First published online June 22, 2006
British Journal of Radiology (2006) 79, 730-733
© 2006 British Institute of Radiology
doi: 10.1259/bjr/75766147
Six years experience in intracoronary brachytherapy procedures: patient doses from fluoroscopy
C Prieto, BSc1,
E Vano, PhD1,4,
J M Fernández, BSc1,4,
C Galvan, MD, PhD2,4,
M Sabate, MD, PhD3,
L Gonzalez, PhD4 and
D Martinez, BSc1
1 Medical Physics Service, 2 Radiotherapy Service and, 3 Interventional Cardiology Service, San Carlos University Hospital, 28040 Madrid, 4 Radiology Department, Medicine School, Complutense University, 28040 Madrid, Spain
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Abstract
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Typical patient dose levels during intracoronary brachytherapy (ICB) procedures using beta sources were determined across a sample of 221 treatments. Dose–area product values, fluoroscopy time and number of frames per procedure, with median values of 62 Gy cm2, 17.0 min and 1493 images, respectively, resulted in a 20% to 50% increase in the values measured for percutaneous transluminal coronary angioplasty procedures in the same medical centre (median values 41 Gy cm2, 14.3 min and 1078 images). Likely reasons for this increase include the additional complexity of ICB, the need for recording and reporting every step of the treatment, getting the essential parameters for the volume determination of the lesion and therapeutic radiation dose calculation and, finally, the learning curve for this kind of procedure. A high concentration skin dose distribution during ICB procedures was measured and in 12% of the patients peak skin doses higher than 1.5 Gy were confirmed. 10 patients were submitted to clinical follow-up and skin injuries were not identified.
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Introduction
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Restenosis, or re-narrowing of a coronary artery after dilatation, is the main limitation of percutaneous transluminal coronary angioplasty (PTCA). Stents have substantially reduced the incidence of restenosis, but neo-intimal proliferation within the stent is still a great problem for a significant percentage of patients. Intracoronary brachytherapy (ICB) is indicated in patients with in-stent restenosis [1].
ICB includes delivery of a local radiation dose to the artery wall (target volume) after stenting. There are several potential problems in the safety aspects for the practice of ICB procedures: (a) procedures more complex than standard PTCA procedures in catheterization laboratories; (b) introduction of brachytherapy equipment not previously used, resulting in a new challenge in radiological protection; and (c) coordination among different specialists (cardiologists, radiotherapists and medical physicists working in the catheterization laboratory). These aspects affect safety of both the patient and staff involved.
Complex interventional cardiology procedures can produce deterministic effects (skin injuries) due to the high radiation doses imparted to some regions of the patient's skin [2–4]. PTCA is one of the most frequent interventional procedures in cardiology, and sometimes requires long fluoroscopy times and a large number of cine frames to document the patient's lesion and the result of the treatment. The radiation field is usually "concentrated" in one or two specific areas of the skin, usually between 60 cm2 and 80 cm2 (depending on the X-ray beam projection, image intensifier field size and collimation carried out by the cardiologist). Because of the high restenosis rate, a certain number of patients require several coronary angiographies and PTCAs, contributing to the increase in skin irradiation, sometimes over intervals of several months or years [5]. ICB procedures exhibit a higher concentration of fields than other interventional cardiology procedures [6], and an increased dose–area product (DAP), fluoroscopy time (FT) and number of frames (NF) [7]. The need for recording and reporting every step of the treatment, the essential parameters for target volume determination, and therapeutic dose calculations [8, 9] are important factors to explain this increment. Therefore, the estimation of the risk of deterministic effects (skin injuries) in ICB procedures is an aspect of radiation protection that should be considered as part of the quality assurance (QA) programme.
In this work, the outcomes in patient radiation protection aspects from our 6 years experience with a sample of 221 ICB procedures are shown.
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Methods and materials
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A total of 221 patients underwent ICB procedures between November 2000 and May 2005 with beta sources from a Novoste system (www.novoste.com, Norcross, Georgia, USA) in 73 cases, and a Guidant system (www.guidant.com, Indianapolis, USA) in 148 cases. Intravascular ultrasound (IVUS) was used in a significant number of patients to quantify the lesion and to determine the clinical dosimetry parameters for the brachytherapy procedures. DAP values, together with FT and NF were recorded in 202 of the ICB patients as relevant dosimetric parameters. ICB procedures were carried out in three dedicated X-ray interventional cardiology rooms equipped with Philips Integris 3000 and 5000 systems (Philips, Best, The Netherlands; all with high filtration fluoroscopy pulsed modes and with the routine use of wedge filters) by one expert interventional cardiologist in collaboration with a radiotherapist and a medical physicist.
