Feasibility study of Modulated Arc Beam TBI treatment rescheduling between two Linear Accelerators
Maria Victoria Gutierrez,
Italy
PO-1643
Abstract
Feasibility study of Modulated Arc Beam TBI treatment rescheduling between two Linear Accelerators
Authors: Maria Victoria Gutierrez1, Paola Ceroni1, Elisa Cenacchi1, Grazia Maria Mistretta1, Luigi Manco1, Luciano Morini1, Lodovica Boni1, Silvia Pratissoli2, Giuseppina De Marco2, Nicola Maffei1, Francesca Itta1, Federica Campanaro1, Gabriele Guidi1
1University Hospital of Modena, Medical Physics, Modena, Italy; 2University Hospital of Modena, Radiotherapy Unit– Oncology and Hematology, Modena, Italy
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Purpose or Objective
In our
department modulated arc beam Total
Body Irradiation (TBI) is performed on two matched beam 6MV Elekta Versa linear
accelerators. If necessary, conventional VMAT treatments are rescheduled
from Linac1, to the other commissioned system, Linac2. For commissioning of Linac 1, originally, a beam profile, parallel to the direction of gantry
rotation and generated by an arc between 290º and 70º using a 40x40cm2
field size, had been characterized.
The aim of the present analysis was to evaluate the variability of VMAT TBI
treatments between two Linacs fully matched for conventional VMAT
treatments without additional profile beam characterization measurements and
replanning for Linac 2.
Material and Methods
Beam characterization measurements were performed
with a PMMA solid phantom (Acrylic Slab T.2967, 30x30x30cm3, ρ=1.19g/cm3) and a 0.6cm3 TM30006 Farmer type
ionization chamber. In order to provide a modulated arc beam, individual
contribution and weighting factors of each sub-arc were determined. Profile
acquisitions were performed for dose estimation at patient`s skin and mid
prescription plane. Acquisition of a PDD curve in TBI setup was performed.
In-Vivo dosimetry was implemented with diodes controlled by the dedicated
Software PTW-VivoSoft. In order to
verify the feasibility of rescheduling
treatment between Linacs, on Linac2 only profile acquisition for dose
estimation at patient`s skin and mid-plane; homogeneity; and PDD curve, were
performed. For every clinical TBI treatment, pre-treatment quality assurance
and dose evaluation were performed in order to validate the calculated monitor
units, as well as in vivo measurements during first fraction.
Results
Theoretical curves proved to be accurate in prediction of weighing factors for determination of modulated arcs for both Linac1 and Linac2, with homogeneities of 2,3% & 1.42% in L-R profiles, respectively. Homogeneities derived from measurements in phantom surface were 7,06% & 4.67% with respect to CAX and 7,11% & 3.94% with respect to prescription mid-plane. PDD curves in TBI conditions of both Linacs matched within 0.4%.
For 22 treated patients, 18 Linac1 and 4 Linac2, pre-treatment dose measurements differed from prescribed doses within 1.1% ± 1.86% on average. For Linac1 & Linac2, pre-treatment dose measurements were within 4.5% & 3.5% of prescribed dose; while clinical in vivo dosimetry, within 3.0% & 4.5% of prescribed dose, respectively. For clinical treatments, pre-treatment dose evaluation and in vivo dosimetry were within 5% of the prescribed dose regardless of the Linac.
Conclusion
VMAT-TBI Dosimetry is robust between Linacs that are fully matched for conventional VMAT treatments, adding therefore easy patient transferability to other advantages of the technique, such as Independence of treatment room size; optimal dose homogeneity throughout the body, comfort and reproducibility of patient position and short treatment times.