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Spot-Scanning Proton Arc (SPArc) Therapy: The First Robust and Delivery-Efficient Spot-Scanning Proton Arc Therapy

PURPOSEnToxa0present a novel robust and delivery-efficient spot-scanning proton arc (SPArc) therapy technique.nnnMETHODS AND MATERIALSnA SPArc optimization algorithm was developed that integrates control point resampling, energy layer redistribution, energy layer filtration, and energy layer resampling. The feasibility of such a technique was evaluated using sample patients: 1 patient with locally advanced head and neck oropharyngeal cancer with bilateral lymph node coverage, and 1 with a nonmobile lung cancer. Plan quality, robustness, and total estimated delivery time were compared with the robust optimized multifield step-and-shoot arc plan without SPArc optimization (Arcmulti-field) and the standard robust optimized intensity modulated proton therapy (IMPT) plan. Dose-volume histograms of target and organs at risk were analyzed, taking into account the setup and range uncertainties. Total delivery time was calculated on the basis of a 360° gantry room with 1 revolutions per minutexa0gantry rotation speed, 2-millisecond spot switching time, 1-nA beam current, 0.01 minimum spot monitor unit, and energy layer switching time of 0.5 to 4xa0seconds.nnnRESULTSnThe SPArc plan showed potential dosimetric advantages for both clinical sample cases. Compared with IMPT, SPArc delivered 8% and 14% less integral dose for oropharyngeal and lung cancer cases, respectively. Furthermore, evaluating the lung cancer plan compared with IMPT, it was evident that the maximum skin dose, the mean lung dose, and the maximum dose to ribs were reduced by 60%, 15%, and 35%, respectively, whereas the conformity index was improved from 7.6 (IMPT) to 4.0 (SPArc). The total treatment delivery time for lung and oropharyngeal cancer patients was reduced by 55% to 60% and 56% to 67%, respectively, when compared with Arcmulti-field plans.nnnCONCLUSIONnThe SPArc plan is the first robust and delivery-efficient proton spot-scanning arc therapy technique, which could potentially be implemented into routine clinical practice.

Improve dosimetric outcome in stage III non-small-cell lung cancer treatment using spot-scanning proton arc (SPArc) therapy

BackgroundTo evaluate spot-scanning proton arc therapy (SPArc) and multi-field robust optimized intensity modulated proton therapy (RO-IMPT) in treating stage III non-small-cell lung cancer (NSCLC) patients.MethodsTwo groups of stage IIIA or IIIB NSCLC patients (group 1: eight patients with tumor motion less than 5xa0mm; group 2: six patients with tumor motion equal to or more than 5xa0mm) were re-planned with SPArc and RO-IMPT. Both plans were generated using robust optimization to achieve an optimal coverage with 99% of internal target volume (ITV) receiving 66xa0Gy (RBE) in 33 fractions. The dosimetric results and plan robustness were compared for both groups. The interplay effect was evaluated based on the ITV coverage by single-fraction 4D dynamic dose. Total delivery time was simulated based on a full gantry rotation with energy-layer-switching-time (ELST) from 0.2 to 4xa0s. Statistical analysis was also evaluated via Wilcoxon signed rank test.ResultsBoth SPArc and RO-IMPT plans achieved similar robust target volume coverage for all patients, while SPArc significantly reduced the doses to critical structures as well as the interplay effect. Specifically, compared to RO-IMPT, SPArc reduced the average integral dose by 7.4% (pu2009=u20090.001), V20, and mean lung dose by an average of 3.2% (pu2009=u20090.001) and 1.6xa0Gy (RBE) (pu2009=u20090.001), the max dose to cord by 4.6xa0Gy (RBE) (pu2009=u20090.04), and the mean dose to heart and esophagus by 0.7xa0Gy (RBE) (pu2009=u20090.01) and 1.7xa0Gy (RBE) (pu2009=u20090.003) respectively. The average total estimated delivery time was 160.1xa0s, 213.8xa0s, 303.4xa0s, 840.8xa0s based on ELST of 0.2xa0s, 0.5xa0s, 1xa0s, and 4xa0s for SPArc plans, compared with the respective values of 182.0xa0s (pu2009=u20090.001), 207.9xa0s (pu2009=u20090.22), 250.9xa0s (pu2009=u20090.001), 509.4xa0s (pu2009=u20090.001) for RO-IMPT plans. Hence, SPArc plans could be clinically feasible when using a shorter ELST.ConclusionsThis study has indicated that SPArc could further improve the dosimetric results in patients with locally advanced stage NSCLC and potentially be implemented into routine clinical practice.

