Pencil Beam Scanning Plans Research Articles

Proton Pencil Beam Scanning Facilitates the Safe Treatment of Extended Radiation Targets for Hodgkin Lymphoma: A Report from the Proton Collaborative Group Registry.

Because proton beam therapy (PBT) can lower the dose of radiation to the heart, lungs, and breast, it is an established radiation modality for patients with Hodgkin lymphoma (HL). Pencil beam scanning (PBS) PBT facilitates the treatment of more extensive targets. This may be especially of value for lymphoma patients who require RT to both mediastinal and axillary targets, defined here as extended target RT (ETRT), given the target distribution and need to minimize the lung, heart, and breast dose. Using the Proton Collaborative Group registry, we identified patients with HL treated with PBT to both their mediastinum and axilla, for which DICOM-RT was available. All patients were treated with PBS. To evaluate the dosimetric impact of PBS, we compared delivered PBS plans with VMAT butterfly photon plans optimized to have the same target volume coverage, when feasible. Between 2016 and 2021, twelve patients (median 26 years) received PBS ETRT (median 30.6 Gy (RBE)). Despite the large superior/inferior (SI, median 22.2 cm) and left/right (LR, median 22.8 cm) extent of the ETRT targets, all patients were treated with one isocenter except for two patients (both with SI and LR > 30 cm). Most commonly, anterior beams, with or without posterior beams, were used. Compared to photons, PBS had greater target coverage, better conformity, and lower dose heterogeneity while achieving lower doses to the lungs and heart, but not to the breast. No acute grade 3+ toxicities were reported, including pneumonitis. Proton ETRT in this small cohort was safely delivered with PBS and was associated with an improved sparing of the heart and lungs compared to VMAT.

Pencil Beam Scanning Proton Therapy as Single Vocal Cord Irradiation for Early-Stage Glottic Cancer

PurposeSingle vocal cord irradiation (SVCI) is a promising technique to maintain excellent oncologic control and potentially improve upon toxicities for treatment of early-stage glottic squamous cell carcinomas. We sought to investigate whether pencil beam scanning (PBS) proton therapy could improve upon the already favorable dose gradients demonstrated with volumetric modulated arc therapy (VMAT) SVCI. Patients and MethodsA 64-year-old gentleman was treated in our department with 6X-flattening filter-free VMAT SVCI to 58.08 Gy in 16 fractions for a T1a well-differentiated squamous cell carcinoma of the left true vocal cord and tolerated it well with good local control. Comparative PBS plans were created in Raystation for the Varian ProBeam with clinical target volume (CTVs) generated to mimic the prescription target volume extent of the VMAT planning target volumes when accounting for PBS plan robustness (±3 mm translational shifts, 3.5% density perturbation). A 3-field single-field optimization plan was selected as dosimetrically preferable. Dosimetric variables were compared. ResultsSeveral organs at risk doses improved with PBS, including the maximum and mean dose to ipsilateral carotids, maximum and mean dose to contralateral carotid, maximum dose to the spinal cord, maximum and mean dose to inferior constrictor/cricopharyngeus, maximum and mean dose to the uninvolved vocal cord, and mean dose to the thyroid gland. There are tradeoffs in skin dose depending on location relative to the target—with the highest and lowest isodoses extending more into the skin with the VMAT plan but with the moderate isodose lines covering a wider area with the PBS plan, but we deemed it tolerable regardless. ConclusionSVCI is a promising strategy for maintaining the oncologic effectiveness of whole-larynx photon radiation while potentially improving upon the historic toxicity profile. The favorable dose distribution with PBS with respect to organs at risk dosimetry for PBS may allow for further improvements upon VMAT SVCI strategies. Clinical implementation of PBS SVCI may be considered.

Patient-specific quality assurance of dynamically-collimated proton therapy treatment plans.

The dynamic collimation system (DCS) provides energy layer-specific collimation for pencil beam scanning (PBS) proton therapy using two pairs of orthogonal nickel trimmer blades. While excellent measurement-to-calculation agreement has been demonstrated for simple cube-shaped DCS-trimmed dose distributions, no comparison of measurement and dose calculation has been made for patient-specific treatment plans. To validate a patient-specific quality assurance (PSQA) process for DCS-trimmed PBS treatment plans and evaluate the agreement between measured and calculated dose distributions. Three intracranial patient cases were considered. Standard uncollimated PBS and DCS-collimated treatment plans were generated for each patient using the Astroid treatment planning system (TPS). Plans were recalculated in a water phantom and delivered at the Miami Cancer Institute (MCI) using an Ion Beam Applications (IBA) dedicated nozzle system and prototype DCS. Planar dose measurements were acquired at two depths within low-gradient regions of the target volume using an IBA MatriXX ion chamber array. Measured and calculated dose distributions were compared using 2D gamma analysis with 3%/3mm criteria and low dose threshold of 10% of the maximum dose. Median gamma pass rates across all plans and measurement depths were 99.0% (PBS) and 98.3% (DCS), with a minimum gamma pass rate of 88.5% (PBS) and 91.2% (DCS). The PSQA process has been validated and experimentally verified for DCS-collimated PBS. Dosimetric agreement between the measured and calculated doses was demonstrated to be similar for DCS-collimated PBS to that achievable with noncollimated PBS.

