Radiotherapy is a cancer treatment intended to deposit the entire prescribed dose homogeneously into a target volume in order to eliminate the tumor and to spare the surrounding healthy tissues. This paper aimed to provide a dosimetric comparison between the treatment planning system (TPS) ECLIPSE from Varian Medical Systems and the internal dosimetric measurements in a breast phantom. The methodology consisted in performing a 3D conformal radiotherapy planning with two tangential opposite parallel fields applied to the synthetic breast in a thoracic phantom. The irradiation was reproduced in the Varian Linear accelerator, model SL -20 Precise, 6 MV energy. EBT2 Radiochromic films, placed into the glandular equivalent tissue of the breast, were used to measure the spatial dose distribution. The absorbed dose was compared to those values predicted by the treatment planning system; besides, the dosimetric uncertainties were analyzed. The modal absorbed dose was in agreement with the prescribed value of 180 cGy, although few high dose points between 180 and 220 cGy were detected. The findings suggested a non-uniform dose distribution in the glandular tissue of the synthetic breast, similar to those found in the TPS, associated with the irregular anatomic breast shape and presence of inhomogeneities next to the thoracic wall generated by the low lung density.Keywords: Radiotherapy, Film Dosimetry; Thoracic Phantom A radioterapia é um tratamento de câncer que visa depositar toda a dose prescrita de forma homogênea no volume alvo, a fim de eliminar o tumor, poupando tecidos sadios adjacente. Este trabalho teve como objetivo fornecer uma comparação dosimétrica entre o sistema de planejamento (TPS) ECLIPSE da Varian Medical Systems e medidas dosimétricas internas realizadas em um phantom de mama. A metodologia consistiu em realizar um planejamento radioterápico conformacional 3D com dois campos paralelos opostos tangenciais aplicados em uma mama sintética de um phantom torácico. A irradiação foi efetuada no acelerador linear da Varian, modelo SL -20 Precise, com 6 MV de energia. Filmes radiocrômicos EBT2 posicionados no interior do tecido equivalente glandular mamário do phantom foram utilizados para mensurar a distribuição espacial dosimétrica. A distribuição da dose foi comparada com a dose prescrita no TPS e as incertezas foram analisadas. A dose absorvida modal ficou de acordo com o valor prescrito no TPS de 180 cGy, embora tenham sido encontrados poucos pontos de altas doses no intervalo de 180 a 220 cGy . Os resultados sugerem uma distribuição de dose não uniforme no tecido glandular da mama sintética, similar aos valores encontrados no TPS, relacionados à anatomia irregular da mama e a presença de não homogeneidades na parede torácica gerada pela baixa densidade do pulmão.

Radiotherapy is an established method of cancer treatment and is widely applied to control this disease. Medical Linear Accelerators (LINAC) are widely used in modern radiotherapy due to their flexibility and high therapeutic reliability. Traditionally, national and international codes of conduct provide guidelines on dosimetry with a reference field of 10x10 cm², considering that for larger fields, dosimetric parameters are well defined and can be accurately measured. In this work, a LINAC X-ray beam of 6 MV was used to irradiate a solid water phantom, using fields of 10x10 and 5x5 cm². X-ray beam was generated in a 6MV linear accelerator (linac) model Synergy Platform from the manufacturer Elektra. Radiochromic film sheets were used to record dose profiles inside the solid water phantom. For irradiation of the phantom loaded with the film, it was positioned twice 1.0 m away from the focus of the X-ray beam for both field sizes. In the first irradiation, the phantom was exposed laterally to obtain the longitudinal dose variation profile and in the second irradiation, the phantom was irradiated frontally. The longitudinal profile of absorbed dose obtained showed the maximum dose value at 1.30 cm depth for both fields. Axial dose profiles were recorded at 1 cm depth, and showed a plateau in the axis Y for both fields. The plateau for the field of 10x10 cm² in the axis X presented a depression in the central area and that did not happen in the 5x5 cm² field.