameters were not altered for patient body weight in either group. In the standard protocol group (kVp = 120, mean mAs = 180), the targeting phase for 13 (26%) of 50 procedures included an ancillary component: a pre-ablation needle biopsy (n = 8) or hydrodissection via normal saline injection through additional fine needles (20–22gauge) to displace adjacent critical structures (e.g., bowel) from the treatment site (n = 5). No procedure involved both biopsy and hydrodissection. Multiple cryoablation probes were used (mean, 4, range, 2–6). CT fluoroscopy was used in 14 (28%) of the 50 procedures during the targeting phase to guide the placement of reference needles (n = 5), biopsy needles (n = 1) or cryoablation probes (n = 8). Monitoring scans were obtained every 3 min during each freeze phase. All 50 procedures were delivered according to the standard protocol plan, and were technically successful, i.e., the iceball included the entire tumor and a minimum margin of 5 mm [3]. In the dose reduction protocol group (kVp = 100, mean mAs = 100), the targeting phase for 16 (34%) of the 47 procedures included an ancillary component: a needle biopsy (n = 11) or hydrodissection (n = 5). No procedure involved both biopsy and hydrodissection. Multiple cryoablation probes were used (mean, 4 ± 1, range, 2–8). CT fluoroscopy was used in 40 (85%) of the 47 procedures in the targeting phase to guide the placement of initial reference needles, biopsy needles, or cryoablation probes. Instead of obtaining monitoring scans every 3 min as in the standard protocol, monitoring scans were obtained during each freeze only as deemed necessary by the interventional radiologists. All 47 procedures were delivered according to the dose reduction protocol plan, and were technically successful with iceball coverage of the entire tumor and a minimum margin of 5 mm [3]. At 12 month post-ablation follow up, technique effectiveness of all procedures was reviewed. Technique effectiveness was defined by “no evidence of recurrent disease” on imaging at 12 months postablation. Any case with evidence of recurrence that could require a second ablation was noted. 2.3. Data analysis Using chi-square tests, with two-sided p-values reported at a preset significance (␣) level of 0.05, the baseline distribution of gender, age, tumor size, number of cryoprobes used, and performance of an ancillary procedure were confirmed as comparable for the standard and dose reduction protocol groups (p > 0.05) (Table 1). Table 1 Radiation dose using a standard protocol and that of a dose reduction protocol in patients undergoing CT-guided percutaneous cryoablation of renal tumors: comparison of study groups.
Gender Female Male Age (years) ≤60 >60 Tumor size (cm) ≤2 >2, <4 ≥4 Number of cryoprobes used ≤3 ≥4 Ancillary procedure performed Yes No
Standard protocol
Dose reduction protocol
17 33
16 31
9 41
14 33
23 24 3
13 30 4
15 35
18 29
13 37
16 31
P 0.996
0.173
0.174
0.389
0.387
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Table 2 Median dose-length product (DLP) for the standard and dose reduction protocols in patients undergoing CT-guided percutaneous cryoablation of renal tumors. Procedure phase
Median standard protocol DLP
Median dose reduction protocol DLP
P
Total Targeting Monitoring
4833.5 mGy*cm 2087 mGy*cm 1733 mGy*cm
2648 mGy*cm 1092 mGy*cm 866 mGy*cm
<0.01 <0.01 <0.01
The total dose-length product (DLP) for each group was calculated using the total radiation dose of each procedure: the sum of the scanner’s calculated DLP of all CT and CT fluoroscopy acquisitions. The DLP was also calculated separately for the individual CT and CT fluoroscopy acquisitions during the targeting and monitoring phases for each group. The median DLPs for the targeting and monitoring phases of the two groups were compared for subanalysis as these were the only phases in which the protocols differed. Multiple Wilcoxon (rank-sum) Mann–Whitney tests were used to compare total DLP, targeting phase DLP, and monitoring phase DLP, between the two groups after confirming that the dose distribution patterns differed from a normal distribution pattern. Fisher’s exact test was used to compare technique effectiveness at 12 months post ablation between the two groups. Only 2-sided p-values at preset significant level (␣) were reported. All statistical analyses were conducted using STATA Version 11.2 (StataCorp LP, College Station, Texas). 3. Results The median overall DLP for the standard protocol group was 4833.5 mGy*cm (range, 1667–8267 mGy*cm). The median overall DLP for the dose reduction protocol group was 2648 mGy*cm (range, 850–7169 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). In the standard protocol group, the median DLP in the targeting phase was 2087 mGy*cm (range 380–4951 mGy*cm). The median DLP in the monitoring phase was 1733 mGy*cm (range 667–3200 mGy*cm) (Table 2). In the dose reduction group, the median DLP in the targeting phase was 1092 mGy*cm (range, 220–4998 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). The median DLP in the monitoring phase was 866 mGy*cm (range, 158–2479 mGy*cm), significantly lower than that of the standard protocol group (p < 0.01) (Table 2). At 12 month follow up, the technique effectiveness for the standard protocol group (n = 50) was 100% (43/43 procedures; five patients were lost to follow up and two patients had no recurrence with follow up of less than 12 months). At 12 month follow up, the technique effectiveness for the dose reduction group (n = 47) was 97% (32/33 procedures; seven patients were lost to follow up and seven patients had no recurrence with follow up of less than 12 months). The technique effectiveness for the dose reduction group was not significantly different from that of the standard protocol group at 12 month follow up (p = 0.434). 4. Discussion Reports describing radiation dose from CT-guided liver tumor ablation procedures suggest an almost two-fold greater radiation exposure during cryoablation compared to RFA procedures [15–16]. One of these reports also found almost three times the radiation exposure from CT-guided renal cryoablation procedures compared to CT-guided liver RFA procedures [16]. In our study, using DLP as an estimate, the radiation dose of CT-guided renal cryoablation was substantial (reaching as high as 8267 mGy*cm) and varied over a wide range. Cryoablation typically requires placement of a greater number of probes than RFA procedures, thereby requiring an increased number of scans. Some
