Arc Plans Research Articles

Treatment planning comparison of volumetric modulated arc therapy with the trilogy and the Halcyon for bilateral breast cancer

BackgroundThe Halcyon is a new machine from the Varian company. The purpose of this study was to evaluate the dosimetry of the Halcyon in treatment of bilateral breast cancer with volumetric modulated arc therapy.MethodsOn CT images of 10 patients with bilateral breast cancer, four Halcyon plans with different setup fields were generated, and dosimetric comparisons using Bonferroni’s multiple comparisons test were conducted among the four plans. Whole and partial arc plans on the Trilogy and the Halcyon, referred to as T-4arc, T-8arc, H-4arc and H-8arc, were designed. The prescription dose was 50 Gy in 2-Gy fractions. All plans were designed with the Eclipse version 15.5 treatment planning system. The dosimetric differences between whole and partial arc plans in the same accelerator were compared using the Mann–Whitney U test. The better Halcyon plan was selected for the further dosimetric comparison of the plan quality and delivery efficiency between the Trilogy and the Halcyon.ResultsHalcyon plans with high‐quality megavoltage cone beam CT setup fields increased the Dmean, D2 and V107 of the planning target volume (PTV) and the V5 and Dmean of the heart, left ventricle (LV) and lungs compared with other Halcyon setup plans. The mean dose and low dose volume of the heart, lungs and liver were significantly decreased in T-8arc plans compared to T-4arc plans. In terms of the V5, V20, V30, V40 and Dmean of the heart, the V20, V30, V40 and Dmean of the LV, the V30, V40, Dmax and Dmean of the left anterior descending artery (LAD), and the V5 and V40 of lungs, H-8arc was significantly higher than H-4arc (p < 0.05). Compared with the Trilogy’s plans, the Halcyon’s plans reduced the high-dose volume of the heart and LV but increased the mean dose of the heart. For the dose of the LAD and the V20 and V30 of lungs, there was no significant difference between the two accelerators. Compared with the Trilogy, plans on the Halcyon significantly increased the skin dose but also significantly reduced the delivery time.ConclusionFor the Halcyon, the whole-arc plans have more dosimetric advantages than partial-arc plans in bilateral breast cancer radiotherapy. Although the mean dose of the heart and the skin dose are increased, the doses of the cardiac substructure and other OARs are comparable to the Trilogy, and the delivery time is significantly reduced.

Proof-of-concept delivery of intensity modulated arc therapy on the Elekta Unity 1.5 T MR-linac

In this work we present the first delivery of intensity modulated arc therapy on the Elekta Unity 1.5 T MR-linac. The machine’s current intensity modulated radiation therapy based control system was modified suitably to enable dynamic delivery of radiation, for the purpose of exploring MRI-guided radiation therapy adaptation modes in a research setting. The proof-of-concept feasibility was demonstrated by planning and delivering two types of plans, each investigating the performance of different parts of a dynamic treatment. A series of fixed-speed arc plans was used to show the high-speed capabilities of the gantry during radiation, while several fully modulated prostate plans—optimised following the volumetric modulated arc therapy approach—were delivered in order to establish the performance of its multi-leaf collimator and diaphragms. These plans were delivered to Delta4 Phantom+ MR and film phantoms passing the clinical quality assurance criteria used in our clinic. In addition, we also performed some initial MR imaging experiments during dynamic therapy, demonstrating that the impact of radiation and moving gantry/collimator components on the image quality is negligible. These results show that arc therapy is feasible on the Elekta Unity system. The machine’s high performance components enable dynamic delivery during fast gantry rotation and can be controlled in a stable fashion to deliver fully modulated plans.

A case study of evaluating bilateral lung dose for VMAT treatment planning using a partial sagittal arc.

Radiation pneumonitis (RP) is a potential toxic side effect of thoracic radiotherapy. Optimal planning techniques must maintain tumor coverage while limiting dose to normal lung tissue to reduce the risk of patients developing RP. The addition of a noncoplanar arc may be beneficial by increasing treatment angles and providing an ideal dose distribution for tumor coverage while decreasing dose to organs at risk (OAR). The purpose of this research was to compare the effects on the normal bilateral lung tissue receiving 20 Gy, 10 Gy and 5 Gy (V20, V10, V5) and the mean lung dose (MLD) values when medial lung tumors are treated with 3 partial coplanar arcs vs 2 partial coplanar arcs combined with a partial sagittal arc. Researchers hypothesized that a beam arrangement of 2 partial coplanar arcs and 1 partial sagittal arc would reduce V20, V10, V5, and MLD values when compared to a 3 partial coplanar arc plan. In a retrospective study of 5 patients with bulky, medial right lung lesions without nodal involvement, cases were planned with both a noncoplanar and a coplanar arc geometry. Results were evaluated using a two-tailed t-test to determine the statistical significance (p < 0.05) of changes to total lung volume analyzation metrics when a noncoplanar sagittal arc was incorporated compared to the standard lung treatment using only coplanar arcs. Although some patient cases showed minor improvement in the V20, V10, V5, and MLD metrics, the study results were not statistically significant and showed no advantage with the introduction of an anterior sagittal arc over a coplanar beam arrangement.

