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Skin dose and dose–area product values in patients undergoing intracoronary brachytherapy

¹ Medical Physics Service and ² Interventional Cardiology Service and ³ Radiothereapy Service, San Carlos University Hospital, 28040 Madrid, Spain and ⁴ Radiology Department, Medicine School, Complutense University, 28040 Madrid, Spain

	Abstract

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Entrance skin doses, dose–area product (DAP) values, fluoroscopy times and digital cine acquisition data were measured for 86 patients undergoing intracoronary brachytherapy procedures with beta sources, to estimate risk of skin injuries. Interventions were carried out in three dedicated X-ray interventional cardiology rooms equipped with X-ray systems operating in pulsed modes, with high filtration and edge filter options. Skin dose distribution was analysed in detail in 56 patients using slow films and thermoluminescent dosimetry. Digital recording of Digital Imaging and Communications in Medicine cine images also allowed analysis of the technical parameters used throughout the procedures. A protocol for clinical follow-up of these patients at the cardiology service is also presented, which prescribes special attention when a threshold dose is reached. Median values for DAP, fluoroscopy time and number of frames were 81.2 Gy cm², 17.5 min and 1569 frames, respectively, and maximum values were 323.3 Gy cm², 46.2 min and 3213 frames, respectively. In two cases, maximum skin doses in a procedure reached 3.5 Gy and 4.6 Gy. Comparing median values in this study, intracoronary brachytherapy involved approximately two-fold the DAP used in percutaneous transluminal coronary angioplasty procedures performed during the same period in the same catheterization laboratories, as a consequence of the need to monitor the radioactive source location used for the treatment of stenoses and the intravascular ultrasound. Special care must be paid in those cases of high dose in relation to potential patient skin injuries and late effects.

	Introduction

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Interventional radiology (IR) and interventional cardiology (IC) procedures can produce skin injuries as typical deterministic effects, resulting from high radiation doses imparted to some areas of the patient's skin [1–5]. This undesirable effect has been of special concern to several national and international organizations [1, 6, 7]. In Europe, the Medical Exposures Directive [8] includes IR as a "special practice" and requires Member States to ensure the use of appropriate radiological equipment, practical techniques and ancillary equipment and to ensure that special attention be given to the quality assurance (QA) programmes, including quality control (QC) measures and patient dose assessment.

In cardiology procedures, patient dosimetry is a difficult task owing to the relatively unpredictable irradiation of skin areas and different X-ray beam projections, field sizes, radiation qualities and focus-to-skin and focus-to-image intensifier distances. Furthermore, some X-ray facilities include the option of introducing high absorption copper filters to reduce patient dose further on the basis of beam hardening. Therefore, the patient skin dose distribution cannot easily be obtained from technical parameters such as the tube potential (kVp), tube current (mA) pulse time and distances used throughout the intervention. New software has been developed [9] to measure, in real time, the dose applied to each 1 cm² area of skin, taking into account movement of the X-ray source and changes in beam characteristics. In the future, this may be one of the best solutions with which to audit skin doses during procedures.

Percutaneous transluminal coronary angioplasty (PTCA), the most frequent intervention in cardiology, is sometimes a very complex procedure requiring long fluoroscopy times and large numbers of cine frames to document the patient's lesion and results of a procedure. The radiation field is usually focused on one or two specific areas of the skin of between 60 cm² and 120 cm², depending upon the projection of the X-ray beam, image intensifier (II) field size and the collimation carried out by the cardiologist to quantify the stenosis. A certain number of patients require several coronary angiographies and PTCAs because of the high restenosis rate, and further skin irradiation can therefore be expected within an interval of some months or a few years [10].

The complexity of this kind of procedure [11, 12] and, sometimes, patient size are the main reasons for large doses to the skin, which can involve risk of skin injuries. The "cutaneous syndrome" refers to a number of pathologies that may be apparent after exposure of the skin to ionizing radiation. Development of lesions may take days or years. The latent period for manifestation of a specific pathology depends upon the characteristics of the target cells and the radiation dose delivered. The intensity and duration of the lesions are also dose dependent. Erythema, loss of tactile sensation and itching have an onset time from hours to 30 days. Blistering, swelling, oedema and desquamation can appear between 5 days and 8 weeks if doses are sufficiently high. Hair loss has an onset time of 2–8 weeks. Hyperpigmentation or depigmentation, atrophy, keratosis, fibrosis and telangiectasia are considered late effects and would appear after 12 months.

