Quantifying lateral penumbra advantages of collimated spot-scanning beam for intensity-modulated proton therapy

Quantifying lateral penumbra advantages of collimated spot-scanning beam for intensity-modulated proton therapy 1 first-author 1 2 1 3 2 corresp Corresponding author, hsfeng@ipp.ac.cn Corresponding author, hsfeng@ipp.ac.cn hsfeng@ipp.ac.cn University of Science and Technology of China, Hefei 230026, China University of Science and Technology of China, Hefei 230026, China Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China Hefei CAS Ion Medical and Technical Devices Co., Ltd, Hefei 230088, China Hefei CAS Ion Medical and Technical Devices Co., Ltd, Hefei 230088, China Intensity-modulated proton therapy (IMPT) is becoming essential for proton therapy and is under rapid development. However, for IMPT, the lateral penumbra of the spot-scanning proton beam is still an urgent issue to be solved. Patient-specific block collimators (PSBCs), which can block unnecessary doses, play a crucial role in passive scattering delivery technology but are rarely used in spot scanning. One objective of this study is to investigate the lateral penumbra variations of intensity-modulated spot scanning with and without a PSBC. For fields with varying degrees of sharpness and at varying depths in a water phantom, the lateral penumbral widths were calculated using a Monte Carlo-based dose engine from RayStation 6. The results suggest that the lateral penumbral widths can be reduced by more than 30% for uniform target volumes, regardless of whether a range-shifter is used, and that the maximum dose beyond the field edges can be reduced significantly. The results of patient cases show that the doses in organs-at-risk near the edge of the target volume decrease if a PSBC is implemented. This study demonstrates that intensity-modulated spot scanning with a PSBC can effectively reduce the lateral penumbra and block unnecessary doses and is therefore promising for clinical applications in spot-scanning proton therapy. Lateral penumbra Patient-specific block collimator Monte Carlo Intensity-modulated proton therapy 1 Introduction 1 Proton therapy, which takes advantage of the sharp Bragg peaks of protons, is becoming a promising treatment method for tumors [ 1 1 - 4 4 1-4 ]. Passive scattering, in which a thin pencil beam is scattered and expanded laterally using one or more scatterers [ 5 5 , 6 6 ], has been commonly employed to deliver proton beams over the last decade. Patient-specific block collimators (PSBCs) and compensators are employed to confine proton beams laterally and reduce unnecessary doses to healthy distal tissues. There are a few disadvantages to passive scattering methods, such as unwanted neutrons, poor conformity, and a complicated nozzle design [ 7 7 - 9 9 7-9 ]. In recent years, spot scanning, in which a narrow beam is driven by scanning magnets to deliver intensity-modulated proton beams into the target, has become overwhelmingly pop ular [ 10 10 , 11 11 ]. Greater accuracy in dose delivery and better protection of organs-at-risk (OARs) are achieved by spot scanning in intensity-modulated proton therapy (IMPT). However, the lateral penumbral width, which is the spatial distance between the distal 80% to 20% dose level, is relatively larger in spot scanning than in passive scattering [ 12 12 ]. Therefore, the healthy tissues around the target volume may receive a higher unnecessary dose. In a practical patient treatment plan, the beam angle is usually chosen to ensure that the OARs are outside of the lateral penumbra [ 13 13 ]. It is important to minimize the lateral penumbra of spot scanning for clinical use. For various types of commercial proton therapy systems, the minimum energy