Proton Beam Evaluation of Common Implantable Port Materials
Abstract
Background. When cancer patients undergo proton beam therapy (PBT), their anatomy is well modeled beforehand with minimum range uncertainty that optimizes therapy and prevents damage to surrounding healthy tissue. Sometimes, previously implanted medical devices, such as ports, unavoidably end up in the path of the beam. The materials used in ports create range uncertainties. Objective. The study objective is to introduce a variety of common port materials in front of a proton beam and measure range pullbacks. Methods. Several raw materials that are commonly featured in commercially available medical ports (silicone, polyether ether ketone [PEEK], polyoxymethylene, and titanium) were placed in front of a proton beam utilizing a Giraffe mutlilayer ion chamber (IBA Dosimetry) to measure average pullback based on 4 separate measurements (100 MeV, 150 MeV, 200 MeV, and 230 MeV). The chamber was calibrated to give range of pullback in terms of water equivalent thickness, and the data were used to calculate a relative proton stopping power ratio (PSPR) for each material. Results. The PSPRs measured 1.06 for silicone, 1.26 for PEEK, 1.35 for black polyoxymethylene, 1.35 for white polyoxymethylene, and 3.17 for titanium. Discussion/Conclusion. Prior to PBT, cancer patients undergo computed tomography scanning to determine precise lesion location(s). When a port or other medical device is located between the proton beam and the lesion, its presence must be factored into the dosimetry calculations. The material compositions and PSPRs are provided in this paper to provide physicists with useful information when treating patients with an implanted port. Given that a PSPR closest to the density of water (1.0) causes the least disruption to the proton beam, this information also has interesting implications on whether the only currently available 100% metal-free, power-injectable ports (ClearVUE™ Slim and ClearVUE™ ISP implantable ports [BD]), which are composed of PEEK (PSPR, 1.26) and polyoxymethylene (PSPR, 1.35), respectively, might be considered the best options for cancer patients who are expected to undergo PBT as part of their cancer treatment plan. Further studies comparing currently available ports are necessary to test this hypothesis.
IO Learning: 2021;9:A1-A4.
Key words: implantable port, proton beam therapy, radiation therapy
Background
Radiation therapy remains an important component of cancer treatment, with approximately 50% of all cancer patients undergoing radiation therapy during their course of illness; it contributes toward 40% of curative treatment for cancer.1 Conventional external beam radiotherapy uses photon beams to deliver a high dose of radiation to the tumor volume to destroy malignant cells.2 This treatment effect must be offset against the irradiation of surrounding normal tissues, as increasing this dose causes more treatment-related toxicity.2,3
Proton beam therapy (PBT) has advantages over conventional radiation therapy due to its unique absorption profile in tissues (the “Bragg’s peak”), which allows deposition of maximum destructive energy at the tumor site while minimizing the damage to healthy tissues along their path.1 PBT is therefore particularly useful in tumors situated next to critical organs, such as spinal cord and skull base tumors, and in the pediatric population, where secondary effects from radiation are especially significant.2-4
In order to optimize PBT and minimize damage to surrounding tissues, physicists use computed tomography (CT) to model the patient’s anatomy beforehand and calculate the range with minimum uncertainties for the proton beam. Ideally, the patient-specific plan is optimized while avoiding any implanted medical devices; however, the lesion location sometimes necessitates shooting the proton beam through an implanted port.4 When we introduce medical device materials in front of the beam, dosimetry changes and results in increased range uncertainties. The physicist must therefore incorporate the effect of the device material into their pullback calculations, which results in reduced range uncertainty. A higher proton energy results in deeper penetration of the proton beams through water (or through human tissue, which is comprised of mostly water). Implantable device materials with proton stopping power ratio (PSPR) values close to water theoretically cause minimal beam perturbations, which reduces uncertainty and increases the effectiveness of the planned therapy.
In order to facilitate current calculations and to lay the framework for potential future studies into the best port devices for use in patients who might undergo PBT, we selected a variety of raw materials commonly utilized in ports, placed them in front of a proton beam, and measured precise range pullback values.
Methods
Study materials (Figure 1). The following materials that are commonly incorporated into commercially available port devices were included in the study:
(1) Silicone sample, comprising 13 silicone membranes stacked on top of one another. Together, they measure 5.5 cm length x 5.7 cm width x 2.09 cm height by caliper.
(2) Polyether ether ketone (PEEK) sample, comprising 4 plaques machined to 6 cm x 6 cm, in thicknesses measuring 5.91 mm, 5.95 mm, 5.92 mm, and 5.94 mm and stacked to a total height of 2.278 cm.
(3) White and black polyoxymethylene (Delrin® plastic; Dupont) blocks. The two materials have identical chemical composition, but difference in colorant.
(4) Two titanium rods, with range pullbacks measured individually. Their dimensions each measured 2.64 cm length x 2.59 cm width x 6 cm height.
