Fixed field technique for hippocampal avoidance whole-brain radiotherapy: A feasibility study using Elekta system

Treatment planning for Hippocampal avoidance whole-brain radiotherapy (ha-WBRT) is demanding and time-consuming due to the position of the Hippocampus and low dose tolerances. This study evaluates the feasibility of fixed field volumetric modulated arc therapy (fVMAT) for ha-WBRT using the Elekta Agility™ collimator system and Monaco treatment planning system.

Fifteen patients treated for WBRT were subjected to fVMAT and conventional VMAT (cVMAT) planning with similar optimisation criteria. Jaws were restricted above and below the Hippocampus for the fVMAT plans with isocenter positioned at the brain's center, dividing the brain target into two. In contrast, Jaws were not restricted for cVMAT plans. Plans were compared in terms of dose constraints, dose conformity, and dose homogeneity. Plan complexity was compared in terms of modulation degree (MD), and delivery efficiency was checked by performing patient-specific quality assurance.

Both plans met the RTOG-0933 dose requirements. The fVMAT plans showed statistically significant improved target coverage (D98%, V30Gy), target homogeneity, and conformity. There was no statistically significant change in hippocampus doses between the two plans. The fVMAT plans showed lesser plan complexity with average MD of 3.34±0.5 compared to cVMAT plans (average MD of 4.21±0.4, p=0.00011). The increased plan complexity was reflected in the delivery efficiency as cVMAT showed higher average gamma failure for patient3.84%(p=0.0004) and a target volume 7.13% (p=0.0359) structures.

According to the obtained results, the Elekta Agility™ collimator system and Monaco treatment planning system can generate better ha-WBRT plans using the fVMAT technique.