Defining the dose-volume criteria for laryngeal sparing in locally advanced oropharyngeal cancer utilizing split-field IMRT, whole-field IMRT and VMAT.

PurposeTo determine the optimal dose‐volume constraint for laryngeal sparing using three commonly employed intensity modulated radiation therapy (IMRT) approaches in patients with oropharyngeal cancer treated to the bilateral neck.Materials and methodsThirty patients with stage II‐IVA oropharynx cancers received definitive radiotherapy with split‐field IMRT (SF‐IMRT) to the bilateral neck between 2008 and 2013. Each case was re‐planned using whole‐field IMRT (WF‐IMRT) and volumetric modulated arc therapy (VMAT) and plan quality metrics and dose to laryngeal structures was evaluated. Two larynx volumes were defined and compared on the current study: the Radiation Therapy Oncology Group (RTOG) larynx as defined per the RTOG 1016 protocol and the MDACC larynx defined as the components of the larynx bounded by the superior and inferior extent of the thyroid cartilage.ResultsTarget coverage, conformity, and heterogeneity indices were similar in all techniques. The RTOG larynx mean dose was lower with WF‐IMRT than SF‐IMRT (22.1 vs 25.8 Gy; P < 0.01). The MDACC larynx mean dose was 17.5 Gy ± 5.4 Gy with no differences between the 3 techniques. WF‐IMRT and VMAT plans were associated with lower mean doses to the supraglottic larynx (42.1 vs 41.2 vs 54.8 Gy; P < 0.01) and esophagus (18.1 vs 18.2 vs 36 Gy; P < 0.01).ConclusionsModern whole field techniques can provide effective laryngeal sparing in patients receiving radiotherapy to the bilateral neck for advanced oropharyngeal cancers.SummaryWe evaluated laryngeal dose in patients with locally advanced oropharyngeal cancer treated to the bilateral neck using split‐field IMRT (SF‐IMRT), whole‐field IMRT (WF‐IMRT) and volumetric arc therapy (VMAT). All three techniques provided good sparing of laryngeal structures and were able to achieve a mean larynx dose < 33 Gy. There were no significant differences in dose to target structures or non‐laryngeal organs at risk among techniques.

Image quality evaluation of intra-irradiation cone-beam computed tomography acquired during one- and two-arc prostate volumetric-modulated arc therapy delivery: A phantom study.

PurposeTo evaluate (a) the effects of megavoltage (MV)‐scatter on concurrent kilovoltage (kV) projections (P MVkV) acquired during rotational delivery, and (b) the image quality of intra‐irradiation cone‐beam computed tomography (ii‐CBCT) images acquired during prostate volumetric‐modulated arc therapy (VMAT) delivery.MethodsExperiment (1): P MVkVs were acquired with various MV beam parameters using a cylindrical phantom: field size (FS), MV energy (6 or 15 MV), dose rate (DR), and gantry speed. The average pixel values were calculated in a region on each P MVkV which were extracted at eight equally spaced gantry angles. Experiment (2): 11 one‐arc and seven two‐arc 15 MV prostate VMAT plans were used along with a pelvis phantom. One plan was selected from each of arc plans and its MV energy was changed to 6 MV. After P MVkVs were acquired, projections consisting of MV‐scatter only (P MVS) were acquired with closing kV blades and subtracted from P MVkV (P MVScorr). Projections by kV beams only were acquired (P kV). The corresponding CBCT images were reconstructed (CBCTMVkV, CBCTMVScorr, and CBCTkV). The root‐mean‐square errors (RMSEs) were calculated in prostate region and 3D gamma analysis was conducted, in which the CBCT‐number was used instead of doses between ii‐CBCT images and CBCTkV (30 HU/1 mm).ResultsExperiment (1): The MV‐scatters were dependent on the FSs, MV energies, and DRs. Experiment (2): The median RMSEs for CBCTMVScorr were decreased by 107.5 HU (1‐arc) and 42.9 HU (2‐arc) compared to those for CBCTMVkV. The median GPRs for CBCTMVScorr were 94.7% (1‐arc) and 93.4% (2‐arc), while those for CBCTMVkV were 61.1% and 79.9%, respectively. GPRs for 6 MV plans were smaller than those for 15 MV plans.ConclusionsThe number of MV‐scatters increased with larger FSs and DRs, and smaller MV energy. The MV‐scatters were corrected on the CBCTMVScorr regardless of the number of arcs.

