Dosimetric Impact of Metallic Hip Prostheses in Pelvic Radiotherapy

Aim

This study aimed to fabricate customized tissue equivalent human pelvic phantom with implanted hip prostheses (HP) model made up of stainless steel and titanium for the dosimetric measurement of routine quality assurance in Radiotherapy. Also to evaluate the dosimetric influence of various HPs during megavoltage conformal and Intensity Modulated Radiotherapy (IMRT) of the pelvic cancers.

Background

Patients undergoing radiotherapy of the pelvic region make up a significant portion of all the patients treated with radiotherapy. During past several decades, the number of people with implanted hip prostheses has increased globally.

Along with this increase, the average life expectancy has also continuously increased which, in combination with improved diagnostic methods, have led to increased cancer detection rates. All these factors together have resulted in increased number of radiotherapy patients having prosthetic devices implanted in their bodies. Metal objects, such as prostheses and dental fillings, can cause artifacts on X-ray computed tomography (CT) images due to a combination of beam hardening, photon starvation, edge gradient effect, and scatter.

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These artifacts may obscure visualization of anatomical structures and affect dose calculation accuracy. High-Z prostheses cause dosimetric calculation errors when setting up a radiotherapy (RT) plan. This can be attributed to metal artifacts in the computed tomography (CT) dataset, which are perceived as starburst streaking and blurring in kilovoltage CT images. Artifacts degrade the image diagnostic quality and impair contour delineation of target and critical organs. The concern is tumor dose reduction due to radiation attenuation through the HP and induced scattered dose near the prosthesis.

These dose perturbations (radiation attenuation and scatter) may not be accurately predicted in conventional treatment planning systems (TPS), especially near tissue- bone-HP interfaces.

Material and Methods

The indigenously developed phantom was used in this work. It can include different tissue-equivalent materials. In order to simulate a pelvis with a double-sided endoprosthesis for the hip, a PMMA phantom was fabricated indigenously that features steel rods (diameter Ø = 2.0 cm, length l = 15.0 cm), Titanium rods (diameter Ø = 2.0 cm, length l = 15.0 cm) and PMMA rods (diameter Ø = 2.0 cm, length l = 15.0 cm). The phantom consists of 30 layers (height 1.0 cm) with an elliptical base (half-axes a = 23.0 cm, b = 30.0 cm). The stacked layers are held together by two laterally arranged rods of PMMA(Ø = 2.5 cm, l = 35.0 cm) that pass through all plates.

Another bore with a diameter of 2.0 cm is used to simulate a rectum filled with air. Furthermore, the phantom can be equipped with an ionization chamber. For the acquisition of images, a 126-slice CT scanner (Siemens AG, Erlangen, Germany) was used. The scans were acquired sequentially with 120 kVp, a field of view of 500 mm, and a slice thickness of 1.25 mm. The images were reconstructed using the filtered backprojection with a ramp filter (FBP). In addition to a dataset with the two metal rods, an image dataset without steel rods was acquired in order to have an artifact-free image set available. This was used for the contouring of the ionization chamber.

Results: Dosimetric Impact of Hip Prostheses

To investigate the accuracy of planning algorithms six 10×10 cm2 static 6 MV, IMRT plan consisting of nine X-ray fields were planned and delivered. The six square fields share a common isocenter, which was positioned on the central axis of the phantom. For each field 200 MU was delivered and the dose was measured for both the sagittal and frontal orientation. The fields vary by the gantry angle, which are 0°, 30°, 60°, 90°, 120°, and 160°, respectively. Hence the variation in dose caused by the fixation device could be measured. Dose errors were significant only for high-Z phantom setting, causing discrepancies up to ≈ 26% in the dose received by the PTV volume.

In this case, it was observed that the PTV had the highest overdosage errors because it was centered on the imaging artifact streaks. By comparing the dose voxel-by-voxel mismatches, up to 6 Gy were found. In particular, hot and cold spots occurred where streak artifacts are present rather than where the maximum nominal dose is optimized and delivered. On average, the dose estimation on the PTV had a relative accuracy of 0.15%, much lower than the clinically accepted threshold for IMRT treatment (1%–2%). These results confirm the proposed algorithm for metal artifacts recovery coming from low- and high-Z is accurate in a fabricated phantom phantom, and encourage the use of the procedure in the clinical framework of radiation therapy. Further studies are needed to investigate on dose calculation accuracy in clinical IMRT treatment planning.

This study's findings highlight the critical dosimetric challenges posed by metallic hip prostheses in pelvic radiotherapy. The fabrication of a tissue-equivalent pelvic phantom with embedded metal implants provided valuable insights into the extent of dose perturbation caused by different materials. The results underscore the necessity for advanced treatment planning strategies that can accurately account for metal artifacts, thereby ensuring precise dose delivery to the target while minimizing exposure to surrounding healthy tissues.

Further research is warranted to refine dose calculation algorithms within TPS, particularly for high-Z materials, to enhance the accuracy of radiotherapy treatment planning for patients with metallic implants. The development and implementation of such methodologies will be pivotal in optimizing therapeutic outcomes and minimizing radiation-induced complications.

Table 1: Dosimetric Deviation Analysis

Dosimetric Calculation Formula:

• Dose Deviation (%) = ((Measured Dose - Planned Dose) / Planned Dose) x 100
• HU Variation = Standard Deviation of HU Differences in Artifact vs. Artifact-free Images

Conclusion

In this work, a novel method implemented i) beam hardening, and ii) photon starvation due to low-Z and high-Z metal implants. Titanium (low-Z) and Stainless steel (high-Z) were additional inserts used with the indigenously developed phantom for this purpose. High-Z implants caused larger HU differences relative to low-Z, when comparing the reconstructed images with the GT (std ≈ 120 HU versus std ≈ 35 HU). The HU differences significantly to std ≈ 10 HU, with high-Z and low-Z having a residual error of the same order. By computing the maximum image difference, it was found 1050 HU and 250 HU for high-Z and low-Z, respectively, thus confirming that the local image corruption was much larger for Stainless steel. In this paper, we simulated a phantom with a unilateral metallic hip prosthesis, as well as a phantom with bilateral metallic hip prostheses. In both simulations, we found CT number differences that were larger than 5 HU due to a residual artifact. However, these differences are not of clinical relevance.