Volumetric Modulated Arc Therapy Treatment Plans Research Articles

Dosimetric impact of dental metallic crown on intensity-modulated radiotherapy and volumetric-modulated arc therapy for head and neck cancer.

Metal dental restoration materials cause dose enhancement upstream and dose disturbance downstream of the high‐density inhomogeneous regions in which these materials are used. In this study, we evaluated the impact of a dental metallic crown (DMC) on intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for head and neck cancer. Additionally, the possibility of sparing the oral mucosa from dose enhancement using an individual intraoral mouthpiece was evaluated. An experimental oral phantom was designed to verify the dosimetric impact of a DMC. We evaluated the effect on single beam, parallel opposing beam, arc beam, IMRT, and VMAT treatment plans. To evaluate the utility of a 3‐mm‐thick intraoral mouthpiece, the doses across the mouthpiece were measured. For single beam irradiation, the measured doses at the entrance and exit planes of the DMC were 51% higher and 21% lower than the calculated dose by the treatment planning system, respectively. The maximum dose enhancements were 22% and 46% for parallel opposing beams and the 90° arc rotation beam, respectively. For IMRT and VMAT, the measured doses adjacent to the DMC were 12.2%±6.3% (mean±1.96 SD) and 12.7%±2.5% higher than the calculated doses, respectively. With regard to the performance of the intraoral mouthpiece for the IMRT and VMAT cases, the disagreement between measured and calculated doses at the outermost surface of the mouthpieces were −2.0%, and 2.0%, respectively. Dose enhancements caused by DMC‐mediated radiation scattering occurred during IMRT and VMAT. Because it is difficult to accurately estimate the dose perturbations, careful consideration is necessary when planning head and neck cancer treatments in patients with DMCs. To spare the oral mucosa from dose enhancement, the use of an individual intraoral mouthpiece should be considered.PACS numbers: 87.55.km, 87.55.N‐, 87.55.Qr

An investigation of the dose distribution effect related with collimator angle in volumetric arc therapy of prostate cancer.

To investigate the dose-volume variations of planning target volume (PTV) and organ at risks (OARs) in eleven prostate cancer patients planned with single and double arc volumetric modulated arc therapy (VMAT) when varying collimator angle. Single and double arc VMAT treatment plans were created using Monaco5.0® with collimator angle set to 0°. All plans were normalized 7600 cGy dose to the 95% of clinical target volume (CTV) volume. The single arc VMAT plans were reoptimized with different collimator angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°), and for double arc VMAT plans (0–0°, 15°–345, 30–330°, 45–315°, 60–300°, 75–285°, 90–270°) using the same optimization parameters. For the comparison the parameters of heterogeneity index (HI), dose-volume histogram and minimum dose to the 95% of PTV volume (D95 PTV) calculated and analyzed. The best plans were verified using 2 dimensional ion chamber array IBA Matrixx® and three-dimensional IBA Compass® program. The comparison between calculation and measurement were made by the γ-index (3%/3 mm) analysis. A higher D95 (PTV) were found for single arc VMAT with 15° collimator angle. For double arc, VMAT with 60–300° and 75–285° collimator angles. However, lower rectum doses obtained for 75–285° collimator angles. There was no significant dose difference, based on other OARs which are bladder and femur head. When we compared single and double arc VMAT's D95 (PTV), we determined 2.44% high coverage and lower HI with double arc VMAT. All plans passed the γ-index (3%/3 mm) analysis with more than 97% of the points and we had an average γ-index for CTV 0.36, for PTV 0.32 with double arc VMAT. These results were significant by Wilcoxon signed rank test statistically. The results show that dose coverage of target and OAR's doses also depend significantly on the collimator angles due to the geometry of target and OARs. Based on the results we have decided to plan prostate cancer patients in our clinic with double arc VMAT and 75°–285° collimator angles.

A comparative dosimetric study of volumetric-modulated arc therapy vs. fixed field intensity-modulated radiotherapy in postoperative irradiation of stage IB-IIA high-risk cervical cancer.