In addition, ionizing radiation is also a carcinogen. Radiation induced cancers do not appear immediately after radiation exposure but require time, the latent period, to become clinically apparent. Mean latent periods are 7 years for non-chronic lymphocytic leukaemia and more than 20 years for most other cancers [13]. Mean age of the patients submitted to procedures studied in this paper (66±11 years in our sample; 78% males, 22% females) and the importance of patient heart pathology usually render this stochastic effect less important.

The use of new treatment techniques, such as intracoronary brachytherapy (ICB), can involve a substantial increase in the risk of skin injuries. ICB with beta radioactive sources does not entail any extra significant skin dose as the result of an increase in emitted radiation. However, because it is necessary to record and report every step of the treatment and the parameters essential for the calculation of volume and dose [14, 15], the duration of the full procedure is extended. First, intravascular ultrasound (IVUS) is recommended [14] to evaluate the lesion in detail and to obtain the basic data for clinical dosimetry, but its use entails an increase in fluoroscopy time. Second, guiding the radioactive source, monitoring its correct positioning in the target lesion and removing it also require extra fluoroscopy time and filming series. Furthermore, when the extension of the lesion plus the appropriate safety margins exceeds the reference isodose length of the radioactive source, irradiation of the lesion is made in several steps (the "stepping" technique), making the procedure even more complex.

The peak skin dose or maximum skin dose (MSD) is very difficult to predict, especially in therapeutic procedures. Therefore, total DAP is not always a good estimator of the risk of deterministic effects [16]. In ICB, the ratio between DAP and MSD is even more difficult to establish. In fact, the specific difficulties of performing IVUS in the vessel to be treated and of positioning the radioactive source, force the X-ray beam to be focused on given skin areas for running the procedure and documenting the source setting. Thus, spread in the irradiated areas, characteristic of IC typical procedures, becomes altered. 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. A "geographic miss" in the location of the radiation source should be avoided, and this requires a detailed fluoroscopically guided procedure and additional cine series to confirm that the radioactive source has fully covered the injured area. Therefore, proper placement of the radioactive source in the lesion is one of the fundamental aspects determining quality of ICB.

To the best of our knowledge, no evaluation of the risk of deterministic effects in patients undergoing ICB procedures is available in the scientific literature. This paper reports values for entrance skin dose, DAP, fluoroscopy time and digital cine acquisition in patients undergoing intracoronary brachytherapy. A protocol for clinical follow-up of these patients that considers potential skin injuries is also presented.

	Materials and methods

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A total of 86 patients were treated with ICB between November 2000 and February 2002 using beta sources from Novoste (Novoste, Norcross, GA) and Guidant (Guidant, Indianapolis, IN) systems. IVUS was used in almost all patients to quantify the lesion and to determine clinical dosimetry parameters for brachytherapy procedures. In addition to DAP values, skin dose distribution has been measured in 56 patients. Procedures were performed in three dedicated X-ray IC rooms equipped with Philips Integris 5000 systems (Philips Medical Systems, Best, the Netherlands), and Integris 3000 systems updated to 5000 systems, operated in fluoroscopy pulsed mode with high filtration and edge filter options. Wedge filters were used routinely by every cardiologist conducting a procedure to improve image quality and reduce the dose applied to the irradiated fields.

Periodic constancy checks included in the QA programme to evaluate doses at the II entrance and doses at the entrance to standard absorbers (copper filters and polymethylmethacrylate (PMMA) were made, following the protocol proposed by the European Digital Imaging: Measures for Optimising Radiological Information and Dose consortium [18].

Skin dose distributions were measured using Kodak X-OMAT-V and EDR2 slow films (Kodak, Rochester, NY) [19]. The operation of the automatic processor (Kodak X-Omat-M6B with Kodak chemicals) used throughout this work was also optimized under the QA programme. Thermoluminiscent dosimetry (TLD) chips from Harshaw TLD/Bicron/NE-Technology (BICRON-NE, Solon, OH) were also placed in contact with each film in fixed positions for dose measurements. DAP was measured with calibrated ionization transmission chambers (PTW, Freiburg, Germany).

