Scanning Beam Spot Preservation for Head and Neck Proton Therapy: Machine, Universal, or 3D-Printed Patient-Related Range Shifter?

Abstract

To investigate whether machine-related range shifter (MRS) should be replaced by universal proton range shifter (UPRS) or personalized 3D-printed patient specific range shifters (PSRS) to preserve the scanning beam spot size and dosimetric advantage of pencil beam scanning (PBS) proton therapy for head and neck and brain (HNB) patients. In PBS proton therapy, it is critical to maintain the smallest possible spot size to achieve the best organs at risk (OARs) sparing and dose conformality to targets for HNB patient. The spot properties are influenced by range shifter material, dimensions and air gap to patient. Currently MRS and UPRS or 3D-printed PSRS may be used in HNB proton therapy. Their ability to minimize the air gap from the range shifter to patient surface varies with MRS, UPRS and PSRS dimensions and location within the treatment geometry to avoid clearance issues. We analyzed the pros and cons of UPRS developed and routinely used in our clinic along with PSRS relative to MRS by means of in proton beam data measurements, analytical calculations (Differential Moliere formula and Eclipse TPS Analytical Algorithm) and Volumetric Imaging. The UPRS (consists of U-shape, thick couch top) can be consistently placed close to the patient (2-8 cm) and maintain the integrity of the proton spot size (within<0.5mm for a 2cm air gap) during HNB treatment, while the MRS often couldn’t be positioned close to the patient (air gap>15 cm), due to clearance issues. As a result, higher quality dose distributions were achieved (heterogeneity index D5/D95 ∼10% lower) by employing the in house developed UPRS which minimizes air gaps and preserves the spot size. A 3D-printed PSRS may create a virtually near zero air gap and fully preserve the integrity of the spot size. Improved dosimetric plan quality relative to MRS was observed in terms of OAR sparing and dose conformality to target when UPRS and 3D printed PSRS were used for HNB patients. However, for a non-removable UPRS (e.g. thick couch top), the quality of in- room volumetric imaging such as cone beam computed tomography (CBCT) was found to be degraded due to high Z elements (Si and Al). The developments in precise 3D printing allowed the use of homogeneous materials with low Z elements for PSRS, minimizing the impact on CBCT image quality, while it could be also designed as a personalized patient immobilization system. UPRS was found to enhance dosimetry plan quality, but it could complicate the clinical workflow and hinder the value of IGRT when it includes high Z-material components, while the low Z 3D printed homogeneous PSRS may further enhance the dose distribution, minimally impact CBCT image quality and simplify clinical processes when acts as a personalized patient immobilization system for HNB patients.