The X-ray systems were under a QA programme, including periodic constancy checks to evaluate incident air kerma at the entrance of the image intensifier and entrance surface air kerma for different thicknesses of polymethylmethacrylate (PMMA) and copper, following the protocol proposed by the European DIMOND consortium [10].
To evaluate the increase in dose and procedure complexity in ICB, DAP, FT and NF were also recorded for 1707 PTCAs during the same period and using the same X-ray systems.
DAP was measured with the built-in calibrated ionization transmission chambers (PTW, Freiburg, Germany). Skin dose distributions were measured using slow film Kodak X-OMAT-V initially, using the procedure previously described [11] and the new EDR2 [12] introduced in 2002 (Kodak, Rochester, NY). For both types of film, thermoluminescent dosimetry (TLD) was used additionally to evaluate some high dose values and for autocalibration of the films. TLD-100 (LiF:Mg,Ti) chips and a Harshaw TLD/Bicron/NE-Technology (BICRON-NE, Solon, OH) reader were used.
Digital recording of all the cine images in DICOM (Digital Imaging and Communications in Medicine) format allows, in some complex cases when saturation of the slow film occurred, a complementary analysis of the skin dose distribution using the technical parameters of all the cine series acquired during the procedures [13]. These parameters were retrieved throughout using an updated version of an ad hoc specific software [14]. DICOM header in the cine series recorded by Philips systems allows us to identify the X-ray beam projection (left–right and craniocaudal angulations), image intensifier field size, kilovolts and milliamperes per frame, and distance from the focus to the image intensifier entrance during the different cine series.
Methodology for the evaluation of skin dose distribution and peak skin dose (PSD) has been developed by the authors [11, 12]. As a part of the clinical follow-up of patients undergoing ICB procedures, a specific protocol has been developed in the framework of the DIMOND III project [15, 16] to detect possible deterministic effects on the skin of patients whose slow film pattern shows densities corresponding to doses above 1.5 Gy. Clinical follow-up was initiated when PSD exceeded 2 Gy, DAP values exceeded 180 Gy cm2 or whenever it was recommended by other medical circumstances (e.g. previous procedures, special skin radiosensitivity). In these cases, the interventionist should arrange for review of the patient between 10 days and 14 days after the procedure. The use of these triggering levels for the clinical follow-up respond to the likely inability to verify whether the maximum skin dose exceeded the threshold for deterministic effects when slow film is clearly saturated and no TLD chips had been placed in the region of high dose. The purpose of this review was to identify skin effects. A record of the FT, NF and DAP is part of the documentation of each patient reviewed.
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Results
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Dosimetric aspects of the X-ray procedure
Table 1
shows a comparison between DAP, FT and NF in PTCA and ICB procedures in the period 2000–2005.
Previous initial data published for the period 2000–2001 [6, 7] were similar for PTCA, but greater for ICB. The reduction of doses during ICB procedures is shown in Figure 1 for the years 2001–2004. The years 2000 and 2005 have been excluded as few patients were treated in those years.
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Figure 1. Evolution of median values of dose–area product (DAP) in intracoronary brachytherapies (ICBs) during the period 2001–2004. Detail of the distribution (frequency histogram) of DAP in ICBs.
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Patient skin dose
Skin dose is generally quite widely distributed in cardiology procedures, but in some procedures a certain area of the skin could receive a higher dose if the visualization of the lesion to be treated requires maintaining a fixed orientation of the X-ray beam. The introduction of a "concentration factor" has been proposed in a previous paper [5] to consider this aspect. The concentration factor has been defined as:
where PSD is the "peak skin dose" and ASD is the "average skin dose", obtained as the quotient of DAP and the total irradiated area S (measured from the slow film).
Figure 2 shows a mosaic of several selected examples of slow film for PTCAs and for ICB. Note the highest concentration of the irradiation (more density in some areas of the films) for the ICB procedures. Conversely, note the greater number of projections (with different C-arm angulations) for PTCA procedures.
