Dose-Averaged LET Evaluation in Head and Neck Adenoid Cystic Carcinoma Patients for Carbon-Ion Radiotherapy

Purpose: In carbon ion radiation therapy (CIRT), the dose-averaged LET (LETd) is one of the important factors in determining clinical outcomes. In order to investigate the local control of adenoid cystic carcinoma (ACC) and the optic neuropathy of optic nerve (ON)s in CIRT, the effects of the LETd were evaluated clinically. We propose novel tumor control probability (TCP) and normal tissue complication probability (NTCP) models with D90-value of RBE-weighted dose and LETd as clinical parameters.Methods and Materials: Total 99 patients with head and neck ACC who received CIRT at the National Institute of Radiological Sciences were investigated. The physical dose and LETd distributions for each patient were recalculated using in-house treatment planning system. For ACC tumor, minimum-value of dose-averaged LET and D90-value of RBE-weighted dose as an indicator of evaluation were estimated. For ONs, maximum-value of LETd and maximum-value of RBE-weighted dose were evaluated.Results: For each prescribed RBE-weighted dose of 57.6 and 64 Gy (RBE), no statistical significance for local control by RBE-weighted dose was found, but for 64 Gy (RBE), we observed the LETd may be a marginally significant. For ONs, it was obvious that the optic neuropathy had a greater effect from RBE-weighted dose rather than dose-averaged LET. The proposed models could be applied to both TCP and NTCP prediction, and was found that the clinical outcomes from low to high LETd region were predictable.Conclusions: We analysed clinically the physical dose and LETd distributions for 99 patients with head and neck ACC. Our results indicated that LETd may be a determining factor for local control for ACC tumors. For ON, it was obvious that the RBE-weighted dose was related to optic neuropathy rather than LETd. The proposed approach may be helpful to predict whether LETd will affect clinical outcomes and be useful when constraining not only RBE-weighted dose but also LETd.AAPM202

Preliminary Clinical Analysis with Dose-Averaged LET Calculation for Head and Neck Cancer Patients Treated by Using Carbon-Ion Radiotherapy

Purpose: For head and neck cancer patients treated with carbon-ion radiotherapy (CIRT), normal tissues such as brain and optic nerve are close to the irradiation field such that those may receive radiation damage. In order to estimate their clinical results, a dose-averaged LET (LETd) calculation tool was developed. We focused on the usefulness of LETd as a clinical analytical parameter.Methods: In-house Heavy Ion Plan (HIPLAN) treatment planning system was used in the CIRT with a wobbler-scattering method. We developed the LETd and its standard deviation (LETdSD) calculation tool, and implemented in the HIPLAN. 33 head and neck patients treated with CIRT, who had follow-up more than 3 years and did not receive additional radiotherapy, were selected in order to evaluate local control and optic neuropathy. LETd and LETdSD for these 33 patients were calculated using the analysis tool.Results: By implementing the clinical analysis tool in HIPLAN, it was possible to efficiently calculate and analyze the LETd and LETdSD for each patient. Of the 33 patients, 13 had local recurrences and 8 had optic neuropathies. Among 33 with and without recurrence cases, the mean LETd values in the PTV were 58.7±5.4 for with recurrence and 61.7±6.9 keV/m for without recurrence, respectively. On the other hand, the mean LETd values were 45.6±12.8 for with optic neuropathy and 29.0±19.0 keV/m for without optic neuropathy (p-value=0.046).Conclusion: We developed the LETd and LETdSD calculation tool and analyzed 33 patients of head and neck cancer for local control and optic neuropathy. Although the mean LETd in the PTV were similar values between cases with and without recurrence, we could observe the usefulness of the LETd analysis for optic neuropathy cases. We will continue to analyze more patients in the future.AAPM202