Proton Therapy for Ocular Melanoma: A Dosimetric Comparison of Three Different Delivery Systems

The purpose of this study is to compare the treatment plans for intraocular tumors with two types of Pencil Beam Scanning (PBS) machine configurations and compare them with the widely used uniform scanning (US)/scattering technique. Two virtual PBS machines were constructed for ocular planning. The first machine (P1) consisted of a standard spot size and a 7.5cm WET range shifter (RS) positioned 50cm upstream from the aperture. The second machine (P2) had a smaller spot size and used a 4cm WET RS but had no aperture support. These PBS delivery systems resemble currently available models. Ten patients with different locations and sizes of intraocular tumors were planned with US using 2-3 conformal fields following our institution's intraocular criteria. PBS spot patterns for each machine were optimized using the same beam geometry as the US plan to achieve identical tumor coverage. PBS distribution was calculated using Monte Carlo. All air gaps were minimized as small as clinically achievable. PBS plans passed robustness criteria with all scenarios meeting CTV D95>95% with 2mm translational offsets and 3.5% range uncertainty. The average conformity index (CI) of 95% reference isodose was better for both PBS techniques (P1: 1.43±0.22, P2:1.5±0.32) versus US (1.72±0.56) for all plans (p = 0.025, p = 0.034). The average CI of 50% reference isodose was worse for the P2 plans (10.4±3.3) compared to US (9.34±2.93). However, CI 50% for P1 plans was better than US (5.21±1.24, p<0.003). There were no significant improvements to OARs using P2. However, the mean dose to anterior structures ciliary body, lacrimal gland, and D0.03cc lens were significantly lower in the P1 plans than in the US plans (p<0.016). Small spot size PBS systems without apertures can achieve similar coverage to US plans but have higher OAR dose. PBS systems with a standard spot size and reconfigured RS and apertures are better for anterior OAR sparing and is non-inferior to US planning of intraocular targets.

A Potential Cause for Unexplained Radiation Toxicity in Proton Therapy

Rare but devastating neurological tissue injury by proton therapy is poorly understood. One consideration is that the transition from passively scattered to pencil beam scanning (PBS) proton radiotherapy over the past decade has affected dose rate. We analyzed clinical data acquired at two institutions on a correlation of voxel-wise dose rates and injury of brain tissue (institution 1) and the optic apparatus (institution 2). All radiation-induced toxicities were delineated (Brain/optic apparatus) on follow-up MRIs. A method to calculate dose rates for PBS plans has been implemented at two institutions independently. Institution 1 reports beam-by-beam time-averaged maximum dose rates per voxel (VMDR) using a 100 ms time interval. Data from four patients who incurred brain tissue injury was included. Dose rate volume histograms were prepared for the target, a 1 cm outer ring to the target, and the necrotic region itself. Prescription doses ranged from 30.6 to 60 Gy delivering either 1.8 or 2.0 Gy per fraction. Tissue injury was generally observed in the dose fall-off at the edge of the target. Additionally, we performed LET weighted dose calculation, robustness analysis, and Monte Carlo calculations. Institution 2 presents voxel-wise dose delivered above a selected dose rate threshold, considering values between 30 and 180 Gy/min. The analysis was performed for 2 cases with optical apparatus injuries and one matching non-toxicity case. Both injuries were observed in the chiasma. Patients received 50.4, 50.4 and 55.8 Gy (non-toxicity) in 1.8 Gy fractions. D1 for chiasma was 51.1, 47.2 and 54 Gy, respectively. Mean and (max) LET in chiasma were 4.02 (4.72), 2.49 (3.16) and 3.44 (4.56) keV/µm accordingly. Institution 1 finds distinct dose rate hot spots in the contoured injured brain regions. Mean VMDR values across all beams are lower in targets than in the 1 cm outer ring (80 Gy/min and 82.8 Gy/min, respectively) and elevated to 113 Gy/min for necrotic regions. Neither of the additional computations showed significant increase of dose in the affected areas. Institution 2 observes that chiasmas, which developed radiation injuries, were exposed to higher dose levels delivered at elevated dose rates (> 72 Gy/min) than the chiasma that did not show any radiation injury. D95 above 72 Gy/min for the two volumes identified as radiation injury was 0.25 and 0.12 Gy/fx, while D98 to chiasmas minus radiation injury or chiasma without any injury for the matching case was 0.06, 0.03 and 0 Gy/fx respectively. Using unique formulations to assess voxel-wise dose rates to brain and optic chiasm, two independent institutions observed that regions with elevated dose rates correlated with radiographic radiation injury. An extended retrospective study including a larger patient cohort with radiation-induced toxicity and asymptomatic patients is under way to confirm or refute the presented observations.

Selecting Optimal Proton Pencil Beam Scanning Plan Parameters to Reduce Dose Discrepancy between Discrete Spot Plan and Continuous Scanning: A Proof-of-Concept Study.

