MV Volumetric Modulated Arc Therapy Research Articles

Non-coplanar lung SABR treatments delivered with a gantry-mounted x-ray tube

Objective. To create two non-coplanar, stereotactic ablative radiotherapy (SABR) lung patient treatment plans compliant with the radiation therapy oncology group (RTOG) 0813 dosimetric criteria using a simple, isocentric, therapy with kilovoltage arcs (SITKA) system designed to provide low cost external radiotherapy treatments for low- and middle-income countries (LMICs). Approach. A treatment machine design has been proposed featuring a 320 kVp x-ray tube mounted on a gantry. A deep learning cone-beam CT (CBCT) to synthetic CT (sCT) method was employed to remove the additional cost of planning CTs. A novel inverse treatment planning approach using GPU backprojection was used to create a highly non-coplanar treatment plan with circular beam shapes generated by an iris collimator. Treatments were planned and simulated using the TOPAS Monte Carlo (MC) code for two lung patients. Dose distributions were compared to 6 MV volumetric modulated arc therapy (VMAT) planned in Eclipse on the same cases for a Truebeam linac as well as obeying the RTOG 0813 protocols for lung SABR treatments with a prescribed dose of 50 Gy. Main results. The low-cost SITKA treatments were compliant with all RTOG 0813 dosimetric criteria. SITKA treatments showed, on average, a 6.7 and 4.9 Gy reduction of the maximum dose in soft tissue organs at risk (OARs) as compared to VMAT, for the two patients respectively. This was accompanied by a small increase in the mean dose of 0.17 and 0.30 Gy in soft tissue OARs. Significance. The proposed SITKA system offers a maximally low-cost, effective alternative to conventional radiotherapy systems for lung cancer patients, particularly in low-income countries. The system’s non-coplanar, isocentric approach, coupled with the deep learning CBCT to sCT and GPU backprojection-based inverse treatment planning, offers lower maximum doses in OARs and comparable conformity to VMAT plans at a fraction of the cost of conventional radiotherapy.

A feasibility study of ultra-high dose rate mini-GRID therapy using very-high-energy electron beams for a simulated pediatric brain case

Ultra-high dose rate (UHDR, >40 Gy/s), spatially-fractionated minibeam GRID (mini-GRID) therapy using very-high-energy electrons (VHEE) was investigated using Monte Carlo simulations. Multi-directional VHEE treatments with and without mini-GRID-fractionation were compared to a clinical 6 MV volumetric modulated arc therapy (VMAT) plan for a pediatric glioblastoma patient using dose-volume histograms, volume-averaged dose rates in critical patient structures, and planning target volume D98s. Peak-to-valley dose ratios (PVDRs) and dose rates in organs at risk (OARs) were evaluated due to their relevance for normal-tissue sparing in FLASH and spatially-fractionated techniques. Depths of convergence, defined where the PVDR is first ≤1.1, and depths at which dose rates fall below the UHDR threshold were also evaluated. In a water phantom, the VHEE mini-GRID treatments presented a surface (5 mm depth) PVDR of (51±2) and a depth of convergence of 42 mm at 150 MeV and a surface PVDR of (33±1) with a depth of convergence of 57 mm at 250 MeV. For a pediatric GBM case, VHEE treatments without mini-GRID-fractionation produced 25% and 22% lower volume-averaged doses to OARs compared to the 6 MV VMAT plan and 8/9 and 9/9 of the patient structures were exposed to volume-averaged dose rates >40 Gy/s for the 150 MeV and 250 MeV plans, respectively. The 150 MeV and 250 MeV mini-GRID treatments produced 17% and 38% higher volume-averaged doses to OARs and 3/9 patient structures had volume-averaged dose rates above 40 Gy/s. VHEE mini-GRID plans produced many comparable dose metrics to the clinical VMAT plan, encouraging further optimization.

Cellular Damage in the Target and Out-Of-Field Peripheral Organs during VMAT SBRT Prostate Radiotherapy: An In Vitro Phantom-Based Study.

