Simultaneous optimization of multiple plans within one treatment course with dosimetric pathfinding for temporally...

Radiotherapy workflows conventionally deliver one treatment plan multiple times throughout the treatment course. Non-coplanar techniques with beam angle optimization or dosimetrically optimized pathfinding (DOP) exploit additional degrees of freedom to improve spatial conformality of the dose distribution compared to widely used techniques like volumetric-modulated arc therapy (VMAT). The temporal dimension of dose delivery can be exploited using multiple plans (sub-plans) within one treatment course. For instance, temporally feathered radiation therapy (TFRT) uses iso-curative sub-plans to deliver an alternance of higher and lower doses compared to a single plan to selected organs-at-risk (OARs), facilitating the dynamic recovery process of healthy tissues.

This study presents a simultaneous optimization framework based on direct aperture optimization with or without DOP to optimize multiple coplanar or non-coplanar sub-plans within one treatment course and demonstrates its use for TFRT planning.

The goal of the framework was to minimize an objective function consisting of weighted upper or lower dose-volume, generalized equivalent uniform dose, and normal tissue objectives set on the dose distribution of each sub-plan or the combined total plan. Reference VMAT and dynamic collimator trajectory radiotherapy (colli-DTRT) single plans were created and used to derive the objectives for TFRT planning. The TFRT high-to-low dose modulation was integrated into the objective list and systematically investigated for a digital academic phantom using three variations ("soft", "medium", "hard"). Additionally, a "super-soft" variation used the same objectives for all five sub-plans (i.e., no high-to-low dose modulation). Furthermore, "medium" TFRT sub-plans with colli-DTRT were created for three more complex clinically motivated head and neck cases.

For the phantom, the sub-plans were iso-curative with target D_98% within 1.6% of the reference plans. High-to-low OAR dose modulation was achieved with median D_mean differences between high and low dose fractions of 2.7% of the prescription dose (soft), 3.3% (medium), and 4.4% (hard) for VMAT. Median OAR D_mean differences were 2.8% of the prescription dose (soft), 6.3% (medium), and 6.1% (hard) for colli-DTRT. The dose distributions of the total plans had higher homogeneity indices (HI = D_98%/D_2%) compared to the reference plans. Lower OAR D_mean were achieved for the soft, medium, and hard TFRT variation in the total plans compared to the reference plan. However, in the super-soft variation only five of the 10 feathered OARs showed this reduction. For the three clinically motivated cases with colli-DTRT, median OAR D_mean differences between the high and low dose fractions were 3.5%, 4.2%, and 7.5% of the prescription dose. The total plans' dose distributions had higher HIs compared to their respective reference plans and lower or equal D_mean for all feathered OARs except for pharynx in one case.

A framework for the simultaneous optimization of multiple coplanar or non-coplanar sub-plans within one treatment course was developed. Simultaneous optimization was investigated with a phantom and three clinically motivated cases for TFRT planning. OAR dose modulation in the sub-plans was achieved while increasing target homogeneity and reducing OAR doses in the dose distribution of the total plans compared to the reference plans.