X-ray sensitometry was carried out to calibrate films for the X-ray beams and filtration usually employed in IR and IC. The packaging of these film types does not need darkroom loading. Automatic processing is recommended. Density readings were made with a digital Victoreen 07-424 densitometer (Nuclear Associates Victorian, Cleveland, OH). The characteristic curve of the X-Omat V film allows evaluating doses from 10 mGy to 500 mGy, with a linear range of optimal behaviour between 20 mGy and 200 mGy, where errors in dose determination are below 20% [19]. A similar curve was obtained for the new EDR-2 film. It shows saturation at approximately 1.2–1.5 Gy, and uncertainty at the highest side of the calibration curve is approximately 30–40%. For doses in this dose range or higher, only thermoluminiscent chips allow a realistic estimation of the MSD, if properly placed in the most irradiated area.

Digital recording of all cine images in Digital Imaging and Communications in Medicine (DICOM) format allowed the analysis of the technical parameters of the acquired cine series.

Of each Philips system, the following data was accessible from the DICOM header of the patient files: X-ray beam projection (left–right and craniocaudal angulations), II field size, kVp and mA and pulse time per frame, and distance from the focus to the II entrance in every cine series. From this data the entrance surface dose may be calculated. Distance from the focus to the patient skin is assumed to be 65 cm on average. This calculation can be performed in some cases to estimate skin doses for some X-ray beam projections, if the slow film is saturated and thermoluminescent dosemeters do not exist in such an irradiated area. In these cases, the corresponding proportional percentage of fluoroscopy dose is added to this skin area. Figure 1 shows a scheme that allows comparison of the skin dose distribution measured using slow film with a plot from the DICOM header, depicting the different X-ray beam angulations.

View larger version (57K): [in this window] [in a new window]   Figure 1. A typical information exploit from the Digital Imaging and Communications in Medicine (DICOM) record of an intervention, showing graphical details. The upper plot shows the angular leaning coordinates of the cine series recorded. (b) The digitized picture of the slow film shows the corresponding fields and other from intermediate fluoroscopy projections. The film image has been adapted in size to approximately match the dimensions of the DICOM header graph. Angular data for every projection are referred to the II entrance position, whereas radiation enters the patient in the reverse direction. Arrows in the image on the left indicate where both information details come from; image (a) from the short arrow and image (b) from the long arrow. The image on the right shows the real size and location of the slow film on the patient. In this example, the entire fields recorded by the film only reach the projections around +50° lateral, -30° and +32° in craniocaudal projection, with reference to the vertical. Beyond the fields located at +50° lateral, -20° and +25° craniocaudal (left side of the slow film, on the horizontal midline) one can see the right border of the about +80° lateral projection (the projection at near +95° lateral, +8° craniocaudal does not appear).

Osiris software (Hopitaux Universitaires de Génève and Hôpital Cantonal, Geneva, Switzerland) was used to analyse the digitized films. Calibration of pixel content with dose was performed by irradiating a film with known doses at the usual X-ray beam qualities. Osiris allows the choice of regions of interest by selecting isodose curves in irradiated areas; the area surface within a given isodose can then be calculated.

A team consisting of an interventional cardiologist, a radiotherapist and a medical physicist participated in each procedure. A clinical follow up protocol for possible skin injuries has been locally adapted from recommendations of the International Commission on Radiological Protection [1] and the World Health Organization [6], also taking into account a recent document on medical management of radiation accidents [20]. The latter reference recommends that patients with suspected radiation exposure of unknown severity should be referred to a medical service capable of identifying radiation induced effects.

According to the QA programme of the centre, the first step in the clinical follow-up to assess possible skin injuries is the responsibility of the radiotherapy team involved in the ICB procedure, and a dermatologist if necessary. Erythema is one of the symptoms considered particularly relevant. Special emphasis is given to inspection, before the procedure of the skin on the patient's back to evaluate possible radiation effects from earlier coronary angiographies and PTCAs.

Another clinical review is made later on and when patients return to the hospital for clinical follow-up at the IC unit. The later reviews of patients' skin are performed if skin doses exceed 2 Gy or DAP values exceed 180 Gy cm² during the procedure. Both parameters must be considered to allow for the inability to verify whether the MSD exceeded the threshold for deterministic effects when slow film is clearly saturated and no TLD chips have been placed properly in a high dose area. In some cases, lower DAP values may also warrant investigation if slow film reveals high concentrations in the radiation fields. Figure 2 and Figure 3 show the skin dose distribution for the ICB procedures described in the last two rows of Table 1, and serve to illustrate the above points.

