Extension of RBE-weighted 4D particle dose calculation for non-periodic motion

Selection of breathing phase number in 4D scanned proton treatment planning optimization for lung tumors

The standard four-dimensional (4D) treatment planning includes all breathing states in the optimization process, which is time-consuming. This work was aimed to optimize the number of intermediate phases needed for 4D proton treatment planning optimization to reduce the computational cost. Five 4D optimization strategies adopting different numbers of intermediate states and one three-dimensional (3D) optimization plan were studied for fifteen lung cancer patients treated with scanned protons, optimizing on all ten phases (4D_10), two extreme phases (4D_2), six phases during the exhalation stage (4D_6EX), six phases during the inhalation stage (4D_6IN), two extreme phases plus an intermediate state (4D_3) and average computed tomography image (3D), respectively. The 4D dose evaluation was conducted on all the ten phases, considering the interplay effect. The resulting doses accumulated on the reference phase were computed and compared. Compared to the 4D optimization plans, the 3D optimization plan performed inferiorly in target coverage, but superiorly in organ at risks (OARs) sparing. For the 4D optimization, all the five 4D plans showed similar performance in OARs protection. However, the 4D_6EX and 4D_6IN strategies out-performed the 4D_2 and 4D_3 plans in dose homogeneity. The computing times of the 4D_2, 4D_3, 4D_6EX and 4D_6IN approaches decreased to 32%, 41%, 66% and 67% of the 4D_10 method, respectively. Thus, our study suggested that the use of all phases during inhalation or exhalation stage might be a feasible approach substituting for the full phase strategy to reduce the calculation load while guaranteeing the plan quality for scanned proton therapy.

High-speed low-noise optical respiratory monitoring for spot scanning proton therapy

PURPOSE

To investigate a novel optical markerless respiratory sensor for surface guided spot scanning proton therapy and to measure its main technical characteristics.

METHODS

The main characteristics of the respiratory sensor including sensitivity, linearity, noise, signal-to-noise, and time delay were measured using a dynamic phantom and electrical measuring equipment on a laboratory stand. The respiratory signals of free breathing and deep-inspiration breath-hold patterns were acquired for various distances with a volunteer. A comparative analysis of this sensor with existing commercially available and experimental respiratory monitoring systems was carried out based on several criteria including principle of operation, patient contact, application to proton therapy, distance range, accuracy (noise, signal-to-noise ratio), and time delay (sampling rate).

RESULTS

The sensor provides optical respiratory monitoring of the chest surface over a distance range of 0.4-1.2 m with the RMS noise of 0.03-0.60 mm, SNR of 40-15 dB (for motion with peak-to-peak of 10 mm), and time delay of 1.2 ± 0.2 ms.

CONCLUSIONS

The investigated optical respiratory sensor was found to be appropriate to use in surface guided spot scanning proton therapy. This sensor combined with a fast respiratory signal processing algorithm may provide accurate beam control and a fast response in patients' irregular breathing movements. A careful study of correlation between the respiratory signal and 4DCT data of tumor position will be required before clinical implementation.

Parameter based 4D dose calculations for proton therapy

Background and purpose Retrospective log file-based analysis provides the actual dose delivered based on the patient's breathing and the daily beam-delivery dynamics. To predict the motion sensitivity of the treatment plan on a patient-specific basis before treatment start a prospective tool is required. Such a parameter-based tool has been investigated with the aim to be used in clinical routine. Materials and Methods 4D dose calculations (4DDC) were performed for seven cancer patients with small breathing motion treated with scanned pulsed proton beams. Validation of the parameter-based 4DDC (p-4DDC) method was performed with an anthropomorphic phantom and patient data employing measurements and a log file-based 4DDC tool. The dose volume histogram parameters (Dx%) were investigated for the target and the organs at risk, compared to static and the file-based approach. Results The difference between the measured and the p-4DDC dose was within the deviation of the measurements. The maximum deviation was 0.4Gy. For the planning target volume D98% varied up to 15% compared to the static scenario, while the results from the log file and p-4DDC agreed within 2%. For the liver patients, D33%liver deviated up to 35% compared to static and 10% comparing the two 4DDC tools, while for the pancreas patients the D1%stomach varied up to 45% and 11%, respectively. Conclusion The results showed that p-4DDC could be used prospectively. The next step will be the clinical implementation of the p-4DDC tool, which can support a decision to either adapt the treatment plan or apply motion mitigation strategies.