Pencil beam scanning delivered with continuous scanning has several advantages over conventional discrete spot scanning. Such advantages include improved beam delivery efficiency and reduced beam delivery time. However, a move dose is delivered between consecutive spots with continuous scanning, and current treatment planning systems do not take this into account. Therefore, continuous scanning and discrete spot plans have an inherent dose discrepancy. Using the operating parameters of the state-of-the-art particle therapy system, we conducted a proof-of-concept study in which we systematically generated 28 plans for cubic targets with different combinations of plan parameters and simulated the dose discrepancies between continuous scanning and a planned one. A nomograph to guide the selection of plan parameters was developed to reduce the dose discrepancy. The effectiveness of the nomograph was evaluated with two clinical cases (one prostate and one liver). Plans with parameters guided by the nomograph decreased dose discrepancy than those used standard plan parameters. Specifically, the 2%/2 mm gamma passing rate increased from 96.3% to 100% for the prostate case and from 97.8% to 99.7% for the liver case. The CTV DVH root mean square error decreased from 2.2% to 0.2% for the prostate case and from 1.8% to 0.9% for the liver case. The decreased dose discrepancy may allow the relaxing of the delivery constraint for some cases, leading to greater benefits in continuous scanning. Further investigation is warranted.

Commissioning dose computation model for proton source in pencil beam scanning therapy by convolution neural networks

Objective. Proton source model commissioning (PSMC) is critical for ensuring accurate dose calculation in pencil beam scanning (PBS) proton therapy using Monte Carlo (MC) simulations. PSMC aims to match the calculated dose to the delivered dose. However, commissioning the ‘nominal energy’ and ‘energy spread’ parameters in PSMC can be challenging, as these parameters cannot be directly obtained from solving equations. To efficiently and accurately commission the nominal energy and energy spread in a proton source model, we developed a convolution neural network (CNN) named ‘PSMC-Net.’ Methods. The PSMC-Net was trained separately for 33 energies (E, 70–225 MeV with a step of 5 MeV plus 226.09 MeV). For each E, a dataset was generated consisting of 150 source model parameters (15 nominal energies ∈ [E, E + 1.5 MeV], ten spreads ∈ [0, 1]) and the corresponding 150 MC integrated depth doses (IDDs). Of these 150 data pairs, 130 were used for training the network, 10 for validation, and 10 for testing. Results. The source model, built by 33 measured IDDs and 33 PSMC-Nets (cost 0.01 s), was used to compute the MC IDDs. The gamma passing rate (GPRs, 1 mm/1%) between MC and measured IDDs was 99.91 ± 0.12%. However, when no commissioning was made, the corresponding GPR was reduced to 54.11 ± 22.36%, highlighting the tremendous significance of our CNN commissioning method. Furthermore, the MC doses of a spread-out Bragg peak and 20 patient PBS plans were also calculated, and average 3D GPRs (2 mm/2% with a 10% threshold) were 99.89% and 99.96 ± 0.06%, respectively. Significance. We proposed a nova commissioning method of the proton source model using CNNs, which made the PSMC process easy, efficient, and accurate.

Validation of pencil beam scanning proton therapy with multi-leaf collimator calculated by a commercial Monte Carlo dose engine.

This study aimed to evaluate the clinical beam commissioning results and lateral penumbra characteristics of our new pencil beam scanning (PBS) proton therapy using a multi-leaf collimator (MLC) calculated by use of a commercial Monte Carlo dose engine. Eighteen collimated uniform dose plans for cubic targets were optimized by the RayStation 9A treatment planning system (TPS), varying scan area, modulation widths, measurement depths, and collimator angles. To test the patient-specific measurements, we also created and verified five clinically realistic PBS plans with the MLC, such as the liver, prostate, base-of-skull, C-shape, and head-and-neck. The verification measurements consist of the depth dose (DD), lateral profile (LP), and absolute dose (AD). We compared the LPs and ADs between the calculation and measurements. For the cubic plans, the gamma index pass rates (γ-passing) were on average 96.5%±4.0% at 3%/3mm for the DD and 95.2%±7.6% at 2%/2mm for the LP. In several LP measurements less than 75mm depths, the γ-passing deteriorated (increased the measured doses) by less than 90% with the scattering such as the MLC edge and range shifter. The deteriorated γ-passing was satisfied by more than 90% at 2%/2mm using uncollimated beams instead of collimated beams except for three planes. The AD differences and the lateral penumbra width (80%-20% distance) were within±1.9% and±1.1mm, respectively. For the clinical plan measurements, the γ-passing of LP at 2%/2mm and the AD differences were 97.7%±4.2% on average and within±1.8%, respectively. The measurements were in good agreement with the calculations of both the cubic and clinical plans inserted in the MLC except for LPs less than 75mm regions of some cubic and clinical plans. The calculation errors in collimated beams can be mitigated by substituting uncollimated beams.

