Simeon Nill

Objective. Treatment of pancreas using Cyberknife is usually accomplished using radio-opaque fiducial markers that can be visualised on the orthogonal kilovoltage imaging system, but implantation of markers can lead to complications. This study therefore describes a method for pancreas tracking without fiducial markers and estimates the improvement in accuracy that can be achieved by additional daily pre-treatment volumetric imaging. Approach. Fiducial markers were artificially removed from the digitally reconstructed radiographs and treatment images of eight previously treated patients. Pancreas position was identified using a combination of local rigid registrations and inter- and intra-fraction motion models. The optimum effect of daily volumetric imaging was simulated by correcting the mean tracking position to match the mean position observed using fiducials. The impact of residual error on the dose distribution was estimated by shifting and recalculating the treatment plan. Main results. Median absolute tracking accuracy without fiducials over the 2823 acquired images is 3.8 mm (range 0.2 mm – 19.9 mm) without daily volumetric imaging. The lower bound of accuracy is 2.3 mm (0.1 mm – 18.5 mm) when daily volumetric imaging is used. The former scenario reduces the D95% of the planning target volume by a median of 2.5 Gy (1.0 Gy – 10.2 Gy) compared to the planned dose distribution, whereas the latter scenario reduces D95% by 1.4 Gy (0.4 Gy – 2.8 Gy). In neither case is the D95% of the clinical target volume substantially lowered below the prescription. Median D0.5cc to critical structures changes by less than 2 Gy. Significance. When tracking the target without fiducial markers, a planning target margin of 3–5 mm is sufficient to ensure that the clinical target volume receives the desired prescribed dose, but the spatial accuracy and target coverage are considerably improved by using daily volumetric imaging.

To fully exploit the prospects presented by the increasing focus on biological approaches for enhancing radiotherapy outcomes, improvements in repeatability and translatability of radiobiological and preclinical studies are required. This requires the development and adoption of appropriate dosimetric standards and reproducible approaches to increase confidence in the studies, enabling inter-laboratory validation and facilitating clinical translation. An Institute of Physics and Engineering in Medicine (IPEM) working party reviewed the current status and challenges associated with dosimetry of medium-energy X-rays and make recommendations with the aim to optimize the potential clinical significance of radiobiological preclinical investigations. The paper discusses the currently available resources with technical recommendations for performing dosimetry in medium-energy X-rays, along with the consequences of lack of standardization and implications of dose inhomogeneity. It is clear that there is still a gap in understanding the needs for standardization of dosimetric aspects of preclinical and radiobiological studies. It is recommended that these radiobiology studies should be conducted in partnership with medical/radiation physicists. This collaboration ensures the correct utilization of suitable dosimetry systems, thus guaranteeing accuracy and consistency of dose delivery. Appropriate calibration and traceability to national/international standards laboratory, along with regular quality assurance of radiation devices, are paramount to reproducibility. Additionally, it is critical that experimental details and associated dosimetry are sufficiently reported to ensure accurate replication that enables reanalysis including evaluation of dose distributions. Increasing awareness among the researchers and the funding bodies was identified as a crucial step to improve translatability and appropriate resources are budgeted to increase the value for money of research proposals. The proposed recommendations will serve as a vital resource for researchers, encouraging uniformity in experimental design and improving the translatability of preclinical research to clinical settings.