Have we reached proton beam therapy dosimetric limitations? – A novel robust, delivery-efficient and continuous spot-scanning proton arc (SPArc) therapy is to improve the dosimetric outcome in treating prostate cancer

During the recent years, proton beam therapy practice has been dramatically advanced from passive-scattering (PS) to Pencil Beam Scanning (PBS) technique [1]. Such evolution not only has improved the dosimetric quality [2], but also has simplified the operation and workflow of proton therapy centers [3]. As a result, most of the new proton therapy centers are equipped with PBS only [4]. Nevertheless, challenges still remain. More recently, there have been several discussions regarding whether the proton beam therapy has reached its dosimetric limitation [5] due to its lateral penumbra (spot size) [6], range uncertainties [7] and deliver efficiency [8]. Although, there have been a lot of efforts on defining the role of proton arc therapy via developing different techniques such as Distal Edge Tracking (DET) [9], mono-energetic arc delivery [10] or static multiple fields [11], none of the above proposed techniques could address the plan robustness, beam delivery efficiency, and the continuous rotation-delivery at the same time. Hence, there have been concerns in our scientific community that whether proton arc therapy is needed or feasible in our clinical practice [12]. Recently, our group proposed a novel Spot-Scanning Proton Arc (SPArc) algorithm to address the above three main challenges in proton arc therapy through an interactive inverse planning approach. A potential dosimetric improvement was presented over current IMPT technique especially resulting in reduction of body integral dose, better target conformity as well as a practical achievable arc treatment delivery time [13]. Herein we propose the first systematic study to exploit this novel technique of SPArc in treating prostate cancer.

Redefine the role of range shifter in treating bilateral head and neck cancer in the era of Intensity Modulated Proton Therapy

The use of proton beam therapy has been increasing rapidly. As the Pencil Beam Scanning (PBS) technology has become commercially available in the recent 10 years, nearly all the new proton centers under the contract or constructions are now configured with only PBS technique. Compared to passive‐scattering technique, Intensity Modulated Proton Therapy (IMPT) based on PBS technique allows for creating a more conformal dose distribution to target volume while resulting in a less body integral dose and ultimately less neutron dose. For a majority of the current commercial proton beam systems, the minimum proton beam energy ranges from 70 to 100 MeV which is about 4.1–7.5 cm in water‐equivalent thickness (WET). In order to treat superficial target volume such as patients with head and neck cancer (HNC), and brain tumors, a range shifter (RS) is normally needed to attenuate the proton beam energy.3–5 The RS which is normally composed of slab of plastics such as acrylonitrile butadiene styrene (ABS) and polyethylene 3–5 which broadens the proton beams due to the secondary scattering. In order to reduce the scattering and keep a smaller spot size, the air gap between the gantry nozzle and patients skin must be reduced. Thus, the proton gantry nozzle causes potential collision concerns with the patients body especially in the vicinity of the shoulder region during the treatment of HNC. In order to avoid using RS, Both et al. introduced a rigid U‐shaped bolus placed close to the patients head and neck region as an alternative. The advantage was to reduce the air gap and therefore the proton beam was able to maintain the spot size. However, as a trade‐off, it also introduced additional workload for the therapists to mount this heavy U‐shaped bolus on the table every time during the Computed Tomography (CT) simulation as well as to the couch following the daily imaging alignment prior to the radiation delivery. Another challenge is the size of bolus which needs to be carefully selected to fit patients anatomy. Additionally, it also introduces WET inhomogeneity at the edge of bolus near the connection area to the couch top which limits the proton beam angle selections due to the range uncertainties. It is very difficult to model a continuous moving RS configuration due to the secondary proton scattering. Shen et al. and Li et al. have been addressing these issues using analytical model as well as an in‐house Monte Carlos simulation. Moreover, commissioning of a RS also requires extensive measurements. To overcome such limitations in proton beam therapy, air gap <10 cm between the patient body and RS is recommended in order to minimize the dose calculation error. In addition, larger air gap results in a larger spot size which makes robust coverage of the CTV while sparing critical structures less likely. Thus, to minimize such air gap, the potential collisions between the gantry nozzle and patients shoulder became a concern in treating bilateral HNC. With so many disadvantages and inconvenient clinical workflow with using RS, it is critical to evaluate the role of RS and possibly eliminating it in treating bilateral HNC patient using IMPT. Normally, three to four field IMPT with RS were used in the bilateral HNC treatment. We hypothesize that by increasing the degree of freedom or beam angle directions, IMPT is able to deliver a robust prescription dose to the bilateral HNC target without

