Sammendrag
The relative biological effectiveness (RBE) of protons is highly variable and difficult to
quantify. However, RBE is related to the local ionization density, which can be related to the
physical measurable dose weighted linear energy transfer (LETD). The aim of this study was
to validate the LETD calculations for proton therapy beams implemented in a commercially
available treatment planning system (TPS) using microdosimetry measurements and
independent LETD calculations (Open-MCsquare (MCS)).
The TPS (RayStation v6R) was used to generate treatment plans on the CIRS-731-HN
anthropomorphic phantom for three anatomical sites (brain, nasopharynx, neck) for a
spherical target (Ø=5 cm) with uniform target dose to calculate the LETD distribution.
Measurements were performed at the University Medical Center Groningen proton therapy
center (Proteus Plus, IBA) using a
+
-probe utilizing silicon on insulator microdosimeters
capable of detecting lineal energies as low as 0.15 keV/m in tissue. Dose averaged mean
lineal energy 𝛾̅̅𝐷̅ depth-profiles were measured for 70 and 130 MeV spots in water and for
the three treatment plans in water and an anthropomorphic phantom. The ̅𝛾̅𝐷̅ measurements
were compared to the LETD calculated in the TPS and MCS independent dose calculation
engine. D⋅𝛾̅̅𝐷̅ was compared to D⋅LETD in terms of a gamma-index with a distance-toagreement criteria of 2 mm and increasing dose difference criteria to determine the criteria
for which a 90% pass rate was accomplished.
Measurements of D⋅𝛾̅̅𝐷̅ were in good agreement with the D⋅LETD calculated in the TPS and
MCS. The 90% passing rate threshold was reached at different D⋅LETD difference criteria for
single spots (TPS: 1% MCS: 1%), treatment plans in water (TPS: 3% MCS: 6%) and
treatment plans in an anthropomorphic phantom (TPS: 6% MCS: 1%).
We conclude that D⋅LETD calculations accuracy in the RayStation TPS and open MCSquare
are within 6%, and sufficient for clinical D⋅LETD evaluation and optimization. These findings
remove an important obstacle in the road towards clinical implementation of D⋅LETD
evaluation and optimization of proton therapy treatment plans.
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