ACPSEM position paper on ROMP scope of practice and staffing levels for magnetic resonance linear accelerators.

The purpose of this position paper is to outline the ACPSEM recommendations on Medical Physicist scope of practice and staffing levels, as they relate to the use of dedicated MRI-Linacs in the treatment of patients. A core function of Medical Physicists is to safely implement changes in medical practice via the introduction of new technology and to ensure high quality radiation oncology services are provided to patients. Determining the feasibility of MRI-Linacs in any existing setting, or in establishing a new site, mandates the knowledge and services of Radiation Oncology Medical Physicists (ROMPs) as the Qualified Experts within this setting. ROMPs are key members of the multi-disciplinary team which will be required to steer the successful establishment of MRI Linac infrastructure within departments. To support efficient implementation, ROMPs must be embedded in the process from the start, including any feasibility study, initiation of the project, and development of the business case. ROMPs must be retained throughout all stages of acquisition, service development, and ongoing clinical use and expansion. The number of MRI-Linacs in Australia and New Zealand is growing. This expansion is occurring in parallel with rapid technological evolution, expanding tumour stream applications, and increasing consumer uptake. Growth and applications of MRI-Linac therapy will continue to occur beyond current known horizons, via development on the MR-Linac platform itself and through the migration of learning from this platform to conventional Linacs (known horizons for example include the use of daily, online image guided adaptive radiotherapy and MRI data informing decision making for planning and treatment before and throughout treatment courses). Clinical use, research and development will be a significant component of expanding patient access to MRI-Linac treatment and there will be an ongoing need to attract and retain ROMPs to initially establish services and in particular to drive service development and delivery for the life of the Linacs. MRI and Linac technologies mean it is necessary to perform a specialized workforce assessment for these devices, distinct from those employed for conventional Linacs and associated services. MRI-Linacs are complex, have a heightened risk profile compared to standard Linacs, and are unique in their treatment of patients. Accordingly, the workforce needs for MRI-Linacs are greater than for standard Linacs. To ensure safe and high-quality Radiation Oncology patient services are provided, it is recommended that staffing levels should be based on the 2021 ACPSEM Australian Radiation Workforce model and calculator using the MRI-Linac specific ROMP workforce modelling guidelines outlined in this paper. The ACPSEM workforce model and calculator are closely aligned with other Australian/New Zealand and international benchmarks.

Linac reported steering error insensitive to 6 MV FFF transverse beam position deviations.

Clinically significant beam position deviations were observed for a 6 MV FFF beam during patient specific QA on an Elekta linear accelerator. There was no significant reported transverse steering error from the machine ion chamber, and routine linac QA practices, including cardinal angle Winston-Lutz test, did not identify the deviations. Subsequent investigation using an electronic portal imaging device (EPID) revealed clinically significant beam position deviations for small steering errors. This prompted investigation into further impact and possible solutions. Testing set-points were established by adjusting transverse steering current to achieve introduced 2 T steering errors. Tests at each set-point included scanning water tank profiles and EPID images. A proposed method for adjusting the 2 T error sensitivity was tested via adjusting the 2 T loop parameter such that a reported error corresponds to specific beam position deviations. The testing set-points resulted in positional deviations of greater than 3 mm for reported errors of less than 1. A new method for improving 2 T error sensitivity was implemented. This work has shown that existing vendor protocol for establishing beam steering error for 6 MV FFF beams can lead to beam position deviations without machine interlocks or significant reported steering errors. Thus, an alternative method of establishing steering error sensitivity based on positional deviations is presented.

Optimising treatment distance and treatment area for HDR surface mould brachytherapy.

The purpose of this study was to quantify the effect of treatment area and treatment distance on dose distributions for geometrically optimised surface mould plans in order to provide guidance in choosing treatment parameters and constructing moulds for individual patients. Geometrically optimised plans were generated with a typical brachytherapy planning system and measurements were taken with radiochromic film over depths of 5-32 mm with an (192)Ir high dose rate source. Films were calibrated with a cylindrical geometry technique utilising the (192)Ir source and readout was performed with a flatbed scanner. The rate of dose fall-off about the prescription plane, as well as the magnitude and extent of local dose maxima superficial to the prescription plane, increased with decreasing treatment areas when inter-catheter spacing and treatment distance were kept constant. The dose fall-off was highly dependent on treatment distance, with a 16 % reduction in dose 4 mm superficial to the treatment depth occurring when the distance was increased from 10 to 20 mm while maintaining a 10 mm inter-catheter spacing. The table generated using the measured planar geometry data, can be used as an initial guide for mould construction and planning. The properties of high dose regions near to the catheter plane are highly dependent on the treatment area, which must be considered when normal tissue dose tolerances are a concern. Treatment distance is a key variable influencing the overall dose distribution and should be adjusted as a function of the desired tumour to skin dose ratio, controlled by mould thickness.