Dose perturbations due to contrast medium and air in MammoSite® treatment: An experimental and Monte Carlo study C.-W. Chenga兲 Arizona Oncology Associates, 2625 N. Craycroft Road, Suite 100, Tucson, Arizona 85712
R. Mitra Ochsner Clinic Foundation, 1516 Jefferson Highway, New Orleans, Louisiana 70121
X. Allen Li Department of Radiation Oncology, Medical College of Milwaukee, Milwaukee, Wisconsin 53226
Indra J. Das Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania 19104
共Received 28 October 2004; revised 26 April 2005; accepted for publication 4 May 2005; published 20 June 2005兲 In the management of early breast cancer, a partial breast irradiation technique called MammoSite® 共Proxima Therapeutic Inc., Alpharetta, GA兲 has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system 共Nucletron PLATO system兲. For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution 共in the range 5%–25% by volume兲 with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances ⬍1 mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances 艌10 mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface 共⬍1 mm兲 due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5% per cm3 of air. Monte Carlo simulation suggests that the interface effect 共enhanced dose near surface兲 is primarily due to Compton electrons of short range 共⬍0.5 mm兲. For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system. © 2005 American Association of Physicists in Medicine. 关DOI: 10.1118/1.1943827兴 Key words: MammoSite, breast, dose perturbation, Monte Carlo simulation I. INTRODUCTION In the management of early breast cancer, lumpectomy followed by external beam irradiation is the most commonly employed treatment modality. Other treatment modalities include multi-planar implants with low dose rate or high dose rate 共HDR兲 brachytherapy to the entire breast. Recently, a partial breast irradiation technique using HDR called MammoSite® is being advocated.1–9 In MammoSite, a balloon filled with saline mixed with an iodine-based contrast material is placed at the surgical cavity of the primary tumor. In the HDR procedure, a single 192Ir source is positioned at the center of the balloon. The goal is to treat the wall of the 2279
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surgical cavity 共or more precisely, a 1 cm rind of tissue extending from the tumor bed兲 with high doses. The treatment regiment is 3.4 Gy per fraction to 1 cm from the surface of the balloon, delivered twice daily, for a total dose of 34 Gy in ten fractions.1,3 Computed tomographic 共CT兲 scans or orthogonal films are taken before the first treatment of each day to ensure that the balloon remains intact. The presence of a high-Z contrast medium and an air cavity adjacent to or inside the balloon may pose a dosimetric problem, depending on concentration and the volume of the air cavity, respectively. The distance between the balloon surface and the skin is another clinical factor of concern for the selection of patients for MammoSite procedure.
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The major advantage of MammoSite over the conventional treatment is its much reduced length of treatment. The treatment of MammoSite completes in one week, whereas the conventional treatment with external beams usually takes six weeks. The long-term clinical outcome of partial breast irradiation with MammoSite in the management of early stage breast cancer is not known and there are conflicting debates about its usefulness.5,7,10–13 Nonetheless, a clear understanding of dosimetry in MammoSite is necessary to correlate the outcome with the treatment technique. The dosimetry at the interface between tissue and a high Z-medium is a well known problem. Dose perturbation exists between two dissimilar media with different atomic compositions14–16 or in the presence of a contrast medium.17–25 For breast irradiation, the dosimetric effect of breast implant has been studied previously. Early studies showed that the magnitude of dose perturbation at the interface is dependent on the beam energy and the atomic number of the medium.14,15,26–28 For the silicone gel breast prosthesis, Klein et al.29 reported dose perturbation of less than 10% for 6 MV x rays. Similar effects have also been noted by other authors30–34 for megavoltage photon and electron beams. The physical placement of a MammoSite balloon is not very dissimilar to a silicone breast implant. Although, the dosimetric implication of a radio-opaque material in combination with a low energy photon source has been reported recently, the physical principle and the interaction mechanisms behind the observed dosimetric variations have not been addressed. Furthermore, the predominantly low energy nature of the ␥ rays in 192Ir, is expected to have a more pronounced dose perturbation effect at the high-Z interface than those from the 6 MV x rays. Another potential issue, which should be addressed in MammoSite dosimetry is the existence of air bubble adjacent to or inside the balloon. Air may be trapped in the vicinity of the balloon as a result of the balloon placement. An air bubble may also be introduced during the placement of the balloon or during