The aim of the present study was to compare the dosimetry features of volumetric-modulated arc therapy (VMAT) and fixed field intensity-modulated radiotherapy (f-IMRT) in postoperative irradiation of stage IB-IIA high-risk cervical cancer. Fifteen patients exhibiting stage IB-IIA high-risk cervical cancer, who had been treated with postoperative adjuvant concurrent radiochemotherapy, were selected. The clinical target volume (CTV) and organs at risk (OARs) were delineated according to contrast computed tomography images. The planning target volume (PTV) was subsequently produced by using 1 cm uniform expansion of the CTV. The treatment plans were intended to deliver 50 Gy in 25 fractions. The OARs that were contoured included the bladder, rectum, small bowel and femoral heads. Dose volume histograms were used to evaluate the dose distribution in the PTV and OARs. VMAT and f-IMRT treatment plans resulted in similar dose coverage of the PTV. VMAT was superior to f-IMRT in conformity (P<0.05), and resulted in a reduction of OARs irradiated at high dose levels (V40 and V50) compared with f-IMRT (P<0.05), particularly for the bladder. However, the doses of low levels (V10 and V20) delivered to OARs with f-IMRT were slightly reduced compared with VMAT (P<0.05). For ambilateral femoral heads, VMAT demonstrated improved sparing compared with f-IMRT, with regard to D5 (P<0.05). Furthermore, VMAT treatment plans revealed a significant reduction in monitor units (MU) and treatment time. VMAT techniques exhibited similar PTV coverage compared with f-IMRT. At doses of high levels delivered to OARs, VMAT demonstrated improved sparing compared with f-IMRT, particularly for the bladder, while significantly reducing treatment time and MU number.

Dose impact of rectal gas on prostatic IMRT and VMAT.

In this study, we compared the dose impact of the heterogeneity caused by rectal gas using two methods of treatment planning for intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT). In addition to the structure set used for the standard treatment plan, we created a structure set for evaluation for each patient. These sets were transferred to the same isocenter as the respective treatment plans for IMRT and VMAT that were to become the standard. Values were then recalculated. During the standard prostatic IMRT and VMAT treatment planning, all study participants met dose restrictions in place at our hospital. Dose restrictions were fulfilled in treatment plans for evaluation, excluding those with a clinical target volume (CTV) of V(100%) and planning target volume (PTV) of D95 when the rectum was excluded. However, in treatment plans for evaluation, IMRT was shown to have a higher concordance rate with standard treatment plans than VMAT. If rectal gas is present during either IMRT or VMAT, a dose decrease will occur in relation to CTV and PTV, suggesting that a plan does not eliminate adverse effects on organs at risk.

The role of complexity metrics in a multi-institutional dosimetry audit of VMAT

To demonstrate the benefit of complexity metrics such as the modulation complexity score (MCS) and monitor units (MUs) in multi-institutional audits of volumetric-modulated arc therapy (VMAT) delivery. 39 VMAT treatment plans were analysed using MCS and MU. A virtual phantom planning exercise was planned and independently measured using the PTW Octavius(®) phantom and seven29(®) 2D array (PTW-Freiburg GmbH, Freiburg, Germany). MCS and MU were compared with the median gamma index pass rates (2%/2 and 3%/3 mm) and plan quality. The treatment planning systems (TPS) were grouped by VMAT modelling being specifically designed for the linear accelerator manufacturer's own treatment delivery system (Type 1) or independent of vendor for VMAT delivery (Type 2). Differences in plan complexity (MCS and MU) between TPS types were compared. For Varian(®) linear accelerators (Varian(®) Medical Systems, Inc., Palo Alto, CA), MCS and MU were significantly correlated with gamma pass rates. Type 2 TPS created poorer quality, more complex plans with significantly higher MUs and MCS than Type 1 TPS. Plan quality was significantly correlated with MU for Type 2 plans. A statistically significant correlation was observed between MU and MCS for all plans (R = -0.84, p < 0.01). MU and MCS have a role in assessing plan complexity in audits along with plan quality metrics. Plan complexity metrics give some indication of plan deliverability but should be analysed with plan quality. Complexity metrics were investigated for a national rotational audit involving 34 institutions and they showed value. The metrics found that more complex plans were created for planning systems which were independent of vendor for VMAT delivery.

Optimization of Craniospinal Irradiation for Pediatric Medulloblastoma Using VMAT and IMRT.

Intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) provide highly conformal target radiation doses, but also expose large volumes of healthy tissue to low-dose radiation. With improving survival, more children with medulloblastoma (MB) are at risk of late adverse effects of radiotherapy, including secondary cancers. We evaluated the characteristics of IMRT and VMAT craniospinal irradiation treatment plans in children with standard-risk MB to compare radiation dose delivery to target organs and organs at risk (OAR). Each of 10 children with standard-risk MB underwent both IMRT and VMAT treatment planning. Dose calculations used inverse planning optimization with a craniospinal dose of 23.4 Gy followed by a posterior fossa boost to 55.8 Gy. Clinical and planning target volumes were demarcated on axial computed tomography images. Dose distributions to target organs and OAR for each planning technique were measured and compared with published dose-volume toxicity data for pediatric patients. All patients completed treatment planning for both techniques. Analyses and comparisons of dose distributions and dose-volume histograms for the planned target volumes, and dose delivery to the OAR for each technique demonstrated the following: (1) VMAT had a modest, but significantly better, planning target volume-dose coverage and homogeneity compared with IMRT; (2) there were different OAR dose-sparing profiles for IMRT versus VMAT; and (3) neither IMRT nor VMAT demonstrated dose reductions to the published pediatric dose limits for the eyes, the lens, the cochlea, the pituitary, and the brain. The use of both IMRT and VMAT provides good target tissue coverage and sparing of the adjacent tissue for MB. Both techniques resulted in OAR dose delivery within published pediatric dose guidelines, except those mentioned above. Pediatric patients with standard-risk MB remain at risk for late endocrinologic, sensory (auditory and visual), and brain functional impairments.

TU-F-CAMPUS-J-01: A Formulation of 4D Treatment Planning for Tumour Tracking Volumetric Modulated Arc Therapy for Lung Cancer

Purpose: To develop a formulation for 4D treatment planning for a tumour tracking volumetric modulated arc therapy treatment (VMAT) plan for lung cancer. Methods: A VMAT plan was optimized based on a reference phase of the 4DCT of a lung cancer patient. The PTV was generated from the GTV of the reference phase. The collimator angle was set to 90 degrees such that the MLC travels along superior-inferior direction which is the main component of movement of a lung tumour. Then, each control point of the VMAT plan was assigned to a particular phase of the 4DCT in chronological order. The MLC positions of each control point were shifted according to the position of the tumour centroid of its assigned phase to form a tumour tracking VMAT plan. The control points of the same phase were grouped to form a pseudo VMAT plan for that particular phase. Dose calculation was performed for each pseudo VMAT plan on the corresponding phase of the 4DCT. The CTs of all phases were registered to the reference phase CT according to the displacement of the tumour centroid. The individual dose distributions of the pseudo VMAT plans were summed up and displayed on the reference phase of the 4DCT. A control VMAT plan was optimized based on a PTV generated from the ITV of all phases and compared with the tumour tracking VMAT plan. Results: Both plans achieved >95% volume coverage at the prescription dose level (96% for the tumour tracking plan and 97% for the control plan). But the normal lung volume irradiated at the prescription dose level was 39% less for the tumour tracking plan than the control plan. Conclusion: A formulation of 4D treatment planning for tumour tracking VMAT plans for lung cancer was developed.

TH‐EF‐BRD‐11: Clinical Skin Toxicity Comparison and Phantom Dose Measurements for Head and Neck Patients Treated with IMRT Vs. VMAT

