# Diurnal variation of the distance between cranium and the third lumbar vertebra and its implications for craniospinal irradiation

## TL;DR

This study shows that the spine shortens during the day, which can affect craniospinal irradiation treatment accuracy.

## Contribution

The study quantifies diurnal spinal length changes and recommends treatment timing to reduce dosimetric errors.

## Key findings

- The spine shortens by 0.8±0.2 mm per hour due to diurnal variation.
- Treatment fractions should be timed within two hours of planning imaging to minimize dosimetric impact.

## Abstract

The spine shortens during the day because of gravity. This study quantified the effect of treatment fraction timing on spinal length in 13 craniospinal irradiation patients. The distance deviation from the base of skull to the third lumbar vertebra in daily planar kilovoltage setup images compared to the treatment planning computed tomography image was determined. The time deviation between the treatment fraction and planning computed tomography image was registered. A distance decrease of 0.8±0.2 mm/hour was observed. Timing the treatment fractions within two hours of the planning imaging session is advisable to minimise the potential dosimetric impact of diurnal variations.