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Bringing 3D portal dosimetry in the clinic
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Resumo(s)
The increasing complexity of radiotherapy treatments requires comprehensive quality assurance (QA) programmes. The clinical standard is pre-treatment verification by portal dosimetry, where measured portal images acquired from electronic portal imaging devices (EPIDs) are compared with those predicted by the treatment planning system. This thesis addresses two challenges: reinstating two-dimmensional (2D) pre-treatment portal dosimetry on a TrueBeam linear accelerator (Study A) and implementing three-dimensional (3D) in-vivo dosimetry using the research software EpiCoreMedPhys (ECMP) (Study B). In Study A, portal dosimetry performance on the TrueBeam was compared to the Edge Linac, where it is routine practice. For 20 breast patients, the Edge achieved better results, i.e., higher gamma passing rates (100% vs 97.8%) and lower mean gamma values (0.196 vs 0.364). Following EPID replacement and a portal dose image prediction (PDIP) upgrade in 2022, further improvements were observed in 28 patients (99.9% vs 95.4%, and 0.200 vs 0.380), on the Edge Linac. Direct comparison of PDIP versions 15.6 and 13.7 in 10 patients showed slightly superior results for the updated version (98.2% vs 97.9%). Jaw tracking did not impact gamma metrics but revealed visual collimator leakage in the portal images when not used. Study B investigated a systematic 5% underdosage in previous in-vivo dosimetry results. Recalibration of the dose conversion (G-value) improved the in-phantom workflow, reducing D50% discrepancies between measurements and planning doses (from -1.9% to -1.29%). However, systematic in-vivo errors persisted, traced to a missing conversion factor in the transmission model. After correction, residual overdose was linked to couch interactions, requiring a 2.5% adjustment for specific irradiation angles. In 12 pelvic patients, the mean Field D50% dose difference improved from -5% to -1.29%. These results support the reinstatement of portal dosimetry in TrueBeam accelerator and justify clinical implementation of 3D in-vivo dosimetry.
Descrição
Tese de Mestrado, Engenharia Biomédica e Biofísica, 2025, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Radiotherapy Quality Assurance EPID Portal dosimetry In-vivo dosimetry