Dosimetric Effects Near Implanted Vascular Access Ports: An Examination Of External Photon Beam Dose Calculations

Vascular access ports are used widely in the administering of drugs for radiation oncology patients. Their dosimetric effect on radiation therapy delivery in photon beams has not been rigorously established. In this work, the effects on external beam fields when any of a variety of vascular access ports were included in the path of a high energy beam are studied. This study specifically identifies sidescatter and backscatter consequences as well as attenuation effects. The study was divided into two parts. First, a total of 18 ports underwent extended HU range CT scanning followed by 3D computer treatment planning, where homogeneous and heterogeneous plans were created for photon beams of energy 6 MV and 18 MV using a Pencil Beam Convolution (PBC) algorithm. Dose points were analyzed at locations around each device. A total of 1,440 points were reviewed in this section of the study. A replicate of the largest vascular access port was created in the treatment planning workspace for further investigation with alternative treatment planning algorithms. Then, plans were generated identical to the above and compared to the results of dose computation between the Pencil Beam Convolution algorithm, the Analytical Anisotropic Algorithm (AAA), and the EGSnrc Monte Carlo algorithm with user code DOSRZnrc (MC). A total of 300 points were reviewed in this part of the study. It was concluded that ports with more bulky construction and those with partial metal composition create the largest changes. Similar effects were observed for similar port configurations. Considerable differences between the PBC and AAA in comparison to MC are noted and discussed. By thorough examination of planning system results, the presented vascular access ports may now be ranked according to the greatest amount of change exhibited within a treatment planning system. Effects of backscatter, lateral scatter, and attenuation are up to 5.0%, 3.4% and 16.8% for 6 MV and 7.0%, 7.7% and 7.2% for 18 MV, respectively.

I. INTRODUCTION

Port catheters are primarily placed in either the subclavian, axillary, or internal jugular veins. Typically, the catheters are tunneled from the venotomy to a place on the upper chest just inferior to the infraclavicular fossa, where the port is positioned subdermally. For example, in Fig. 1 we show the clinical presentation of the device in a patient from past treatment documentation. A patient diagnosed with non-small cell lung cancer was prescribed a Bard Access Systems model 0602660 MRI plastic lumen port (Bard Access Systems, Inc., Salt Lake City, UT). A photograph of the patient�s chest indicates the underlying placement of the access port as it distends the skin surface (Fig. 1(a)). A CT image reveals the exact placement of the port in the axial view (Fig. 1(b)).

These devices vary in construction and are available in many different shapes and sizes.⁽⁴⁾ There are single and dual injectable ports as well as recently available power injectable ports. The plastic construction materials include Delrin (polyacetal resin), silicone, polycarbonate plastic, and polyurethane. Delrin is used to make the port body. The septum and suture plugs are made from silicone. The catheter locking collar is made from polycarbonate plastics for rigidity. Additionally, some polyurethane and silicone catheters are doped with barium sulfate to make the catheter well-observed in diagnostic radiology. These construction materials provide more stiffness than silicone, and result in smoother positioning and thus less irritation.^(5,6,7) The only significant scattering material identified in the construction of ports presented here is the titanium metal alloy. Identified as Ti (6Al4V), it is composed of 6% aluminum and 4% vanadium; the resulting metal is very dense. No stainless steel is found in any of the Bard ports.

Of course, it is important to examine radiation dosimetric effects near foreign materials implanted in the body. Concerning titanium, Mian et al.⁽⁸⁾ found backscattering in 1.1 mm of titanium at the interface may be as much as 14% greater for 6 MV and 11% for 25 MV. Similarly for mandibular bridging plates, they observed that at 6 MV, the backscatter dose was greatest. It is noteworthy that bridging plates contain more metal than ports in general. Still, this study provided early evidence that therapy beam dose distributions should be considered, as some vascular access ports are now constructed with such metals.

Noriega et al.(9) used film, and observed that the attenuation was as much as 17.5% for 6MV and 10% for 15 MV in a very small selection of ports. Attenuation was found to decrease with increasing energy. The devices studied were very similar in design and shape.

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