Monte Carlo investigation of collapsed versus rotated IMRT plan verification

Abstract

IMRT QA requires, among other tests, a time‐consuming process of measuring the absorbed dose, at least to a point, in a high‐dose, low‐dose‐gradient region. Some clinics use a technique of measuring this dose with all beams delivered at a single gantry angle (collapsed delivery), as opposed to the beams delivered at the planned gantry angle (rotated delivery). We examined, established, and optimized Monte Carlo simulations of the dosimetry for IMRT verification of treatment plans for these two different delivery modes (collapsed versus rotated). The results of the simulations were compared to the treatment planning system dose calculations for the two delivery modes, as well as to measurements taken. This was done in order to investigate the validity of the use of a collapsed delivery technique for IMRT QA. The BEAMnrc, DOSXYZnrc, and egs_chamber codes were utilized for the Monte Carlo simulations along with the MMCTP system. A number of different plan complexity metrics were also used in the analysis of the dose distributions in a bid to qualify why verification in a collapsed delivery may or may not be optimal for IMRT QA. Following the Alfonso et al. (1) formalism, the kQclin,Qfclin,fref correction factor was calculated to correct the deviation of small fields from the reference conditions used for beam calibration. We report on the results obtained for a cohort of 20 patients. The plan complexity was investigated for each plan using the complexity metrics of homogeneity index, conformity index, modulation complexity score, and the fraction of beams from a particular plan that intersect the chamber when performing the QA. Rotated QA gives more consistent results than the collapsed QA technique. The kQclin,Qfclin,fref factor deviates less from 1 for rotated QA than for collapsed QA. If the homogeneity index is less than 0.05 then the kQclin,Qfclin,fref factor does not deviate from unity by more than 1%. A value this low for the homogeneity index can only be obtained with the rotated QA technique.PACS number: 87.55.Qr

Desenvolvimento e avaliação de um sistema de cálculo de dose independente para controle de qualidade de IMRT do tipo jaws-only

“…Além da preocupação com a entrega da dose planejada, há a preocupação com as previsões dosimétricas do SPT e com a transferência de dados para o tratamento. Essas preocupações motivaram diversas publicações sobre o assunto, entretanto, ainda não há consenso de como deve ser realizado o CQ de IMRT [12,14,16,17,18,29,25,19].…”

“…Devido a sua complexidade, o uso na rotina clínica desta técnica de tratamento requer extensivos testes de aceite, comissionamento e um programa de controle de qualidade [12,13,14,15]. Entretanto, não há consenso sobre quais testes devem ser realizados [12,14,16,17,18,19]. Questões como a necessidade de controle individual para cada paciente, de verificação da distribuição de dose para cada campo de tratamento (ou o somatório dos campos) e se esta verificação deve ser realizada em uḿ unico plano (ou de forma tridimensional) ainda estão em discussão.…”