Purpose:Observations in our clinic have suggested a trend towards increased skin‐toxicity for head and neck (HN) patients treated with Volumetric Modulated Arc Therapy (VMAT) compared with Intensity Modulated Radiation Therapy (IMRT). Here, we report on these observations and quantify surface dose differences between VMAT and IMRT treatment plans for HN cancer patients.Methods:We retrospectively compared skin‐toxicity scores gathered by the treating physician according to the Common Terminology Criteria for Adverse Events (CTCAE v4.0) for head and neck squamous cell carcinoma (HNSCC) patients treated with IMRT (102) and VMAT (88) . A Cochran‐Armitage test evaluated the relationship between treatment modality, chemotherapy and toxicity. Six patients with grade 3 skin‐toxicities were selected from this cohort and the target/organ at risk volumes were transferred onto an anthropomorphic phantom using a deformable image registration based atlas (SmartSegmentation, Varian Medical). Two‐arc VMAT and 9‐field IMRT plans were optimized and delivered to the anthropomorphic phantom to produce similar, clinically‐acceptable, dose distributions. Surface dose was measured using optically‐stimulated luminescent dosimeters placed at 2 positions on the phantom's neck which were identical between VMAT and IMRT deliveries. N‐factor ANOVA was performed to identify statistically significant differences in surface dose.Results:Our retrospective study showed a marginally significant higher skin‐toxicity (Grade≥ 2) for VMAT compared with IMRT (35%vs.20%, p=0.06) for patients treated with radiation alone. Phantom measurements showed a significant effect of treatment modality on surface dose (F=42.5,p&lt;0.001) with VMAT delivering 8% higher surface doses on average. No interaction was found between use of a thermoplastic mask and treatment with VMAT (F=0.02,p=0.884).Conclusion:This work indicates that marginal increases in skin dose and subsequent toxicity may be expected from HN patients treated with VMAT compared with IMRT. Our results motivate the need for techniques to spare the skin during VMAT treatment planning and for the early assessment of skin‐toxicity.

SU‐E‐T‐216: Comparison of Volumetrically Modulated Arc Therapy Treatment Using Flattening Filter Free Beams Vs. Flattened Beams for Partial Brain Irradiation

Purpose:Flattening Filter Free (FFF) beams offer the potential for higher dose rates, short treatment time, and lower out of field dose. Therefore, the aim of this study was to investigate the dosimetric effects and out of field dose of Volumetric Modulated Arc Therapy (VMAT) plans using FFF vs Flattening Filtering (FF) beams for partial brain irradiation.Methods:Ten brain patients treated with a 6FF beam from a Truebeam STX were analyzed retrospectively for this study. These plans (46Gy at 2 Gy per fraction) were re‐optimized for 6FFF beams using the same dose constraints as the original plans. PTV coverage, PTV Dmax, total MUs, and mean dose to organs‐at‐risk (OAR) were evaluated. In addition, the out‐of‐field dose for 6FF and 6FFF plans for one patient was measured on an anthropomorphic phantom. TLDs were placed inside (central axis) and outside (surface) the phantom at distances ranging from 0.5 cm to 17 cm from the field edge. Paired T‐test was used for statistical analysis.Results:PTV coverage and PTV Dmax were comparable for the FF and FFF plans with 95.9% versus 95.6% and 111.2% versus 111.9%, respectively. Mean dose to the OARs were 3.7% less for FFF than FF plans (p&lt;0.0001). Total MUs were, on average, 12.5% greater for FFF than FF plans with 481±55 MU (FFF) versus 429±50 MU (FF), p=0.0003. On average, the measured out of field dose was 24% less for FFF compared to FF, p&lt;0.0001. A similar beam‐on time was observed for the FFF and FF treatment.Conclusion:It is beneficial to use 6FFF beams for regular fractionated brain VMAT treatments. VMAT treatment plans using FFF beams can achieve comparable PTV coverage but with more OAR sparing. The out of field dose is significant less with mean reduction of 24%.

SU‐F‐BRB‐04: Comparison of Coplanar VMAT, Non‐Coplanar VMAT, and 4π Treatment Plans