the injection of the saline and contrast solution into the balloon with a syringe. In the former case, air pockets could exist in any location around the balloon. In the latter case, the air bubble stays at the anterior aspect of the balloon as the patient is lying on a treatment couch, thus the dosimetric effect of the air bubble is almost exclusively confined to the anterior aspect of the breast tissue. The location of the air bubble may also vary from treatment to treatment due to the patient positioning. To our knowledge, most treatment planning systems in brachytherapy do not account for the inhomogeneities in the dose calculation. Thus the effect on the dose distribution of an iodine-based contrast medium inside the balloon may not be correctly predicted by the treatment planning system. This may be especially significant when there is little breast tissue between the balloon and the skin surface, or when the balloon is in proximity to the lung.35 In practice, the finite size of an ion chamber imposes a physical limitation on the measurement of interface dosimetry. To obtain a better understanding of the dose variation at the interface and the physical mechanism governing the dose Medical Physics, Vol. 32, No. 7, July 2005
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absorption process, Monte Carlo simulation is an ideal tool and is used to investigate the interface dosimetry in this study. The objective in this study is threefold: 共1兲 to investigate experimentally the extent of dose perturbation for various concentrations of a contrast medium in a MammoSite balloon using ion chamber and diode measurements, and to examine the dosimetric effect of the air bubble inside the balloon, 共2兲 using Monte Carlo 共MC兲 simulation to provide a fundamental understanding of the mechanism of dose perturbation due to the contrast solution, and 共3兲 to investigate the effect of the lack of inhomogeneity correction in a brachytherapy planning system.
II. MATERIALS AND METHODS To quantify the effect on dose distribution from the contrast medium, a dose perturbation factor 共DPF兲, defined as the ratio of the doses at the same point in tissue with and without the presence of a contrast medium, was introduced by Mitra et al.36 Mathematically, DPF is given by DPF =
D共E,d,x,C兲 , D共E,d,x,0兲
共1兲
where D is the dose expressed as a function of several variables: E 共photon energy兲, d 共the diameter of the balloon兲, x 共distance from the balloon surface兲, and C 共concentration of the radio-opaque solution兲. A dose enhancement is observed when DPF⬎ 1 and a dose reduction occurs when DPF⬍ 1.
A. Experimental study
1. DPF versus iodine concentrations Measurements were performed in a scanning water tank with full scatter condition as recommended in MammoSite treatment. In all experimental measurements, a MammoSite balloon 42 mm in diameter is used. The size of the balloon is independently verified by measuring the volume of the liquid injected inside the balloon. It is held stationary inside a water phantom with a metal clamp so that the catheter is parallel to the bottom of the water tank. A Markus chamber 共PTW, NY兲 fitted with an O-ring sealed acrylic cap is used in the measurements. Dose is measured at various distances from the balloon surface in the direction perpendicular to the catheter. Due to the thickness of the acrylic cap 共1 mm兲, the closest point of measurement to the balloon surface is 1 mm. A stereotactic field diode detector 共SFD兲 共Welhoffer, Germany兲 with an active volume of 0.2 mm diameter was also used to measure dose at selected points. The balloon is first filled with water and the dose variation with distance from the balloon surface is measured in 1 mm steps. The water in the balloon is then replaced with an equal volume of a 5% 共by volume兲 radio-opaque solution 共iodine concentration 0.3% by volume兲 and the dose variation with distance is measured. The DPF is then calculated at each point of measurement. This process is repeated for various concentrations of the radio-opaque solution in the range 0–25% by volume 共corresponding to an iodine concentration
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of 0–1.5% by volume兲. To verify the consistency of the measurements, some of the measurements are repeated with the SFD. No attempts were made to account for the decay of the source activity during relatively short time of the measurements. The inherent uncertainty due to source decay in the measurement is 艋1%. The major uncertainty in the measurement is in determining the distance to the balloon surface. It is estimated by repeating measurements at selected points and also with the two different types of detectors. The positional accuracy is estimated to be ⫾1 mm at the surface of the balloon. This uncertainty will be compounded with the positioning accuracy of the scanning water phantom system 共⫾0.1 mm兲 in all subsequent positioning of the detector, giving a total positioning uncertainty of ⫾1.1 mm. Thus at distances close to the balloon surface 共⬍5 mm兲, the combined experimental uncertainty 共distance and source activity兲 is estimated to be about ⫾3%, and at distances ⬎5 mm, the combined uncertainty is ⫾2%.
