Can dose convolution modelling explain bath and shower effects in rat spinal cord?

Area of Science:

• Radiation oncology
• Biophysics
• Radiobiology

Background:

• The 'bath and shower' effect in proton irradiations suggested spinal cord sensitization, which conflicted with existing tissue complication models.
• Previous models struggled to explain the disproportionate response observed with varying radiation field sizes.

Purpose of the Study:

• To explore a novel dose convolution algorithm for modeling the 50% isoeffect endpoint in radiation therapy.
• To reconcile the 'bath and shower' effect with established radiobiological principles.

Main Methods:

• Dose distributions from 'bath and shower' radiation fields were convolved with Gaussian functions (parameterized by sigma).
• The hypothesis centered on the constancy of maximum convolved dose values for isoeffect across different scenarios.
• A field length-dependent relative biological effectiveness (FLRBE) approach was used for validation.

Main Results:

• Maximum convolved dose values remained consistent within ±17% across experiments for sigma = 3.5 mm, despite a four-fold variation in maximum dose.
• FLRBE analysis yielded results within ±14%, confirming the validity of the dose convolution modeling.
• Convolution modeling aligned with bystander effect mechanisms, suggesting a signaling factor diffusion.

Conclusions:

• Dose convolution modeling of 50% isotoxicity provides a consistent explanation for 'bath and shower' effects.
• Biological effectiveness decreases with very small field sizes, necessitating higher isoeffect doses.
• The 'bath' dose does not sensitize the spinal cord; rather, accounting for biological effectiveness requires significant shower dose reduction with increased bath dose.