ESTRO 2024 - Abstract Book

S6025

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

following ventral and dorsal language tracts: fasciculus arcuatus, superior longitudinal fasciculus, inferior fronto occipital fasciculus, inferior longitudinal fasciculus and uncinate fasciculus. Language DTI-FTTMS of 27 patients harbouring HGG were incorporated into the RT planning system and merged with planning imaging. Then, tracts were delineated as partial risk volume (PRV-FTTMS) and defined as OAR. Postoperative shift was compensated with elastic fusion (EF). Originally applied Intensity modulated RT plans were optimized to preserve PRV-FTTMS beyond the planning target volume (PTV) within the 20 Gy-Isodose line. Areas within the PTV were not spared (overlap) to ensure therapeutic coverage. Mean values ±SD are given if not otherwise indicated. Level of significance was set at p < 0.05.

Results:

Mean dose (Dmean) of the complete PRV-FTTMS was 37.3±11.8 Gy. Given to the high overlap with the PTV only a moderate dose reduction was observed with -10.9% (range, 0- 27.2 Gy) to a Dmean of 33.6±12.2 Gy (p<0.01). However, the dose saving for the total PRV-FTTMS portions outside the PTV (exPTV) was higher, from 25.1±9.8 Gy to 18.7±7.7 Gy corresponding to an average reduction of 24.7% (range, 0 – 46.9%). For high-risk PRV-FTTMS portions within the 20%-isodose-line, the mean dose avoidance to be achieved was – 25.0% (range, 0-48.5%). Dose-volume-histogram parameters for PRV-FTTMS exPTV were analysed: V54Gy and V20Gy were 4.1±5.1% and 59.3±25.6 % vs. 1.6±3.4% and 36.0±22.3% (p<0.01 each) with optimization, respectively. PTV coverage was not impaired: V95% and V90% were 95.8±3.7% and 98.8±1.9% vs. 95.7±4.2% and 98.7±1.9% (p=0.364 and 0.173). Minimal dose and maximum dose within the PTV were not affected with 46.8 Gy vs. 46.7 Gy and 61.7 Gy vs. 61.5 Gy (p=0.873 and p= 0.162). Accordingly, no change of the homogeneity index D2-D98/Dmax was observed (0.093 vs. 0.091; p=0,19). Dose constraints for organs at risk were all met.

Conclusion:

This study demonstrates that nTMS-derived DTI-FT can be used in radiotherapy planning to reduce dose exposure of speech-eloquent brain areas without compromising PTV coverage. However, due to the partially high overlap with the PTV, parts of the language tracts run within the high dose area that can´t be spared. If a concept of tract-based sparing of language function has clinical impact for patient´s language function needs to be further evaluated within prospective trials.

Keywords: Glioma, Fibertractography, Language-function

References:

1) Connor M. et al, Int J Radiat Oncol Biol Phys. 2023 May 6:S0360-3016(23)00436-4

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