is 70 MeV (CIM SC200 [ 14 14 ], Hitachi ProBeat [ 15 15 ], and Mevion S250/double-scattering system [ 11 11 ]) or 100 MeV (Ion Beam Applications (IBA) ProteusPlus [ 16 16 ] and Varian ProBeam [ 17 17 ]NE.Cms_InsertNE.Cms_Insert), with corresponding water equivalent thicknesses (WETs) of 4.1 cm and 7.5 cm [ 18 18 ], respectively. A range-shifter (RS) is necessary for superficial tumors, such as breast cancer and head and neck cancer, to pull back the proton range. To study the influence of an RS, two scenarios with and without the use of an RS in the beam line were studied in our work. We employed a treatment planning system using RayStation 6 (RaySearch Laboratories, Sweden), in which a proton pencil-beam (PPB) dose engine and a dose engine based on Monte Carlo (MC) are embedded. Compared with the PPB dose engine, the MC-based dose engine can calculate and optimize dose distributions with higher accuracy [ 18 18 ]. To quantitatively evaluate the lateral penumbral widths of the spot-scanning beam with and without a PSBC, two uniform targets with regular shapes and three patient cases were investigated. We used the MC-based dose engine of RayStation 6 with a 0.1% statistical uncertainty throughout this work, which means that more than 1 billion primary protons were simulated for each case. 2 Materials and Methods 1 2.1 Nozzle and Monte Carlo model 2 A spot-scanning beam line with an IBA universal nozzle was commissioned for RayStation 6. The commissioned model supports a proton energy range of 100-226 MeV, which corresponds with WETs of between 7.71 cm and 31.9 cm [ 19 19 ]. The virtual source-to-axis distances in the horizontal and vertical directions were 230 cm and 193 cm, respectively. The simplified nozzle model and the beam spot size (1δ at the isocenter) as a function of energy are shown in Fig. 1a Fig. 1a and Fig. 1b Fig. 1b , respectively. The snout position can be adjusted from 8.5 cm to 57.5 cm relative to the isocenter. A polymethylmethacrylate RS with a thickness of 6.5 cm (and density of 1.19 g/cm 3 , corresponding to a 7.5 cm WET) was available, which can shift the range of a 100 MeV proton beam close to the skin. A brass PSBC (with a density of 8.4 g/cm 3 and a thickness of 6.5 cm) was mounted on the source side and an RS was mounted on the patient side (the location sequence of the PSBC and RS was unchangeable because it was fixed by IBA). The inner edge shape of the PSBC was automatically confirmed by RayStation 6 according to the target’s shape. The MC-based dose engine [ 20 20 ], which is a fast MC algorithm in RayStation 6, was developed independently by RaySearch. In the MC code, primary protons and secondary ions (protons, deuterons, and alphas) can be accurately simulated and transported. Neutral reaction products (neutrons and gammas) are not transported but the induced absorbed energies are counted by given fractions. The proton source is modelled as it arrives at the nozzle entrance. The RS and PSBC can be modelled in the beam line such that proton transportation in the RS and PBSC can be correctly simulated [ 16 16 ]. The high accuracy of the MC dose engine in RayStation 6 has been studied and verified in clinical circumstances [ 18 18 , 21 21 ]. 