Dose Optimization of Electron Arc Treatment Technique in Chest Wall Beams after Mastectomy

In breast cancer, electron energies are preferred over photon energies because they provide minimal lung exposure when conditions allow for thin chest wall irradiation after mastectomy. In patients with irregular chest wall, it is difficult to perform a homogeneous irradiation with fixed electron beam therapy due to reasons such as thickness differences, irregular contour and lack of tissue, long scar and area joint problems. Electron arc therapy is propose as an alternative method in such patients. The study was carried out with electron energies of 6, 9, 12, 13.5 and 16 MeV. First, in order to be able to use it in electron arc planning in the planning system, after determining the dose characteristics of all available electron energies of the electron arc technique, the accuracy of these dose distributions was verified with film and TLD dosimetry. After the suitability was determined, electron arc plans were made on the CT simulation image of 20 patients selected due to the difficulty of homogeneous irradiation with the classical method. While the chosen reference dose of 85% covered the PTV homogeneously, it was found that the dose was decreased by an average of 50% compared to photon and classical electron therapy in the examination performed in terms of radiation dose to which the lung volume was exposed. During the planning, a homogeneous dose and bolus of different thicknesses were required depending on the energy in most patients to regulate the reduction in surface dose depending on the arc angle. Bolus prevents the lung and heart from overdosing while ensuring that the dose in the deeper parts of the target volume is more uniform. The use of tertiary block in electron-arc dose distributions prevented unwanted dose reduction in the field edges and provided a more homogeneous dose distribution at 85% reference isodose. If the structure of the patient's contour is very irregular, the dose distribution is not smooth due to the depth difference. In this context, it has been determined that during optimization, the isocenter depth should be chosen for the homogeneity of the dose distribution and to be as equal as possible from the surface at all beam angles. In addition, in the study, it was determined that more appropriate dose distributions were obtaine when the isocenter depth is greater than the maximum reach of electrons. Even if multiple electron fields of different energies are used, more homogeneous dose distributions have been achieve by eliminating field combination problems with the use of electron arc therapy.

Radiosurgery treatment planning using conformal arc informed volumetric modulated arc therapy

Linac based radiosurgery to multiple metastases is commonly planned with volumetric modulated arc therapy (VMAT) as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT can struggle to block between targets, which can lead to highly modulated and/or nonconformal multi-leaf collimator (MLC) trajectories that unnecessarily irradiation of healthy tissue. In this study we introduce, describe, and evaluate a treatment planning technique called Conformal Arc Informed VMAT (CAVMAT), which aims to reduce the dose to healthy tissue while generating highly conformal treatment plans. CAVMAT is a hybrid technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has 3 main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were replanned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5Gy[cm3], 6Gy[cm3], 12Gy[cm3], and 16Gy[cm3], conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. In addition, CAVMAT was compared to a broad range of planning strategies from various institutions (108 linear accelerator based plans, 14 plans using other modalities) for a 5-target case utilized in a recent plan challenge. For the linear accelerator-based plans, a plan complexity metric based on aperture opening area and perimeter, total monitor units (MU), and MU for a given aperture opening was utilized in the plan challenge scoring algorithm to compare the submitted plans to CAVMAT. After re-planning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cm3] by 25.25 ± 19.23%, V6Gy[cm3] by 13.68 ± 18.97%, V12Gy[cm3] by 11.40 ± 19.44%, and V16Gy[cm3] by 6.38 ± 19.11%. CAVMAT improved conformity by 3.81 ± 7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35 ± 24.66%, leading to an increased treatment time of 2 minutes. For the plan challenge case, CAVMAT was 1 of 12 linac based plans that met all plan challenge scoring criteria. Compared to the average submitted VMAT plan, CAVMAT increased the V10%Gy[%] of healthy tissue (Brain-PTV) by roughly 3.42%, but in doing so was able to reduce the V25%Gy[%] by roughly 3.73%, while also reducing V50%Gy[%], V75%Gy[%], and V100%Gy[%]. The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. Tissue sparing, improved conformity, and decreased plan complexity at the cost of slight increase in treatment delivery time indicates CAVMAT to be a promising method to treat brain metastases.

Clinical implementation of a wide-field electron arc technique with a scatterer for widespread Kaposi\u2019s sarcoma in the distal extremities

A novel wide-field electron arc technique with a scatterer is implemented for widespread Kaposi’s sarcoma (KS) in the distal extremities. Monte Carlo beam modeling for electron arc beams was established to achieve <2% deviation from the measurements, and used for dose calculation. MC-based electron arc plan was performed using CT images of a foot and leg mimicking phantom and compared with in-vivo measurement data. We enrolled one patient with recurrent KS on the lower extremities who had been treated with photon radiation therapy. The 4- and 6-MeV electron arc plans were created, and then compared to two photon plans: two opposite photon beam and volumetric modulated arc with bolus. Compared to the two photon techniques, the electron arc plans resulted in superior dose saving to normal organs beneath the skin region, although it shows inferior coverage and homogeneity for PTV. The electron arc treatment technique with scatterer was successfully implemented for the treatment of widespread KS in the distal extremities with lower radiation exposure to the normal organs beyond the skin lesions, which could be a treatment option for recurrent skin cancer in the extremities.