Location, Location, Location: A Treatment Planning Study Comparing OAR Sparing between IMRT and PBS Plans Based on Disease Location Using the Ann Arbor Staging System for Hodgkin and Aggressive Non-Hodgkin Lymphoma

<h3>Purpose/Objective(s)</h3> Radiotherapy (RT) is often used as a component pf combined-modality therapy for patients with Hodgkin (HL) and aggressive non-Hodgkin(aNHL) lymphoma. RT options include photon intensity-modulated (IMRT) or proton (PT) RT. While the latter offers improved organ-at-risk (OAR) sparing, PT is a limited resource. As such, to guide provider and patient decision-making, it is important to identify cases in which PT has the greatest impact. Accordingly, we sought to assess the difference in OAR sparing between photon IMRT and pencil-beam-scanning (PBS) PT based on disease location using the Ann Arbor staging system. <h3>Materials/Methods</h3> Patients with HL or aNHL treated using either photon or proton RT at a single institution between 2007 and 2020 were considered for analysis. Of these, a preliminary subset of 43 patients, all with mediastinal disease, was selected for a comparative dosimetric analysis. Two plans—IMRT and PBS PT—were developed for each patient using consistent planning techniques. OAR differences were assessed based on disease location using the Ann Arbor staging system. Lower mediastinal disease referred to disease below the aortic valve. <h3>Results</h3> The initial dosimetric analysis focused on 6 disease locations and 5 OARs, including the heart, two cardiac substructures, lungs, and breast (female patients only). Table 1 shows PBS plans relative to IMRT, providing the percent decrease in mean dose to the 5 OARs when employing PBS. <h3>Conclusion</h3> The use of PBS resulted in a substantial decrease in the mean dose to OARs, the magnitude of which depended on the disease location. Sparing in the left hilar and right hilar locations did not vary substantially from the general cohort, while patients with left axillary disease did receive a meaningful increase in sparing to the left ventricle when compared to the general cohort. Sparing was less prominent in patients with lower mediastinal disease. However, the absolute dose to the OARs was higher for these patients and sparing may still be clinically valuable. This preliminary analysis represents a portion of the total cohort available to the study. Further work will be performed to fully establish patterns and evaluate statistical significance.

Surgical Resection and Intensity Modulated Proton Therapy for Esthesioneuroblastoma

<h3>Purpose/Objective(s)</h3> Esthesioneuroblastoma (ENB) is a rare locally aggressive neuroendocrine sinonasal malignancy commonly presenting with skull base and intracranial invasion, which is often managed using multimodality therapy with surgery and radiotherapy (RT). Advances in surgical techniques as well as RT have improved outcomes for these complex tumors. Proton therapy has dosimetric advantages over conventional X-ray based approaches and may further improve the therapeutic ratio. We report our early experience with intensity modulated proton therapy (IMPT) in the management of ENB. <h3>Materials/Methods</h3> We retrospectively reviewed treatment records of patients with ENB treated at our center with IMPT from 2015-2021. IMPT was delivered with a 2 to 5 field pencil-beam scanning plan, initially with single-field optimization, then multi-field optimization after 2018. All pathology was internally reviewed. Event outcomes were calculated from date of biopsy to time of locoregional recurrence or death. Survival statistics were calculated using the Kaplan-Meier (KM) method using a scientific 2-D graphing and statistics software. <h3>Results</h3> 15 patients were identified, median age 52 (range 32-78), gender (male 12, female 3), Kadish stage (B – 2; C – 12; D – 1), AJCC T classification (T3 – 3, T4b – 12), 2 had nodal metastasis at diagnosis, Hyams (grade 2 – 10; grade 3-4 – 5). Local disease extent included: anterior cranial fossa – 11, middle cranial fossa – 2, dura – 12, brain parenchyma 1, nasopharynx – 2, orbit – 6. The majority (n=14) were managed with surgical resection followed by postoperative RT, and one underwent primary chemoradiation. Surgery included bifrontal craniotomy + extended endoscopic resection (n=7) and extended endoscopic resection alone (n=7). IMPT was delivered in conventional fractionation (n=12) or hyperfractionated accelerated course (n=3), median dose 68 CGE (range 60-72), neck coverage (n=11). Four received concurrent platinum-based chemotherapy. At median follow up of 20 months (IQR 16-51), 2 patients have died, unrelated to disease progression. Three patients have recurred including 1 local, 1 regional, and 1 distant metastasis (out of field dural metastases). Both patients with locoregional recurrence have undergone surgical salvage and have no evidence of disease. IMPT was well tolerated with expected acute toxicities. One patient developed frontal lobe brain necrosis at 3 years post-treatment managed conservatively, and one underwent functional sinus surgery for sinus obstruction. No de novo high grade visual toxicity was observed. KM 2-year locoregional recurrence-free survival was 83% and OS 88%. <h3>Conclusion</h3> IMPT is feasible in ENB with early outcomes demonstrating excellent local control and favorable toxicity profile in this cohort of patients with very locally advanced disease. IMPT may allow for improved coverage of tumor volumes in close proximity to numerous critical structures. Additional follow up is needed to track long term disease control outcomes and late toxicities.

