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ABSTRACT
Università degli Studi di Torino
Scuola di Specializzazione in Fisica Medica (A.A. 2014-2015) Roberto Cirio
Azienda Ospedaliera Ordine Mauriziano di Torino
Direttore: Relatore: Michele Stasi
Tesi di Specialità
EFFECT OF MU REDUCTION IN VMAT: DOSIMETRIC, DVH AND SEGMENT ANALYSIS
Lorenzo Radici
INTRODUCTION. Intensity Modulated Radiation Therapy is nowadays the state of art solution for external beam radiotherapy. In last years Volumetric Modulated Arc Therapy (VMAT) is emerging among the IMRT techniques mainly due to lower overall treatment time. High conformity in IMRT techniques is obtained by means of small segments. It is counterweighted by high plan complexity and a possible loss of accuracy due to various factors: uncertainty in dosimetric measurements used as input to the Treatment Planning System (TPS), inaccuracy of TPS dose calculation, beam delivery system output variation. Scientific publications suggest that better accuracy in dose delivery can be achieved by reducing plan complexity and segments broadening [1][2][3], that are both related with monitor unit (MU) reduction[4][5]. Furthermore, higher MU may increase the risk of radiation-induced cancers. Raystation TPS (RaySearch Laboratories, Stockholm, Sweden) allows the user to set a limit on maximum number of monitor unit for each arc in VMAT optimization. Aim of the work is to investigate the feasibility of increasing plan accuracy in VMAT by means of MU reduction. The study was conducted at A.O. Ordine Mauriziano Hospital (Turin, Italy). Statistical analysis of plans with or without MU reduction was performed for Head and Neck and prostate plans. The effectiveness of MU limit was verified. Segment broadening was evaluated; dosimetric accuracy of plans with and without MU reduction was compared. It was also verified that a suitable limit in MU does not affect DVH and dose distribution quality.
MATERIALS AND METHODS. VMAT treatments were performed with an Elekta Synergy Linac delivering 6 MV photon beam. The system was equipped with Agility multileaf collimator. Treatment plans was elaborated with Raystation TPS (version 4.7) using a collapsed-cone algorithm.
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