Dosimetric impact of positron emission tomography-based gross tumour volume (GTV) delineation over conventional CT-based GTV delineation for carcinoma oesophagus

AbstractAim:The aim of the study was to find the dosimetric impact of positron emission tomography (PET)-based gross tumour volume (GTV) delineation over computed tomography (CT)-based GTV delineation for carcinoma oesophagus.Methods:Fifteen patients with carcinoma oesophagus were retrospectively selected. Two sets of GTVs in CT plain images were generated, one with the help of intravenous and oral contrast (GTV CT) and the other with only using PET uptake with the standardised uptake value (simple way of determining the activity in PET) (SUV) &gt; 2.5 (GTV PET). Corresponding PTVs were generated. For all patients, rapid arc plans were generated. Changes in target volumes and critical structure doses were evaluated. The Wilcoxon signed-rank test was used for statistical analysis, and p value &lt; 0.05 was assumed as statistically significant.Results:Mean reduction in GTV was 5.76 ± 19.35 cc. Mean reduction in PTV 45 Gy was 42.40 ± 76.39 cc. Mean reduction in heart mean dose was 1.53 ± 2.16 Gy. Mean reductions in left lung V20% and V10% were 2.43 ± 4.28 and 3.25 ± 5.09 Gy, respectively. Mean reductions in right lung V20% and V10% were 3.11 ± 4.91 and 2.80 ± 4.51 Gy, respectively. Mean reduction in total lung mean dose was 1.00 ± 1.19 Gy.Finding:PET-based GTV contouring reduces the treatment volume and critical structure doses significantly over CT-based GTV contouring for carcinoma oesophagus.

Retrospective quality metrics review of stereotactic radiosurgery plans treating multiple targets using single-isocenter volumetric modulated arc therapy.

PurposeTo characterize key plan quality metrics in multi‐target stereotactic radiosurgery (SRS) plans treated using single‐isocenter volumetric modulated arc therapy (VMAT) in comparison to dynamic conformal arc (DCA) plans treating single target. To investigate the feasibility of quality improvement in VMAT planning based on previous planning knowledge.Materials and methods97 VMAT plans of multi‐target and 156 DCA plans of single‐target treated in 2017 at a single institution were reviewed. A total of 605 targets were treated with these SRS plans. The prescription dose was normalized to 20 Gy in all plans for this analysis. Two plan quality metrics, target conformity index (CI) and normal tissue volume receiving more than 12 Gy (V12Gy), were calculated for each target. The distribution of V12Gy per target was plotted as a function of the target volume. For multi‐target VMAT plans, the number of targets being treated in the same plan and the distance between targets were calculated to evaluate their impact on V12Gy. VMAT plans that had a large deviation of V12Gy from the average level were re‐optimized to determine the possibility of reducing the variation of V12Gy in VMAT planning.ResultsConformity index of multi‐target VMAT plans were lower than that of DCA plans while the mean values of 12 Gy were comparable. The V12Gy for a target in VMAT plan did not show apparent dependence on the total number of targets or the distance between targets. The distribution of V12Gy exhibited a larger variation in VMAT plans compared to DCA plans. Re‐optimization of outlier plans reduced V12 Gy by 33.9% and resulted in the V12Gy distribution in VMAT plans more closely resembling that of DCA plans.ConclusionThe benchmark data on key plan quality metrics were established for single‐isocenter multi‐target SRS planning. It is feasible to use this knowledge to guide VMAT planning and reduce high V12Gy outliers.

Development of a Fluence Generating Programme for Patient-Specific Quality Assurance and Calibration of an Amorphous Silicon EPID

Patient-specific quality assurance (QA) is an essential part of intensity-modulated radiotherapy (IMRT) and volumetric arc therapy (VMAT) treatments. This study aims to develop a Patient-specific QA system through the development of a fluence generating programme and utilization of amorphous silicon (aSi) electronic portal imaging device (EPID). The EPID was investigated on the dependency of dose rate, uniformity of detector sensitivity and output factor. A fluence generating programme was written using MATLAB. Five simple dynamic arc plans and one patient VMAT plan were created using the Monaco treatment planning system which were used in the programme to generate fluence maps. The accuracy of the programme was evaluated by performing gamma analysis between the generated and measured fluence. EPID signal was found to be linearly proportional to dose (1-1000 MU) and independent of dose rate. The mean uniformity ratio was 1.07 ± 0.04%. The measured fluence maps were corrected for uniformity before gamma analysis. Output factor and dose deposition were taken into account in the process. The average result of the gamma analysis passed the 95% threshold. A fluence generating programme was developed, serving as the foundation of the Patient-specific QA system using aSi EPID for IMRT/VMAT delivery in this centre.