Experimental Validation of a Real-Time Adaptive 4D-Optimized Particle Radiotherapy Approach to Treat Irregularly Moving Tumors

PURPOSE

Treatment of locally advanced lung cancer is limited by toxicity and insufficient local control. Particle therapy could enable more conformal treatment than intensity modulated photon therapy but is challenged by irregular tumor motion, associated range changes, and tumor deformations. We propose a new strategy for robust, online adaptive particle therapy, synergizing 4-dimensional optimization with real-time adaptive beam tracking. The strategy was tested and the required motion monitoring precision was determined.

METHODS AND MATERIALS

In multiphase 4-dimensional dose delivery (MP4D), a dedicated quasistatic treatment plan is delivered to each motion phase of periodic 4-dimensional computed tomography (4DCT). In the new extension, "MP4D with residual tracking" (MP4DRT), lateral beam tracking compensates for the displacement of the tumor center-of-mass relative to the current phase in the planning 4DCT. We implemented this method in the dose delivery system of a clinical carbon facility and tested it experimentally for a lung cancer plan based on a periodic subset of a virtual lung 4DCT (planned motion amplitude 20 mm). Treatments were delivered in a quality assurance-like setting to a moving ionization chamber array. We considered variable motion amplitudes and baseline drifts. The required motion monitoring precision was evaluated by adding noise to the motion signal. Log-file-based dose reconstructions were performed in silico on the entire 4DCT phantom data set capable of simulating nonperiodic motion. MP4DRT was compared with MP4D, rescanned beam tracking, and internal target volume plans. Treatment quality was assessed in terms of target coverage (D95), dose homogeneity (D5-D95), conformity number, and dose to heart and lung.

RESULTS

For all considered motion scenarios and metrics, MP4DRT produced the most favorable metrics among the tested motion mitigation strategies and delivered high-quality treatments. The conformity was similar to static treatments. The motion monitoring precision required for D95 >95% was 1.9 mm.

CONCLUSIONS

With clinically feasible motion monitoring, MP4DRT can deliver highly conformal dose distributions to irregularly moving targets.

Inter- and intrafractional 4D dose accumulation for evaluating ΔNTCP robustness in lung cancer

BACKGROUND AND PURPOSE

Model-based selection of proton therapy patients relies on a predefined reduction in normal tissue complication probability (NTCP) with respect to photon therapy. The decision is necessarily made based on the treatment plan, but NTCP can be affected when the delivered treatment deviates from the plan due to delivery inaccuracies. Especially for proton therapy of lung cancer, this can be important because of tissue density changes and, with pencil beam scanning, the interplay effect between the proton beam and breathing motion.

MATERIALS AND METHODS

In this work, we verified whether the expected benefit of proton therapy is retained despite delivery inaccuracies by reconstructing the delivered treatment using log-file based dose reconstruction and inter- and intrafractional accumulation. Additionally, the importance of two uncertain parameters for treatment reconstruction, namely deformable image registration (DIR) algorithm and α/β ratio, was assessed.

RESULTS

The expected benefit or proton therapy was confirmed in 97% of all studied cases, despite regular differences up to 2 percent point (p.p.) NTCP between the delivered and planned treatments. The choice of DIR algorithm affected NTCP up to 1.6 p.p., an order of magnitude higher than the effect of α/β ratio.

CONCLUSION

For the patient population and treatment technique employed, the predicted clinical benefit for patients selected for proton therapy was confirmed for 97.0% percent of all cases, although the NTCP based proton selection was subject to 2 p.p. variations due to delivery inaccuracies.

