Predicting radiotoxic effects after BNCT for brain cancer using a novel dose calculation model

Purpose:

The normal brain is an important dose-limiting organ for brain cancer patients undergoing radiotherapy. This study aims to develop a model to calculate photon isoeffective doses ($D_{IsoE}$) to normal brain that can explain the incidence of grade 2 or higher somnolence syndrome (SS$⩾$2) after Boron Neutron Capture Therapy (BNCT).

Methods:

A $D_{IsoE}$ model was constructed to find the reference photon dose that equals the Normal Tissue Complication Probability (NTCP) of the absorbed dose from BNCT. Limb paralysis rates from the rat spinal cord model exposed to conventional or BNCT irradiation were used to determine model parameters. NTCP expressions for both irradiations were constructed based on Lyman’s model accordingly. $D_{IsoE}$ values were calculated for BNCT treatments performed in Finland and USA. An equivalent uniform dose (EUD) based on peak and average whole-brain doses and treatment fields was also introduced. Combining $D_{IsoE}$ and EUD models, a dose–response curve for SS$⩾$2 in BNCT patients was constructed and compared to conventional radiotherapy outcomes.

Results:

The $D_{IsoE}$ model reveals higher than expected photon-equivalent doses in the brain, indicating the need to modify standard dose calculation methods. Neither peak dose nor average whole-brain dose alone predicts SS$⩾$2 development. However, the dose–response curve derived from combining $D_{IsoE}$ and EUD models effectively explains the incidence of SS$⩾$2 after BNCT.

Conclusions:

The introduced $D_{IsoE}$ and EUD models predict the incidence of somnolence syndrome after BNCT. The first dose–response relationship for SS$⩾$2 derived entirely from brain tumour patients treated with BNCT, consistent with photon radiotherapy responses, is presented.