RBE-weighted dose conversions for patients with recurrent nasopharyngeal carcinoma receiving carbon-ion radiotherapy from the local effect model to the microdosimetric kinetic model

Abstract

Background We sought to establish a conversion curve to convert the local effect model I (LEM) based RBE-weighted doses (LEM doses) in patients with locally recurrent nasopharyngeal carcinoma (rNPC) to the microdosimetric kinetic model (MKM) based RBE-weighted dose (MKM doses) model. We also converted the relevant organ at risk (OAR) constraints based on this curve. Methods Data from 13 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. Clinical target volume CTV1(GTV+5mm)was given 63 Gy (RBE) in 21 fractions. 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 the ratio of LEM dose to MKM dose was obtained as the conversion factor to obtain the conversion curve by using an isovolumetric dose method. Using prescriptions and OAR, constraints were converted to MKM doses with this curve. Results Conversion factors (LEM dose/MKM dose) from 1.37±0.02 to 3.09±0.09 corresponded to the LEM fractionated doses from 0.24 Gy (RBE) to 2.86 Gy (RBE), including the doses constraining upon OARs. LEM doses of 30 Gy (RBE) and 45 Gy (RBE) in 21 fractions were converted to MKM doses of 16.64 Gy (RBE) and 30.98 Gy (RBE) in 16 fractions. Conclusions This conversion curve could be used to convert LEM doses to MKM doses for patients with rNPC receiving CIRT, providing dose references for re-irradiation therapy.