SU-F-J-192: A Quick and Effective Method to Validate Patient's Daily Setup and Geometry Changes Prior to Proton Treatment Delivery Based On Water Equivalent Thickness Projection Imaging (WETPI) for Head Neck Cancer (HNC) Patient

PURPOSEnWith the implementation of Cone-beam Computed-Tomography (CBCT) in proton treatment, we introduces a quick and effective tool to verify the patients daily setup and geometry changes based on the Water-Equivalent-Thickness Projection-Image(WETPI) from individual beam angle.nnnMETHODSnA bilateral head neck cancer(HNC) patient previously treated via VMAT was used in this study. The patient received 35 daily CBCT during the whole treatment and there is no significant weight change. The CT numbers of daily CBCTs were corrected by mapping the CT numbers from simulation CT via Deformable Image Registration(DIR). IMPT plan was generated using 4-field IMPT robust optimization (3.5% range and 3mm setup uncertainties) with beam angle 60, 135, 300, 225 degree. WETPI within CTV through all beam directions were calculated. 3%/3mm gamma index(GI) were used to provide a quantitative comparison between initial sim-CT and mapped daily CBCT. To simulate an extreme case where human error is involved, a couch bar was manually inserted in front of beam angle 225 degree of one CBCT. WETPI was compared in this scenario.nnnRESULTSnThe average of GI passing rate of this patient from different beam angles throughout the treatment course is 91.5 ± 8.6. In the cases with low passing rate, it was found that the difference between shoulder and neck angle as well as the head rest often causes major deviation. This indicates that the most challenge in treating HNC is the setup around neck area. In the extreme case where a couch bar is accidently inserted in the beam line, GI passing rate drops to 52 from 95.nnnCONCLUSIONnWETPI and quantitative gamma analysis give clinicians, therapists and physicists a quick feedback of the patients setup accuracy or geometry changes. The tool could effectively avoid some human errors. Furthermore, this tool could be used potentially as an initial signal to trigger plan adaptation.

SU-F-T-191: 4D Dose Reconstruction of Intensity Modulated Proton Therapy (IMPT) Based On Breathing Probability Density Function (PDF) From 4D Cone Beam Projection Images: A Study for Lung Treatment

PURPOSEnWith energy repainting in lung IMPT, the dose delivered is approximate to the convolution of dose in each phase with corresponding breathing PDF. This study is to compute breathing PDF weighted 4D dose in lung IMPT treatment and compare to its initial robust plan.nnnMETHODSnSix lung patients were evaluated in this study. Amsterdam shroud image were generated from pre-treatment 4D cone-beam projections. Diaphragm motion curve was extract from the shroud image and the breathing PDF was generated. Each patient was planned to 60 Gy (12GyX5). In initial plans, ITV density on average CT was overridden with its maximum value for planning, using two IMPT beams with robust optimization (5mm uncertainty in patient position and 3.5% range uncertainty). The plan was applied to all 4D CT phases. The dose in each phase was deformed to a reference phase. 4D dose is reconstructed by summing all these doses based on corresponding weighting from the PDF. Plan parameters, including maximum dose (Dmax), ITV V100, homogeneity index (HI=D2/D98), R50 (50%IDL/ITV), and the lung-GTVs V12.5 and V5 were compared between the reconstructed 4D dose to initial plans.nnnRESULTSnThe Dmax is significantly less dose in the reconstructed 4D dose, 68.12±3.5Gy, vs. 70.1±4.3Gy in the initial plans (p=0.015). No significant difference is found for the ITV V100, HI, and R50, 92.2%±15.4% vs. 96.3%±2.5% (p=0.565), 1.033±0.016 vs. 1.038±0.017 (p=0.548), 19.2±12.1 vs. 18.1±11.6 (p=0.265), for the 4D dose and initial plans, respectively. The lung-GTV V12.5 and V5 are significantly high in the 4D dose, 13.9%±4.8% vs. 13.0%±4.6% (p=0.021) and 17.6%±5.4% vs. 16.9%±5.2% (p=0.011), respectively.nnnCONCLUSIONn4D dose reconstruction based on phase PDF can be used to evaluate the dose received by the patient. A robust optimization based on the phase PDF may even further improve patient care.

