The influence of bone density on the radiotherapy of cervix cancer

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Abstract

Until the 1970s the irradiated region of a patient undergoing external beam radiotherapy was considered a homogeneous volume and a regular surface, with physical characteristics similar to water. With the improvement of medical imaging equipment, it has become possible to conduct planning in radiotherapy treatment that considers the heterogeneities and irregularities of a patient's anatomy. Consequently, such technological resources have brought greater accuracy to radiotherapy. In this study, we determined the variation in the average amount of absorbed dose on the target volume and at the point of prescription treatment by comparing the doses which were calculated in a planning system considering the patient both as a homogeneous, and as a heterogeneous medium. The results showed that when we take into account the volume of the upper vagina and cervix, and consider the pelvis as a heterogeneous medium, the calculated dose was under-estimated at some points in the studied volume with respect to the dose when this region was considered homogeneous.

Introduction

With approximately 500,000 new cases worldwide annually, invasive cervical cancer is the second most commonly diagnosed cancer in women. Its incidence is higher in developing countries. A peculiarity in these countries is that the majority of diagnosed cases are at a relatively advanced stage [1], [2].

Currently, high-dose rate (HDR) brachytherapy combined with external beam radiation therapy (EBRT) and chemotherapy technique have been used to treat cervical cancer. Generally, EBRT uses the four orthogonal fields technique to irradiate all tumour cell infiltrations, including the target volume, and deliver low doses of radiation to healthy adjacent tissues to the planning target volume (PTV) or inside of it. The disease stages are defined by the International Federation of Gynecology and Obstetrics. Each stage has sub-stages according to the clinical course of the disease in the affected organ [3], [4].

Until the 1970s the irradiated region of a patient undergoing external beam radiotherapy was considered a homogeneous volume and a regular surface, with physical characteristics similar to water [5]. With the improvement of medical imaging equipment, it has become possible to conduct a better planning in radiotherapy treatment that considers the heterogeneities and irregularities of a patient's anatomy. Consequently, such technological resources have brought greater accuracy to radiotherapy [6].

Many cancer centres have now 3D treatment planning systems (TPS). This software allows the creation of 3D plans that digitally reconstruct a patient's anatomy and provide data on the volumes and densities of organs to be irradiated as well as their vicinity. The result makes possible an improved analysis and better comparison between different treatment plans, in addition to evaluating the distribution of the prescribed dose in the target volume and protecting adjacent structures. The main tools used for these analyses are isodose curves and dose–volume histograms (DVHs), with the latter being the most currently used [6].

In radiotherapy, depending on the anatomical region being treated in the patient, the radiation beam can span several structures of different densities, such as lung and bone within a soft tissue. However, such variations in density can alter the distribution of the absorbed dose, resulting in modifications to the radiation beam absorption. Such a change in dose depends on the nature of the materials which are present in the beam path and its energy [4].

Despite these technological advances in radiotherapy and the technical resources to improve quality control, treatment planning in radiation therapy departments in underdeveloped or developing countries continues to be made by considering the pelvis as a homogeneous medium with a regular surface, thus ignoring the existence of bone tissue in the path of the beam, and this can, in turn, affect the dose distribution [4].

This study aims at determining the variation in the average amount of absorbed dose on the target volume and at the point of the prescribed treatment by comparing the calculated doses in a planning system which considers the patient as a homogeneous as well as a heterogeneous medium. Furthermore, it will also investigate whether the dose gradients in the target volume, considering the variations in the density of the pelvic region in both types of planning systems, are consistent with the limit recommended by the International Commission on Radiation Units and Measurements (ICRU). The commission´s document ICRU-50 recommends that the delivery dose variation to the target volume be at most between 95% and 107% of the prescribed dose [7], [8].

Section snippets

Materials and methods

For this study, 20 radiotherapy treatments planning of diagnosed patients with cervical cancer were selected. These plans were based on homogeneous volumes. In this group, 12 patients had the disease at stage IIB and eight at stages IIIA and IIIB. The cases were originally planned for radiation therapies that were performed at a public hospital in Brazil between 2006 and 2008.

The adopted criteria for the selection of the treatment plans was that there were available data on the computed

Results

The increase of bone density resulted in a reduction in the absorbed dose to the reference point (Fig. 1). We observed a standard deviation of 0.15% compared with the absorbed dose at the reference point.

ICRU-50 recommends a variation of up to –5% of the prescribed delivered dose to the target volume [7]. In this case, even when assessing only a reference point, one can see that bone density affects the dose calculation at the point of prescription. By considering the homogeneous medium, we

Conclusion

The results show that when we take into account the volume of the upper vagina and cervix, and consider the pelvis as a homogeneous medium, the calculated dose to the target volume and dose gradient in the entire volume were within the recommended limits by ICRU-50. However, when we consider the pelvic region as a heterogeneous medium, the calculated dose was below 95% of the prescribed dose at some points in the studied volume, in other words, outside the lower recommended limit.

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Acknowledgment

This work was partially supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa e à Inovação Tecnológica do Estado de Sergipe (FAPITEC/SE), Brazil.

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