Desenvolvimento de um fantoma de tórax e mama para comparações de protocolos de radioterapia mamária

Abstract

Radiotherapy (RT) is one of the therapeutic modalities used in the treatment of breast neoplasms, disease that is one of the main causes of cancer death in the world. The success of RT depends on the correct use of the prescribed doses. In this way, each service must maintain a strict quality control (QC) that guarantees such specificity. Objects simulating human body tissues (phantoms) can be used to aid in the dosimetry and QC of services. Thus, the central objective of this work was to develop a phantom with removable breast to be used exclusively in dosimetric evaluations in radiotherapeutic breast treatments. In addition, to evaluate and experimentally compare the dosimetry from treatment planning systems (TPS) of 4 techniques of mammary RT: Volumetric arc therapy with complementary fields using the field-in-field technique, opposite parallel fields with field-in-field technique, volumetric arch theraphy and conformational therapy with parallel fields of 4 and 6 MeV, performed in 5 RT centers in Spain and Brazil, using the phantom developed in this research. This phantom was constructed based on the available literature, and presented anthropomorphic, anthropometric equivalents compatibility to the human body. Its elemental composition, Housfield number, density, mass attenuation coefficients and flow / Kerma coefficients were evaluated and demonstrated similarity with the references available in the literature and equivalence with female and male biotypes, since the use of removable breasts allows such specification. It has been shown that the phantom is a suitable tool to be employed in female breast RT and QC, and after adjustments male. The irradiation methodology consisted in performing TPS and exposure in the mammary substitute tissue (TS) of the phantom, indicated by each service. Dose measurement was performed by radiocromic films positioned inside the mammary TS of the simulator. The dose distribution was compared to the prescribed dose (PD) by the TPS of the evaluated services. From the results, in the Volumetric arc therapy with complementary fields using the field-infield technique the mean dose was 20% lower than the PD of 200cGy, due to the portion of the film that was outside the mammary TS at the air-skin interface; however, the modal dose in the portion of the internal film reached PD. The opposite parallel fields with field-in-field technique provided dosimetric data that did not corroborate for a good comparison with the experimental dosimetric findings, however within the PD value of 200 cGy. In the volumetric arch theraphy, in a test for the application of the phantom as male breast, there were values up to 68% lower than the PD (200 cGy). In this case, it was verified the need for adjustments in the simulator, as the measured dose represented values of entrance dose in the skin, since the simulator did not present adequate depth. In the two exposures using the conformational therapy with parallel fields, of 4 and 6 MeV, the measured values attended to PD (180 cGy). All services presented random points of high doses, attributed to non- uniformity of the film, and similar behavior in the profile of deep dose, with presence of the build-up effect. It was found that the RT techniques in Spain provide doses more homogeneous, however the techniques in Brazilian RT centers deliver doses within the RT patterns. In addition to the presence of more modern equipment abroad, another perceived difference between RT in Spain and Brazil was the greater freedom of the hospital staff to define the number of fields to be employed, without reducing treatment time. The findings in this research demonstrated the feasibility of phantom use in QC of RT female and male, after adjustment. The experimental dosimetry in phantoms assists in quality assurance and brings interesting results, which contribute to guarantee the quality of the treatments