Purpose:The 4π non‐coplanar radiotherapy delivery technique has demonstrated significantly better normal tissue sparing and dose conformality than the clinically used volumetric modulated arc therapy (VMAT). It is unclear whether this is a fundamental limitation of VMAT delivery or the coplanar nature of its typical clinical plans. The non‐coplanar basis of 4π is incorporated into VMAT treatment planning to compare its effect on plan quality.Methods:Clinical stereotactic body radiation therapy plans for 9 liver patients treated with 30–60 Gy using coplanar VMAT (cVMAT) were re‐planned using non‐coplanar VMAT (nVMAT) with 3 arcs and 4 π with 20 intensity‐modulated non‐coplanar fields. All plans were optimized to deliver 100% of the prescribed dose to 95% of the planning target volume (PTV), and nVMAT and 4π plans were tailored to match the maximum and mean PTV dose from the clinical plan. The conformality index (CI), 50% dose spillage volume (R50), normal liver volume receiving &gt;15 Gy (VL&gt;15), and doses to organs at risk (OARs) were compared for all three treatment plans.Results:Compared to cVMAT, the nVMAT and 4π plans reduced VL&gt;15 by an average of 30.6 cm3 and 96.3 cm3, respectively. The average CI was also reduced from 1.22 (cVMAT) to 1.17 (nVMAT) and 1.14 (4π), indicating higher conformality in the same order. Similarly, R50 was reduced from 3.87 (cVMAT) to 3.58 (nVMAT) and 2.74 (4π). With the exception of the mean right kidney dose, which increased by an average of only 0.6 Gy for nVMAT, the dose differences to OARs were not statistically significant between the two VMAT plans. 4π plans either significantly decreased or maintained OAR doses.Conclusion:While the manual selection of intuitive non‐coplanar arcs does show some improvement over coplanar VMAT, the automated beam selection for 4π still results in superior plan quality.This project is supported in part by Varian Medical Systems and NIH R43 CA183390.

SU‐E‐T‐343: Dynamic Collimator Rotation for Volumetric Modulated Arc Therapy

Purpose:Volumetric modulated arc therapy (VMAT) utilizes dynamic gantry rotation, dynamic MLC and varying dose rate. However, in addition the collimator angle could be changed dynamically, thus, increasing the degrees of freedom for the optimization, which might lead to improved dose distributions. This work investigates the feasibility of including a dynamic collimator rotation for VMAT.Methods:In this work a 20×20×20 cm3 water phantom with a cigar shaped target volume and a spherical shaped critical structure was used. By means of the Eclipse Research Scripting a predefined collimator rotation was included in a not yet optimized treatment plan, consisting of one partial arc, by assigning for each dicom control point a different collimator angle. Thereby the collimator rotation takes into account the physical limitations for the delivery. This treatment plan was then imported into Eclipse using the Eclipse Research Scripting interface and VMAT PRO3 optimization was performed in a research version of Eclipse. For the dose calculation of the optimized treatment plan the Swiss Monte Carlo Plan (SMCP) was used. Similarly, a dose distribution was determined using a static collimator angle as applied in VMAT. The resulting DVHs for the target and the critical structure were compared for the two treatment plans.Results:A VMAT treatment plan with dynamically moving collimator angle was successfully generated. The comparison of the DVHs for the target volume shows a slight improvement of the coverage and dose homogeneity for the treatment plan using dynamic collimator rotation. Additionally, the dose to the critical structure could be reduced when using the moving collimator rotation.Conclusion:The usage of dynamic collimator rotation for VMAT is feasible and has the potential to improve the dose distribution for the target while reducing the dose to critical structures. This work was supported by Varian Medical Systems.This work was supported by Varian Medical Systems.

SU‐E‐T‐700: Single Isocenter Volumetric Modulated Arc Therapy (VMAT) and Intensity Modulated Radiotherapy (IMRT) for Intact Left Breast Cancer Patients with Internal Mammary (IM) and Supraclavicular (S/C) Nodes ‐ a Treatment Planning Study

