The study of characterization of proton beams through contacted range shifter and machine range shifter and implementation in clinical uses

Abstract

Intensity-modulated proton therapy (IMPT) for superficial lesions commonly requires a range shifter; however, conventional range shifters can increase proton spot size and radiation dose to adjacent organs at risk (OARs) due to multiple Coulomb scattering. Although minimizing the air gap between the range shifter and the patient is dosimetrically beneficial, it is often difficult to achieve clinically. This study developed and evaluated a novel Chula Range Shifter (CURS) to improve proton beam delivery and support clinical implementation in head and neck proton therapy.The effect of air gap on proton spot size was evaluated for air gaps ranging from 5 to 42.1 cm at beam energies of 80, 100, 150, and 200 MeV using treatment planning system (TPS) calculations, FLUKA Monte Carlo simulations, and measurements. CURS was fabricated from acrylic material selected based on its relative stopping power ratio (RSPR), comparable to the machine range shifter (MRS). Material properties, including R80, RSPR, and water-equivalent thickness (WET), were evaluated using a multilayer ionization chamber. CT and cone-beam CT (CBCT) image quality were assessed using phantom studies. Dosimetric evaluation was performed using simple and complex treatment scenarios in the head and neck region. FLUKA simulations agreed well with measurements, with R80 differences within 0.5 mm. Proton spot size increased with increasing air gap, particularly at lower energies. CURS maintained comparable target coverage, conformity index, and homogeneity index to MRS plans, while reducing lateral penumbra and mean OAR dose. Compared with MRS plans at 5 cm and 42.1 cm air gaps, CURS reduced lateral penumbra by 19.7% and 35.7%, and mean OAR dose by 37.1% and 61.8%, respectively. Image quality with CURS remained clinically acceptable for image-guided proton therapy workflows.These findings demonstrate that CURS effectively reduces proton spot size and improves OAR sparing while maintaining target coverage and image quality. CURS shows strong potential for clinical implementation in head and neck proton therapy, particularly for superficial, orbital, paranasal sinus, and re-irradiation cases requiring sharp dose fall-off and enhanced OAR protection.