So sánh các thông số liều lượng của kỹ thuật vmat và 3d-crt trong xạ trị ung thư vú trái và hạch vùng

COMPARISON OF DOSIMETRIC PARAMETERS OF VOLUMETRIC MODULATED ARC THERAPY AND THREE-DIMENSIONAL CONFORMAL RADIOTHERAPY FOR LEFT SIDE BREAST AND REGIONAL NODES Objective: Comparative study of dose parameters of Volumetric modulated arc therapy (VMAT) and three-dimension radiotherapy (3D-CRT) in patients after left breast surgery with radiation therapy with regional lymph nodes. Materials and Methods: Twenty-four plans of 12 left-breast cancer patients were treated with VMAT with 4 partial arc and 3D-CRT plans were added to compare each other, from 1/2018 to 3/2019 at the Vinmec TimesCity International Hospital. Results: The average coverage dose to the treatment volume (PTV) of VMAT plans is better than the 3D-CRT plan (99.10 ± 0.17 % compared to 98.7 ± 0.34 %; p = 0.01). Conformity index (CI), homogeneity index (HI) for VMAT plans also better results CI: 0.99 vs. 0.98; p = 0.26 and HI: 0.096 compared to 0.119; p = 0.02). Organs at risk, at low doses V5Gy, V10Gy VMAT plans do not avoid good doses by 3D-CRT plans: V5Gy for left lung (78.8 ± 4.6 % compared to 55.37 ± 4.38 %) and heart (77.15 ± 9.52 % compared to 19.15 ± 7.93 %) (p &lt;0.001). However, at high doses, 3D-CRT plans are much higher than VMAT: Dose to left lung V20Gy (36.22 ± 3.9 % compared to 22.62 ± 2.43 %), V30Gy (30.6 ± 5.97 % compared to 11.77 ± 2.53 %) and V40Gy (24.73 ± 8.59 % compared to 3.63 ± 2.02 %) (P &lt;0.001). Similarly, the cardiac arrival dose of the 3D-CRT plan is higher than that of VMAT: V20Gy (11.54 ± 5.83 % compared to 9.49 ± 6.00 %, p&gt; 0.05), V30Gy (9.25 ± 5.56 % compared to 2.13 ± 1.89 %) and V40 Gy (7.54 ± 5.03 % compared with 0.2 ± 0.25 %). Dose to coronary arteries also decreased significantly for VMAT plans compared to 3D-CRT: V20Gy (0.8 ± 1.01 % compared to 27.29 ± 15.19 %) V30Gy (0.00 % compared to 23.09 ± 10.73 %). Conclusion: VMAT is dosimetrically superior to the 3D-CRT for left‑sided breast cancer patients with regional nodes by dose coverage to PTV and good ability to avoid heart and lung and coronary arteries than. Key words: Breast cancer, Postmastectomy radiotherapy, Three-dimension conformal radiotherapy, Volumetric modulated arc therapy.

Simulation study of proton arc therapy with the compact single-room MEVION-S250 proton therapy system

AbstractAim:The purpose of this study is to investigate the feasibility of proton arc therapy (PAT) using the double-scattering MEVION-S250 proton system. The treatment planning and dose delivery parameters from PAT were compared with conventional treatment planning techniques.Materials and methods:PAT was simulated with multiple conformal and fixed-aperture beams (5–15) using the MEVION-S250-double-scattering proton system. Conformal apertures were simulated with the Eclipse-treatment-planning system: (a) using a static single aperture that provides the best average conformal circular or rectangular apertures to cover the tumour from different angular views (SPAT), and (b) dynamic conformal apertures of the tumour shape at each irradiation angle that can be obtained from a multi-leaf-collimator system (DPAT).Results:The DPAT and SPAT plans provided superior dose coverage and sparing of normal tissues in comparison with conventional plans (CPT). The entrance normal tissue and skin doses (&lt;40%) were lowered significantly by delivering dose from different directions over a wider angular view compared to conventional plans that have large entrance dose from only two fields. While the mean and minimum doses from PAT and CPT were comparable, the maximum doses from arc plans were lower than the maximum doses in conventional plans. The SPAT and DPAT plans had comparable dose parameters for regularly shaped targets. The heterogeneity index (HI) was superior in PAT plans which improved with increasing number of beams in arc plans for the different treatment sites. The conformality index (CI) depends on the treatment site and complexity of the shape of the planning target volume where for brain, pancreatic and lung tumours, PAT plans conformality was comparable and sometimes superior to CPT; and HI and CI were generally better in DPAT compared to SPAT.Conclusions:PAT plans have superior dose coverage and sparing of normal tissues compared to CPT plans using the MEVION double-scattering system as shown in this simulation study. Ideally, conformal proton arcs require beam shaping and dose delivery with the gantry moving; however, the MEVION double-scattering system lacks a multi-leaf collimator system and cannot deliver dose during gantry rotation. The single aperture conformal proton therapy technique is more time and cost effective compared with conventional techniques that are used currently with the MEVION proton therapy system because of the elimination of the need for patient-specific compensators. In present study, PAT was simulated with the MEVION double-scattering proton therapy system; however, it can be performed also with other proton therapy systems.

