EP-1305: Management of the interruptions in the conventional treatment of prostate cancer upon a database analysis

Abstract

Purpose/Objective: The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments requires knowledge of the position of the isocentre to sub millimetre accuracy. The deviation between the radiation and mechanical isocentres must be less than 1 mm. The use of an add-on micromultileaf collimator (μMLC) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam centre of the cone-beam CT system was included in the study. Materials and Methods: The quantification of the gantry rotational performance was done using a pointer supported by an inhouse made rigid holder attached to the gantry head. The pointer positions were measured using a digital theodolite. The displacement of the isocentre due to an add-on μMLC of 50 kg was investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocentre. Additionally, the displacement of the radiation isocentre was measured using a Winston-Lutz phantom and the electronic portal image device (EPID) system. These measurements were based on eight MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. Results: The displacement of the mechanical isocentre caused by a 50 kg heavy μMLC was found to be (-0.01±0.06, -0.10±0.03, -0.26±0.05) mm in lateral, longitudinal and vertical direction, respectively. Similarly, the displacement of the radiation isocentre was found to be (0.00±0.03, -0.08±0.06, -0.32±0.02) mm. Good agreement was found between the displacement of the two isocentres. A displacement of the kV cone-beam CT beam centre due to the attached weight of 50 kg could not be detected. By comparing the CW and CCW data the presence of the effect of backlash appears. The effect of backlash was found to be < 0.14 mm and < 0.10 mm in the lateral and vertical direction, respectively. Conclusions: General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in an isocentre displacement downward of 0.26 to 0.32 mm. We recommend that the beam centre of the kV cone-beam CT image system should be matched to the isocentre related to the weight of the μMLC. Consequently, the imperfections in isocentre localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the sub millimetre regime are negligible.