2.2 Regular targets 2 A computed tomography dataset, which was generated with an empty phantom image set during commissioning, was imported into RayStation 6. A 40 × 40 × 40 cm region of interest (ROI) was constructed and filled with water. The dose grid resolution used for the dose calculation was set to 2 × 2 × 2 mm. For superficial targets with an RS, two uniform targets with regular shapes and uniform densities were defined: a cubic target of 6 × 6 cm in the coronal plane and extending to 5 cm in depth, and a spherical target with a diameter of 6 cm. The geometric centers of both targets were placed at a depth of 6 cm from the water phantom surface. For comparison with cases without an RS, the same targets (cube and sphere) were located at a depth of 15 cm. The gantry angle of the proton beam was selected to be 0° and the air gap between the RS and the water phantom was 5 cm throughout all plans. In accordance with the settings above, eight plans were created, as shown in Table 1 Table 1 . For the plans using the PSBC, the external expansion distance of the PSBC inner edge was 1 cm. All plans were optimized and evaluated using an intensity-modulated spot-scanning proton beam under the MC-based dose engine. The spot spacing, weight, and number of spots at each energy layer were determined by the optimization algorithm of RayStation 6. The optimized beam energies and the numbers of layers shown in Table 1 Table 1 were kept constant for targets of the same shape and at the same depth, regardless of whether the PSBC was placed. The mean dose was 200 cGy within the target volume. 1 1   Plan ID 1 1 Depth (cm) 1 1 Target shape 1 1 PSBC 1 1 RS 1 1 Optimized energies (MeV) 1 1 1 1 1 6 1 1 cube 1 1 ✔ 1 1 ✔ 1 1 121.2-156.0/ 13 layers 1 1 2 1 1 1 1 1 1 ✘ 1 1 ✔ 1 1 3 1 1 1 1 sphere 1 1 ✔ 1 1 ✔ 1 1 118.2-158.6/ 15 layers 1 1 4 1 1 1 1 1 1 ✘ 1 1 ✔ 1 1 5 1 1 15 1 1 cube 1 1 ✔ 1 1 ✘ 1 1 130.5-163.9/ 12 layers 1 1 6 1 1 1 1 1 1 ✘ 1 1 ✘ 1 1 7 1 1 1 1 sphere 1 1 ✔ 1 1 ✘ 1 1 127.1-166.5/ 14 layers 1 1 8 1 1 1 1 1 1 ✘ 1 1 ✘ 1 1 2.3 Clinical cases 2 Three patient cases including brain, abdomen, and prostate tumors were selected to verify the dose distributions with and without the PSBC. For all patient plans, the dose grid was set to 2 × 2 × 2 mm and two beams with well-chosen gantry angles for each plan were employed. The primary parameters are shown in Table 2 Table 2 . Six plans including plan 1 and plan 2 for brain tumors, plan 3 and plan 4 for abdomen tumors, and plan 5 and plan 6 for prostate tumors were optimized using the MC dose engine. The dose–volume histograms (DVHs) were acquired and the mean doses to the ROIs were calculated for evaluation purposes. 1 1 Plan ID 1 1 Tumor Site 1 1 Beam Angle (°) 1 1 RS 1 1 PSBC 1 1 Optimized energies (MeV) 1 1 1 1 1 Brain 1 1 100, 260 1 1 ✔ 1 1 ✘ 1 1 136.6-168.0/ 11 layers 1 1 2 1 1 1 1 1 1 1 1 ✔ 1 1 3 1 1 Abdomen 1 1 90, 270 1 1 ✘ 1 1 ✘ 1 1 116.2-172.6/ 20 layers 1 1 4 1 1 1 1 1 1 1 1 ✔ 1 1 5 1 1 Prostate 1 1 90, 270 1 1 ✘ 1 1 ✘ 1 1 126.8-186.6/ 20 layers 1 1 6 1 1 1 1 1 1 1 1 ✔ 1 1 3 Results 1 3.1 Homogeneous targets 2 Fig. 2 Fig. 2 shows the cross-section views of the two-dimensional (2D) dose distributions at the isocenter for the cubic and spherical targets with homogeneous densities. The maximum dose level was about 200 cGy, which was spread over the whole target volume. The edge shadows were relatively larger for the uncollimated beams, as illustrated in Fig. 2a1 Fig. 2a1 , Fig. 2b1 Fig. 2b1 , Fig. 2c1 Fig. 2c1 , and Fig. 2d1 Fig. 2d1 . When the PSBC was used, the dose levels of the edge shadows were obviously lower than those without the PSBC and the widths were reduced, as shown in Fig. 2a2 Fig. 2a2 , Fig. 2b2 Fig. 2b2 , Fig. 2c2 Fig. 2c2 , and Fig. 2d2 Fig. 2d2 . The dose differences are shown in Fig. 2a3 Fig. 2a3 , Fig. 2b3 Fig. 2b3 , Fig. 2c3 Fig. 2c3 , and Fig. 2d3 Fig. 2d3 . A reduction in the maximum dose at the outer edge of target area reached 40 cGy, which is significant (~20%) relative to the overall prescription dose (200 cGy). The dose differences at the field edges decreased with increasing depth. Fig. 3a Fig. 3a shows the lateral penumbral width variation as a function of depth for the cubic targets. The penumbral width increased as the depth increased