Simple SummaryNew developments show that patients with prostate cancer can benefit from radiotherapy delivered with a hypo-fractionated regimen. The aim of our study was to investigate the effect of hypo-fractionated stereotactic body radiation therapy (SBRT) of prostate cancer on out-of-field organs. We used a humanoid phantom to irradiate prostate cells in conditions similar to patient therapy, using SBRT planning. Our results show that radiation doses in the location of the intestine and lung resulted in significantly higher radiation doses than the further locations. We observed a high radiotoxic effect in the cells irradiated in the prostate, and a small increase in DNA damage and cell killing in the intestine location. Gene expression analysis revealed significant enrichment of the biological processes related to the radiation response in the prostate. In the lung and thyroid, the enrichment of several gene groups was revealed, however the processes were not clearly related to the response to radiation. Our study provides extensive data on out-of-field safety of prostate SBRT.Hypo-fractionated stereotactic body radiation therapy (SBRT) is an effective treatment for prostate cancer (PCa). Although many studies have investigated the effects of SBRT on the prostate and adjacent organs, little is known about the effects further out-of-field. The aim of this study was to investigate, both in vitro and in a quasi-humanoid phantom, the biological effects (using a dose-scaling approach) of radiation in the out-of-field peripheral organs delivered by 6 MV volumetric modulated arc therapy (VMAT) SBRT in a prostate cancer model. Healthy prostate cells were irradiated in a phantom at locations corresponding to the prostate, intestine, lung, thyroid, and brain. Seven 10 Gy fractions of VMAT SBRT were delivered to the target in a single session without intermission (scaled-up method). Radiochromic films were used to measure the doses. The radiobiological response was assessed by measuring DNA breaks, the cell survival fraction, and differences in gene expression profile. Our results showed a strong, multiparametric radiobiological response of the cells in the prostate. Outside of the radiation field, the highest doses were observed in the intestine and lung. A small increase (not statistically significant) in DNA damage and cell death was observed in the intestines. Several gene groups (cell cycle, DNA replication) were depleted in the lung and thyroid (DNA replication, endocytosis), but further analysis revealed no changes in the relevant biological processes. This study provides extensive evidence of the types and extent of radiobiological responses during VMAT SBRT in a prostate cancer model. Additional research is needed to determine whether the radiobiological effects observed in the peripheral organs are validated in a clinical context.

A transit portal dosimetry method for respiratory gating quality assurance with a dynamic 3D printed tumor phantom.

BackgroundsRespiratory gating is one of the motion management techniques that is used to deliver radiation dose to a tumor at a specific position under free breathing. However, due to the dynamic feedback process of this approach, regular equipment quality assurance (QA) and patient‐specific QA checks need to be performed. This work proposes a new QA methodology using electronic portal imaging detector (EPID) to determine the target localization accuracy of phase gating.MethodsQA tools comprising 3D printed spherical tumor phantoms, programmable stages, and an EPID detector are characterized and assembled. Algorithms for predicting portal dose (PD) through moving phantoms are developed and verified using gamma analysis for two spherical tumor phantoms (2 cm and 4 cm), two different 6 MV volumetric modulated arc therapy plans, and two different gating windows (30%–70% and 40%–60%). Comparison between the two gating windows is then performed using the Wilcoxon signed‐rank test. An optimizer routine, which is used to determine the optimal window, based on maximal gamma passing rate (GPR), was applied to an actual breathing curve and breathing plan. This was done to ascertain if our method yielded a similar result with the actual gating window.ResultsHigh GPRs of more than 97% and 91% were observed when comparing the predicted PD with the measured PD in moving phantom at 2 mm/2% and 1 mm/1% levels, respectively. Analysis of gamma heatmaps shows an excellent agreement with the tumor phantom.The GPR of 40%–60% PD was significantly lower than that of the 30%–70% PD at the 1 mm/1% level (p = 0.0064). At the 2 mm/2% level, no significant differences were observed. The optimizer routine could accurately predict the center of the gating window to within a 10% range.ConclusionWe have successfully performed and verified a new method for QA with the use of a moving phantom with EPID for phase gating with real‐time position management.

In vivo dosimetry for patients with prostate cancer to assess possible impact of bladder and rectum preparation.

Purpose/objectiveIn all treatment sites of our radiotherapy network, in vivo dosimetry (PerFRACTION™) was fully implemented in February 2018. We hypothesized that additional help with bladder and rectum preparation by home nursing would improve patients’ preparation and investigated if this could be assessed using in vivo dosimetry (IVD).Materials/methodsA retrospective study was conducted with a test group who received additional help with bladder and rectum preparation by home nurses and a control group who only received information on bladder and rectum preparation according to the standard protocol. Patients were treated with a 6 MV Volumetric Modulated Arc Therapy (VMAT) technique. Electronic portal imaging device (EPID)-based integrated transit dose images were acquired on the first 3 days of treatment and weekly thereafter or more if failed fractions (FF) occurred. Results were analyzed using a global gamma analysis with a threshold of 20%, tolerance of 5% (dose difference) and 5 mm (distance to agreement), and a passing level of 95%.ResultsData of 462 prostate patients was analyzed: 39 and 423 in a test and control group respectively with a comparable number of measurements (on average 8.0 (σ = 4.8) and 7.1 (σ = 4.5) respectively per treatment course). Of the FF, 39% and 31% were related to variations in bladder and rectum filling for the test and control group respectively. Subgroups were created based on the number of FF, no statistically significant differences were observed.ConclusionTwo dimensional EPID-based IVD successfully detected deviations due to variations in bladder and rectum filling, however it could not confirm the hypothesis.