Bone versus soft-tissue setup in proton therapy for patients with oesophageal cancer

Background The aim of this study was to investigate the effect of patient positioning based on either bone or soft-tissue matching for PT in oesophageal cancer and its impact on plan adaptation. Materials and methods Two retrospective patient cohorts treated with radiotherapy were included in the study. Cohort A consisted of 26 consecutive patients with a planning 4DCT scan (CT1) and a surveillance 4DCT scan (CT2) at fraction ten. Cohort B consisted of 17 patients selected based on large anatomical changes identified during treatment resulting in a rescan (CT2). Mean dose to the iCTV (sum of the CTVs in all respiratory phases) was 50.4 Gy (RBE) in 28 fractions or 41.4 Gy (RBE) in 23 fractions. A nominal pencil beam scanning plan was created using two posterior beams and robust optimization (5 mm setup, 3.5% range). For each patient, two rigid registrations were made between average (avg) CT1 and CT2: a match on the vertebral column (bone match) and a match on the iCTV (soft-tissue match). Robustness towards setup (5 mm) and range (3.5%) errors was evaluated at CT2. Robustness towards respiration was evaluated by recalculation of the plan on all phases of the CT2 scan. Dose coverage <96% would trigger adaptation. The statistical significance (p-value <0.05) between dose coverage for the two registration methods was assessed using the Wilcoxon signed rank test. Results All plans fulfilled V95%iCTV>99% for the nominal plan and V95%iCTV>97% for all respiratory phases and robustness scenarios at CT1. In two (8%) and three (18%) patients, V95%iCTV<96% on CT2 for Cohort A and B, respectively when bone match was used. For soft-tissue match, V95%iCTV >96% for all patients. V95%iCTV was significantly higher (p-value = 0.0001) for soft-tissue match than bone match. Conclusion Anatomical changes during the treatment course led to target dose deterioration and a need for plan adaptation when using a bone match.

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Proton and Photon Dosimetry of Carbon Fiber-PEEK Implants for Paraspinal Stereotactic Body Radiation Therapy

<h3>Purpose/Objective(s)</h3> Carbon fiber composite (CFR-PEEK) implants have been shown to improve image quality in volumetric imaging compared with titanium (Ti). We perform a dosimetric comparison of proton and photon SBRT plans with Ti and CFR-PEEK implants for paraspinal SBRT treatments. <h3>Materials/Methods</h3> CT scans of a spine phantom embedded with CFR-PEEK and Ti implants were obtained. PTV and OAR contours for a 24 Gy single fraction plan were generated with d_max spinal cord < 14 Gy. Nine-field IMRT photon plans were optimized on both scans. Three-field pencil beam scanning (PBS) plans using multi-field optimization (MFO) were generated. Robust evaluation for 2 mm setup, ± 3.5% range and ± 10% material density override uncertainties were performed. To evaluate the MR image quality of the carbon fiber implants, an implant set was placed in a water phantom. Spine MRI imaging sequences were acquired at 1.5T and 3T and associated artifacts were evaluated. <h3>Results</h3> Target V95 coverage statistics for all plans are shown in Table 1. Photon plans with CFR-PEEK vs. Ti implants were not significantly different. Target coverage goals and OAR constraints were met. When the CFR-PEEK optimized photon plan was recalculated on Ti CT, PTV V100% was reduced by 12.4%, demonstrating the dose perturbation induced by Ti implants. The MFO proton plans for both implant scenarios had lower coverage than the IMRT plans due to the sharper gradients on the IMRT plans. The PBS plan for CFR-PEEK had slightly better coverage than for Ti. Cord-constraint-free proton plans were generated to investigate how the cord constraint impacted the proton plan. Without the constraint, PTV V95% was 99.0% for CFR-PEEK and 94.9% for Ti plans. The increased coverage for CFR-PEEK plan suggests that the potential advantage of CFR-PEEK is masked by the tight cord constraints. In robustness analysis, no clear benefit was shown for range and setup uncertainties. However, advantage in target coverage robustness was seen under material density perturbation analysis. Specifically, the perturbed PTV V95% ranged from 88.7% to 88.4%. In contrast, for Ti plan, PTV V95% varied from 86.5% to 82.3% in perturbed scenarios. In MRI studies, no metal signal loss image artifacts were observed around the CFR-PEEK screws at either field strengths. Minor signal pile-up artifacts of < 5 mm can be seen on one side of the screw in high field 3 T studies but were negligible in 1.5 T studies. <h3>Conclusion</h3> For paraspinal SBRT, utilizing CFR-PEEK compared to Ti implants do not significantly affect plan quality for both PBS and IMRT. Proton plans are more robust for material composition uncertainties only. The largest advantage of CFR-PEEK vs. Ti will be in MR image quality improvements, which can translate to more accurate target delineations and improved follow-up image quality.

