Purpose: To evaluate intrafractional motion effects as a function of peak-to-peak motion and period during single -field, single-fraction and single -field, multifraction irradiation of the moving target in spot-scanning proton therapy. Materials and Methods: An in -house dynamic phantom was used to simulate peak-to-peak motion of 5, 10, and 20 mm with periods of 2, 4, and 8 seconds. The dose distribution in the moving target was measured using radiochromic films. During the perpendicular motion, the film was fixed and moved perpendicular to the beam direction without changing the water equivalent thickness (WET). During longitudinal motion, the film was fixed and moved along the beam direction, causing a change in WET. Gamma index analysis was used with criteria of 3%/3 mm and 3%/2 mm to analyze the dose distributions. Results: For single-fraction irradiation, varying the period did not result in a significant difference in any of the metrics used ( P > .05), except for the local dose within the planning target volume ( P < .001). In contrast, varying peak-to-peak motion was significant ( P < .001) for all metrics except for the mean planning target volume dose ( P approximate to .88) and the local dose ( P approximate to .47). The perpendicular motion caused a greater decrease in gamma passing rate (3%/3 mm) than WET variations (65% +/- 5% vs 85% +/- 4%) at 20 mm peak-to-peak motion. Conclusion: The implementation of multifraction irradiation allowed to reduce hot and cold spots but did not reduce dose blurring. The motion threshold varied from 7 to 11 mm and depended on the number of fractions, the type of motion, the acceptance criteria, and the calculation method used.

The interplay effect is a challenge when using proton scanning beams for the treatment of thoracic and abdominal cancers. The aim of this study was to evaluate the facility-specific interplay effect in terms of dose distortion and irradiation time for different beam delivery modalities, including free breathing (FB) irradiation, rescanning, deep inspiration breath-hold (DIBH), and respiratory gating. This study was carried out at a synchrotron-based facility with spot-scanning beam delivery. A motion phantom with a radiochromic film was used to measure dose distributions. Regular and irregular motion patterns were studied. Dose homogeneity and the gamma index were calculated to quantify the interplay effect. The interplay effect significantly decreased the homogeneity and gamma passing rate by 12% and 46%, respectively, when FB irradiation without motion mitigation was used for 20 mm peak-to-peak motion. Rescanning and DIBH partially mitigated the distortions but doubled the irradiation time, while gating provided the superior dose distribution with only a 25% increase in time compared to FB irradiation without mitigation. The interplay effect was a function of motion amplitude and varied with the beam delivery modality. Gating may be a more preferable technique for the synchrotron-based facility in terms of minimizing dose distortion and treatment time.

Background and purpose: The inherent uncertainties associated with proton therapy (PT) may present a challenge to the optimal clinical utilization of this radiotherapy modality. The possibility to overcome the uncertainties associated with temporal fluctuations in some parameters of the PT system by selecting an additional clinical target volume to planning parget volume (CTV‐PTV) margin or by modifying the planning approach was suggested. The objective of this work was to verify the sufficiency of the calculated additional margin for overcoming beam positioning uncertainties in spot‐scanning PT. Methods: Treatment plans for eight patients were calculated using two methods (robust optimization and PTV‐based approaches) and evaluated with regard to dose‐volume histogram (DVH)‐based dosimetric metrics. The robustness of each plan was evaluated under six scenarios in linear directions ±4 mm and two scenarios with a ±3.5% range uncertainty. A comparison between robust optimization and PTV‐based approaches was performed in the context of additional 1‐mm setup uncertainty associated with facility‐specific beam uncertainties. Results: No statistically significant difference was identified between the initial plans of the PTV‐based and robust approaches with regard to investigated dosimetric metrics. But robust optimization shows better results in comparison to PTV approach in majority of investigated scenarios. CTV was fully encompassed by the 100% dose in the plans created using the robust optimization approach. The Robust approach demonstrated superior dose conformity to the CTV, both for the 100% and 95% prescribed doses. Although both the PTV and Robust approaches permitted homogeneity in all scenarios, the superiority of the Robust approach was evident. Conclusions: It is necessary to consider the facility‐specific beam uncertainty as additional margin during treatment planning in PT. It was demonstrated that the robust optimization approach is an effective method for eliminating beam‐related facility‐specific uncertainties order of 1 mm in comparison to the PTV planning approach.

Purpose: To investigate a novel bioimpedance‐based respiratory gating system (BRGS) designed for external beam radiotherapy and to evaluate its technical characteristics in comparison with existing similar systems.Materials and methods: The BRGS was tested on three healthy volunteers in free breathing and breath‐hold patterns under laboratory conditions. Its parameters, including the time delay (TD) between the actual impedance change and the gating signal, temperature drift, root mean square (RMS) noise, and signal‐to‐noise ratio (SNR), were measured and analyzed.Results: The gate‐on TD and the gate‐off TD were found to be 9.0 ± 2.0 ms [mean ± standard deviation (M ± SD)] and 7.2 ± 1.3 ms, respectively. The temperature drift of the BRGS output signal was 0.02 Ω after 30 min of operation. RMS noise averaged 0.14 ± 0.05 Ω (M ± SD) for all subjects and varied from 0.08 to 0.20 Ω with repeated measurements. A significant difference in SNR (p < 0.001) was observed between subjects, ranging from 4 to 15.Conclusion: The evaluated bioimpedance‐based gating system showed a high performance in real‐time respiratory monitoring and may potentially be used as an external surrogate guidance for respiratory‐gated external beam radiotherapy. Direct comparison with commercially available systems, 4D correlation studies, and expansion of the patient sample are goals for future preclinical studies.