Investigation of elastomeric materials for bolus using stereolithography printing technology in radiotherapy

Purpose: An investigation was conducted of an elastomeric material, VisiJet M2 (3D systems, USA) for use as 3D bolus within high energy photon beams for radiotherapy. Personalized conformal bolus material on complex structures like the nose can be challenging. This material was evaluated for its clinical feasibility due to its pliability and comfort compared to alternatives. Method: Regular slabs of bolus were created of various thicknesses for dosimetric and non-dosimetric characterization. Verification culminated with the creation of a custom nose bolus for an end to end verification using an anthropomorphic head phantom. In vivo dosimetry using Gafchromic EBT3 (Ashland, USA) film validated delivered doses from a 6 MV conformal field and a pair of 6 MV volumetric modulated arc therapy (VMAT) beams. Results & Conclusion: Non-dosimetric and dosimetric tests were conducted to assess clinical suitability. The bolus was precisely created using stereolithographic (SLA) methods and presented a compliant and uniform water equivalent material with elastic memory. Measurement yielded a physical density of 1.10 g cm−3 and 1.06 relative to water electron density, and the bolus to skin distance was measured to be a maximum of 3 mm. A maximum measured dose difference of <2% was observed for dynamic treatment. Based on the investigation conducted, and the benefits presented for patient comfort while being uniform and water equivalent, and correctly represented within the treatment planning system (TPS), this material has the potential for clinical use for patient specific custom bolus.

The effect of photon energy on dose distribution in volumetric-modulated arc therapy planning for head and neck cancer

AbstractAim:To investigate the effect of different energies on dose distribution in volumetric-modulated arc therapy (VMAT) plans for head and neck cancer.Materials and methods:Data from nine patients undergoing VMAT plans using 6 MV, 10 MV and dual-energy X-ray beams with the Pinnacle 3 V 9.10 treatment planning system (Philips Medical System, Fitchburg, WI, USA) were analysed for quality using the conformity index (CI) and homogeneity index (HI) for planning target volume (PTV), and for mean and maximum dose to the organs at risk (OARs): parotid glands, brainstem, spinal cord and optic nerves.Results:There were no clear differences in the HIs of the PTV dose among the different plans. The CIs for 10 MV and dual-energy VMAT plans were superior to that of the 6 MV VMAT plan (0·8 ± 0·3, 0·8 ± 0·3, and 0·7 ± 0·2, respectively; p = 0·001). There were no significant differences in mean/maximum dose to the OARs among the three VMAT plans.Findings:Compared with the 6 MV VMAT plan, the dual-energy VMAT plan slightly increased the coverage of the PTV with the prescribed dose but did not decrease dose to the OARs.

Tratamiento del meduloblastoma: comparación entre arcoterapia volumétrica modulada y radioterapia conformada tridimensional

Objective: To present the feasibility of the treatment of medulloblastoma with volumetric-modulated arc therapy (VMAT) planned with RayStation for Elekta linacs and compare the results with conventional treatment. Materials and methods: 3 adult patients in prone position treated with 3D-conformal radiotherapy (3DCRT) with 6 MV and 18 MV were replanned with 6 MV VMAT. The dose prescription was 1.8 Gy/fraction. The evaluation of the techniques was carried out with dose-volume histograms and isodoses in target volumes and healthy tissues, and with conformity (CI) and homogeneity (HI) indexes. Results: VMAT CI (0.79 ± 0.02) and HI (0.08 ± 0.02) of the tended to be better than those of 3DCRT (CI = 0.58 ± 0.03 and HI = 0.14 ± 0.07). For healthy tissues, a decrease in the maximum absorbed dose was observed (VMAT: D1% = 1.76 ± 0.04 Gy/fraction and 3DCRT: D1% = 1.93 ± 0.04 Gy/fraction) as well as in the overdoses (VMAT: V107% = 0.05 ± 0.08% and 3DCRT: V107% = 1.0 ± 0.5%). The dosimetric results of organs at risk showed no trend when both techniques were compared. Conclusion: The performance with RayStation of the medulloblastoma treatment by VMAT is viable for Elekta linacs, and a greater coverage, homogeneity and conformation in the target is achieved. Overdoses were also markedly reduced compared with those caused by 3DCRT treatments.