Intra-fractional per-beam adaptive workflow to mitigate the need for a rotating gantry during MRI-guided proton therapy

The integration of real-time magnetic resonance imaging (MRI) guidance and proton therapy would potentially improve the proton dose steering capability by reducing daily uncertainties due to anatomical variations. The use of a fixed beamline coupled with an axial patient couch rotation would greatly simplify the proton delivery with MRI guidance. Nonetheless, it is mandatory to assure that the plan quality is not deteriorated by the anatomical deformations due to patient rotation. In this work, an in-house tool allowing for intra-fractional per-beam adaptation of intensity-modulated proton plans (BeamAdapt) was implemented through features available in RayStation. A set of three MRIs was acquired for two healthy volunteers (V 1, V 2): (1) no rotation/static, (2) rotation to the right and (3) left. V 1 was rotated by 15°, to simulate a clinical pediatric abdominal case and V 2 by 45°, to simulate an extreme patient rotation case. For each volunteer, a total of four intensity-modulated pencil beam scanning plans were optimized on the static MRI using virtual abdominal targets and two-three posterior-oblique beams. Beam angles were defined according to the angulations on the rotated MRIs. With BeamAdapt, each original plan was initially converted into separate plans with one beam per plan. In an iterative order, individual beam doses were non-rigidly deformed to the rotated anatomies and re-optimized accounting for the consequent deformations and the beam doses delivered so far. For evaluation, the final accumulated dose distribution was propagated back to the static MRI. Planned and adapted dose distributions were compared by computing relative differences between dose-volume histogram metrics. Absolute target dose differences were on average below 1% and organs-at-risk mean dose differences were below 3%. With BeamAdapt, not only intra-fractional per-beam proton plan adaptation coupled with axial patient rotation is possible but also the need for a rotating gantry during MRI guidance might be mitigated.

Optimization of Field Design in the Treatment of Rectal Cancer with Intensity Modulated Proton Beam Radiation Therapy: How Many Fields Are Needed to Account for Rectal Distension Uncertainty?

PurposePreoperative chemoradiation represents the standard of care in patients with locally advanced rectal cancer. Robustness is often compromised in the setting of proton beam therapy owing to the sensitivity of proton particles to tissue heterogeneity, such as with intestinal gas. The ideal beam arrangement to mitigate the anatomic uncertainty caused by intestinal gas is not well defined. Methods and MaterialsWe developed pencil beam scanning plans using (1) 1-beam posteroanterior (PA) plans, (2) 2-beam with right and left posterior oblique (RPO and LPO) plans, (3) 3-beam with PA and opposed lateral plans, and (4) 5-beam with PA, RPO, LPO, and opposed lateral plans. We created 12 plans with robustness optimization and ran a total of 60 plan evaluations for varying degrees of intestinal gas distension to evaluate which plans would maintain clinical goals to the greatest degree. ResultsA single PA beam resulted in considerable loss of target coverage to the clinical target volume prescribed 50 Gy (volume receiving 100% of the prescribed dose [V100%] < 90%) with rectal distension ≥3 cm in diameter in the short axis. In contrast, the other field designs maintained coverage with up to 5 cm of distension. On plans generated based on a 5-cm distended rectum with air medium, the 1-beam, 3-beam, and 5-beam arrangements resulted in loss of target coverage (V100% < 90%) with rectal contraction ≤3 cm, whereas the 2-beam arrangement maintained coverage to as low as 2 cm. On plans generated based on a 3-cm distension of the rectum, both the 2-beam and 3-beam arrangements maintained V100% > 90% even with collapsed rectum to as low as 1 cm, simulating a patient treatment scenario without any rectal gas. ConclusionsA single PA beam should be avoided when using proton beam therapy for rectal cancer. RPO/LPO and PA/opposed lateral arrangements may both be considered; RPO/LPO is favored to reduce integral dose and avoid beams traversing the hips. In patients for whom the plan CT has rectal distension of ≥3 cm, resimulation or strategies to reduce intestinal gas should be strongly considered.

Plan quality effects of maximum monitor unit constraints in pencil beam scanning proton therapy for central nervous system and skull base tumors

Purpose/objective(s)With reports of CNS toxicity in patients treated with proton therapy at doses lower than would be expected based on photon data, it has been proposed that heavy monitor unit (MU) weighting of pencil beam scanning (PBS) proton therapy spots may potentially increase the risk of toxicity. We evaluated the impact of maximum MU weighting per spot (maxMU/spot) restrictions on PBS plan quality, prior to implementing clinic-wide maxMU/spot restrictions. Materials/methodsPBS plans of 11 patients, of which 3 plans included boosts, for a total of 14 PBS sample cases were included. Per sample case, a single dosimetrist created 4 test plans, gradually reducing the maxMU/spot in the plan. Test Plan 1, unrestricted in maxMU/spot, was the reference for all restricted plan comparisons (comparison sets 2 vs. 1; 3 vs. 1; and 4 vs. 1). The impact of MU/spot restrictions on plan quality metrics were analyzed with Wilcoxon signed rank test analyses. Treatment delivery time was modeled for a representative case. ResultsA total of 14 PBS sample cases, 7 (50%) single-field optimized, 7 (50%) multi-field optimized, 9 (64%) delivering > 3500 cGy, 9 (64%) with 3 beams, and 7 (50%) without a range shifter were included. There were no differences in plan quality metrics of target coverage (V95% and V100% prescription), conformality and gradient indices, hot spot volume (V105% prescription), and dose to normal brain (V10%/30%/50%/70%/90%/100% prescription) with reductions of allowable maxMU/spot across all comparison sets (p > 0.05). Max MU/spot restrictions did not increase treatment delivery time when analyzed for a representative case. ConclusionMaxMU/spot restrictions within the thresholds evaluated in this study did not degrade overall plan quality metrics. Future studies should evaluate spot weighting with linear energy transfer/relative biologic effectiveness-informed planning to determine if spot weighting manipulation impacts clinical outcomes and mitigates toxicity.