procedures required only two or three probes to treat smaller tumors with no adjacent critical structures. Others required five or six probes to treat larger tumors that were close to critical structures such as the bowel and ureter. The number of probes was determined by the need to achieve an iceball of sufficient size to cover the tumor plus a minimum margin of 5 mm [3]. Some small tumors were difficult to target requiring additional scans because of their small size or particular location. Other small tumors were not sufficiently well defined on unenhanced CT scans and therefore, warranted more probes and larger iceballs. Others were located centrally near vasculature, and more probes were used to counteract heat sink effects. We analyzed both the total DLP values incurred during the entire procedure, and separately, the component DLP values incurred during the targeting and monitoring phases in both groups. The targeting and monitoring phases were the portions of the procedures primarily impacted by the dose reduction protocol. We did not compare the DLP values in the planning and surveying phases of the two groups because the protocol for these phases was not changed and these phases contributed a substantially lower radiation dose than the targeting and monitoring phases. Radiation during the targeting phase was an important contributor to total radiation dose. Using CT fluoroscopy during the targeting phase helped minimize the number of helical CT scans required. Indeed, the increased use of CT fluoroscopy during targeting in the dose reduction protocol was accompanied by a significant decrease in radiation dose compared to that of the standard protocol group. CT fluoroscopy was used in 85% of cases in the dose reduction protocol group, but only in 28% of cases in the standard protocol group, suggesting that the judicious use of CT fluoroscopy during the targeting phase can reduce both the number of helical CT scans required and the overall radiation dose to the patient. A key advantage of cryoablation relative to other ablation technologies is the ability to visualize the entire extent of the ablation throughout the procedure, allowing interventional radiologists to treat the tumor completely while minimizing damage to adjacent critical structures. However, this advantage carries with it the added radiation dose of monitoring CT scans. The radiation dose reduction protocol employed in this study provides a markedly reduced radiation dose to patients. The dose reduction was attributable to lower CT tube potential and fewer CT scan acquisitions in the dose reduction protocol group. During the monitoring phase of procedures in the dose reduction protocol group, the iceballs remained well-depicted using the lower tube potential, and did not compromise the ability to achieve a technically successful ablation as the interventional radiologist always had the option of scanning more frequently when necessary. Technique effectiveness for both groups was high and demonstrated that our dose reduction protocol reduced radiation dose to the patient without sacrificing long term desirable outcomes. Although not used in this study, ultrasound can be used adjunctively to expedite the real-time targeting of a tumor without ionizing radiation exposure to the patient. Some tumors are not visible with ultrasound, and ultrasound is not ideal for monitoring cryoablation because the iceball is largely obscured by acoustic shadowing. Other strategies to reduce radiation dose during CTguided cryoablations may include the use of a computer-based navigational device to reduce the number of targeting scans [17,18].
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021
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Since the entire iceball is visible also with MRI, ionizing radiation exposure can be totally eliminated by performing the entire procedure using MRI-guidance [6,11,19]. Finally, it should be noted that the radiation dose incurred during an ablation procedure may constitute only a fraction of the total cumulative radiation dose a patient receives in the course of care. It is likely that many patients treated with CT-guided cryoablation also undergo a diagnostic CT scan, a pre-ablation CT-guided biopsy, and several follow-up CT scans [20,21]. Furthermore, some patients may require additional cryoablation procedures, either to retreat a recurrent tumor or to treat other renal tumors. In view of our findings, we suggest that radiologists performing CT-guided percutaneous cryoablations assess their techniques and implement methods to limit radiation exposures to levels that are as low as reasonably achievable. A limitation of this study was that radiation dose was estimated retrospectively using DLP. We acknowledge that DLP does not accurately quantify dose to an individual patient. However, we believe DLP can serve as a reference value against which other studies of radiation dose can be compared, particularly those that evaluate the effects of dose reduction strategies. DLP has been used in a recent study to estimate radiation dose to patients undergoing CT-guided interventional procedures [22]. 5. Conclusion The findings of this study can be expected to contribute to the existing knowledge in the field of CT-guided percutaneous cryoablation of renal tumors by demonstrating that (a) potentially substantial radiation doses are received by patients during CTguided percutaneous cryoablation of renal tumors; (b) radiation dose reductions are achievable by decreasing tube current and kilovoltage of targeting and monitoring scans and reducing the number of monitoring scans; and, (c) further dose reductions are possible through the use of CT fluoroscopy during the targeting phase so as to limit the number of helical CT scans acquired. The findings of this study have an important implication for patient care: the potential radiation doses received by patients during CT-guided percutaneous renal cryoablation procedures should alert interventional radiologists of the need for adopting procedural techniques and dose reduction protocols that limit radiation dose. Conflict of interest None
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Acknowledgments The authors thank Nina Geller, PhD, and the late Paul R. Morrison, MS, for their assistance in preparations of portions of the manuscript.
Please cite this article in press as: V.M. Levesque, et al., Radiation dose during CT-guided percutaneous cryoablation of renal tumors: Effect of a dose reduction protocol, Eur J Radiol (2015), http://dx.doi.org/10.1016/j.ejrad.2015.07.021