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Figure 2. Mosaic of several selected examples of slow films used to measure skin dose distribution for percutaneous transluminal coronary angioplasties(PTCAs) and for intracoronary brachytherapies (ICBs).
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DAP greater than 180 Gy cm2 was measured in 13% of all the procedures over the whole study period, although this threshold was surpassed more frequently during the introduction of this kind of procedure (27% of the cases in the period 2000–2001).
For 172 patients (78% of patients treated) skin dose distribution was measured. In 12% of the patients, peak skin doses higher than 1.5 Gy were measured (one patient with 4.6 Gy), but only 10 of these patients were effectively reviewed, as some patients belonged to other health areas and some had died. No radiation skin injuries in ICB procedures were found during follow-up examinations.
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Discussion
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It is clear that ICB produces a higher DAP, FT and NF, due to the complexity of the procedure, the need for recording and reporting every step of the treatment and essential parameters for volume determination of the lesion, and therapeutic dose calculation to the target volume [8, 9]. IVUS is advisable [8] to evaluate in detail the lesion and to obtain the basic data for clinical dosimetry of the brachytherapy treatment. However, its use implies an increase in the fluoroscopy time. The control of the right position of the radioactive source in the lesion and the removal of the source also require extra fluoroscopy time and filming series. Sometime, when "stepping" (irradiation of the lesion in several steps because of the extension of the lesion plus appropriate safety margins) is required, the procedure could become even longer.
The correct position of the radioactive source in the lesion is one of the basic aspects of the quality of ICB. Sabate et al [17] state that a recognized limitation of endovascular beta-radiation therapy is the development of new stenoses at the edges of the irradiated area. The term "geographic miss" is used to define cases in which the radiation source did not fully cover the injured area. To avoid this problem, fluoroscopy guidance throughout the procedure and additional cine series to confirm the correct position of the radioactive source are required. Images recorded allowed explanation of some unsuccessful treatments as due to geographical miss.
The difficulty in performing IVUS in the vessel to be treated, the difficulty in positioning the radioactive source, and the need to use some specific and fixed projections to correctly document the position of the source, increase the "concentration factor" of radiation in some specific skin areas [6]. In these situations, the total DAP could not always be a good indicator of the level of risk for deterministic effects (skin injuries), as previously highlighted [5].
The preliminary European reference level [18] for PTCA is 94 Gy cm2, which is well above our median values for both PTCAs and ICBs (Table 1
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The reduction of median DAP for ICB along the period studied may be due to greater experience and confidence by the staff involved, the effect of periodical training in radiation protection highlighting special aspects of ICB [19], and the use of IVUS in fewer patients (73% of patients in 2000–2001 compared with 53% in 2002–2005).
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Conclusions
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ICB leads to an increase in patient dose during the procedure in comparison with PTCA. The patient dose and skin dose increase because of the increase in the complexity of the procedure, the need for recording and reporting every step of the treatment and essential parameters for volume determination and the therapeutic dose calculation. The use of IVUS to quantify the lesion and to determine the clinical dosimetry parameters for the brachytherapy procedures may also involve an increase in patient dose due to the increased fluoroscopy time. Finally, the learning curve with new techniques implied a greater increment with respect to conventional PTCA procedures in the first period of this study.
Continuous training in radiation protection highlighting special aspects of ICB together with greater experience and ability in these kinds of procedures also contributed to shortening the difference in patient dose between ICB and PTCA procedures.
The extra dose associated with ICB procedures and greater skin dose concentration factor may lead to further skin injury problems. Notwithstanding, in our institution, with the X-ray systems submitted to rigorous quality assurance programmes and with the optimized technical and clinical protocol, this extra dose has not been of special concern for patient skin injuries during the reported period. Nevertheless, in those cases in which a threshold dose is reached, a specific clinical follow-up protocol is advisable.
During the study period, satisfactory outcomes and acceptably low radiation doses to the skin of the patients were confirmed.
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Acknowledgments
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This study was partially funded under the European Commission DIMOND III project (FIGM-CT-2000-00061), and Coordination Action SENTINEL (FI6R-012909). Funding was also provided by the National Program for Scientific Research, Development and Technological Innovation of the Spanish Department for Science and Technology (project BFI2003-09434) and by the Autonomous Community of Madrid (project GR/SAL/0272/2004).
Received for publication August 18, 2005.
Revision received October 17, 2005.
Accepted for publication October 31, 2005.
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References
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Abstract
Introduction