Deep Inspiration Breath-Hold (DIBH) for Locoregional Breast Irradiation using Volumetric Modulated Arc Therapy (VMAT) and Intensity Modulated Proton Therapy (IMPT): A Dosimetric Comparison to Cardiac Substructure

Conventional breast irradiation with tangent fields poses cardiotoxicity risks but recent techniques significantly reduce heart exposure, such as rotational intensity modulated radiation therapy or proton therapy. This study evaluates the potential of deep inspiration breath-hold (DIBH) combination with volumetric modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) for further cardiac sparing, at the cardiac substructure level. Following breast conserving surgery, 14 left-sided breast cancer patients treated with free breathing (FB)-VMAT (n = 7) or DIBH-VMAT (n = 7) were retrospectively included. Patients were treated to a dose of 51.8Gy to the whole left breast, 50.4 Gy to the axillary and internal mammary nodes, and 63 Gy to the tumor bed. Pencil beam scanning plans were generated on the initial simulation FB or DIBH computed tomography scans with similar PTV coverage constraints. The endpoint was the mean dose to the heart, to the cardiac cavities, to the left ventricle walls and to the coronary arteries. Table 1 presents the mean doses to cardiac substructures using VMAT and IMPT, with or without DIBH. When combined with VMAT, DIBH significantly reduces dose to nearly all cardiac substructures and to all coronary artery segments. In contrast, when combined with IMPT, DIBH does not show any dosimetric benefit. FB-IMPT significantly spares cardiac substructures compared with DIBH-VMAT. Contrasting with VMAT irradiation where DIBH significantly reduces doses delivered to nearly all cardiac substructures, DIBH does not seem to demonstrate a dosimetric benefit when combined with IMPT. DIBH is thus probably unnecessary for locoregional breast irradiation with IMPT.Abstract 2091; TableStructureVMAT (mean dose Gy)IMPT (mean dose Gy)Free BreathingDIBHp valueFree BreathingDIBHp valueHeart7.123.500.010.800.320.05Left ventricle7.403.19<0.010.090.220.09Anterior wall12.475.23<0.010.260.720.18Apical wall12.384.390.010.400.880.20Lateral wall6.603.37<0.010.040.160.02Inferior wall4.131.65<0.02<0.010.010.47Septal wall6.622.77<0.030.020.010.36Right ventricle7.053.55<0.040.790.070.14Left auricle3.971.910.02<0.01<0.010.08Right auricle3.892.440.040.060.030.13Left main coronary artery6.342.85<0.010.02<0.010.37Left anterior descending artery21.198.32<0.012.481.650.47Proximal segment12.995.170.040.190.290.63Mid segment24.1010.88<0.014.003.130.68Distal segment23.528.370.012.151.380.35Circumflex artery5.122.540.03<0.01<0.010.11Proximal segment7.343.090.02<0.010.010.22Distal segment4.532.410.06<0.01<0.010.10Right coronary artery5.402.870.010.030.060.62Proximal segment6.454.580.220.060.140.59Mid segment4.152.270.050.01<0.010.29Distal segment3.011.170.01<0.01<0.010.36Posterior descending segment6.151.560.01<0.01<0.010.18Author Disclosure: P. Loap: None. F. Goudjil: None. B. Baron: None. M. Ribeiro: None. A. Fourquet: None. Y. Kirova: None. Open table in a new tab

Dosimetric Comparison of Volumetric-Modulated Arc Therapy and Proton Therapy for Postmastectomy Radiation Using the New ESTRO-ACROP Breast Reconstruction Contouring Guidelines