Purpose:To investigate treatment plan quality and feasibility of VMAT for treatment delivery of intact left breast cancer patients with involved internal mammary (IM) and supraclavicular (S/C) nodes.Methods:Ten patients were selected for this retrospective study. All PTVs (Intact left breast, IM &amp; S/C nodes) were contoured by a single radiation oncologist. Four 200° partial arcs with 2cm overlapping jaws (two clockwise and two counterclockwise; gantry angle 300°–140°) and ten fixed‐field equally spaced coplanar modulated fields were used to optimize single isocenter VMAT (RapidArc) and IMRT treatment plans, respectively, using Eclipse TPS. Prescribed dose to PTV was 46Gy in 23 fractions with prescription dose covering 95% of the PTV.Results:Four of the 10 patients had PTV &gt; 1200cc. The mean PTV volume was 1200.17±571.51cc (findings are reported as mean ± 1 SD). All plans were judged clinically acceptable while satisfying dose‐volume constraints, with VMAT showing significantly better conformity than IMRT. The mean Paddick conformity index (PCI) was 0.88±0.03 for VMAT vs. 0.82±0.06 for IMRT plans (p=0.001). However, maximum doses were within ±1.0% of 111% of the prescription dose for both plans. The heart mean dose was 5.00±0.99Gy for VMAT vs. 7.09±1.70Gy for IMRT (p&lt;0.001), heart volume receiving &gt;30Gy was 0.18±0.29% for VMAT vs. 0.68±1.24% for IMRT (p=0.19), and V5Gy and V7Gy were 31.67±11.56% and 14.93±7.41% for VMAT vs. 67.15±17.66% and 36.55±16.59% for IMRT plans, respectively (p&lt;0.001). The mean total lung dose was 8.95±0.52Gy vs. 8.48±0.81Gy (p=0.06) and V20Gy was 10.65±2.00% vs. 8.12±1.37% (p=0.055) for VMAT and IMRT plans, respectively. The right breast mean dose was 2.83±0.46Gy for VMAT vs. 4.06±0.61Gy for IMRT plans (p&lt;0.001).Conclusion:VMAT plans achieved intended dose coverage to the intact left breast, IM &amp; S/C nodes at ≥ 95% with significantly lower dose to critical structures, especially, to heart and contralateral breast compared to IMRT plans.

SU‐E‐T‐411: Evaluation of Treatment Plan Quality Between Two Treatment Planning Systems for VMAT

Purpose:To investigate the treatment plan quality between two treatment planning systems (TPS) for volumetric modulated Arc therapy (VMAT).Methods:VMAT plans based on Varian Eclipse (Ver10.0) and Philips Pinnacle (Ver9.10) TPS were respectively generated by 10 cervical cancer patients by using 2, 3 arcs, controlling the gantry movement with the space of 2 degrees and setting the same optimization constraint as soon as possible. The delivery system with VMAT was Varian Clinac iX machine. The prescription dose was 50Gy/23f for regions at high risk (PTV1), and 45Gy/23f for regions at low risk (PTV). The difference of treatment plans was evaluated by dose distribution and delivery efficiency.Results:The VMAT 2‐arcs plans based on Pinnacle in the conformal indexes of PTV(CI: 0.77±0.05 vs 0.75±0.02, p&gt;0.05), and OARs (such as V30, V40 of rectum and bladder) showed slightly better than those of Eclipse TPS, and the homogeneity indexes (HI) of in Pinnacle were slightly worse than the Eclipse (PTV: 0.17±0.01 vs 0.14±0.01, p &lt;0.05; PTV1: 0.06±0.01 vs 0.04±0.002, p &lt;0.05), there were statistical difference. The MUs with 2‐arcs (750±91 vs 1118±84, p &lt;0.05) and 3‐arcs (837±86 vs 1215±208, p &lt;0.05) of the Eclipse plans had a clear advantages over the Pinnacle ones, with the statistical difference. The 3‐arc plans in Eclipse showed an advantage over the 2‐arcs ones and the plan quality of 3‐arcs in Pinnacle had no advantage comparing with 2 arcs, but the MUs of 3 Arcs in both Eclipse (837±86 vs 750±92, p &lt;0.05) and Pinnacle (1215±208 vs 1118±84, p &gt;0.05) were more than those of 2‐arcs.Conclusion:For patients with cervical cancer, the VMAT treatment plans of two TPSs could achieve clinical acceptable dose distribution. However, we will still need more cases to further study and determine the optimization and performance characteristics of the two commercial TPSs.

