# Effects of intrafractional diaphragm motion on dose perturbation in stereotactic body radiation therapy for lower thoracic vertebrae

## TL;DR

This study shows that diaphragm motion during spine radiation therapy can cause up to 4.3% dose variation, and using time-averaged CT scans improves dose accuracy.

## Contribution

The study introduces dynamic dose calculation incorporating intrafractional diaphragm motion for SBRT of the lower thoracic vertebrae.

## Key findings

- A 4.3% dose variation was observed in cases with the greatest diaphragm motion.
- Mid-ventilation and time-averaged CT scans reduced dose variation to less than 0.7%.
- Dose variation correlated significantly with the magnitude of diaphragm motion.

## Abstract

•The doses considering diaphragm motion during spine irradiation were assessed.•A 4.3% variation in target dose due to diaphragm motion (DM) was observed.•Improper planning commuted tomography (CT) caused dose variation from DM.•Dose variation was predicted using correlations with DM.•Time-averaged CT provided the most robust planning dose for accuracy.

The doses considering diaphragm motion during spine irradiation were assessed.

A 4.3% variation in target dose due to diaphragm motion (DM) was observed.

Improper planning commuted tomography (CT) caused dose variation from DM.

Dose variation was predicted using correlations with DM.

Time-averaged CT provided the most robust planning dose for accuracy.

This study aimed to evaluate the impact of intrafractional diaphragm motion (IFDM) on dose accuracy in stereotactic body radiation therapy (SBRT) for lower thoracic vertebrae.

A retrospective analysis was conducted on 10 patients who underwent SBRT using volumetric-modulated arc therapy (SBRT-VMAT) for the lower thoracic vertebrae. For all patients, dynamic dose calculation (DDC) was performed, incorporating IFDM using arc-divided VMAT plans, respiratory waveforms, and four-dimensional computed tomography (4DCT). The DDC results were compared with doses calculated using time-averaging CT (AveCT) and individual-phase CT scans. Diaphragm motion was quantified using 4DCT, and the correlation between IFDM and dose perturbation was assessed.

The minimum gross tumor volume (GTV) dose was overestimated by 1.8 % in phase 0 % and underestimated by − 1.0 % in phase 50 %. A statistically significant correlation was observed between dose variation and the magnitude of IFDM. In the case with the greatest magnitude of diaphragm motion, a 4.3 % variation in GTV was observed compared with the DDC. By contrast, mid-ventilation CT and AveCT showed a mean dose variation of < 0.7 %.

This study incorporated IFDM into dose calculation for SBRT-VMAT. Static planning based on CT scans acquired at a specific phase may result in unexpected dose variations. Mid-ventilation CT and AveCT demonstrated utility in mitigating dose variations associated with IFDM. Considering the correlation between dose variation and diaphragm motion magnitude is crucial for developing effective dose perturbation strategies for IFDM.