Extended CT density range: Acuros XB algorithm performance

Modern treatment planning systems (TPS) offer photon algorithms capable of enhanced performance in heterogeneous media and high density materials, such as metal implants. Acuros XB was implemented by Varian Medical Systems in their Eclipse TPS. Computed tomography (CT) reconstruction can be performed in two ways: a 16 bits depth for HU or 12 bits. A 12 bit study would have a reduced HU scale, describing normal tissues accurately, but underestimating metal objects. Whenever metal objects can be found in the images, 16 bits are needed to have a realistic estimate of their densities. This work addresses the calculation differences between Acuros XB 13.6 using these two CT configurations. A Philips Brilliant Big Bore CT scanner has been used to scan phantoms with different inserts mimicking different metal implants and both configurations. Head and Neck VMAT and conformal plans (two full arc plans, as per local protocol) were prepared and optimized in Eclipse v13.6. Coverage and dose figures of merit were assessed, and DVH curves compared. Dose to water was used throughout this study. Acuros XB results show an increase in maximum dose for distributions obtained in 16 bits image sets: for instance, from 111.0% (12 bits) to 138.2% for PTV60, in a phantom with Ti dental implants (insert) along the mandible. Differences between Accuros XB results with 12 or 16 bits reconstructions are statistically significant (Student’s T test, p = 0.05). Therefore, the use of extended HU ranges helps address the effect of metal objects inside a patient.

Automated Planning to Improve Linac-Based Stereotactic Radiosurgery Plan Quality in Patients with 4 up to 10 Brain Metastases

Stereotactic radiosurgery (SRS) is a promising treatment option for patients with multiple brain metastases (BM). We studied whether automated planning can improve LINAC-based SRS plan quality for multiple BM. Additionally, the use of one isocenter instead of separate isocentres for each metastasis was studied. For 12 patients with 4 up to 10 brain metastases, five non-coplanar LINAC-based SRS plans were created for 6MV photons: a manually planed dynamic conformal arc (DCA) plan with a separate isocenter for each metastasis, a dynamic IMRT plan with one isocenter, a VMAT plan with one isocenter, two DCA plans with one isocenter for 3 and 5 couch rotations. The last three plans were automatically generated. The maximum allowed gross tumor volume to planning target volume (PTV) margin was 1 mm. The prescription dose was 21 Gy or 18 Gy in a single fraction or 8.5 Gy in 3 fractions depending on the PTV volume. All plans were calculated with Monte Carlo, except the VMAT which was calculated with Collapsed Cone algorithm. The PTV coverage should be at least 98%. To assess SRS plan quality, the Paddick conformity index (CI), the Paddick gradient index (GI), the total V12Gy and V5Gy were studied and reported as MEAN±1SD. The number of monitor units (MU) and irradiation time were also compared. The mean total CI was the highest for dynamic IMRT (see table). The lowest GI was for DCA plans with the separate-isocenter approach (3.7±0.6) and automatically generated DCA plans with one isocenter, whereas the highest GI was for VMAT plans (7.0±3.0). Moreover, the GI was 4.4±0.6 and 4.2±0.6 for automatically generated DCA plans with 3 couch and 5 couch rotations, respectively. The total V12Gy of automatically generated DCA plans with one isocenter (4.7±5.6% and 4.1±4.3% for 3 and 5 couch rotations, respectively) and dynamic IMRT plans (4.4±4.5%) were comparable with the manual DCA plans (3.6±3.7%). For the total V5Gy, quite the same behaviour as for the GI was found. The number of MU was the smallest for VMAT plans, followed by automatically generated DCA and IMRT plans and much lower than the manual DCA plans. The irradiation time was the smallest for automatically planned DCA plans followed by VMAT and IMRT plans. Automatically generated LINAC-based SRS plans for multiple BM in one isocenter can make the number of MU and irradiation time smaller with a comparable plan quality to manual DCA plans with the separate-isocenter approach. Based on all compared parameters, dynamic IMRT and DCA plans with one isocenter were the best and comparable with manual DCA plans with separate isocentres for each metastasis. Moreover, we can conclude that automated DCA plans with 5 couch and 3 couch rotations were comparable.Abstract 3798; Table 1Conf IGIV12Gy [%]V5Gy [%]MUIrradiation time [s]Separate-isocenter DCA0.713.73.619146641832IMRT 1 isoc0.724.84.42675171235VMAT 1 isoc0.707.05.7374123976Multimets DCA 1 isoc 3 table rotations0.694.44.7206130820Multimets DCA 1 isoc 5 table rotations0.714.24.1185403724 Open table in a new tab