2. Effect of air bubble inside balloon To investigate the effect of air bubble inside a balloon, dose variation through the bubble as a function of distance from the balloon surface is compared with the corresponding measurements without the air bubble. The SFD detector is used for the measurements because of the small size of the air bubbles. Dose measurements are first obtained without the presence of air bubble, in the direction perpendicular to the axis of the balloon at three distances: 0, 5, and 10 mm. Using a syringe, a known amount of water in the balloon is replaced with an equal volume of air. Dose 共measured charge兲 values at the same three distances are then acquired. The measurements are repeated with different air volumes over the range 2–10 cm3. The dose values at each distance are then plotted as a function of the volume of air bubble. B. Monte Carlo calculation
The EGSNRC Monte Carlo system37 is used in this study. In the Monte Carlo calculation, electron transport is turned on and a set of transport parameters is carefully chosen for the geometry and the low energy photon source in question 共for example, AE= ACUT= 0.521 MeV, AP= PCUT = 0.01 MeV兲. Up to 1012 incident particle histories are evaluated for each study. The 192Ir source contains a cylindrical active iridium core 0.65 mm in diameter and 3.6 mm in length 共micro-Selectron HDR source, Nucletron兲. The core with rounded edges is encapsulated by a stainless steel cylinder of 0.9 mm outer diameter and 3.6 mm in length. The distal end of the cylinder is a 0.2-mm-thick stainless steel spherical segment 0.8 mm in diameter, and the proximal end consists of a 0.55-mm-thick steel disk 0.9 mm in diameter. The encapsulated source is welded to a steel cable of 0.7 mm diameter and 20 mm long via a transited cone segment of 0.15 mm long. The geometry and material data for the source are derived by Daskalov et al.38 The ␥-ray energy spectrum for the 192Ir source is obtained from a published report.39 Medical Physics, Vol. 32, No. 7, July 2005
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FIG. 1. A schematic diagram of the cross-sectional view of the geometry used in Monte Carlo simulation.
In the MC simulation, a 192Ir source is placed at the center of the balloon with its axis aligned with the balloon axis. The dose distributions for the 192Ir source with and without the presence of a contrast medium in a MammoSite balloon is calculated with the user-code DOSRZNRC. Doses are scored in concentric shell-shaped regions outside the balloon. The scoring region is 0.1 mm thick for distances 共d兲 less than 3.0 mm from the balloon surface. Thicker scoring regions 共up to 0.5 mm兲 are used in the distal region in order to improve the statistics. A schematic diagram of the cross-sectional view of the calculation geometry is shown in Fig. 1. The statistical uncertainty in the doses is less than 2%, and it is within 1.0% for d ⬍ 1.0 mm. The overall uncertainty on the DPF data presented is estimated to be ⬍3%. Although the actual MammoSite balloons are more or less spherical in shape, cylindrically shaped balloons are chosen for convenience in the DOSRZNRC Monte Carlo code. To examine the validity of using the cylindrical shape, a calculation using a separate Monte Carlo code, MCNP,40 is performed for a spherical balloon with a radius of 3 cm, with and without the presence of the contrast medium. The DPF values outside this spherical balloon are found to agree well, within 1%, to those calculated for a cylindrical balloon of 3 cm in radius and 8 cm in height. Much more comprehensive comparisons between the EGSNRC/DOSRZNRC and the MCNP codes under different geometric configurations and in the presence of heterogeneous media have been reported.41,42 Generally good agreement between the two Monte Carlo codes are observed for the 192Ir source. Thus the use of cylindrical balloon in the current calculation of DPF is justified. Because of its higher calculation efficiency, the EGSNRC/ DOSRZNRC code is selected over the MCNP code. In the present study, the cylindrical balloons have a height of 8 cm and radii of 2, 3, and 4 cm respectively. Although the 3 and 4 cm radii balloons are not used for MammoSite applications, they are included in the study since they provide a more complete picture of interface dosimetry in MammoSite geometry than only those used clinically, which are typically in the range 2.0–2.5 cm radius. A commonly used contrast medium, Visipaque 共Nycomed Inc.兲, is used to study the variation of DPF with balloon diameter. This medium is a dimeric, isomolar, nonionic, water-soluble, iodinated x-ray contrast agent. Visipaque has a
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molecular composition of C35H44I6N6O15 and a molecular weight of 1550.2 of which 49.1% of the mass is iodine. The Visipaque has a measured density of 1.356 g/ml 共at 37 °C兲 for the iodine concentration of 320 mg/ml 共or 6.5% by volume兲. In order to study the effect of various iodine concentrations on the DPF, further calculations are performed for a hypothetical mixture of water and iodine with various iodine concentrations by volume 共0.4%, 1%, 2%, 4%, 6%, and 10%兲. C. Comparison between PLATO treatment planning system and measurements
To allow quantitative comparison of the measured dose values with those predicted by a brachytherapy planning system, dose distributions are generated from the Nucletron PLATO treatment planning system 共Version 14.2.4兲 for the same balloon size of 42 mm in diameter 共diameter of the prescription sphere is 62 mm兲. Marker points are set up along the same radial line as in the measurements in 1 mm spacing from the surface of the balloon. The calculated doses at the marker points are then compared with measurements for 0% and 0.6% iodine concentrations 共corresponding to 0% and 10% by volume of radio-opaque concentration, respectively兲, and Monte Carlo simulations for the 0%, 0.4%, and 4% iodine concentration.
