| Περίληψη: | Dosimetric verification of Volumetric Modulated Arc Therapy (VMAT) based Total Marrow Irradiation (TMI) treatment therapy, actualized in an anthropomorphic phantom. Verification achieved by utilizing both patient specific Quality Assurance (QA) protocol along with in vivo dosimetry with two different types of dosimeters.
Materials and Methods: Initially, an experimental phase was conducted based on a set of CT images previously acquired, in an effort to get familiar with the demands of this treatment characteristics. Several arrangements of isocenter positions and arc geometries were utilized, until concluding that a final plan, that was originated from PTV division into 4 sub-volumes, was going to be implemented. Treatment plan selected for the irradiation of the phantom was aimed towards target coverage and the prescribed dose was 12 Gy in 6 fractions. Subsequently, the anthropomorphic phantom followed all steps of a treating patient in the Radiotherapy Department of Attikon General University Hospital: at first, a set of CT images was acquired, followed by the contouring and treatment planning processes, and eventually a patient specific QA protocol was performed prior to the irradiation phase. In this study, in vivo dosimetry was also performed prior to the irradiation, where the two different types of dosimeters placed inside the phantom in predetermined places. Calibration process was conducted for both types of dosimeters. After the irradiation phase was completed, the readout process of the dosimeters was performed, and the results were compared to the dose estimated by the algorithm of the Treatment Planning System (TPS).
Results: TMI Treatment plan implemented in this study, achieved sufficient dose coverage, with 98% of the target receiving almost 12 Gy (11.9 Gy), while managing to spare the organs at risk and especially lungs which received a mean dose 7.3 Gy (60% of the prescribed dose). Patient specific QA protocol results were satisfactory, with gamma (γ) passing rates ranging from 98.6% to 100%. In vivo dosimetry resulted in an average difference between TPS calculated dose and OSL dosimeter recorded dose of -0.5%, while the corresponding average difference with Radiochromic Films of -3.6%. Differences encountered in the interfaces between air and bone tissue, were anticipated, given the inability of the dose calculation algorithm to replicate dose variations near tissue heterogeneities.
Discussion: By evaluating the results of this dosimetric study, it is feasible that a VMAT based TMI treatment, could be planned and accurately delivered using the existing software and hardware capabilities at Attikon General University Hospital.
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