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 (LET) and, consequently, the biological potential in the target. This aspect becomes worse in larger tumors for which inferior clinical outcomes and corresponding lower LET 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 LET in large volume tumors while maintaining the relative biological effectiveness (RBE)-weighted dose, utilizing the LET 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 were considered as large targets. The total RBE-weighted median dose prescription with the local effect model (LEM) was in 16 fractions (nine to low dose and seven to high dose planning target volume). No LET optimization was performed in the reference plans, while LET optimized plans used the minimum LET (L) optimization function in RayStation 2023B. Three different L values were investigated and specified for the seven boost fractions: , and . To compare the LET optimized against reference plans, LET 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 ( range uncertainty and setup uncertainty along the main three axes).

RESULTS

The optimization method with resulted in an optimal LET distribution with an average increase of (and ) in the CTV-HD by ( ) (and ( )), without significant difference in the RBE-weighted dose. By allowing over- and under-dosage in the target, the (and ) can be increased by ( ) (and ( )), using the optimization parameters . The pass rate for the OAR goals in the LET optimized plans was in the same level as the reference plans. LET optimization lead to less robust plans compared to reference plans.

CONCLUSIONS

Compared to conventionally optimized treatment plans, the LET in the target was increased while maintaining the RBE-weighted dose using TPS LET optimization functionalities. Regularly assessing RBE-weighted dose robustness and acquiring more in-room images remain crucial and inevitable aspects during treatment.