Analyzation of the local confidence limits for IMRT and VMAT based on AAPM TG119 report

The aim of this study was to generate a local confidence limit (CL) for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques for Clinac IX linear accelerator using the American Association of Physicists in Medicine (AAPM) Task Group (TG119) protocol. The results were compared with the published studies to test the capability and quality of the VMAT technique in our clinic. We used TG119 cases to create plans for IMRT and VMAT in Eclipse Treatment Planning System for Clinac IX using 6 MV and 10 MV photons. Two preliminary and 5 clinical test cases were created based on the dose prescriptions and planning objectives provided by TG119. Verification plans were created in a planning slab phantom, 2D matrix dosimetry system (MatriXX) with multicube phantom and a volumetric phantom (Delta4). Radiation absorption doses to high-dose points in the planning target volume region and low-dose points in avoidance structures were measured with a 0.125 cc semiflex thimble ionization chamber (PTW). The measured and planned doses were normalized with respect to their prescription doses and intercompared with each other. The gamma analysis was carried out for MatriXX and Delta4 adopting the acceptance criteria of 3% dose difference and 3 mm distance to agreement with 10% threshold dose, respectively. The local CLs with the bench mark set by TG119 were obtained for point, composite planar and field-by-field measurements for IMRT and VMAT with different energies. In this study, the CLs for the high-dose regions of IMRT with 6 MV and 10 MV were 0.025 and 0.014, respectively. For VMAT, they were 0.032 and 0.018. The counterpart CL was 0.045 in TG119. And in organs at risk region, the CLs of IMRT with 6 MV and 10 MV beam were 0.022 and 0.019, respectively, with the counterpart CL indicated by TG119 was 0.047. For VMAT with 6 MV and 10 MV photon beams, the CLs were measured 0.030 and 0.027 with Delta4, respectively. The CLs of the maximum gamma passing for all values were 2.0 in 6 MV VMAT plan, which however recommended in TG119 was 12.4. The data presented here showed all the CLs in our clinic meet the criteria of TG119 report well. All these local CLs reached the goals mentioned in AAPM TG119, which indicated that the local clinic had commissioned IMRT and VMAT techniques with adequate accuracy. Prior to the clinical application practice, it is essential to verify with the TG119 test cases for IMRT and VMAT, which allows us to better understand the basic capability of VMAT technology.

Whole-body dose equivalent including neutrons is similar for 6 MV and 15 MV IMRT, VMAT, and 3D conformal radiotherapy.

PurposeThis study investigates the difference in whole‐body dose equivalent between 6 and 15 MV image‐guided radiotherapy (IGRT) for the treatment of a rhabdomyosarcoma in the prostate.MethodsA previously developed model for stray radiation of the primary beam was improved and used to calculate the photon dose and photon energy in the out‐of‐field region for a radiotherapy patient. The dose calculated by the treatment planning system was fused with the model‐calculated out‐of‐field dose, resulting in a whole‐body photon dose distribution. The peripheral neutron dose equivalent was calculated using an analytical model from the literature. A daily cone beam CT dose was added to the neutron and photon dose equivalents. The calculated 3D dose distributions were compared to independent measurements conducted with thermoluminescence dosimeters and an anthropomorphic phantom. The dose contributions from the IGRT treatments of three different techniques applied with two nominal X‐ray energies were compared using dose equivalent volume histograms (DEVHs).ResultsThe calculated and measured out‐of‐field whole‐body dose equivalents for the IGRT treatments agreed within (9 ± 10) % (mean and type A SD). The neutron dose equivalent was a minor contribution to the total out‐of‐field dose up to 50 cm from the isocenter. Further from the isocenter, head leakage was dominating inside the patient body, whereas the neutron dose equivalent contribution was important close to the surface. There were small differences between the whole‐body DEVHs of the 6 and 15 MV treatments applied with the same technique, although the single scatter contributions showed large differences. Independent of the beam energy, the out‐of‐field dose of the volumetric‐modulated arc therapy (VMAT) treatment was significantly lower than the dynamic intensity‐modulated radiation therapy (IMRT) treatment.ConclusionThe calculated whole‐body dose helped to understand the importance of the dose contributions in different areas of the patient. Regarding radiation protection of the patient for IGRT treatments, the choice of beam energy is not important, whereas the treatment technique has a large influence on the out‐of‐field dose. If the patient is treated with intensity‐modulated beams, VMAT should be used instead of dynamic IMRT in terms of radiation protection of the patient. In general, the developed models for photon and neutron dose equivalent calculation can be used for any patient geometry, tumor location, and linear accelerator.

Phantom Verification of AAA and Acuros Dose Calculations for Lung Cancer: Do Tumor Size and Regression Matter?