III. RESULTS A. Experimental study
1. Dependence of DPF on iodine concentration The variation of dose with distance from the balloon surface for the water-filled balloon 共42 mm diameter, 0% concentration兲 is shown in Fig. 2共a兲 for both the Markus chamber and the SFD measurements. The agreement between the two measurements is excellent for the 5 and 10 mm distances. A fifth-order polynomial fitted to the Markus chamber data extrapolates to a y intercept of 1.969, which agrees with the SFD value at 0 mm to within 0.4%, indicating the consistency of the dose measurements. Figure 2共b兲 shows the variation of DPF for various iodine concentrations. Despite the highly fluctuating nature of the DPF curves, they are all within the ⫾3% experimental uncertainty 共error bar not shown兲, indicating that the DPF is relatively constant for the iodine concentration included in the study. It is interesting to note that the corresponding ionization curves for the different iodine concentration exhibit only small fluctuations as shown in Fig. 2共c兲. These small fluctuations are magnified in the calculation of DPF due to the mathematical operation of dividing each ionization value obtained with a certain iodine concentration with the ionization value in water at the same depth. The uncertainty in the DPF is correspondingly increased due to compounding errors by quadrature. The average DPF is 0.965± 0.011, at distances 艌1 mm from the balloon surface. This value is in agreement with Medical Physics, Vol. 32, No. 7, July 2005
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recent published data.35,43,44 Due to the physical limitation of the ion chamber, the dose variation in the first 1 mm from the balloon surface cannot be measured.
2. The effect of air bubble For a given distance from the balloon surface, the dose varies linearly as a function of the air volume as shown in Fig. 3. A linear regression analysis on the three sets of data points indicates that the air volume effect on dose is almost independent of distance from the surface of the balloon and the volume of the air bubble inside the balloon. For example, compared to a water-filled balloon, the presence of a 2 cm3 air volume increases the dose at the surface by about 1%, while a 10 cm3 air bubble increases the dose at the surface by about 5%. At 5 and 10 mm from the surface of the balloon, a similar dose increase due to air bubble is observed for the 2 and 10 cm3 air bubbles, respectively. B. Monte Carlo calculations
1. DPF versus balloon diameter at interface „d < 1 mm from surface… Figure 4 presents the DPF as a function of distance from a Visipaque-filled balloon surface 共iodine concentration 6.5% by volume兲 for three balloon radii of 2, 3, and 4 cm, respectively. It is clear from Fig. 4 that a dose enhancement is observed at the immediate vicinity of the balloon surface. It is interesting to note that the DPF varies inversely with the balloon diameter. For example, at 0.1 mm from the surface, the DPF is about 1.30 for the 2 cm radius balloon whereas it is about 1.18 for the 4 cm radius. The DPF falls rapidly within a short distance from the balloon surface. For the 2 cm radius balloon, the DPF falls below unity 共i.e., dose reduction兲 at approximately 0.5 mm from the surface, while it is ⬍0.2 mm from the balloon surface for the 4 cm radius balloon. These observations are consistent with the DPF values obtained by various authors with different beam energies.14,25,45,46
2. DPF versus concentration „d < 1 mm… Figure 5 shows the variation of the DPF for various iodine concentrations for the 2 cm radius balloon. Clearly, the dose enhancement at the surface increases with the iodine concentration. It is interesting to note that the variation of DPF with iodine concentration reverses at d 艌 0.5 mm from the balloon surface. This reversal is probably due to the change of interaction mechanisms with distance from the balloon surface. C. Comparison of dosimetry between planning system and measurements
PLATO
treatment
Figure 6 compares the relative dose as a function of distance from the balloon surface between the PLATO calculation and measurements for 0% and 0.6% iodine concentrations that may be used clinically. Superimposed on the curves are the Monte Carlo simulations for three different iodine concentrations: 0%, 0.4%, and 4%. The Monte Carlo data have
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FIG. 2. 共a兲 Comparison of Markus chamber measurement with SFD for 0% iodine concentration normalized to 10 mm distance from the balloon surface. A fifth-order polynomial is fitted to the Markus chamber data. 共b兲 Variation of DPF with distance from surface of the balloon, for different iodine concentrations 共with corresponding contrast concentration in parentheses兲. 共c兲 Variation of the ionization curves 共dose兲 with distance from surface of the balloon for various iodine concentrations.
