ESTRO 2024 - Abstract Book

S5412

Radiobiology - Tumour biology

ESTRO 2024

underdose or overdose the target, potentially leading to reduced tumor control or increased normal tissue toxicity, respectively. In the past, our study investigated the effects of the duration and timing of the irradiation interruption on the reduction of the biological equivalent dose (BED). Since the dose accumulation history differs in each dose voxel in practice, the reduction of the BED differs in each dose voxel. While biological dose optimization has been equipped in the recent treatment planning system, the effects of the interruption to the BED have not yet been considered. The current study proposes Biological Adaptive Radiotherapy (BART), an evolved ART system that integrates the interpretation of these biological effects. It could compensate for the BED in 3D by generating a base plan for optimizing the modified plan using the integration system of an existing function of the TPS and in-house program.

Material/Methods:

We used patients with stage III lung cancer treated using a volumetric-modulated arc therapy with two full arcs. Partial treatment plans and dose distributions assuming the irradiation interruption were created in the treatment planning system (TPS). A compensated physical dose distribution that provides a homogeneous BED distribution considering the interruption was generated by in-house software developed using Python. The reduction in the BED due to the interruption was calculated using a biological model based on the microdosimetric kinetic model. The BED was converted to the physical dose, and which was incorporated into the TPS. A compensated plan and dose distribution were created in the TPS by a single-time optimization using a base plan created in the in-house software and existing optimization objectives for the original VMAT plan. The treatment suspension was assumed 2-hour interval after 1 arc irradiation. The dose-volume histogram with biological effectiveness was evaluated with and without the dose compensation. The difference of the BED at 98% of the dose (BD 98% ), 50% of the dose (BD 50% ), and 2% of the dose (BD 2% ) due to irradiation interruption in the planning target volume (PTV) ranged between 15.7-16.5%, 15.9-16.5%, and 5.2-14.5%, respectively. For the organ at risk, the difference of the volume at 5 Gy (BV 5Gy ) and 20 Gy (BV 20Gy ) of the normal lung due to irradiation interruption were 0.7-2.2% and 0.3-4.7%, respectively. On the other hand, the difference of the BED at BD 98% , BD 50% , and BD 2% with and without the dose compensation due to the irradiation interruption were 0.8-1.4%, 0.6-1.0%, and 1.4-8.3%, respectively. The difference of the BV 5Gy and BV 20Gy of the normal lung with and without the dose compensation due to the irradiation interruption were 1.0-4.1% and 0.4-2.6%, respectively. The planning organ at risk volume of the spinal cord satisfied the dose constraint of 48 Gy for all plans. These results indicated that the dose compensation was essential for the irradiation interruption and the BART could provide the enhancing the target coverage while keeping the risk organs within the tolerance dose. Results:

Conclusion:

We developed a framework to compensate for the reduced BED due to the irradiation interruption and demonstrated its performance. The compensated plan successfully achieved the equivalent target dose and satisfied the dose constraint for the short time interruption. This innovative approach could be a transformative shift in the field of adaptive radiation therapy.

Keywords: ART, BART, irradiation interruption