PurposeThis study aimed to evaluate dose calculation accuracy for the Eclipse Analytical Anisotropic Algorithm (AAA) and Acuros XB algorithm for various lung tumor sizes and to investigate dosimetric changes associated with treatment of regressing tumors. Methods and materialsA water phantom with cylindrical cork inserts (lung surrogates) was fabricated. Large (202 cm3), medium (54 cm3), and small (3 cm3) solid water tumors were implanted within cork inserts. A plain cork insert was used to simulate a lung without a tumor. The cork inserts and tumors were cut along the long axis, and Gafchromic film was placed between the sections to measure dose distributions. Three-dimensional conformal plans were created using 6 MV and 10 MV beams, and volumetric modulated arc therapy plans were created using 6 MV beams for each tumor size. Doses were calculated using Eclipse AAA and Acuros XB. The measured and calculated dose distributions were compared for each tumor size and treatment algorithm. To simulate a regressing tumor, the original plans created for the large tumor were separately delivered to the phantom that contained a small, medium, or no tumor. The dosimetric effects were evaluated using gamma passing rates with a 2%/2 mm criterion and dose profile comparisons. ResultsAgreement between the measurements and AAA calculations decreased as tumor size decreased, but Acuros XB showed better agreement for all tumor sizes. The largest difference was observed for a 6 MV volumetric modulated arc therapy plan created to treat the smallest tumor. The gamma passing rate was 89.7% but that of Acuros was 99.5%. For the tumor regression evaluation, the gamma passing rates ranged from 53% to 99% for AAA. For Acuros XB, the gamma passing rates were >98% for all scenarios. ConclusionBoth AAA and Acuros XB calculated the dose accurately for the largest lung tumor. For the smallest and regressing tumors, Acuros XB more accurately modelled the dose distribution compared with AAA.

P169] Can lead shielding during paediatric brain radiotherapy reduce the risk of secondary cancer?

Purpose Radiotherapy of paediatric patients comes at the expense of increased secondary cancer risk. This study examines whether the use of a 1 mm lead shielding during volumetric-modulated arc therapy (VMAT) for paediatric brain cancer can reduce the secondary cancer risk for the thyroid and breast. Methods Coplanar 6 MV VMAT plans were prepared for two anthropomorphic phantoms resembling a 1-year and 5-year old girl, for two different PTV sizes (2 cm and 5 cm diameter), simulating post-operative radiotherapy of an ependymoma in the posterior fossa. Out-of-field doses to the thyroid and breast were measured using thermoluminescence dosimeters. The phantoms were subject to three fractions (1,8 Gy each), with and without 1 mm lead shielding of the torso. The measured doses were used to calculate lifetime attributable risks for cancer incidence, for the total treatment dose of 54 Gy, in accordance with the BEIR VII report. 1 Results Applying lead shielding yielded dose reductions of 8–18% for the thyroid and 12–18% for the breast. The results were statistically significant (Student’s t-test, significance level p Conclusions Lead shielding of the phantoms resulted in a systematic decrease in the measured doses to the thyroid and breast, reducing the lifetime attributable risks for cancer incidence. The results hereby suggest that a 1 mm thick lead rubber blanket might be a clinically feasible solution for reducing the risk of secondary cancer in paediatric radiotherapy patients.

Investigating the generalisation of an atlas-based synthetic-CT algorithm to another centre and MR scanner for prostate MR-only radiotherapy

There is increasing interest in MR-only radiotherapy planning since it provides superb soft-tissue contrast without the registration uncertainties inherent in a CT–MR registration. However, MR images cannot readily provide the electron density information necessary for radiotherapy dose calculation. An algorithm which generates synthetic CTs for dose calculations from MR images of the prostate using an atlas of 3 T MR images has been previously reported by two of the authors. This paper aimed to evaluate this algorithm using MR data acquired at a different field strength and a different centre to the algorithm atlas. Twenty-one prostate patients received planning 1.5 T MR and CT scans with routine immobilisation devices on a flat-top couch set-up using external lasers. The MR receive coils were supported by a coil bridge. Synthetic CTs were generated from the planning MR images with () and without (sCT) a one voxel body contour expansion included in the algorithm. This was to test whether this expansion was required for 1.5 T images. Both synthetic CTs were rigidly registered to the planning CT (pCT). A 6 MV volumetric modulated arc therapy plan was created on the pCT and recalculated on the sCT and . The synthetic CTs’ dose distributions were compared to the dose distribution calculated on the pCT.The percentage dose difference at isocentre without the body contour expansion (sCT–pCT) was and with (–pCT) was (mean ± one standard deviation). The result was within one standard deviation of zero and agreed with the result reported previously using 3 T MR data. The sCT dose difference only agreed within two standard deviations. The mean ± one standard deviation gamma pass rate was for the sCT and for the (with global dose difference and distance to agreement gamma criteria). The one voxel body contour expansion improves the synthetic CT accuracy for MR images acquired at 1.5 T but requires the MR voxel size to be similar to the atlas MR voxel size. This study suggests that the atlas-based algorithm can be generalised to MR data acquired using a different field strength at a different centre.

