Physics contribution
Radiotherapy dose perturbation of metallic esophageal stents

https://doi.org/10.1016/S0360-3016(02)03803-8Get rights and content

Abstract

Purpose: Metallic esophageal stents frequently present during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy. The dosimetric effects due to these metallic stents have not been reported. This work investigates these dose effects for various stent models presented during a radiotherapy procedure.

Methods and Materials: Two types of representative stent models, shell and ring stents, with various designs (e.g., composition and shell thickness or ring spacing), were studied. Three Monte Carlo code systems (EGS4/BEAM, EGSnrc/DOSRZnrc, and MCNP) were used to calculate the dose distributions for 6- and 15-MV external photon beams and for a 192Ir brachytherapy source with and without a metallic esophageal stent in place.

Results: For a single external beam, a dose enhancement is generally observed in front of the stent (upstream) in the region within 4-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 20%. The dose behind the stent (downstream) is generally reduced. For a parallel-opposed pair (POP), a dose enhancement is always observed in the region within 3-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 10% for the 15-MV POP and 8% for the 6-MV POP. The dose effects depend on stent design (e.g., composition, thickness of shell stent, or ring spacing in ring stents). This dependence is reduced for a POP. In the case of the 192Ir brachytherapy source, a dose enhancement is observed in the region within 1-mm distance from the stent surface. The dose enhancement is approximately 5% at 0.5-mm distance from the stent surface.

Conclusion: The dose perturbations due to the presence of a metallic esophageal stent during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy should be recognized. These perturbations result in an overdose in esophageal mucosa. The overdose is within 5%–10% at a depth of 0.5 mm in the esophageal wall.

Introduction

Esophageal cancer is an important problem in the United States. It results in over 10,000 deaths annually. Furthermore, the incidence of esophageal adenocarcinoma is increasing at a rate faster than that of nearly any other cancer, and the reasons for the increase are not well understood (1). Despite improvements in surgical techniques and perioperative mortality, only slight improvement in the 5-year survival of patients with esophageal cancer have been observed in the last 20 years. Many patients with apparently localized cancer will have recurrences or metastatic disease despite surgery with curative resection. Consequently, multimodal therapies, including chemotherapy and radiotherapy, were introduced. In recent years, self-expanding metallic stents have assumed importance in the palliative treatment of inoperable esophageal cancer and of extrinsic compression of the esophagus by other malignant masses 2, 3, 4, 5, 6, 7. Stents are also used occasionally as a last resort in benign disease. They are relatively easy for practicing endoscopists to insert and have low rates of early complications. The majority of stents are placed with the help of endoscopy and fluoroscopy while the patient is under sedation. Once deployed, stents expand in the esophagus, causing pressure necrosis on the wall of the esophagus. A variety of stents with different designs and compositions are available on the market with newer stents continuing to emerge. Figure 1 shows five examples of metallic esophageal stents used in the clinic. The major characteristics of these stents are tabulated in Table 1. These stents are made of stainless steel or Nitinol (mainly made of nickel and titanium). The choice of stent seems random among clinicians. Combination therapy using stenting followed by radiotherapy and/or chemotherapy increases quality survival 2, 7. Two forms of radiotherapy may be used: external beam or brachytherapy (mainly high-dose-rate brachytherapy).

The radiotherapy dose will inevitably be perturbed by the presence of a metallic stent, and this perturbation has not been reported. The purpose of this work is to calculate the dosimetric perturbations caused by the presence of a metallic esophageal stent during the treatment of esophageal cancer using either external beam radiotherapy or brachytherapy. The calculation is performed using Monte Carlo techniques for various representative stent models.

Section snippets

Stents

Exact modeling of a stent’s physical design is normally difficult with Monte Carlo calculations. In this work, we used two stent models, ring stent and shell stent, to approximate typical designs representing actual esophageal stents. A mesh or coil type of stent (e.g., Wallstent, Ultraflex) may be considered a ring stent, provided the stent has the same metallic surface area. The shell stent with 100% metallic surface area may be used to represent an EsophaCoil stent, because its metallic

External beams

Figure 3 presents the DPF values plotted against the distance from the stent axis for (a) a single 6-MV photon beam and (b) a single 15-MV photon beam. The calculation was carried out for three steel shell stents with a shell thickness of 0.2, 0.5, and 1.0 mm. Only the data points within 10 mm of the stent surface in both the upstream and downstream regions are shown. It is clear from Fig. 3 that a dose enhancement is generally observed in the upstream region, whereas a dose reduction is found

Discussion and conclusions

The dose effects due to the presence of a metallic esophageal stent during the treatment of esophageal cancer with either external beam radiotherapy or brachytherapy have been calculated using Monte Carlo techniques. The results obtained for various representative stent models irradiated by 6- and 15-MV external photon beams and by a 192Ir HDR brachytherapy source are presented.

For treatment with a single external photon beam, a dose enhancement is generally observed in the upstream region (in

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