SU‐F‐T‐186: A Treatment Planning Study of Normal Tissue Sparing with Robustness Optimized IMPT, 4Pi IMRT, and VMAT for Head and Neck Cases

PURPOSEnWe performed a retrospective dosimetric comparison study between the robustness optimized Intensity Modulated Proton Therapy (RO-IMPT), volumetric-modulated arc therapy (VMAT), and the non-coplanar 4? intensity modulated radiation therapy (IMRT). These methods represent the most advanced radiation treatment methods clinically available. We compare their dosimetric performance for head and neck cancer treatments with special focus on the OAR sparing near the tumor volumes.nnnMETHODSnA total of 11 head and neck cases, which include 10 recurrent cases and one bilateral case, were selected for the study. Different dose levels were prescribed to tumor target depending on disease and location. Three treatment plans were created on commercial TPS systems for a novel noncoplanar 4π method (20 beams), VMAT, and RO-IMPT technique (maximum 4 fields). The maximum patient positioning error was set to 3 mm and the maximum proton range uncertainty was set to 3% for the robustness optimization. Line dose profiles were investigated for OARs close to tumor volumes.nnnRESULTSnAll three techniques achieved 98% coverage of the CTV target and most photon plans had less than 110% of the hot spots. The RO-IMPT plans show superior tumor dose homogeneity than 4? and VMAT plans. Although RO-IMPT has greater R50 dose spillage to the surrounding normal tissue than 4π and VMAT, the RO-IMPT plans demonstrate better or comparable OAR (parotid, mandible, carotid, oral cavity, pharynx, and etc.) sparing for structures closely abutting tumor targets.nnnCONCLUSIONnThe RO-IMPTs ability of OAR sparing is benchmarked against the C-arm linac based non-coplanar 4π technique and the standard VMAT method. RO-IMPT consistently shows better or comparable OAR sparing even for tissue structures closely abutting treatment target volume. RO-IMPT further reduces treatment uncertainty associated with proton therapy and delivers robust treatment plans to both unilateral and bilateral head and neck cancer patients with desirable treatment time.

SU-F-T-210: The Variable Virtual Source-To-Axis Distance Effect On A Compact Proton Pencil Beam Scanning System

PURPOSEnWe investigate the spot characteristic and dose profiles properties from a compact gantry proton therapy system. This compact design features a dedicated pencil beam scanning nozzle with the scanning magnet located upstream of the final 60 degree bending magnet. Due to the unique beam line design, uncertainty has been raised in the virtual source-to-axis distance (SAD). We investigate its potential clinical impact through measurements and simulation.nnnMETHODSnA scintillator camera based detector was used to measure spot characteristics and position accuracy. An ion chamber array device was used to measure planar dose profile. Dose profile in-air simulation was performed using in-house built MATLAB program based on additional spot parameters directly from measurements. Spot characteristics such as position and in-air sigma values were used to general simulated 2D elliptical Gaussian spots. The virtual SAD distance changes in the longitudinal direction were also simulated. Planar dose profiles were generated by summation of simulated spots at the isocenter, 15 cm above the isocenter, and 15 cm below the isocenter for evaluation of potential clinical dosimetric impact.nnnRESULTSnWe found that the virtual SAD varies depending on the spot location on the longitudinal axis. Measurements have shown that the variable SAD changes from 7 to 12 meters from one end to the other end of the treatment field in the longitudinal direction. The simulation shows that the planer dose profiles differences between the fixed SAD and variable SAD are within 3% from the isocenter profile and the lateral penumbras are within 1 mm difference.nnnCONCLUSIONnOur measurements and simulations show that there are minimum effects on the spot characteristics and dose profiles for this up-stream scanning compact system proton system. Further treatment planning study is needed with the variable virtual SAD accounted for in the planning system to show minimum dosimetric impact.

