Diseases of the esophagus include motility disorders (achalasia, diffuse spam), hiatal hernias, diverticula, perforation, foreign bodies, chemical burns, gastroesophageal reflux disease (GERD), Barrett’s esophagus, benign tumors, and carcinoma, usually occurring in the fifth or sixth decade of life . Cancer of the esophagus, which has a much higher extent (10 to 100 times higher) in some parts of the world including China and the north of Iran, can be of two categorized types: adenocarcinoma and squamous cell carcinoma. The rate of adenocarcinoma is rapidly increasing in the USA as well as in other western countries. It is found primarily in the distal esophagus and gastroesophageal junction . Risk factors for squamous cell carcinoma of the esophagus include chronic ingestion of hot liquids or foods, nutritional deficiencies, poor oral hygiene, exposure to nitrosamines in the environment or food, cigarette smoking or chronic alcohol exposure, and some esophageal medical conditions such as caustic injury. The risk factor for esophageal cancer includes chronic esophageal irritation. There is an apparent association between GERD and adenocarcinoma of the esophagus. People with Barrett’s esophagus, which is caused by chronic irritation of the mucous membrane due to reflux of gastric and duodenal contents, are more likely to have esophageal cancer.
While detection of esophageal cancer at the early stages can make treatment effective, it is often identified at late stages, making relief of symptoms the only reasonable goal of therapy. Treatment can include surgery, radiotherapy, chemotherapy, or a combination of these modalities, depending on the type of cancer cell, the extent of the disease, and the patient’s condition. A standard treatment plan for a person who is newly diagnosed with esophageal cancer includes the following: preoperative combination chemotherapy and radiation therapy for 4 to 6 weeks, and lastly, surgical resection of the esophagus.
It has been found that locally advanced esophageal cancers are resistant to the conventional X- or gamma-ray radiotherapy with low linear energy transfer (LET). Furthermore, the place of these tumors in the human body has made it easily accessible to use the brachytherapy sources . Neutron brachytherapy (NBT) is a form of high-LET radiotherapy and is expected to be effective in killing the radioresistant esophageal cancer cells. NBT using 252Cf sources has been the subject of interest in recent years for treating advanced-stage cancers [3, 4]. It has been shown that NBT is more effective than conventional photon brachytherapy in treating radioresistant tumors such as bulky, late-stage tumors, melanomas, and glioblastomas .
Clinical NBT sources are designed in three forms: seed, needle, and applicator tube (AT) . Needle type and seeds are generally used for interstitial soft tissue implants and surface applicators. Owing to the fact that manual loading of radioactive sources, which refers to the insertion of radioactive sealed sources into the patient by a staff member (usually the radiation oncologist), involves radiation protection concerns, the safety guide implementers should be considered. The tube sources containing more than 1 mg of 252Cf can produce considerably high neutron flux which is of interest in this treatment method. 252Cf remote-afterloading devices with three sources (two ovoids and tandem with initial source strength of 0.4 and 1.3 μg of Cf, respectively) have been adapted for gynecological applications . Due to their compact size, these sources not only can be used for the treatment of esophageal cancers, but also for treating rectum and brain tumors .
252Cf has a half-life of 2.645 years. While this source emits alpha particles with a probability of 96.9%, they cannot escape from the source capsule arisen from their short range in the medium. The remainder, 3.1% of 252Cf, decays through spontaneous fission which each of them produces two or three fission fragments as well as an average of 3.77 neutrons . One microgram of 252Cf equals 0.536 mCi and emits 2.31 × 106 n/s. These neutrons provide an energy spectrum that is often modeled as either Maxwellian or Watt fission spectrums, with a peak at about 0.7 MeV and a rapid decrement at both higher and lower energies [10, 11]. 252Cf also emits photons and beta particles. Nearly half of the photons emitted by 252Cf (each microgram emits 1.32 × 107 photons/s) are prompt fission gamma rays, and the others belong to the delayed gamma rays. Though they are quite different in the spectrum, their mean energy is about 0.8 MeV. While the fission product gamma-ray spectrum peaks near the mean energy, the prompt gamma-ray spectrum has a considerable component above 3 MeV and increases exponentially by the decrement of the energy. The gamma ray associated with alpha-particle decay is negligible (< 0.1%). Because the fission products gradually build up in a sealed 252Cf source, it is expected that the absorbed dose of gamma rays compared with those of the neutrons emitted from the source would change in time .
Considering the importance of esophageal cancers and the advantages of NBT, the effectiveness of using 252Cf sources for the treatment of esophagus tumors needs to be investigated in detail. The accuracy and great flexibility have made Monte Carlo a consistent method for simulating the transport of particles through matter to achieve accurate data for designing a treatment plan and pre-clinical tests. There are several worthwhile studies dealing with Monte Carlo simulation of AT Model 252Cf source [4, 9, 12,13,14,15,16]. However, to our knowledge, there is not a study devoted to the investigation of the effect of this source in the treatment of esophagus tumors and the dose delivered to the neighboring organs. In the present study, the Monte Carlo method is used to carry out the dose delivered to the esophagus tumor, and the healthy surrounding organs as well, due to the irradiation of 252Cf neutrons implanted in a simulated human phantom. For the evaluation and examination of the accuracy of the simulated source (known as benchmarking), the American Association of Physicists in Medicine (AAPM) Task Group No.43 Report in 1995 (TG-43)  is used. In order to reduce the dose to the non-target healthy tissues, various materials and thicknesses are tested to optimize an appropriate shield for this source. For dose evaluations, the MIRD phantom containing various organs is used, which had not been taken into account in the previously related published works .
The study has been organized in the following sections: a brief introduction of the 252Cf source and detailed description of the model used in the present work, explanations about the human phantom employed in the simulations and the methods for dose evaluation as well, balloon and shield designing, and discussion on the results obtained. The simulations and radiation transport calculations in this study are performed with the MCNPX (Version 2.6.0)  Monte Carlo code. The results reported correspond to the adequate number of histories with the relative errors of about 1%.