Towards proton arc therapy: physical and biologically equivalent doses with increasing number of beams in pediatric brain irradiation

Background: Proton arc therapy may improve physical dose conformity and reduce concerns of elevated linear energy transfer (LET) and relative biological effectiveness (RBE) at the end of the proton range, while offering more degrees of freedom for normal tissue sparing. To explore the potential of proton arc therapy, we studied the effect of increasing the number of beams on physical and biologically equivalent dose conformity in the setting of pediatric brain tumors.Material and methods: A cylindrical phantom (Ø = 150 mm) with central cylindrical targets (Ø = 25 and 30 mm) was planned with increasing number of equiangular coplanar proton beams (from 3 to 36). For four anonymized pediatric brain tumor patients, two ‘surrogate’ proton arc plans (18 equiangular coplanar or sagittal beams) and a reference plan with 3 non-coplanar beams were constructed. Biologically equivalent doses were calculated using two RBE scenarios: RBE1.1; and RBELET, the physical dose weighted by the LET. For both RBE scenarios, dose gradients were assessed, and doses to cognitive brain structures were reported.Results: Increasing the number of beams resulted in an improved dose gradient and reduced volume exposed to intermediate LET levels, at the expense of increased low-dose and low-LET volumes. Most of the differences between the two RBE scenarios were seen around the prescription dose level, where the isodose volumes increased with the RBELET plans, e.g. up to 63% in the 3-beam plan for the smallest phantom target. Overall, the temporal lobes were better spared with the sagittal proton arc surrogate plans, e.g. a mean dose of 3.9 Gy compared to 6 Gy in the reference 3-beam plan (median value, RBE1.1).Conclusion: Proton arc therapy has the potential to improve dose gradients to better spare cognitive brain structures. However, this is at the expense of increased low-dose/low-LET volumes, with possible implications for secondary cancer risks.

Towards FLASH proton therapy: the impact of treatment planning and machine characteristics on achievable dose rates

Background: This study aimed at evaluating spatially varying instantaneous dose rates for different intensity-modulated proton therapy (IMPT) planning strategies and delivery scenarios, and comparing these with FLASH dose rates (>40 Gy/s).Material and methods: In order to quantify dose rates in three-dimensions, we proposed the ‘dose-averaged dose rate’ (DADR) metric, defined for each voxel as the dose-weighted mean of the instantaneous dose rates of all spots (i.e., pencil beams). This concept was applied to four head-and-neck cases, each planned with clinical (4 fields) and various spot-reduced IMPT techniques: ‘standard’ (4 fields), ‘arc’ (120 fields) and ‘arc-shoot-through’ (120 fields; 229 MeV only). For all plans, different delivery scenarios were simulated: constant beam intensity, variable beam intensity for a clinical Varian ProBeam system, varied per energy layer or per spot, and theoretical spot-wise variable beam intensity (i.e., no monitor/safety limitations). DADR distributions were calculated assuming 2-Gy or 6-Gy fractions.Results: Spot-reduced plans contained 17–52 times fewer spots than clinical plans, with no deterioration of plan quality. For the clinical plans, the mean DADR in normal tissue for 2-Gy fractionation was 1.7 Gy/s (median over all patients) at maximum, whereas in standard spot-reduced plans it was 0.7, 4.4, 7.1, and 12.1 Gy/s, for the constant, energy-layer-wise, spot-wise, and theoretical spot-wise delivery scenarios, respectively. Similar values were observed for arc plans. Arc-shoot-through planning resulted in DADR values of 3.0, 6.0, 14.1, and 24.4 Gy/s, for the abovementioned scenarios. Hypofractionation (3×) generally resulted in higher dose rates, up to 73.2 Gy/s for arc-shoot-through plans. The DADR was inhomogeneously distributed with highest values at beam entrance and at the Bragg peak.Conclusion: FLASH dose rates were not achieved for conventional planning and clinical spot-scanning machines. As such, increased spot-wise beam intensities, spot-reduced planning, hypofractionation and arc-shoot-through plans were required to achieve FLASH compatible dose rates.

