Planning Dosimetry Near Vascular Ports

New research shows the effects of electron beams on implanted vascular access ports composed of plastic, determining how they impair the fluence of radiation around them.

Since their first appearance in medicine in 1969, subcutaneous vascular access ports have been used regularly by healthcare providers to provide chemotherapy and antibiotics and to monitor blood chemistry without having to tap into veins continuously.[1,2,3} A vascular access port is most simply described as a hollow reservoir with a silicone septum on top, which is attached to a small catheter. The penetrable septum allows for the direct access of a needle for injection or collection of fluids through the reservoir.

Since these devices are in such common use, they often are being used in patients referred for radiation therapy. In such instances, radiation oncologists often find it difficult when the device is in the way of a targeted tumor. For this reason, it is scientifically important for medical physicists to examine how foreign materials implanted in the body affect radiation treatment dosimetry.

Researchers have investigated consequences of attenuation and scatter through various metals and involving highdensity prosthetics. Research in one group showed that back-scattering from bone-titanium interfaces of mandibular implants can be appreciated to nearly -14% in a 6 MY X-ray beam.[4} Bridging plates contain more metal than vascular access ports do. However, this prior study provided early evidence that therapy beam dose distributions should be considered, as some vascular access ports are now constructed with such metals.

Previous Studies Identify Attenuation Levels

The first study of the dosimetric effects of X-rays and electron beams for vascular access ports was conducted by Bagne et al in I 990.[5} The ports available at that time were made mostly of titanium or steel. Using a variety of radiation detectors, his group showed there is a sizeable decrease in dose if the port is located directly in the path of the beam. A significant discovery of -51.5% attenuation was announced for one port in a 9 MeV electron beam. Xray attenuation was determined to be -17% at 6 MV and -14.5% at 10 MY for stainless steel devices.

In 1994, Noriega et al used film to continue this work and plotted the off-axis profile of X-ray beams when metal ports were imbedded in a phantom. For photons only, attenuation was found to be -17.5% for 6 MV and -10% for IS MY.6 For each of these efforts, attenuation was found to decrease with increasing energy.

Since then, technology has improved, and a new generation of ports containing plastic is now available. In 2006, a research team published an extensive study of the IS different classifications of vascular access ports for their affect in an X-ray beam.[3} The research detailed changes in dose distributions as recognized by pencil-beam and convolution superposition algorithms in comparison to Monte Carlo simulations.

For locations beyond the device, the maximum change in dose was found to be as much as -16.8% in attenuation for 6 MV photons and -7.2% for 18 MY photons. Dose increases of7% for back-scatter and side-scatter locations were discovered for a titanium port. It has only recently been suggested that electron beam attenuation through nonmetal ports may be clinically feasible.

New Research Evaluates Plastic Ports

The scope of this new study was to characterize the dosimetric effect on external beam electron radiation when directed upon plastic vascular access ports. The study also determines whether there is a difference between similar port designs affecting radiation treatment regimens, and if port placement alternatives or radiation therapy should be avoided.[8. 9.1 O} This research integrated a variety of seven representative models from Bard Access Systems, a company which encompasses approximately 30% of all vascular access ports used in medicine in the United States.

The ports used in this work were made primarily of plastic, except for one made oftitanium. Noteworthy specimens included the MRI Powerport and the Rosenblatt port. The Rosenblatt model had a little metal on the base and existed with a dual lumen design. The MRl Powerport is made of radio translu cent material, so it is specially designed not to interfere with imaging techniques. This research incorporated devices of diverse design and varying dimension (see Figure 1).

The experimental phantom setup included a 420 cm3 miniature water phantom made of polystyrene. This phantom was placed on top of a Sun Nuclear MapCheck 1175 silicon diode array. Natural water at 1.5 cm depth was added to completely submerge each vascular access port under investigation. The MapCheck diode array was centered in the electron beam using mounted room lasers, an optical distance indicator light field projection cross-hair and bubble level. In order to simulate the saline solution that pulmonologists use to flush all air from the inside cavity of the vascular access port, water was injected identically for all devices.

Attenuated absorbed dose results were acquired using a MapCheck diode array with version 5.0 software. The electrometer software was capable of cumulative charge collection for up to 445 diodes during irradiation. It also was proficient to profile results across any two-dimensional plane. This setup was incorporated for attenuation measurements at electron energies of9-20 MeV.

We wanted the detectors to be at a depth ideal for measurement at the energy chosen. Given that the silicone diodes in the MapCheck device are inherently sealed in 2 cm plastic, this depth was determined inappropriate for 6 MeV electron beams, as it was near to the practical range. Consequently, the attenuation exhibited in a 6 MeV beam required the use ofa Kodak EDR-2 ReadyPak film instead of the MapCheck device.

To measure lateral scatter, a PTW TN31 014 ionization chamber with sensitive volume 0.015 cm3 was connected to a Capintec 192 electrometer. The chamber was placed 1 cm from the port and affixed with an adhesive to the bottom of the container. No build-up cap was needed, since the thimble was entirely submerged.

To measure back-scatter with a port in place, the chamber was directly positioned on top of the port, immediately above the dense rim. A build-up cap was then used, where a portion of the thimble protruded anterior to the water surface. The distance from the ionization chamber to the point of measurement was 0.3 cm, as determined by the build-up cap thickness and thimble radius (see Figure 2).

The particle accelerator was a Varian 21 EX, programmed to deliver electron beams with energies of 6, 9, 12, 16 and 20 MeV. Machine calibration was achieved for each electron beam to produce 1 cGy/MU at the depth of maxinnum dose in a lOx 10 cm2 field. A 100 cm source-to-surface distance defined by the center of rotation of the accelerator was maintained for all measurements.

A larger 20 x 20 cm2 open field cone was selected for use with all experiments, since it was observed in prior studies to have ideal properties of flatness and symmetry. This aperture defined the electron beam collimation for all measurements. Attenuation, lateral scatter and back-scatter were each determined for all seven vascular access ports by comparing results with and without them being in the path of the beam. Measurements were averaged over multiple runs at 100 monitor units and with radiation being delivered at a dose rate of 600 cGy/min.

Dose Profiling

From diode array dose profiling data saved and analyzed at 9-20 MeV electron beam energies, the comparison of diode readings with the port in place to those when the port was removed provided the beam blocking effect magnitude. The maximum changes in absorbed dose were qualitatively evaluated by overlaying each profile on the same plot. The numerical percentage variation was computed given the spreadsheet values at each diode location.

Numerical tabulated data for the 6 MeV electron beam were characteristic of the dose response to the ReadyPak film. Each calibration film and test film was developed using an AFP ImageWorks Mini-Medical 90 processor and analyzed using an X-Rite 301 point densitometer. A Hurter & Driffield calibration curve was generated by relating the optical density of the same batch offilm observed at different dose levels. Dose profile attenuation plots were fashioned similarly by scaling these results along a graduated path.

Scatter measurements were conducted identically for all vascular access ports investigated. Although dose profile plots were not made possible in the lateral and backward direction, various readings were achieved around each device to ensure the result represented the location of maximum scatter effect.

The equation found at the end of this article provides the calculation used for all attenuation and scatter measurements.

Discussing the Findings and Implications

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