In 2019, ESTRO-ACROP released new consensus guidelines for postmastectomy radiation therapy (PMRT) in the setting of breast reconstruction. In particular, the chest wall (CW) target was redefined to exclude the breast prosthesis and underlying tissue. In this study, we assess the impact of these changes on treatment planning and compare the dosimetric outcomes using both volumetric-modulated arc therapy (VMAT) and pencil-beam scanning (PBS) proton therapy. We performed a treatment planning study of 10 women with left sided breast cancer who received PMRT after implant-based breast reconstruction. All target structures and organs-at-risk (OARs) were delineated first using current RTOG breast contouring guidelines and then separately delineated using the updated ESTRO-ACROP consensus guidelines. Four sets of plans were then generated; 1) VMAT using RTOG contours, 2) VMAT using new ESTRO-ACROP contours, 3) PBS using RTOG contours, and 4) PBS using new ESTRO-ACROP contours. All plans prescribed a dose of 50.4 Gy or GyRBE to the CW and 45 Gy or GyRBE to the regional lymphatics, inclusive of the internal mammary nodes. Similar target coverage objectives and normal tissue (OAR) constraints were utilized, applying the concept of ALARA. Dosimetric goals between the RTOG and updated ESTRO-ACROP contoured plans and across treatment modalities were compared. Utilizing the updated ESTRO-ACROP contouring guidelines, VMAT and PBS plans were able to generate equivalent coverage for all targets. The mean CW D95 was 49.9 Gy for VMAT vs. 48.4 GyRBE with PBS, and the mean regional lymphatic D95 was 45.1Gy (VMAT) vs. 42.8 GyRBE (PBS). Despite the more limited CW volume, proton PBS plans still demonstrated superior OAR metrics compared to VMAT: ipsilateral lung V20 of 16.1% (VMAT) vs. 5.3% (PBS), mean heart doses of 4.6 Gy (VMAT) vs. 0.51 GyRBE (PBS), LAD 1cc dose of 7.43 Gy (VMAT) vs. 5.95 GyRBE (PBS). Compared to plans generated with RTOG guidelines, the updated guideline plans were able to achieve equivalent target coverage with similar, but modestly improved sparing of most OARs: ipsilateral lung V20 7.3% (RTOG PBS) vs. 5.3% (ESTRO PBS) and 24% (RTOG VMAT) vs. 20% (ESTRO VMAT), mean heart dose 0.49 GyRBE (RTOG PBS) vs. 0.51 GyRBE (ESTRO PBS) and 4.95 Gy (RTOG VMAT) vs. 4.82 Gy (ESTRO VMAT). Notably, contralateral lung doses with VMAT were higher using the updated ESTRO-ACROP guidelines compared to RTOG guidelines with a mean V10 of 19.6%. PMRT with reconstruction utilizing the updated ESTRO-ACROP consensus contouring guidelines is feasible with both VMAT and PBS proton therapy. Treatment plans utilizing the updated contouring guidelines provided equivalent target coverage compared to conventional guidelines, while modestly further sparing cardiopulmonary structures. Care must be taken with new contouring guidelines when utilizing VMAT to minimize undue contralateral lung exposure.

Characterization of a new scintillation imaging system for proton pencil beam dose rate measurements

The goal of this work was to create a technique that could measure all possible spatial and temporal delivery rates used in pencil-beam scanning (PBS) proton therapy. The proposed system used a fast scintillation screen for full-field imaging to resolve temporal and spatial patterns as it was delivered. A fast intensified CMOS camera used continuous mode with 10 ms temporal frame rate and 1 × 1 mm2 spatial resolution, imaging a scintillation screen during clinical proton PBS delivery. PBS plans with varying dose, dose rate, energy, field size, and spot-spacing were generated, delivered and imaged. The captured images were post processed to provide dose and dose rate values after background subtraction, perspective transformation, uniformity correction for the camera and the scintillation screen, and calibration into dose. The linearity in scintillation response with respect to varying dose rate, dose, and field size was within 2%. The quenching corrected response with varying energy was also within 2%. Large spatio-temporal variations in dose rate were observed, even for plans delivered with similar dose distributions. Dose and dose rate histograms and maximum dose rate maps were generated for quantitative evaluations. With the fastest PBS delivery on a clinical system, dose rates up to 26.0 Gy s−1 were resolved. The scintillation imaging technique was able to quantify proton PBS dose rate profiles with spot weight as low as 2 MU, with spot-spacing of 2.5 mm, having a 1 × 1 mm2 spatial resolution. These dose rate temporal profiles, spatial maps, and cumulative dose rate histograms provide useful metrics for the potential evaluation and optimization of dose rate in treatment plans.

Proton monoenergetic arc therapy (PMAT) to enhance LETd within the target

We show the performance and feasibility of a proton arc technique so-called proton monoenergetic arc therapy (PMAT). Monoenergetic partial arcs are selected to place spots at the middle of a target and its potential to enhance the dose-averaged linear energy transfer (LETd) distribution within the target. Single-energy partial arcs in a single 360 degree gantry rotation are selected to deposit Bragg’s peaks at the central part of the target to increase LETd values. An in-house inverse planning optimizer seeks for homogeneous doses at the target while keeping the dose to organs at risk (OARs) within constraints. The optimization consists of balancing the weights of spots coming out of selected partial arcs. A simple case of a cylindrical target in a phantom is shown to illustrate the method. Three different brain cancer cases are then considered to produce actual clinical plans, compared to those clinically used with pencil beam scanning (PBS). The relative biological effectiveness (RBE) is calculated according to the microdosimetric kinetic model (MKM). For the ideal case of a cylindrical target placed in a cylindrical phantom, the mean LETd in the target increases from 2.8 keV μm−1 to 4.0 keV μm−1 when comparing a three-field PBS plan with PMAT. This is replicated for clinical plans, increasing the mean RBE-weighted doses to the CTV by 3.1%, 1.7% and 2.5%, respectively, assuming an ratio equal to 10 Gy in the CTV. In parallel, LETd to OARs near the distal edge of the tumor decrease for all cases and metrics (mean LETd, LD,2% and LD,98%). The PMAT technique increases the LETd within the target, being feasible for the production of clinical plans meeting physical dosimetric requirements for both target and OARs. Thus, PMAT increases the RBE within the target, which may lead to a widening of the therapeutic index in proton radiotherapy that would be highlighted for low ratios and hyperfractionated schedules.