View larger version (104K): [in this window] [in a new window]   Figure 2. Slow film from a procedure showing saturation (darkest region), as well as a fairly high radiation field concentration. The image corresponds to patient GCM260701 (Table 1) with dose–area product of 193 Gy cm2 and maximum skin dose of 3.5 Gy.

View larger version (48K): [in this window] [in a new window]   Figure 3. (a) Slow film corresponding to patient MIIM011212 (Table 1) with dose–area product of 185 Gy cm2 and maximum skin dose of 4.6 Gy. (b) Isocontours of 30% and 99% (film saturation) corresponding to the analysis of the digitized slow film of patient MIIM011212 (Table 1).

Finally, concentration factor was calculated in some skin dose distributions as a way of evaluating the potential risk of deterministic effects. Concentration factor is defined as the ratio between MSD and "average skin dose" [16]. This last parameter is the quotient of DAP and the total irradiated area, which is measured on slow film. To evaluate the total irradiated area, the 30% isocontour of total pixel content in the digitized slow film is used in all cases (Figure 3b), making some approximations when needed to allow for lateral projections not included and other projections not fully included in the film (see top in Figure 3b).

	Results

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Table 2 shows relevant data from this study for DAP, fluoroscopy time and number of frames. Figure 4 shows frequency plots of the same parameters.

View larger version (29K): [in this window] [in a new window]   Figure 4. (a–c) Frequency plots of dose–area product, fluoroscopy time and number of frames from Table 2.

	Discussion

Top Abstract Introduction Materials and methods Results Discussion References

 
In general, skin dose distributions for ICB exhibit a "concentration factor" higher than those of other IC procedures (previously measured values [16] show a mean value of 2.9 and a maximum of 10). This parameter can provide a useful risk estimate for each particular intervention, as it embraces information regarding both MSD and DAP. However, further experience is required in relating concentration factor to probable deterministic effects. It should be kept in mind that many of these patients undergo several procedures in short time periods, thus a single concentration factor data would be unable to explain some likely deterministic damages.

The case shown in the last row of Table 1 illustrates a situation in which DAP had to be estimated as the measurement was only possible during a part of the procedure. Until such a moment, the DAP was approximately due 50% to fluoroscopy and 50% to cine. In addition, some series for this patient were deleted before sending the study to the Picture Archiving and Communication System (PACS). As the total number of series is known, by the cardinal number of the series stored, the total DAP and total number of frames have been estimated by assuming for the missing series the average values found in the stored series (number of frames per series, dose per frame and corresponding percent of fluoroscopy time and dose).

For the total sample of 86 patients, DAP median values for ICB are approximately two-fold higher than those measured for PTCA at our centre during 2000–2001. However, median DAP values remain below values reported elsewhere. For instance, Van de Putte et al [22] have recently reported DAP values for 100 patients undergoing four types of IC procedures. Median values for coronary catheterization (single), coronary catheterization with left ventricle investigation, and PTCA without and with stenting were 56.82 Gy cm², 106.32 Gy cm², 108.80 Gy cm² and 131.61 Gy cm², respectively. The values presented here are nearly 50% lower. Neofotistou [23] proposed reference values of 67 Gy cm² and 110 Gy cm² for coronary angiography and PTCA, respectively. In 1997, Germany adopted a nationwide reference level of 100 Gy cm² for PTCA [24].

It is obvious that very different DAP values will be measured depending upon the X-ray system settings and on the protocol used throughout intervention, e.g. in relation to the radiation fields used. However, a quality assurance programme and an optimized technical and clinical protocol help to ensure that doses from this kind of intervention do not cause special concern for probable patient skin injuries in the vast majority of cases, although a specific clinical follow-up protocol must be followed when a threshold dose is reached. In this sense, special care should be paid in complex procedures and with overweight patients to avoid unsuitably high concentration factors arising from overlapping of too large radiation fields. Moreover, in aiming to optimize a procedure, cardiologists should know the different dose rates and dose/image for the different projections, to avoid excessive concentration of highly irradiating projections to the same area of the patient's skin. Awareness of the importance of routinely using wedge filters will improve image quality and additionally reduce field size and the possibility of field overlap, one of the main causes of high skin values.

	Footnotes

 
The present work was performed under the research sponsorship of the DIMOND III programme. Authors thank the European Commission for financial support.

Received for publication April 8, 2002. Revision received July 31, 2002. Accepted for publication August 14, 2002.

	References

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