C.W. Stevens

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

PURPOSE With 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. METHODS A 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. RESULTS The 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. CONCLUSION WETPI 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.

TH-CD-209-10: Scanning Proton Arc Therapy (SPArc) - The First Robust and Delivery-Efficient Spot Scanning Proton Arc Therapy

PURPOSE To develop a delivery-efficient proton spot-scanning arc therapy technique with robust plan quality. METHODS We developed a Scanning Proton Arc(SPArc) optimization algorithm integrated with (1)Control point re-sampling by splitting control point into adjacent sub-control points; (2)Energy layer re-distribution by assigning the original energy layers to the new sub-control points; (3)Energy layer filtration by deleting low MU weighting energy layers; (4)Energy layer re-sampling by sampling additional layers to ensure the optimal solution. A bilateral head and neck oropharynx case and a non-mobile lung target case were tested. Plan quality and total estimated delivery time were compared to original robust optimized multi-field step-and-shoot arc plan without SPArc optimization (Arcmulti-field) and standard robust optimized Intensity Modulated Proton Therapy(IMPT) plans. Dose-Volume-Histograms (DVH) of target and Organ-at-Risks (OARs) were analyzed along with all worst case scenarios. Total delivery time was calculated based on the assumption of a 360 degree gantry room with 1 RPM rotation speed, 2ms spot switching time, beam current 1nA, minimum spot weighting 0.01 MU, energy-layer-switching-time (ELST) from 0.5 to 4s. RESULTS Compared to IMPT, SPArc delivered less integral dose(-14% lung and -8% oropharynx). For lung case, SPArc reduced 60% of skin max dose, 35% of rib max dose and 15% of lung mean dose. Conformity Index is improved from 7.6(IMPT) to 4.0(SPArc). Compared to Arcmulti-field, SPArc reduced number of energy layers by 61%(276 layers in lung) and 80%(1008 layers in oropharynx) while kept the same robust plan quality. With ELST from 0.5s to 4s, it reduced 55%-60% of Arcmulti-field delivery time for the lung case and 56%-67% for the oropharynx case. CONCLUSION SPArc is the first robust and delivery-efficient proton spot-scanning arc therapy technique which could be implemented in routine clinic. For modern proton machine with ELST close to 0.5s, SPArc would be a popular treatment option for both single and multi-room center.