Applications of various range shifters for proton pencil beam scanning radiotherapy

Background A range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors. Methods Three UPRS made by QFix (Avondale, PA, USA) allow treating targets across the body: U-shaped bolus (UB), anterior lateral bolus (ALB), and couch top bolus. Head-and-neck (HN) patients who used the UPRS were tested. The in-air spot sizes were measured and compared in this study at air gaps: 6 cm, 16 cm, and 26 cm. Measurements were performed in a solid water phantom using a single-field optimization pencil beam scanning field with the ALB placed at 0, 10, and 20 cm air gaps. The two-dimensional dose maps at the middle of the spread-out Bragg peak were measured using ion chamber array MatriXX PT (IBA-Dosimetry, Schwarzenbruck, Germany) located at isocenter and compared with the treatment planning system. Results A UPRS can be consistently placed close to the patient and maintains a relatively small spot size resulting in improved dose distributions. However, when a UPRS is non-removable (e.g. thick couch top), the quality of volumetric imaging is degraded due to their high Z material construction, hindering the value of Image-Guided Radiation Therapy (IGRT). Limitations of using UPRS with small air gaps include reduced couch weight limit, potential collision with patient or immobilization devices, and challenges using non-coplanar fields with certain UPRS. Our experience showed the combination of a U-shaped bolus exclusively for an HN target and an MRS as the complimentary device for head-and-neck targets as well as for all other treatment sites may be ideal to preserve the dosimetric advantages of pencil beam scanning proton treatments across the body. Conclusion We have described how to implement UPRS and MRS for various clinical indications using the PBS technique, and comprehensively reviewed the advantage and disadvantages of UPRS and MRS. We recommend the removable UB only to be employed for the brain and HN treatments while an automated MRS is used for all proton beams that require RS but not convenient or feasible to use UB.

Clinical Experience Using the SRS MapCHECK for Patient Specific Plan Verification of Flattening Filter Free, Non-Coplanar Stereotactic Brain Plans Using HyperARC.

HyperArc (HA) treatment planning from Varian is a stereotactic specific planning tool enabling quick and efficient optimisation of treatment planning, and delivery. HA was commissioned and implemented at Waikato Regional Cancer Centre (WRCC) in 2019 to fulfil the demands of dose delivery for stereotactic radiosurgery (SRS), allowing for treatment of multiple targets with a single isocenter at non-coplanar angles. The extra levels of plan complexity involved in creating and verifying HA SRS plans required extensive checks and verifications using film and an ion chamber, along with a significant allocation of time and resources. The Sun Nuclear SRS MapCHECK (SRSMC) offered an alternative to the cumbersome film measurements. It is an all-encompassing tool meeting the requirements of TG 218 and ICRU 91 for complex treatment plan verification, claiming to save time and effort, without sacrificing accuracy, enabling for a smoother planning and verification process. SRSMC was initially commissioned on 6MV single target treatments using standard planning, then updated and commissioned for 6FFF multi-target non-coplanar treatments using HA. The SRSMC gamma pass rates were compared to film measurements in the same plane, and the central diode CAX reading compared to ionisation chamber measurements at the same position for a range of plans covering a range of PTV sizes and plan complexities. Pass rates on the SRSMC were comparable to measurements using film (Gamma 3%/1mm, 99.41%, 96.39% SRSMC and film respectively). The central diode is an adequate surrogate for a chamber measurement if the SRSMC is positioned in a similar position as that of the ionisation chamber would be – high dose homogenous region, avoiding steep gradients (mean dose difference Diode vs Chamber: -0.73%). Differences between exposing non-coplanar plans at couch 0 and at planned couch angles were negligible (Gamma 3%/1mm 99.28 coplanar, 99.41% non-coplanar on SRSMC). At WRCC the SRSMC has replaced film and chamber measurements for plan verifications of 6FFF HA multiple metastatic brain treatments at a single isocenter and we are currently investigating its use in other treatment sites.

A prototype anti-scatter detector for megavoltage X-ray imaging

In this work, a prototype anti-scatter detector based on Cherenkov radiation is developed by using glass rods. Scattering lends deleterious effects to the megavoltage x-ray portal imaging and anti-scatter detector can effectively reduce these effects. A 10 cm long glass rod with 1 mm in diameter is used as a Cherenkov detector prototype and it is studied for its response to x-ray scattering from, e.g., machine head and patient. It is subjected to 6 MV x-ray beam generated by linear accelerator (LINAC) with different field sizes (from 3 X 3 to 20 X 20 cm2) at different air gaps such as 10, 30 and 46 cm. The Cherenkov signal created by the detector is transmitted through optical fiber to photomultiplier tube (PMT) and measured by electrometer. The patient scattering is studied by placing a solid water phantom at isocenter. The response of single pixel Cherenkov detector is compared with the conventional ionization chamber detector. It has been observed that glass rod based Cherenkov detector is less sensitive to scatter radiation than ion-chamber for air gap of 10 cm. The Cherenkov signal created by glass rod is quite weak for larger air gaps and the uncertainties are quite high. Moreover, the coupling between Cherenkov detector and optical fiber is quite crucial for transmitting the Cherenkov signal from glass rod into optical fiber.

A prototype anti-scatter detector for megavoltage X-ray imaging

In this work, a prototype anti-scatter detector based on Cherenkov radiation is developed by using glass rods. Scattering lends deleterious effects to the megavoltage x-ray portal imaging and anti-scatter detector can effectively reduce these effects. A 10 cm long glass rod with 1 mm in diameter is used as a Cherenkov detector prototype and it is studied for its response to x-ray scattering from, e.g., machine head and patient. It is subjected to 6 MV x-ray beam generated by linear accelerator (LINAC) with different field sizes (from 3 X 3 to 20 X 20 cm2) at different air gaps such as 10, 30 and 46 cm. The Cherenkov signal created by the detector is transmitted through optical fiber to photomultiplier tube (PMT) and measured by electrometer. The patient scattering is studied by placing a solid water phantom at isocenter. The response of single pixel Cherenkov detector is compared with the conventional ionization chamber detector. It has been observed that glass rod based Cherenkov detector is less sensitive to scatter radiation than ion-chamber for air gap of 10 cm. The Cherenkov signal created by glass rod is quite weak for larger air gaps and the uncertainties are quite high. Moreover, the coupling between Cherenkov detector and optical fiber is quite crucial for transmitting the Cherenkov signal from glass rod into optical fiber.