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 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. Methods Data 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. Results For 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. Conclusions This 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.

Treatment of Recurrent Nasopharyngeal Carcinoma: A Sequential Challenge

“…For recurrent NPC, there are other methods for diagnosis and prognosis in addition to the diagnostic and differential roles of conventional imaging approaches described above. In recent years, some researchers have also explored radiomics-related differential diagnostic methods or means of predicting prognosis [ 36 , 45 , 46 , 47 , 48 ]. There have been many reports on the use of imaging histology in the diagnosis of head and neck tumors as well as imaging histology combined with molecular studies to explore the prognosis of malignant tumors.…”

The rationale for a carbon ion radiation therapy facility in Australia

“…European centres have mainly used the local effect model (LEM), developed by the first German CIRT centre (the Helmholtz Center for Heavy Ion Research (GSI), Darmstadt) 76–78 . The models can give different values of RBE and biologically equivalent doses, and between different model versions as they evolve with time, leading to work on translating between the approaches 37,69,78–81 and to recommendations for international standardisation of reporting ion doses 37 . However, this uncertainty requires care in comparing clinical doses and outcomes between centres and clinical trials and in transferring experience between the different systems 31,36,37,56,79,82 …”

Dose Calculation Algorithms for External Radiation Therapy: An Overview for Practitioners

“…The light ion dose calculation in RayStation is based on PBA. The computation of RBE-weighted values is implemented according to the local effect model (LEM) or to the microdosimetric kinetic model (MKM) [93,94].…”