Study protocol. Each sample was placed in front of a proton beam and range pullbacks were measured at 100 MeV, 150 MeV, 200 MeV, and 230 MeV at the McLaren Proton Therapy Center. Measured data were obtained by utilizing a Giraffe multilayer ionization chamber (MLIC) detector, which is a device that can be used for dosimetric characterization of clinical proton fields.5 When we introduce material in front of the beam, protons traverse the material first before entering the MLIC. In theory, 1.0 cm of material should pull back the measured range in the MLIC by 1.0 cm if the density of that material is equal to water. If the inserted material is more dense than water, it will pull back the range in the MLIC by >1.0 cm; conversely, if the density of material is less dense than water, it will pull back the range by <1.0 cm. When materials are placed in front of the proton beam, the protons lose energy due to scattering. The less energetic beam enters the MLIC, and hence protons do not travel as far as they would if no material was inserted. Before measurements were undertaken with the study materials, the MLIC was calibrated to give the range in water equivalent thickness (WET). Relative proton stopping power ratio (PSPR) was calculated as:
PSPR = average range pullback (WET) / thickness of material (cm)
Study measurements. The raw study data are presented in Table 1. A sample calculation of PSPR using the measurements for PEEK is below:
• 230 MeV range of proton beam in water = 32.70 cm
• 230 MeV range of proton beam in water with PEEK inserted in the beam = 29.80 cm
• The block of black polyoxymethylene material pulled the range back in the MLIC by 32.70–29.80 = 2.90 cm
• Measurements were also taken at 100 MeV (2.80 cm), 150 MeV (2.92 cm), and 200 MeV (2.89 cm) and the four values were averaged (2.80 + 2.92 + 2.89 + 2.90)/4 = to obtain an average pullback of 2.88 cm
• The block of material measured 2.278 cm thick
Therefore, the PSPR of PEEK = 2.88 / 2.278 = 1.264
Results
Table 2 shows the PSPRs for each study material, and measured 1.06 for silicone, 1.26 for PEEK, 1.35 for black polyoxymethylene, 1.35 for white polyoxymethylene, and 3.17 for the titanium.
Discussion
Conventional radiotherapy uses photon beams to deliver a high dose of radiation to the tumor to eradicate cancerous cells. This benefit must be offset against the irradiation of surrounding non-cancerous tissues.2 The chief advantage of PBT over conventional external beam radiotherapy is that the dose of protons is deposited over a narrow range of depth, which results in minimal entry, no exit, or scattered radiation dose to healthy nearby tissues.4 Prior to PBT, the patient anatomy is well modeled with computed tomography, and the treatment path is optimally planned to avoid any non-tissue structures, such as an implanted port. However, effective tumor treatment sometimes requires PBT to be performed through a port. In these cases, the physicist must estimate the effect of the port material on the proton beam and factor this information into the dosimetry calculations. In other words, the calculations must “override” the density of the port material.
Computed tomography does not provide accurate density measurement for non-tissue structures. Prior to this study, PBT treatment in patients with implanted ports required estimations based on chemical composition and density. While these estimations were comparable to the results in the current study, incorporating PSPR offers a more precise measurement that optimizes dosimetry calculations. The PSPRs for common port materials provided herein offer physicists worldwide useful information for their own calculations.
Study implications. Port materials that have densities closest to water (which makes up the majority of human tissue) affect the PBT dosimetry calculations the least. It is preferred not to have high-Z elements in the proton beam path, as protons scatter at large angles in the presence of heavy elements. Protons have greater radiobiological effect at the end of the proton beam range (eg, when protons reach the end of their range). As noted by Kirk et al in 2017,6 the port presents an issue for treatment with protons because the range of the proton beam is sensitive to uncertainties caused by high-density objects. Beam range ambiguity may translate to a degradation of target coverage and/or increase in damage to the surrounding non-cancerous tissue.6 To date, efforts have focused on methods to obviate the effects of metal ports on PBT.7 However, there are only two 100% metal-free, MR-safe, power-injectable ports currently available. The first is the PowerPort™ ClearVUE™ Slim implantable port (BD), which is manufactured with PEEK material in the port body, and the second is the PowerPort™ ClearVUE™ ISP implantable port (BD), which is made from white polyoxymethylene (Delrin™) (Figure 2).8 The ClearVUE™ ports minimally affect the radiation dose and provide optimal treatment because the physicist need not “shoot around” the port. Based on these factors, radiation oncologists and other cancer team members who assist in the creation of treatment plans for patients who potentially require PBT may opt to specify the placement of a ClearVUE™ port before treatment commences.
Study limitations and future directions. The pullback measurements and information provided herein are based on raw materials, rather than on the ports themselves, which are constructed of materials that have been altered during the manufacturing process to some extent (for example, to increase opacity or to increase pliability). The PSPR values for the ports themselves must be ascertained in future investigations. Such a study might place several different commercially available ports in a gel phantom, shoot proton beams through each port, and examine the dosimetry obtained just behind the port using Monte Carlo simulations. The catheter attached to the port is expected to offer minimal effect, but must also be taken into consideration in future studies.
Conclusion
The PSPR values for several common port materials offer useful real-world information to physicians who utilize PBT. These data may be useful to optimize dosimetry, thereby limiting damage to surrounding healthy tissues. While these are preliminary data on raw materials, our results nonetheless suggest that metal-free ports, such as the ClearVUE™ Slim and ISP implantable ports may be the best choice for patients who may potentially undergo PBT. However, this hypothesis must be investigated in future trials that directly compare currently available port devices and their effects upon the proton beam.
Affiliations and Disclosures
From Karmanos Cancer Institute, McLaren Flint, McLaren Proton Therapy Center, Flint, Michigan.
Funding: This study was funded by BD.
Address for Correspondence: Basit Athar, PhD, DABR, Karmanos Cancer Institute, McLaren Flint, McLaren Proton Therapy Center, 4100 Beecher Road, Suite A, Flint, MI 48532. Email: Basit.Athar@mclaren.org
References
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