been corrected for the inverse square effect due to its smaller radius 共2.1 vs 2 cm in PLATO and measurement兲. As shown inFig. 6, the measured curves are in agreement with the PLATO and Monte Carlo calculations for distances greater than 1 mm from the balloon surface and iodine concentrations less than 0.6%. At distances ⬍1 mm from the balloon Medical Physics, Vol. 32, No. 7, July 2005
surface, the Monte Carlo curves agree with the PLATO calculation except for much higher iodine concentrations. As shown in Fig. 6, the dose enhancement at distances close to the balloon surface is not accounted for by the PLATO system. However, the enhancement effect due to iodine is only significant at much shorter distances 共d ⬍ 1 mm兲.
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FIG. 3. The effect of air bubble inside the balloon on dose at different distances from the balloon surface.
FIG. 4. Monte Carlo simulation showing the variation of DPF as a function of distance from surface of balloon for three different balloon radii.
FIG. 5. Monte Carlo simulation showing the variation of DPF as a function of distance 共d ⬍ 1 mm兲 from surface of balloon for different iodine concentrations.
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FIG. 6. Comparison of the variation of dose as a function of distance between the PLATO system, ion chamber measurements, and Monte Carlo simulations for different iodine concentrations.
IV. DISCUSSION Radiation outcome in breast treatment is related to dose, dose inhomogeneity, and volume among other factors such as chemotherapy,12,13,47 the contrast medium and the air bubble in MammoSite balloon. Radiation dose is affected by the contrast medium and the air bubble in MammoSite balloon. The effects of iodine-based contrast solution in dose distribution in the partial breast irradiation with a MammoSite balloon have been studied experimentally. To understand the physics at the interface and its vicinity, Monte Carlo simulations have been carried out to calculate the DPF within the first millimeter from the balloon surface. Our measurements show that, within experimental uncertainty, the DPF remains relatively constant with distance from the balloon surface for the range of iodine concentration used in the MammoSite procedures. The presence of a contrast material inside the balloon produces a dose enhancement effect at the interface as revealed from the MC simulation. The dose enhancement is higher for smaller balloons. For example, for a 2 cm radius balloon with a 6.5% iodine concentration by volume, the dose enhancement at 0.1 mm from the balloon surface is ⬎30% and approximately 18% for the 4 cm radius balloon. The DPF decreases rapidly from the balloon surface. The transition from dose enhancement to dose reduction 共i.e., DPF ⬍1兲 occurs at d ⬃ 0.2 mm for the 4 cm radius and ⬃0.5 mm for the 2 cm radius balloon. The dose enhancement at the immediate vicinity of the balloon surface is probably due to the Compton electrons from the iodine atoms as a result of large angle Compton scatterings of the Ir-192 ␥ rays. A simple calculation shows that large angle Compton scattering of the dominant Ir-192 ␥ rays 共300–480 keV兲 produces electrons in the range ⬃75–300 keV, although a very small portion 共⬃1%兲 of the high energy ␥ rays 共600–900 keV兲 in Ir-192 could produce Compton electrons above 500 keV. The range of a 300 keV electron in Visipaque is about 0.6 mm, and in water is 0.8 mm. Thus as the balloon radius increases, more and more of the lower energy electrons are absorbed inside the balloon. Only those which originate near the inside surface of the Medical Physics, Vol. 32, No. 7, July 2005
balloon may pass through but which will also lose all their energy in the close proximity of the surface. Hence the larger the radius of the balloon, the faster is the fall-off of DPF from the surface. For a given balloon radius, the DPF increases with concentration at the immediate vicinity of the balloon surface, but reverses its trend at distances ⬎0.5 mm, and stays constant at distances beyond the 1 cm prescription depth. Since the number of Compton electrons increases with iodine concentration, the DPF increases with the concentration. At distances ⬎0.5 mm, dose absorption is due to Compton electrons produced in water. As the iodine concentration increases, the attenuation of the Ir-192 ␥ rays also increases, and hence the DPF decreases with increasing iodine concentration at distances beyond the range of Compton electrons from iodine atoms. It should be