Dose profile evaluation for small fields of a LINAC 6 MV beam using a solid water phantom

Abstract

Radiotherapy is basically used in oncology and consists of the use of ionizing radiation to stop neoplastic cells from reproducing or to kill cells. Radiotherapy is one of the three main modalities of treatment for malignant diseases such as cancer, the other two are chemotherapy and radiosurgery. Unlike other medical specialties that rely primarily on the clinical knowledge and experience of specialist physicians, radiotherapy relies heavily on modern technology and the collaborative effort of several professionals whose coordinated team approach greatly influences the outcome of treatment. Linear accelerators with jaw or multileaf (MLC) field colimators are used to irradiate patients undergoing therapy and use fields with dimensions ranging from 4×4 to 40×40 cm². However, looking for the delivery of prescribed doses increasingly conforming to the target volume, small field beam openings are being used in advanced photon beam radiotherapy. In this work, the absorbed dose distribution of an X-ray beam was recorded using a solid water phantom. This phantom was irradiated using small fields with 1×1, 2×2, 3×3 and 5×5 cm². The 6 MV X-ray beam was generated on a Synergy Platform model linear accelerator from the manufacturer Elekta, and sheets of radiochromic film were used to record the dose profiles within the phantom. The solid water phantom loaded with radiochromic film was positioned 1.0 m away from the focus of the X-ray beam. The longitudinal profile of absorbed dose obtained showed the maximum dose value at 1.26 cm (peak) depth inside the phantom. The smaller field size (1×1) generated a smaller maximum absorbed dose (peak). Axial dose profiles were recorded at 1 cm depth and showed a plateau in the central region of the field. The mean absorbed dose at the plateau measured at 1 cm depth ranged from 98% to 90% of the maximum dose for the 5x5 and 1x1 cm² fields, respectively. The experiments performed showed a variations in the absorbed dose values due to the size of the fields. These variations occurred as in the amplitude of peak value in the depth profile as in the axial plane of distribution.