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The use of the simultaneous integrated boost (SIB) in IMRT has been one of the major technical photon-based RT innovations in the last 20 years, while in carbon ion radiotherapy (CIRT) a SIB approach has not been fully exploited so far. Until now, at the National Center for Oncological Hadrontherapy (CNAO, Italy) the standard approach for the treatment of head and neck (HN) adenoid cystic carcinoma (ACC) has been a sequential (SEQ) strategy consisting of a first phase of 10 fractions to the low risk (LR)-clinical target volume (CTV) followed by a second phase of 6 fractions to the high risk (HR)-CTV. In order to improve treatment planning dose distributions, in this in silico study we investigated the feasibility of a CIRT-SIB strategy for HN-ACC patients.

A dataset of 10 ACC patients previously treated with SEQ irradiation was used. The prescribed dose of the first phase was 41.0 Gy(RBE)/10 fractions to the LR-CTV. A sequential boost phase of 24.6 Gy(RBE)/6 fractions was then delivered in 6 more fractions to the HR-CTV (total prescription of 65.6 Gy(RBE)). In the plan optimization, the highest priority was sparing the brainstem and optic pathways. The delivered treatments were re-planned with two SIB dose levels to the LR-CTV: namely 48.0 Gy(RBE) and 54.4 Gy(RBE). While planning with SIB, the HR-CTV coverage had higher priority, with fixed organs at risk dose constraints among the SIB and SEQ plans. The near-minimum (D98%), the median (D50%) and the near-maximum (D2%) doses were chosen as dose-summarizing parameters for CTV coverage evaluation together with the homogeneity (HI) and conformity indexes (CI). The biological effective dose (BED) was calculated to compare the different fractionation schemes. Statistical significance was assessed with the non-parameter Wilcoxon signed-rank test.

All optimized plans respected the organs at risk dose constraints. For the HR-CTV, comparable coverage was achieved between the SEQ and SIB_54.4, while with the SIB_48.0 the dose inhomogeneity slightly increased (see Table 1).  The HI and CI values of the LR-CTV decreased significantly for both SIB dose levels, indicating a better conformality of this technique with respect to the SEQ. With the SEQ, SIB_48.0 and SIB_54.4 the LR-CTV median doses were 50.3%, 11.9% and 6.0% higher than the prescriptions.

In the LR-CTV, the near-minimum BED(D98%) was comparable between the SEQ and SIB_48.0 (p = 0.19), while a statistically significant increase was found for SIB_54.4 plans (p = 0.002). Significant reduction (p = 0.002) of the median BED(D50%) and near-maximum BED(D2%) was achieved with both SIB dose levels (see Table 2).  

The SIB technique resulted in highly conformal dose distributions with the reduction of the unintended dose to the LR-CTV. A prescription dose range for the LR-CTV will be clinically defined to offer tailored personalized treatments, according to the clinical and imaging characteristic of the patients.