pointed out that in MammoSite, the entire ring of tissue surrounding the balloon is of clinical interest as reported in early studies.1,48 Indeed, a quality factor has been defined to compare the volume of tissues receiving at least the prescribed dose to the volume receiving, say 150% of the prescribed dose. Thus it is useful to examine and understand the dose distributions at very close range from the balloon surface and extend beyond several centimeters from the surface. Our measured results are in general, in agreement with recent Monte Carlo studies on the dose effect of the contrast material in MammoSite.35,43,44 Both studies reported a dose reduction at the prescription distance 1%–5%, depending on the concentration. The slight discrepancies are probably due to geometry and some inherent difference between measurement and calculation. We have also studied the effect of air bubble, which may be introduced during the placement of the balloon or during injection of the contrast solution inside the balloon. Our measurement shows that there is about a 1% increase in dose at the balloon surface for a 2 cm3 bubble and 5% dose in
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crease at the surface for a 10 cm3 bubble. The dose increase remains constant 5 and 10 mm away from the surface. While a 10 cm3 bubble is not clinically relevant, its inclusion in our study helps to generalize our observation, that for a given air bubble size, the dose increase due to the air bubble appears to be constant at least within the 1 cm ring of tissue surrounding the sphere. The effect of the dose increase is almost exclusively confined to the anterior aspect of the breast tissue if the air bubble is inside the balloon. However, when air is trapped at the surgical bed during the placement of the balloon, there could be more than one air pocket formed and the locations of these air pockets can be anywhere outside of the balloon. The MammoSite protocol requires that there be at least 5 mm tissue between balloon surface and the skin to avoid giving excessively high skin dose. A 5% dose increase 共with a 10 cm3 bubble兲 corresponds to roughly 1 cm of missing tissue for a 400 keV photon. Even a 1% increase corresponds to roughly 0.3 cm of missing tissue. Thus the presence of an air bubble effectively reduces the amount of tissue between balloon and skin. In practice, the amount of air volume inside a balloon may be estimated from the CT scan 共but not from plain radiographs兲 and the tissue thickness allowed could be adjusted accordingly. The presence of an air pocket will also effectively reduce the volume of the tissue treated since tissue in the outer region of the 1 cm ring will be pushed outside of the ring. This tissue will then receive less than the prescribed dose. Our comparison between the measured dose distribution, Monte Carlo calculation, and PLATO calculation indicates that for distances 艌1 mm from the balloon surface, the presence 共or absence兲 of heterogeneity correction in the dose algorithm has no significant effect on the distribution, for the iodine concentrations used clinically in MammoSite procedures. However, at higher iodine concentration, the larger dose enhancement in the vicinity of the balloon surface 共⬍0.5 mm兲 cannot be predicted by a dose algorithm that does not account for the presence of an inhomogeneity. For example, with a 4% iodine concentration, PLATO underestimates the dose at the balloon surface by about 10%. V. CONCLUSIONS The contrast-filled balloon in MammoSite results in a dose reduction at the prescription distance of 10 mm from the surface. The amount of dose reduction depends on the concentration. The most significant effect of the contrast solution occurs at close proximity to the balloon surface. The interface effect as suggested cannot be measured due to short range 共1 mm兲 of this effect. The presence of an air bubble inside the balloon effectively reduces the tissue thickness between balloon and skin, which could be an issue and of clinical concern. The absence of inhomogeneity correction in brachytherapy planning systems seems to affect mostly dose distributions in close proximity to the balloon. Brachytherapy treatment planning systems should provide inhomogeneity corrections to account for air, lung, and high-Z contrast medium. Medical Physics, Vol. 32, No. 7, July 2005
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Author to whom correspondence should be addressed. Electronic mail:
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