Dose averaged linear energy transfer optimization for large sacral chordomas in carbon ion therapy.

Background: Carbon ion beams are well accepted as densely ionizing radiation with a high linear energy transfer (LET). However, the current clinical practice does not fully exploit the highest possible dose‐averaged LET (LETd) and, consequently, the biological potential in the target. This aspect becomes worse in larger tumors for which inferior clinical outcomes and corresponding lower LETd was reported. Purpose: The vicinity to critical organs in general and the inferior overall survival reported for larger sacral chordomas treated with carbon ion radiotherapy (CIRT), makes the treatment of such tumors challenging. In this work it was aimed to increase the LETd in large volume tumors while maintaining the relative biological effectiveness (RBE)‐weighted dose, utilizing the LETd optimization functions of a commercial treatment planning system (TPS). Methods: Ten reference sequential boost carbon ion treatment plans, designed to mimic clinical plans for large sacral chordoma tumors, were generated. High dose clinical target volumes (CTV‐HD) larger than 250cm3$250 \,{\rm cm}^{3}$ were considered as large targets. The total RBE‐weighted median dose prescription with the local effect model (LEM) was DRBE,50%=73.6Gy$\textrm {D}_{\rm RBE, 50\%}=73.6 \,{\rm Gy}$ in 16 fractions (nine to low dose and seven to high dose planning target volume). No LETd optimization was performed in the reference plans, while LETd optimized plans used the minimum LETd (Lmin) optimization function in RayStation 2023B. Three different Lmin values were investigated and specified for the seven boost fractions: Lmin=60keV/μm$\textrm {L}_{\rm min}=60 \,{\rm keV}/{\umu }{\rm m}$, Lmin=80keV/μm$\textrm {L}_{\rm min}=80 \,{\rm keV}/{\umu }{\rm m}$ and Lmin=100keV/μm$\textrm {L}_{\rm min}=100 \,{\rm keV}/{\umu }{\rm m}$. To compare the LETd optimized against reference plans, LETd and RBE‐weighted dose based goals similar to and less strict than clinical ones were specified for the target. The goals for the organs at risk (OAR) remained unchanged. Robustness evaluation was studied for eight scenarios (±3.5%$\pm 3.5\%$ range uncertainty and ±3mm$\pm 3 \,{\rm mm}$ setup uncertainty along the main three axes). Results: The optimization method with Lmin=60keV/μm$\textrm {L}_{\rm min}=60 \,{\rm keV}/{\umu }{\rm m}$ resulted in an optimal LETd distribution with an average increase of LETd,98%${\rm {LET}}_{{\rm {d,}}98\%}$ (and LETd,50%${\rm {LET}}_{{\rm {d,}}50\%}$) in the CTV‐HD by 8.9±1.5keV/μm$8.9\pm 1.5 \,{\rm keV}/{\umu }{\rm m}$ (27%$27\%$) (and 6.9±1.3keV/μm$6.9\pm 1.3 \,{\rm keV}/{\umu }{\rm m}$ (17%$17\%$)), without significant difference in the RBE‐weighted dose. By allowing ±5%$\pm 5\%$ over‐ and under‐dosage in the target, the LETd,98%${\rm {LET}}_{{\rm {d,}}98\%}$ (and LETd,50%${\rm {LET}}_{{\rm {d,}}50\%}$) can be increased by 11.3±1.2keV/μm$11.3\pm 1.2 \,{\rm keV}/{\umu }{\rm m}$ (34%$34\%$) (and 11.7±3.4keV/μm$11.7\pm 3.4 \,{\rm keV}/{\umu }{\rm m}$ (29%$29\%$)), using the optimization parameters Lmin=80keV/μm$\textrm {L}_{\rm min}=80 \,{\rm keV}/{\umu }{\rm m}$. The pass rate for the OAR goals in the LETd optimized plans was in the same level as the reference plans. LETd optimization lead to less robust plans compared to reference plans. Conclusions: Compared to conventionally optimized treatment plans, the LETd in the target was increased while maintaining the RBE‐weighted dose using TPS LETd optimization functionalities. Regularly assessing RBE‐weighted dose robustness and acquiring more in‐room images remain crucial and inevitable aspects during treatment. [ABSTRACT FROM AUTHOR]