Preclinical studies for implementing flattening filter-free beams in hypofractionated volumetric-modulated arc therapy (VMAT) for prostate cancer radiotherapy

AbstractBackgroundVolumetric-modulated arc therapy (VMAT) has emerged as one of the most favourable techniques for radiotherapy treatment in recent years because of its conformal dose distribution to the planning target volume (PTV), lower doses to adjacent normal organs at risk (OARs) and faster and easier dose delivery. A typical conventional VMAT protocol for low-intermediate risk prostate cancer uses a flattened 6 MV photon beam to deliver 78 Gy in 39 fractions, however, a recent Radiation Therapy Oncology Group study investigated prostate cancer radiotherapy with a hypofractionated dose scheme of 36·25 Gy in 5 fractions. One advantage of flattening filter-free (FFF) beams in radiotherapy is the higher doses in the central region on the dose profile and much higher dose delivery rates.Methods and materialsThis paper reports the investigation of preclinical studies for implementing FFF beams in hypofractionated VMAT for prostate cancer radiotherapy. All treatment planning were accomplished using Varian EclipseTM treatment planning system version 11 and delivered on Varian Truebeam linear accelerators. The studies compared the biological-effective dose–volume histograms and dose–volume histograms of PTV and OARs for 20 patients using conventional and hypofractionated dose schemes. The study also evaluated the 6 and 10 MV FFF by comparing 6 and 10 MV VMAT plans with the FFF beams. The treatment time was investigated using plans with 6 MV beams and doses of 2, 4, 5, 6, 7·25 Gy/fraction and plans with 10 MV FFF with a dose of 7·25 Gy/fraction. We also investigated an angular monitor unit (MU) quantity (MU/deg) and its threshold value for RapidArcTM plans, beyond which FFF beams can be considered superior to flattened beams in terms of treatment time increased caused by higher dose per fraction.ResultsThe results show that the hypofractionated plans resulted in greater biological equivalent doses to PTV and lower doses to OARs. The 10 MV FFF plans have statistically lower mean doses to all the OARs, whereas PTV homogeneity index remains the same compared with other beam energies. The mean body integral dose for the 20 patients is 8·7% lower using 10 MV FFF compared with 6 MV FFF mainly because of the higher energy and less required MUs with the 10 MV FFF beam. The hypofractionated scheme with 10 MV FFF plan has the same treatment time as that of the 6 MV plan at 2 Gy/fraction, as the higher dose delivery rates at 10 MV FFF can compensate for the higher prescribed dose per fraction without the need of extra treatment time.ConclusionIn this study, we observed that the 10 MV FFF beam is better for hypofractionated prostate cancer VMAT plan delivery. The threshold value of MU/deg is found to be 2·083 MU/deg based on our machine configurations.

Feasibility of external beam radiation therapy to deep-seated targets with kilovoltage x-rays.

Radiation therapy to deep-seated targets is typically delivered with megavoltage x-ray beams generated by medical linear accelerators or 60 Co sources. Here, we used computer simulations to design and optimize a lower energy kilovoltage x-ray source generating acceptable dose distributions to a deep-seated target. The kilovoltage arc therapy (KVAT) x-ray source was designed to treat a 4-cm diameter target located at a 10-cm depth in a 40-cm diameter homogeneous cylindrical phantom. These parameters were chosen as an example of a clinical scenario for testing the performance of the kilovoltage source. A Monte Carlo (MC) model of the source was built in the EGSnrc/BEAMnrc code and source parameters, such as beam energy, tungsten anode thickness, beam filtration, number of collimator holes, collimator hole size and thickness, and source extent were varied. Dose to the phantom was calculated in the EGSnrc/DOSXYZnrc code for varying treatment parameters, such as the source-to-axis distance and the treatment arc angle. The quality of dose distributions was quantified by means of target-to-skin ratio and dose output expressed in D50 (50% isodose line) for a 30-min irradiation in the homogeneous phantom as well as a lung phantom. Additionally, a patient KVAT dose distribution to a left pararenal lesion (~1.6 cm in diameter) was calculated and compared to a 15 MV volumetric modulated arc therapy (VMAT) plan. In the design of the KVAT x-ray source, the beam energy, beam filtration, collimator hole size, source-to-isocenter distance, and treatment arc had the largest effect on the source output and the quality of dose distributions. For the 4-cm target at 10-cm depth, the optimized KVAT dose distribution generated a conformal plan with target-to-skin ratio of 5.1 and D50 in 30 min of 24.1 Gy in the homogeneous phantom. In the lung phantom, a target-to-skin ratio of 7.5 and D50 in 30 min of 25.3 Gy were achieved. High dose conformity of the 200 kV KVAT left pararenal plan was comparable to the 15 MV VMAT plan. The volume irradiated to at least 10% (<240 cGy) of the prescription dose was 2.2 × larger in the 200 kV KVAT plan than in the 15 MV VMAT plan, but considered clinically insignificant. This study demonstrated that conformal treatments of deep-seated targets were achievable with kilovoltage x-rays with dose distributions comparable to MV beams. However, due to the larger volumes irradiated to clinically tolerated low doses, KVAT x-ray source usage for deep-seated lesions will be further evaluated to determine optimal target size.

