Abstract
The interactive adjustment of the optimization objectives during the treatment planning process has made it difficult to evaluate the impact of beam quality exclusively in radiotherapy. Without consensus in the published results, the arbitrary selection of photon energies increased the probability of suboptimal plans. This work aims to evaluate the dosimetric impact of various photon energies on the sparing of normal tissues by applying a preconfigured knowledge-based planning (RapidPlan) model to various clinically available photon energies for rectal cancer patients, based on model-generated optimization objectives, which provide a comparison basis with less human interference. A RapidPlan model based on 81 historical VMAT plans for pre-surgical rectal cancer patients using 10MV flattened beam (10X) was used to generate patient-specific objectives for the automated optimization of other 20 patients using 6X, 8X, 10X (reference), 6MV flattening-filter-free (6F) and 10F beams respectively on a TrueBeam accelerator. It was observed that flattened beams produced very comparable target dose coverage yet the conformity index using 6F and 10F were clinically unacceptable (>1.29). Therefore, dose to organs-at-risk (OARs) and normal tissues were only evaluated for flattened beams. RapidPlan-generated objectives for 6X and 8X beams can achieve comparable target dose coverage as that of 10X, yet the dose to normal tissues increased monotonically with decreased energies. Differences were statistically significant except femoral heads. From the radiological perspective of view, higher beam energy is still preferable for deep seated tumors, even if multiple field entries such as VMAT technique can accumulate enough dose to the target using lower energies, as reported in the literature. In conclusion, RapidPlan model configured for flattened beams cannot optimize un-flattened beams before adjusting the target objectives, yet works for flattened beams of other energies. For the investigated 10X, 8X and 6X photons, higher energies provide better normal tissue sparing.
Highlights
The inherent photon beam characteristics of different energies, such as penetrating power and penumbra, have provided planners with more options[1], but the choices vary subjectively among different centers
Analysis (PCA) regression to fit the correlation between the geometric features and the historical ‘achievable’ dose distribution without using the actual beam energy as regression input at the training stage
This study aims to evaluate the dosimetric impact of beam qualities on the sparing of normal tissues by applying a preconfigured RapidPlan model to various clinically available photon energies for rectal cancer patients
Summary
The inherent photon beam characteristics of different energies, such as penetrating power and penumbra, have provided planners with more options[1], but the choices vary subjectively among different centers. Coming to the era of knowledge-based planning, the impact of beam energies on the model configuration and applications remains unknown, and is worthy of more investigations. Analysis (PCA) regression to fit the correlation between the geometric features (including patient anatomy and beam geometry) and the historical ‘achievable’ dose distribution without using the actual beam energy as regression input at the training stage. When the trained model is used to predict DVHs for upcoming cases, Geometry-Based Expected Dose (GED) metric was used to estimate the achievable dose to a voxel by considering patient anatomy (such as distance from the targets surfaces), dose prescriptions, and field geometry (such as in- vs outof-field) [6,7] and beam energy, etc. According to the manufacturer[8], the total dose-distance value in voxel v (gedv) is defined as: Xm
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