both with and without the RS and PSBC. As Table 3 Table 3 shows, if the RS was employed, the penumbral width increased from 4.6 mm to 5.5 mm as the depth increased and from 2.5 mm to 3.6 mm if the PSBC was added; the improvement (reduction of penumbral width) was 46%-35% relative to the non-PSBC case. For targets at a depth of 15 cm, a similar trend was found, as shown by the blue solid line in Fig. 3 Fig. 3 . The penumbral widths were 3.6 mm to 5.1 mm in the absence of the PSBC, 1.3 mm to 3.1 mm in the presence of the PSBC, and the improvement was from 64% to 39%. The role of the PSBC is evident, and the improvement gradually attenuated with increasing depth in water. 1 1 TS 1 1 TD (cm) 1 1 Phase 3 1 PSBC 1 1   1 1   1 1   1 1 No 1 1 Yes 1 1 1 1 1 1 1 1 LP (mm) 1 1 1 1 ΔLP (%) 1 1 cube 1 1 6 1 1 - 1 1 4.6-5.5 1 1 2.5-3.6 1 1 46%-35% 1 1 1 1 15 1 1 - 1 1 3.6-5.1 1 1 1.3-3.1 1 1 64%-39% 1 1 sphere 1 1 6 1 1 1 1 1 5.8-6.6 1 1 3.7-4.6 1 1 36%-30% 1 1 1 1 1 1 2 1 1 6.6-5.3 1 1 4.6-3.6 1 1 30%-32% 1 1 1 1 1 1 3 1 1 5.3-6.3 1 1 3.6-6.3 1 1 32%-0% 1 1 1 1 15 1 1 1 1 1 4.8-6.4 1 1 2.9-4.8 1 1 40%-25% 1 1 1 1 1 1 2 1 1 6.4-4.8 1 1 4.8-3.2 1 1 25%-33% 1 1 1 1 1 1 3 1 1 4.8-5.9 1 1 3.2-5.9 1 1 33%-0% The variations in the lateral penumbral widths for the spherical targets at the two depths are shown in Fig. 3b Fig. 3b . Because the diameter of the circularly shaped cross-section varied as a function of depth, the width of lateral penumbra first increased (phase 1), then decreased (phase 2), and then increased again (phase 3), which spatially corresponds to the region from the surface of the water phantom to the upper surface of the spherical target, the upper surface to the center of the spherical target, and the center to the lower surface of the spherical target, respectively. As Table 3 Table 3 lists, the width of the lateral penumbra of the spherical target at a depth of 6 cm was reduced by 36%-30% at phase 1, 30%-32% at phase 2, and 32%-0% at phase 3. For spherical targets at a depth of 15 cm, improvements of 40%-25% at phase 1, 25%-33% at phase 2, and 33%-0% at phase 3 were obtained. Therefore, using a PSBC in IMPT can lead to a decreased lateral penumbra in the field, which could be helpful for protecting the OARs adjacent to the target volume. 3.2 Clinical cases 2 Fig. 4 Fig. 4 shows the 2D dose distributions and the differences between the uncollimated and collimated beams for a brain tumor patient (plan 1 and plan 2). The differences at the fields’ edges were up to 25% of the maximum dose of the planning target volume (PTV). The DVHs are shown in Fig. 4j Fig. 4j , in which the dose curves of the PTV and OARs near the target edges are displayed. The dose in the PTV was consistent and the prescription was well-maintained in the PTV regardless of whether the PSBC was used. The doses in the OARs adjacent to the target area, such as in the brain stem, brain stem core, and left cochlea, were clearly reduced, as Fig. 4j Fig. 4j illustrates. The mean doses in the left cochlea decreased by as much as 50%, and the other OAR doses were also variously reduced. In general, with the collimated beam, the OARs near the target were better protected than with the uncollimated beam. Fig. 5a Fig. 5a and Fig. 5b Fig. 5b show the DVHs of the abdomen tumor and prostate tumor patient plans, respectively. An obvious decrease in unnecessary doses outside the target volumes were found when the PSBC was employed. For the abdomen tumor case, a dummy ROI (block) was established to ensure a low dose to the end of the stomach. When the PSBC was used, the mean doses to the kidney, stomach, and liver were reduced by 35%, 10%, and 9%, respectively. In the prostate tumor patient plans, the maximum reduction of the mean dose was 35% for the bulb of the penis and 20% and 6% for the rectum and bladder, respectively. Above all, there was little change in the target volume but an obvious dose reduction in the ROIs, which was beneficial to the patients. 