Role of Volumetric-Modulated Arc Therapy with Flattening Filter Free Delivery in Lung Stereotactic Body Radiotherapy

PurposeTo evaluate the role of volumetric-modulated arc therapy (VMAT) combined with flattening filter free (FFF) beam delivery in lung stereotactic body radiotherapy (SBRT), using Acuros XB algorithm. Materials and methodsNinety-eight stage I lung cancer cases treated with SBRT were included. Retrospectively, single arc, 6 MV VMAT with FFF and flattened field (FF) beam plans were generated using the identical optimization criteria with Acuros XB dose computation algorithm. Optimization constraints included target dose ratio of 50% prescription isodose volume to target (R50%) and maximal dose at 2 cm from the target (D2cm). Study parameters, including the number of monitor units (MUs), mean target dose, conformality number, and heterogeneity index of the target, R50%, D2cm, and mean dose to normal tissues were determined and compared for the two delivery modes. A paired matched t-test was used to compare the difference. The effect of tumor, patient and treatment mode (FFF vs. FF) on the difference of MUs obtained from the two delivery systems was statistically determined using linear regression analyses. ResultsFFF beams required 6.9% (range −3.4% to 15.6%) more MUs and resulted in slightly higher values for mean target dose, R50% and D2cm (all P < .0001) compared with FF beams. Conformality number, homogeneity index of target, and mean dose to the normal body were similar between the two plans. Logistic regression analysis confirmed that VMAT with FFF mode, not the tumor or patient-specific parameters, caused the increase in MUs compared with FF beam delivery (P = .043). ConclusionsIn lung SBRT, using VMAT with FFF beam required 7% more MUs compared with a VMAT with FF beam plan and provided similar target coverage. The low leakage dose advantage of FFF delivery is offset by the increased number of MU. The only advantage available to the FFF beams is their estimated faster delivery.

Radiation-induced second primary cancer risks from modern external beam radiotherapy for early prostate cancer: impact of stereotactic ablative radiotherapy (SABR), volumetric modulated arc therapy (VMAT) and flattening filter free (FFF) radiotherapy

Risks of radiation-induced second primary cancer following prostate radiotherapy using 3D-conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), flattening filter free (FFF) and stereotactic ablative radiotherapy (SABR) were evaluated. Prostate plans were created using 10 MV 3D-CRT (78 Gy in 39 fractions) and 6 MV 5-field IMRT (78 Gy in 39 fractions), VMAT (78 Gy in 39 fractions, with standard flattened and energy-matched FFF beams) and SABR (42.7 Gy in 7 fractions with standard flattened and energy-matched FFF beams). Dose-volume histograms from pelvic planning CT scans of three prostate patients, each planned using all 6 techniques, were used to calculate organ equivalent doses (OED) and excess absolute risks (EAR) of second rectal and bladder cancers, and pelvic bone and soft tissue sarcomas, using mechanistic, bell-shaped and plateau models. For organs distant to the treatment field, chamber measurements recorded in an anthropomorphic phantom were used to calculate OEDs and EARs using a linear model. Ratios of OED give relative radiation-induced second cancer risks.SABR resulted in lower second cancer risks at all sites relative to 3D-CRT. FFF resulted in lower second cancer risks in out-of-field tissues relative to equivalent flattened techniques, with increasing impact in organs at greater distances from the field. For example, FFF reduced second cancer risk by up to 20% in the stomach and up to 56% in the brain, relative to the equivalent flattened technique. Relative to 10 MV 3D-CRT, 6 MV IMRT or VMAT with flattening filter increased second cancer risks in several out-of-field organs, by up to 26% and 55%, respectively. For all techniques, EARs were consistently low. The observed large relative differences between techniques, in absolute terms, were very low, highlighting the importance of considering absolute risks alongside the corresponding relative risks, since when absolute risks are very low, large relative risks become less meaningful.A calculated relative radiation-induced second cancer risk benefit from SABR and FFF techniques was theoretically predicted, although absolute radiation-induced second cancer risks were low for all techniques, and absolute differences between techniques were small.

Time-resolved dose reconstruction by motion encoding of volumetric modulated arc therapy fields delivered with and without dynamic multi-leaf collimator tracking

Background. Organ motion during treatment delivery in radiotherapy (RT) may lead to deterioration of the planned dose, but can be mitigated by dynamic multi-leaf collimator (DMLC) tracking. The purpose of this study was to implement and experimentally validate a method for time-resolved motion including dose reconstruction for volumetric modulated arc therapy (VMAT) treatments delivered with and without DMLC tracking. Material and methods. Tracking experiments were carried out on a linear accelerator (Trilogy, Varian) with a prototype DMLC tracking system. A motion stage carrying a biplanar dosimeter phantom (Delta4PT, Scandidos) reproduced eight representative clinical tumor trajectories (four lung, four prostate). For each trajectory, two single-arc 6 MV VMAT treatments with low and high modulation were delivered to the moving phantom with and without DMLC tracking. An existing in-house developed program that adds target motion to treatment plans was extended with the ability to split an arc plan into any number of sub-arcs, allowing the calculated dose for different parts of the treatment to be examined individually. For each VMAT sub-arc, reconstructed and measured doses were compared using dose differences and 3%/3 mm γ-tests. Results. For VMAT sub-arcs the reconstructed dose distributions had a mean root-mean-square (rms) dose difference of 2.1% and mean γ failure rate of 2.0% when compared with the measured doses. For final accumulated doses the mean rms dose difference was 1.6% and the γ failure rate was 0.7%. Conclusion. The time-resolved motion including dose reconstruction was experimentally validated for complex tracking and non-tracking treatments with patient-measured tumor motion trajectories. The reconstructed dose will be of high value for evaluation of treatment plan robustness facing organ motion and adaptive RT.

