Total monitor units influence on plan quality parameters in volumetric modulated arc therapy for breast case.
OBJECTIVE
To investigate the correlation between total monitor units (MU), dosimetric findings, and pre-treatment quality assurance for volumetric modulated arc therapy (VMAT) by RapidArc (RA).


METHODS
Ten patients with breast cancer were considered. Dose prescriptions were: 48 Gy and 40.5 Gy in 15 fractions to, respectively, PTV(Boost) and PTVWholeBreast. A reference plan was optimized and four more plans using the "MU Objective", a tool for total MU controlling, were prepared imposing ± 20 and ± 50% total MU for inducing different complexities. Plan objectives were: D95% > 95% for both PTVs, and D2% < 107% for PTV(Boost); mean dose < 9.5 Gy and V20 Gy < 10% for ipsilateral lung; V18 Gy < 5% for heart; mean dose <3 Gy for controlateral breast; furthermore V5 Gy, V10 Gy, V20 Gy, and V30 Gy to body were minimized. Plans were evaluated in terms of technical parameters, dosimetric plan objectives findings and pre-treatment quality assurance (QA).


RESULTS
Concerning PTVs, there were no significant differences for target coverage (D95%); mean doses for ipsilateral lung and controlateral breast, and V18 Gy for heart decreased with MUs increasing, reaching a plateau with reference plan. Body volume receiving low dose (V5-10 Gy) was minimized for reference plans. All plans had GAI (3 mm, 3%) > 95%.


CONCLUSIONS
The data suggest that the best plan is the reference one, where the "MU Objective" tool was not used during optimisation. Nevertheless, it is advisable to use the "MU Objective" tool for re-planning when low GAI is found to increase its value. In this case, attention should be paid to OARs dose limits, since their values may be increased.
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy. Electronic address: pietro.mancosu@humanitas.it.
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Mancosu
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Reggiori
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Alongi
Medical Physics Unit, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland.
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Cozzi
Medical Physics Unit, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland.
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Fogliata
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Lobefalo
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Navarria
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Stravato
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Tomatis
Radiotherapy and Radiosurgery Humanitas Cancer Center, Rozzano, Milano, Italy.
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Scorsetti
D001943:Breast Neoplasms / Q000532:radiotherapy*; D006801:Humans; D058958:Organs at Risk / Q000528:radiation effects; D011786:Quality Control; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects / Q000379:methods*; D012189:Retrospective Studies
Plan quality; Radiotherapy; RapidArc; Volumetric modulated arc therapy