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Kilovoltage radiotherapy for companion animals: dosimetric comparison of 300 kV, 450 kV, and 6 MV X-ray beams

July 6, 2018

From the Journal of Veterinary Science

Abstract

Radiotherapy for the treatment of cancer in companion animals is currently administered by using megavoltage X-ray machines. Because these machines are expensive, most animal hospitals do not perform radiotherapy. This study evaluated the ability of relatively inexpensive kilovoltage X-ray machines to treat companion animals. A simulation study based on a commercial treatment-planning system was performed for tumors of the brain (non-infectious meningoencephalitis), nasal cavity (malignant nasal tumors), forefoot (malignant muscular tumors), and abdomen (malignant intestinal tumors). The results of kilovoltage (300 kV and 450 kV) and megavoltage (6 MV) X-ray beams were compared. Whereas the 300 kV and 6 MV X-ray beams provided optimal radiation dose homogeneity and conformity, respectively, for brain tumors, the 6 MV X-rays provided optimal homogeneity and radiation conformity for nasal cavity, forefoot, and abdominal tumors. Although megavoltage X-ray beams provided better radiation dose distribution in most treated animals, the differences between megavoltage and kilovoltage X-ray beams were relatively small. The similar therapeutic effects of the kilovoltage and 6 MV X-ray beams suggest that kilovoltage X-ray beams may be effective alternatives to megavoltage X-ray beams in treating cancers in companion animals.

Introduction

Radiotherapy is important in cancer treatment [6], including the treatment of cancer in companion animals. For example, radiotherapy has prolonged survival in dogs with nasal adenocarcinomas, when compared with results from surgery or chemotherapy [12]. Moreover, radiotherapy, both alone and in combination with surgery, has produced good therapeutic outcomes in animals with intracranial meningiomas [2], and radiotherapy has provided excellent therapeutic efficacy for animals with difficult-to-treat mandibular squamous cell carcinomas and genitourinary carcinomas [3, 4, 7, 11, 13, 23, 26].

Although the incidence of cancer is generally higher in companion animals than in humans [8, 9, 25, 27], methods of treating cancers in companion animals are limited [20]. Radiotherapy of companion animals usually involves the use of a linear accelerator designed to treat humans. Clinical linear accelerators accelerate electrons by using high-frequency electromagnetic waves, producing high-energy X-ray beams when the electrons hit their target. Although these X-rays are effective in treating deeply located tumors, the use of these devices to treat cancers in companion animals is limited by their large size, high cost, and complicated protocols. Moreover, these devices require shielding to protect people in the vicinity against high-energy radiation. At present, few animal hospitals in the USA, Europe, and Japan use linear accelerators to treat companion animals.

Tumors are generally located at a shallower depth in companion animals than in humans, largely because these animals are smaller. Because X-ray beams in animals are not required to penetrate as deeply as the high-energy high-penetration beams used to treat human cancers, lower energy X-ray beams may be sufficient to treat cancers in these animals. Kilovoltage X-ray beams of 150 to 500 kV may be effective for treating cancers in animals. These beams require less energy than conventional megavoltage X-ray beams used to treat human cancers and do not require a device to create microwaves, making kilovoltage devices easier to manufacture and more cost-effective. In addition, kilovoltage devices are smaller and require less shielding than megavoltage devices, making kilovoltage devices easier to install. In this study, a commercial treatment-planning system was used to simulate treatment of various tumors in dogs with 300 kV, 450 kV, and 6 MV X-ray beams. The ability of kilovoltage X-ray machines to treat companion animals was evaluated by comparing the results of kilovoltage and megavoltage X-ray beam treatment.

Discussion

The present study compared the results of 300 kV, 450 kV, and 6 MV X-ray beams for radiotherapy of tumors in companion animals. The comparisons included assessment of radiation doses (via DVH evaluation) delivered to tumor and adjacent organs, as well as assessment of the associated HI and CI results.

The build-up distances for the 300 kV (0–0.4 cm) and 450 kV (0–0.8 cm) X-ray beams were much shorter than that for the 6 MV X-ray beam (1.6 cm), which may result in weaker skin protection. Analyses based on DVHs and V30, V60, and V90 doses suggested that all three X-ray beam energies yielded similar dose distributions when treating brain and nasal tumors. For brain tumors, the 300 kV X-ray beam yielded the optimal HI (6.3), whereas the 6 MV X-ray beam yielded the optimal CI (3.37). For the nasal, forefoot, and abdominal tumors, the 6 MV X-ray beam provided the optimal HI (9.1, 10.0, and 6.4, respectively) and CI (2.38, 1.17, and 1.22, respectively) values. The differences among the various beam energies for abdominal tumors may be associated with their relative depth; as well, abdominal tumors are larger (170.2 cm3) than brain (1.9 cm3), nasal (7.4 cm3), and forefoot (142.7 cm3) tumors.

Compared with the results of the kilovoltage X-ray beams, the megavoltage X-ray beam generally delivered lower doses to surrounding organs. These differences, however, were not statistically significant and may have negligible effect in the treatment of cancers in animals. The results indicate that radiotherapy using kilovoltage X-ray beams may be feasible in companion animals. Our results suggest that the manufacturing, management, and shielding advantages of kilovoltage X-ray beams could contribute to the more widespread use of radiotherapy for companion animals.

The present study was limited by the small number of tumors examined, suggesting the need for further research on the effects and effectiveness of kilovoltage X-ray beam treatment for companion animals.

View original publication

Jaehyeon Seo,1 Jaeman Son,1 Yeona Cho,2 Nohwon Park,3 Dong Wook Kim,4 Jinsung Kim,2 and Myonggeun Yoon1
1Department of Bio-Convergence Engineering, Korea University, Seoul 02841, Korea.
2Department of Radiation Oncology, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul 03722, Korea.
3Korea Animal Cancer Center, Seoul 01684, Korea.
4Department of Radiation Oncology, Kyung Hee University Hospital at Gangdong, Seoul 05278, Korea.

Corresponding author: Tel: +82-2-3290-5651; Fax: +82-2-940-2829; Email: radioyoon@korea.ac.kr (M Yoon) Corresponding author: Tel: +82-2-3290-5651; Fax: +82-2-940-2829; Email: jinsung.k@gmail.com (J Kim)

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