Dosimetric comparison of short and full arc in spinal PTV in volumetric-modulated arc therapy-based craniospinal irradiation

Since 2011 when it was first described, the volumetric-modulated arc therapy (VMAT) technique for craniospinal irradiation (CSI) has always seen the use of large arc lengths for the spine fields ranging from 200° to 360°. This study was aimed to do a dosimetric comparison between the large and shorter spinal arc for CSI. For a cohort of 10 patients, 2 VMAT CSI plans were created for each patient, one using the conventional full 360° arc (VMAT_FA) for the spine and the other using 100° posterior arc (VMAT_PA) for 23.4 Gy and 35 Gy prescriptions. In both the plans, 360° arc fields were employed for treating cranial volume. Spillage dose (DBody-PTV) to Body-PTV (DBody-PTV: dose to body excluding planning target volume) was compared with VMAT_FA and VMAT_PA plans. In addition to these VMAT plans, a 3-dimensional conformal radiotherapy plan was also created for all these patients to compare the DBody-PTV and target volume related dose constraints. Mean D95% difference between the two VMAT plans did not exceed 1.3% for cranial and spinal targets for both prescription levels. The conformity index (CI) was averaged over both prescription doses. Average CI shows a similar value for VMAT_FA (0.84 ± 0.04) and VMAT_PA (0.82 ± 0.05) plans. D95%, V110% and CI did not exhibit a statistically significant difference between partial and full-arc VMAT plans. However, the VMAT_PA plan exhibited a lower DBody-PTV compared to VMAT_FA plans (0.007 ≤ p < 0.05) in the 1 to 5 Gy range. Nevertheless, partial arc plans could not offer a statistically significant dose reduction for delineated organs compared to full arc plans, except for bilateral kidneys.

Dose Distribution Analysis of Rapid Arc and Intensity Modulated Radiotherapy Plan in Head and Neck Cancer

Introduction: Globally, intensity-modulated radiation therapy (IMRT) is considered as highly precise and accurate method of radiotherapy planning. This technique amplifies spatial dose distribution conformity by modulating the intensity of radiation beams in each sub-volume. Additionally, it can reduces the dose to surrounding critical organs and deliver the planned dose to targets with the nominal risk of side effects. Material and Methods: In this study, 13 patients with head and neck cancer were randomly taken for analysis. The IMRT and Rapid Arc plans were generated for each case in the Eclipse treatment planning system, version 11.0. There were seven to nine beams deployed in IMRT plan, while Rapid Arc plans were performed using two arcs with opposite direction of rotation. Portal dosimetry plans were created and analyzed before executing the plan on the patient. Results: The mean of V95%(Target’s volume covered by 95% isodose line)was 97.89% and 97.47% for Rapid Arc and IMRT plans, respectively. Moreover, mean standard deviations were found 1.93 and 1.70 in Rapid Arc and IMRT plans, respectively. The mean gamma index was97.55% and 98.43% in Rapid Arc and IMRT, respectively. Conclusion: IMRT technique was slightly better in the treatment of head and neck cancer compared to the Rapid Arc method. The only advantage of Rapid Arc was saving the treatment time by two to three times on an average compared with IMRT. It is prudent to use IMRT technique in head and neck cancer treatment.

Mild hyperthermia as a localized radiosensitizer for deep-seated tumors: investigation in an orthotopic prostate cancer model in mice.

Non-ablative or mild hyperthermia (HT) has been shown in preclinical (and clinical) studies as a localized radiosensitizer that enhances the tumoricidal effects of radiation. Most preclinical in vivo HT studies use subcutaneous tumor models which do not adequately represent clinical conditions (e.g. proximity of normal/critical organs) or replicate the tumor microenvironment-both of which are important factors for eventual clinical translation. The purpose of this work is to demonstrate proof-of-concept of locoregional radiosensitization with superficially applied, radiofrequency (RF)-induced HT in an orthotopic mouse model of prostate cancer. In a 4-arm study, 40 athymic male nude mice were inoculated in the prostate with luciferase-transfected human prostate cancer cells (PC3). Tumor volumes were allowed to reach 150-250 mm3 (as measured by ultrasound) following which, mice were randomized into (i) control (no intervention); (ii) HT alone; (iii) RT alone; and (iv) HT + RT. RF-induced HT was administered (Groups ii and iv) using the Oncotherm LAB EHY-100 device to achieve a target temperature of 41 °C in the prostate. RT was administered ~30 min following HT, using an image-guided small animal radiotherapy research platform. In each case, a dual arc plan was used to deliver 12 Gy to the target in a single fraction. One animal from each cohort was euthanized on Day 10 or 11 after treatment for caspase-9 and caspase-3 Western blot analysis. The inoculation success rate was 89%. Mean tumor size at randomization (~16 days post-inoculation) was ~189 mm3 . Following the administration of RT and HT, mean tumor doubling times in days were: control = 4.2; HT = 4.5; RT = 30.4; and HT + RT = 33.4. A significant difference (p = 0.036) was noted between normalized nadir volumes for the RT alone (0.76) and the HT + RT (0.40) groups. Increased caspase-3 expression was seen in the combination treatment group compared to the other treatment groups. These early results demonstrate the successful use of external mild HT as a localized radiosensitizer for deep-seated tumors. We successfully demonstrated the feasibility of administering external mild HT in an orthotopic tumor model and demonstrated preclinical proof-of-concept of HT-based localized radiosensitization in prostate cancer radiotherapy.