Abstract
BackgroundWe sought to establish a conversion curve to convert the RBE-weighted doses calculated by local effect model I (LEM) (LEM RBE-weighted doses) in patients with locally recurrent nasopharyngeal carcinoma (rNPC) to the RBE-weighted doses calculated by microdosimetric kinetic model (MKM) (MKM RBE-weighted doses). We also converted the LEM dose constraints (RBE-weighted dose constraints in LEM plans) for the brain stem, spinal cord, and optic nerve based on this curve.MethodsData from 20 patients with rNPC receiving carbon-ion radiotherapy (CIRT) in our hospital were collected. LEM in Raystation (V8A, Raystation, Sweden) was used to generate treatment plans. The clinical target volume CTV1 (GTV + 5 mm) was given 3 Gy (RBE) per fraction. Ninety-nine percent of target volumes should be covered by 95% of the prescriptions; the maximum doses of the brainstem and spinal cord were < 45 Gy (RBE) and < 30 Gy (RBE), respectively. The doses covering 20% volumes of optical nerves/chiasms D20 were < 30 Gy (RBE). Then physical doses of the LEM plans were recalculated by using MKM in Raystation to generate MKM plans. A series of conversion factors (i.e., the ratio of LEM RBE-weighted dose to MKM RBE-weighted dose) was then obtained by using an isovolumetric dose method. The LEM plan prescriptions (LEM prescription) and dose constraints of the organs at risk (OARs) (OAR constraints) were converted to the corresponding MKM prescriptions and dose constraints using this conversion curve.ResultsFor the CTV1 fractional RBE-weighted dose prescription of 3.00 Gy (RBE) and CTV2 of 2.70 Gy (RBE) in LEM plans, the conversion factors (LEM RBE-weighted dose/MKM RBE-weighted dose) were 1.37 (CI 95% 1.35–1.39) and 1.46 (1.41–1.51), respectively. The average conversion factors from 1.37 (CI 95% 1.33–1.41) to 3.09 (2.94–3.24) corresponded to the LEM fractionated doses from 2.86 Gy (RBE) to 0.24 Gy (RBE), including the doses constraining upon OARs. LEM RBE-weighted doses of 30 Gy (RBE) and 45 Gy (RBE) in 21 fractions were converted to MKM RBE-weighted doses of 16.64 Gy (RBE) and 30.72 Gy (RBE) in 16 fractions.ConclusionsThis conversion curve could be used to convert LEM RBE-weighted doses to MKM RBE-weighted doses for patients with rNPC receiving CIRT, providing dose references for re-irradiation therapy.
Highlights
We sought to establish a conversion curve to convert the RBE-weighted dose prescription of 3.00 Gy (RBE)-weighted doses calculated by local effect model I (LEM) (LEM RBE-weighted doses) in patients with locally recurrent nasopharyngeal carcinoma to the RBE-weighted doses calculated by microdosimetric kinetic model (MKM) (MKM RBE-weighted doses)
The conversion curve outside Clinical target volume (CTV) is consistent with the previous findings of the other scholars [13, 15]: as the prescription dose increases, the difference between the LEM RBE-weighted dose and the MKM RBE-weighted dose gradually decreases
The LEM RBE-weighted doses of some patients could not find the corresponding MKM RBE-weighted dose value in our study since the calculated LEM RBE-weighted doses are always greater than the corresponding MKM RBE-weighted dose within dose range based on the same physical dose
Summary
We sought to establish a conversion curve to convert the RBE-weighted doses calculated by local effect model I (LEM) (LEM RBE-weighted doses) in patients with locally recurrent nasopharyngeal carcinoma (rNPC) to the RBE-weighted doses calculated by microdosimetric kinetic model (MKM) (MKM RBE-weighted doses). We converted the LEM dose constraints (RBE-weighted dose constraints in LEM plans) for the brain stem, spinal cord, and optic nerve based on this curve. Our center—The Shanghai Proton and Heavy Ion Center (SPHIC)—has been using carbon ions to treat patients with locally recurrent nasopharyngeal carcinoma (rNPC) since May 2015. A follow-up of 75 patients to 2017 showed a survival rate of 82.2% without local progression [4]. Our institute uses the local effect model I (LEM) [11, 12] for treatment planning. LEM assumes that RBE relies on the spatial dose distributions of carbon ion and depends on the delivered doses [11]. The two systems calculate different RBE-weighted doses, even based on the same physical doses
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