Experimentally studied dynamic dose interplay does not meaningfully affect target dose in VMAT SBRT lung treatments

The effects of respiratory motion on the tumor dose can be divided into the gradient and interplay effects. While the interplay effect is likely to average out over a large number of fractions, it may play a role in hypofractionated [stereotactic body radiation therapy (SBRT)] treatments. This subject has been extensively studied for intensity modulated radiation therapy but less so for volumetric modulated arc therapy (VMAT), particularly in application to hypofractionated regimens. Also, no experimental study has provided full four-dimensional (4D) dose reconstruction in this scenario. The authors demonstrate how a recently described motion perturbation method, with full 4D dose reconstruction, is applied to describe the gradient and interplay effects during VMAT lung SBRT treatments. VMAT dose delivered to a moving target in a patient can be reconstructed by applying perturbations to the treatment planning system-calculated static 3D dose. Ten SBRT patients treated with 6 MV VMAT beams in five fractions were selected. The target motion (motion kernel) was approximated by 3D rigid body translation, with the tumor centroids defined on the ten phases of the 4DCT. The motion was assumed to be periodic, with the period T being an average from the empirical 4DCT respiratory trace. The real observed tumor motion (total displacement ≤ 8 mm) was evaluated first. Then, the motion range was artificially increased to 2 or 3 cm. Finally, T was increased to 60 s. While not realistic, making T comparable to the delivery time elucidates if the interplay effect can be observed. For a single fraction, the authors quantified the interplay effect as the maximum difference in the target dosimetric indices, most importantly the near-minimum dose (D99%), between all possible starting phases. For the three- and five-fractions, statistical simulations were performed when substantial interplay was found. For the motion amplitudes and periods obtained from the 4DCT, the interplay effect is negligible (<0.2%). It is also small (0.9% average, 2.2% maximum) when the target excursion increased to 2-3 cm. Only with large motion and increased period (60 s) was a significant interplay effect observed, with D99% ranging from 16% low to 17% high. The interplay effect was statistically significantly lower for the three- and five-fraction statistical simulations. Overall, the gradient effect dominates the clinical situation. A novel method was used to reconstruct the volumetric dose to a moving tumor during lung SBRT VMAT deliveries. With the studied planning and treatment technique for realistic motion periods, regardless of the amplitude, the interplay has nearly no impact on the near-minimum dose. The interplay effect was observed, for study purposes only, with the period comparable to the VMAT delivery time.

Effect of Photon-Beam Energy on VMAT and IMRT Treatment Plan Quality and Dosimetric Accuracy for Advanced Prostate Cancer

The goal of the research was to evaluate treatment plan quality and dosimetric accuracy of volumetric modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) plans using 6, 10, and 15 MV photon beams for prostate cancer including lymph nodes. In this retrospective study, VMAT and IMRT plans were generated with the Pinnacle© treatment planning system (TPS) (V9.0) for 10 prostate cancer cases. Each plan consisted of two target volumes: PTV(B) included the prostate bed, PTV(PC+LN) contained PTV(B) and lymph nodes. For plan evaluation statistics, the homogeneity index, conformity index, mean doses, and near-max doses to organs at risk (OAR) were analyzed. Treatment time and number of monitor units were assessed to compare delivery efficiency. Dosimetric plan verification was performed with a 2D ionization chamber array placed in a full scatter phantom. No differences were found for target and OAR parameters in low and high energy photon beam plans for both VMAT and IMRT. A slightly higher low dose volume was detected for 6 MV VMAT plans (normal tissue: D(mean) = 16.47 Gy) compared to 10 and 15 MV VMAT plans (D(mean) = 15.90 Gy and 15.74 Gy, respectively), similar to the findings in IMRT. In VMAT, > 96% of detector points passed the 3%/ 3 mm γ criterion; marginally better accuracy was found in IMRT (> 97%). For static and rotational IMRT, 15 MV photons did not show advantages over 6 and 10 MV high energy photon beams in large volume pelvic plans. For the investigated TPS and linac combination, 10 MV photon beams can be used as the general purpose energy for intensity modulation.