Limitations of phase-sorting based pencil beam scanned 4D proton dose calculations under irregular motion

Objective.4D dose calculation (4DDC) for pencil beam scanned (PBS) proton therapy is typically based on phase-sorting of individual pencil beams onto phases of a single breathing cycle 4DCT. Understanding the dosimetric limitations and uncertainties of this approach is essential, especially for the realistic treatment scenario with irregular free breathing motion.Approach.For three liver and three lung cancer patient CTs, the deformable multi-cycle motion from 4DMRIs was used to generate six synthetic 4DCT(MRI)s, providing irregular motion (11/15 cycles for liver/lung; tumor amplitudes ∼4-18 mm). 4DDCs for two-field plans were performed, with the temporal resolution of the pencil beam delivery (4-200 ms) or with 8 phases per breathing cycle (500-1000 ms). For the phase-sorting approach, the tumor center motion was used to determine the phase assignment of each spot. The dose was calculated either using the full free breathing motion or individually repeating each single cycle. Additionally, the use of an irregular surrogate signal prior to 4DDC on a repeated cycle was simulated. The CTV volume with absolute dose differences >5% (V_dosediff>5%) and differences in CTVV_95%andD_5%-D_95%compared to the free breathing scenario were evaluated.Main results.Compared to 4DDC considering the full free breathing motion with finer spot-wise temporal resolution, 4DDC based on a repeated single 4DCT resulted inV_dosediff>5%of on average 34%, which resulted in an overestimation ofV_95%up to 24%. However, surrogate based phase-sorting prior to 4DDC on a single cycle 4DCT, reduced the averageV_dosediff>5%to 16% (overestimationV_95%up to 19%). The 4DDC results were greatly influenced by the choice of reference cycle (V_dosediff>5%up to 55%) and differences due to temporal resolution were much smaller (V_dosediff>5%up to 10%).Significance.It is important to properly consider motion irregularity in 4D dosimetric evaluations of PBS proton treatments, as 4DDC based on a single 4DCT can lead to an underestimation of motion effects.

The Influence of Motion on the Delivery Accuracy When Comparing Actively Scanned Carbon Ions versus Protons at a Synchrotron-Based Radiotherapy Facility

Simple Summary

The interplay of breathing and beam motion reduces the efficacy of particle irradiation in moving tumours. The effect of motion on protons and carbon ion treatments was investigated dosimetrically and the results were benchmarked against each other by employing an anthropomorphic thorax phantom that was able to simulate tumour, rib, and lung motion. The critical question was whether target coverage and organ-at-risk sparing could be maintained when the application of simple motion mitigation was addressed. Special focus was put on unique synchrotron characteristics, such as pulsed beam delivery and beam intensity variations. It could be demonstrated that the effect of motion was greater for carbon ions than for protons. These findings demonstrated the need for applying motion mitigation techniques depending on the motion amplitude, particle type, and treatment prescription considering complex time correlations.

Abstract

Motion amplitudes, in need of mitigation for moving targets irradiated with pulsed carbon ions and protons, were identified to guide the decision on treatment and motion mitigation strategy. Measurements with PinPoint ionisation chambers positioned in an anthropomorphic breathing phantom were acquired to investigate different tumour motion scenarios, including rib and lung movements. The effect of beam delivery dynamics and spot characteristics was considered. The dose in the tumour centre was deteriorated up to 10% for carbon ions but only up to 5% for protons. Dose deviations in the penumbra increased by a factor of two when comparing carbon ions to protons, ranging from 2 to 30% for an increasing motion amplitude that was strongly dependent on the beam intensity. Layer rescanning was able to diminish the dose distortion caused by tumour motion, but an increase in spot size could reduce it even further to 5% within the target and 10% at the penumbra. An increased need for motion mitigation of carbon ions compared to protons was identified to assure target coverage and sparing of adjacent organs at risk in the penumbra region and outside the target. For the clinical implementation of moving target treatments at a synchrotron-based particle facility complex, time dependencies needed to be considered.