ESTRO 2022 - Abstract Book

S1453

Abstract book

ESTRO 2022

Conclusion Real-time assessment of the quality of DIBH via the internal surrogates, the LD and SD parameters at multiple levels in the radiation field, can help identify situations where the patient arches their back instead of taking a sufficiently deep breath- hold in DIBH breast cancer treatments. References 1. Vasina E.N., Greer P., Thwaites D., Kron T., Lehmann J., A system for real-time monitoring of breath-hold via assessment of internal anatomy in tangential breast radiotherapy. Accepted to JACMP. 2. Bossuyt E, Weytjens R, Nevens D, De Vos S, Verellen D. Evaluation of automated pre-treatment and transit in-vivo dosimetry in radiotherapy using empirically determined parameters. phiRO. 2020 Oct 1;16:113-29.

Acknowledgements This work was supported by NHMRC grant 1147533.

PO-1655 Simultaneous integrated boost in carbon ion radiotherapy: a feasibility study

E. Mastella 1 , S. Molinelli 2 , G. Magro 2 , S. Russo 2 , M. Bonora 2 , S. Ronchi 2 , R. Ingargiola 2 , M. Ciocca 2 , B. Vischioni 2 , E. Orlandi 2

1 CNAO - National Center for Oncological Hadrontherapy, Medical Physics Unit, Pavia, Italy; 2 CNAO - National Center for Oncological Hadrontherapy , Clinical Department, Pavia, Italy Purpose or Objective 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. Materials and Methods 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. Results 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.