Impact of extended source-to-surface distances and respiratory motion on the precision of electron beam therapy

In electron beam therapy targeting superficial thoracic and abdominal lesions, the source-to-surface distance (SSD) varies due to patient respiration. This study aimed to investigate the percentage depth dose (PDD) and off-center ratio (OCR) of 6, 9, and 12 MeV electron beams at extended SSDs; and to assess the variations in dose caused by respiratory motion.

PDDs and OCRs for 6, 9, and 12 MeV electron beams were measured using a Blue Phantom 2 with 5 cm circular aperture and 10 × 10 cm² fields at SSDs ranging from 98 to 105 cm. The dose differences at the maximum depth (d_max) were calculated relative to an SSD of 100 cm. We simulated patient respiratory motion of 0-4 cm during irradiation with 5 cm circular aperture and 10 × 10 cm² fields by placing a water-equivalent phantom with a parallel-plate ionization chamber on a QUASAR Platform. The gantry angle was set to 270°, and the SSD was set to 100 cm. The dose differences at d_max were calculated relative to a motion amplitude of 0 cm.

Extending the SSD by 3 cm reduced the dose by > 5% for all energies. For a 6 MeV electron beam with a 5 cm circular aperture, extending the SSD to 105 cm resulted in a dose reduction of 13.42% and expansions of the field size by 2.0 mm and the penumbra by 4.2 mm. A respiratory motion amplitude of 3 cm resulted in dose variations of > 3% for all energies and field sizes. The largest dose difference of 5.36% was observed for a 6 MeV electron beam with a 5 cm circular aperture.

The results demonstrate that extending the SSD by 3 cm reduced the dose by >5% for all energies, providing a useful quantitative benchmark for quality assurance in superficial electron therapy. Respiratory motion management may be warranted for electron beam therapy when respiratory-induced SSD variations exceed 3 cm.