4 Discussion 1 In spot scanning for IMPT, the nozzle is generally placed as close as possible to the patient’s skin to minimize the in-air scattering when an RS is mounted. However, an undersized air gap between the patient and nozzle may induce a collision hazard. To ensure safety while maintaining a low in-air scattering, we used an air gap of 5 cm for all plans. Homogeneous targets were used, and although regular cubes or spheres cannot perfectly simulate actual tumors, they were nevertheless sufficient for evaluating the dimension and evolution of the lateral penumbral widths. In a previous study [ 22 22 ], Charlwood et al. reported on investigations into the penumbras of homogeneous targets using Gate (the Geant4 application for tomographic emissions) and employed hypothetical and untuned proton beams. In our study, the trend of the penumbral widths was monotonic, and the results are accurate because the clinical beam parameters included a monitor unit, spot position, and spot spacing, which were well-optimized by RayStation 6. The portion of the study using homogeneous targets demonstrated the advantages of collimated proton beams. For the patient cases, we analyzed three clinical proton treatment plans. The advantages of using a collimated beam have been demonstrated. Although a collimated spot-scanning beam has advantages, such as reducing the penumbral width and doses in OARs, some shortcomings are worthy of notice. First, when energetic protons hit the inner edge of a block collimator, neutrons are inevitably produced by nuclear reactions. As reported, the dose due to secondary neutrons was negligible in IMPT with a PSBC [ 23 23 ]. Second, the processing of the PSBC is relatively time-consuming and labor-intensive. A processing workshop is necessary and various PSBCs need to be manufactured specially for each field of each treatment plan. Multi-layer collimators (MLCs) or dynamic multi-layer collimators (DMLCs) are alternative options; however, more iss ues may arise. For instance, MLCs and DMLCs are complicated and heavy components. Unlike adding a block, it is difficult to add an MLC to a spot-scanning nozzle. In general, although there are some shortcomings, adding a PSBC to the nozzle is still an alternative option to reduce the lateral penumbra and protect OARs from spot-scanning proton beams in IMPT. 5 Conclusion 1 In this work, diversified strategies were used to evaluate a collimated spot-scanning beam for uniform targets of regular shapes and for clinical cases. MC-based dose calculations were performed for all cases to ensure accuracy. The lateral penumbral width increased as the depth increased for the cubic targets and decreased by more than 30% if the beam was collimated. 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Culberson , Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation . Int. J. Radiat. Oncol. Biol. Phys . 103 , 241 - 250 ( 2019 ). doi: 10.1016/j.ijrobp.2018.08.012 http://doi.org/10.1016/j.ijrobp.2018.08.012 Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation 10.1007/s41365-019-0687-y This work was supported by the Key Program of the 13 th Five-Year Plan, the Hefei Institutes of Physical Science of the Chinese Academy of Sciences (No. KP-2017-24). 2019-02-17 2019-05-07 2019-06-06 2019-10-24 2019-11-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2019 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-019-0687-y http://dx.doi.org/10.1007/s41365-019-0687-y . 2019 Nuclear Science and Techniques 11 30