SU-F-T-215: An Investigation Of Multi-Scanner CT Hounsfield Unit Calibration for Pencil Beam Scanning Proton Therapy Using 3D Gamma Analysis

PURPOSEnWe compare and investigate the dosimetric impacts on pencil beam scanning (PBS) proton treatment plans generated with CT calibration curves from four different CT scanners and one averaged global CT calibration curve.nnnMETHODSnThe four CT scanners are located at three different hospital locations within the same health system. CT density calibration curves were collected from these scanners using the same CT calibration phantom and acquisition parameters. Mass density to HU value tables were then commissioned in a commercial treatment planning system. Five disease sites were chosen for dosimetric comparisons at brain, lung, head and neck, adrenal, and prostate. Three types of PBS plans were generated at each treatment site using SFUD, IMPT, and robustness optimized IMPT techniques. 3D dose differences were investigated using 3D Gamma analysis.nnnRESULTSnThe CT calibration curves for all four scanners display very similar shapes. Large HU differences were observed at both the high HU and low HU regions of the curves. Large dose differences were generally observed at the distal edges of the beams and they are beam angle dependent. Out of the five treatment sites, lung plans exhibits the most overall range uncertainties and prostate plans have the greatest dose discrepancy. There are no significant differences between the SFUD, IMPT, and the RO-IMPT methods. 3D gamma analysis with 3%, 3 mm criteria showed all plans with greater than 95% passing rate. Two of the scanners with close HU values have negligible dose difference except for lung.nnnCONCLUSIONnOur study shows that there are more than 5% dosimetric differences between different CT calibration curves. PBS treatment plans generated with SFUD, IMPT, and the robustness optimized IMPT has similar sensitivity to the CT density uncertainty. More patient data and tighter gamma criteria based on structure location and size will be used for further investigation.

SU-F-T-382: Volumetric Modulated Arc Therapy (VMAT) Beam Angle Optimization in Pulsed Partial Brain Irradiation (PPBI) for Newly Diagnosed Glioblastoma

PURPOSEnTo optimize VMAT beam parameters in PPBI to minimize treatment time. We investigate the coverage and organs at risk (OR) avoidance capability of shorter arcs with shorter treatment times.nnnMETHODSnWe evaluated the treatment plans for eleven previously treated PPBI patients. Each patient received 46Gy (2Gy×23) to the initial target and an additional 14Gy (2Gy×7) as a sequential boost. Each daily 2-Gy fraction was delivered as ten 0.2-Gy pulses separated by 3-minute intervals using VMAT. Each pulse was delivered using the same arc and covered at least 95% of the PTV with at least 95% of the prescription dose. To optimize the VMAT beam angle, an initial 360° full-arc VMAT plan was implemented. Beam control points and their corresponding dose rates were exported. A curve of the product of control point and dose rate was plotted against treatment beam angle. The optimum angle range was determined from this relationship. We chose the minimum continuous angle range that covered 85% of the area under the curve. Planning parameters, including treatment time for each pulse (T-pulse), PTV coverage, maximum dose (Dmax), homogeneity index (HI=D5/D95), R50 (50%IDL/PTV), and Dmax to ORs, were compared.nnnRESULTSnMean PTV volume was 364.1±181.5cc. Mean T-pulse of partial-arc beams was 34.3±10.6s, vs. 63.0±1.7s (p<0.001) for that of full-arc beams. No significant differences were found for PTV V95, Dmax and R50, 99.4%±1.2% vs. 99.7%±0.5% (p=0.066), 108.0%±1.2% vs. 107.5%±1.1% (p=0.107), 2.95±0.38 vs. 2.87±0.35 (p=0.165), for the plans with partial-arc and full-arc beams, respectively. However, plans using full-arc do provide better PTV V100 and HI, 96.0%±3.0% vs. 97.2%±2.0% (p=0.025) and 1.06±0.03 vs. 1.04±0.01 (p=0.009). No significant difference was found on Dmax to ORs.nnnCONCLUSIONnPPBI with optimized partial-arc plans are clinically comparable to full-arc plans, while treatment time be significantly reduced, average saving of 287s for a 10-pulse treatment.