SU‐E‐T‐701: Single Isocenter Volumetric Modulated Arc Therapy (VMAT) With Or Without Field Overlap in the Treatment of Left Sided Breast and Regional Lymphatic Tissue: Impact On Surrounding OAR Doses

Purpose:To investigate the impact of field overlap on dosimetric outcomes for patients receiving radiation therapy to the whole breast as well as the supraclavicular and internal mammary nodal regions.Methods:Seven patients with left sided breast cancer were evaluated on this IRB approved study. Four 200° partial arcs were used to optimize single isocenter VMAT plans using 2 different techniques: Technique 1 utilized treatment plans with no field overlap (NFOL) and 30 or 330 degree collimation. Technique 2 utilized 2cm field overlap (FOL) and 30 degree collimation. A single physician contoured all PTV and contralateral breast volumes. The same PTV and OAR volumes were used for both treatment planning techniques. The prescription dose was 46Gy delivered in 23 fractions with a minimum of 95% of the volume receiving 95% of the dose. A two‐sided paired t‐test was used to compare evaluated dosimetric parameters.Results:The mean PTV volume was 923cc (range 589–1218cc). In general, use of FOL resulted in a reduction (though not statistically significant, unless otherwise stated) in OAR dosimetric parameters when compared to NFOL. For left lung, mean dose was 14.3 vs. 13.6Gy, the V20Gy was 23.4% vs. 20.8%, and V30Gy was 10.8% vs. 7.1% (p &lt; 0.01), for NFOL and FOL, respectively. Similarly, for total lung, mean dose was 9.2 vs. 8.8Gy, V20Gy was 11.2% vs. 9.9%, and V30Gy was 5.1 vs. 3.4% (p &lt; 0.01), for heart, mean dose was 4.9 vs. 4.6Gy, V5Gy was 29.7 vs. 27.9%, and the V10Gy was 4.7 vs. 4.0%. Right breast was least impacted with the use of FOL: mean dose 2.6Gy vs. 2.6Gy, D2% of 9.0Gy vs. 9.4Gy (p = 0.5).Conclusion:Use of a 2cm FOL with VMAT treatment plans resulted in reduced doses to the lung and heart with minimal increase in right breast dose.

SU‐E‐T‐315: Dosimetric Effects of Couch Top Shift On VMAT Delivery in Absence of Indexing

Purpose:To investigate dosimetric effects of couch top shift for volumetric‐modulated arc therapy (VMAT) in absence of indexing of immobilization devices.Methods:A total of twelve VMAT treatment plans were selected from three regions (lung, abdomen, and pelvis) to account for the variation of the patient position relative to the couch top. The treatment plans were generated using the Varian Eclipse system. A pinpoint ionization chamber (PTW TN31014) was placed at the center of 16‐cm solid water phantom and the dose was delivered using the Varian TrueBeam STx with BrainLAB ExacTrac couch top. To simulate the day‐to‐day variation of the patient position relative to couch top, the couch top was laterally shifted up to 50 mm, with an increment of 5 mm from 0 to 20 mm; and of 10 mm afterwards, and the phantom was moved back to 0 cm shift for measurement. The dose was also delivered using a Varian tennis racket grid insert at 0 cm shift to simulate the absence of couch top. The treatment plans were delivered with 6, 10, and 15 MV photons using the same leaf sequencing to investigate the energy dependence of couch top shift. The dose difference was normalized to 0 cm shift for the regular couch top for comparison.Results:The percent difference of dose was found to increase with lateral shift for all energies; however, the average differences were close to 0% and the maximum difference was within 1% along the lateral shifts. The differences with the absence of couch top were 2.2±0.5% (6MV), 1.7±0.3% (10MV), and 1.6±0.2% (15MV), respectively.Conclusion:The inclusion of couch top is recommended in treatment planning to minimize the dosimetric uncertainty between calculated and delivered dose even in absence of indexing of immobilization devices in VMAT delivery.

Multi-GPU implementation of a VMAT treatment plan optimization algorithm

Volumetric modulated arc therapy (VMAT) optimization is a computationally challenging problem due to its large data size, high degrees of freedom, and many hardware constraints. High-performance graphics processing units (GPUs) have been used to speed up the computations. However, GPU's relatively small memory size cannot handle cases with a large dose-deposition coefficient (DDC) matrix in cases of, e.g., those with a large target size, multiple targets, multiple arcs, and/or small beamlet size. The main purpose of this paper is to report an implementation of a column-generation-based VMAT algorithm, previously developed in the authors' group, on a multi-GPU platform to solve the memory limitation problem. While the column-generation-based VMAT algorithm has been previously developed, the GPU implementation details have not been reported. Hence, another purpose is to present detailed techniques employed for GPU implementation. The authors also would like to utilize this particular problem as an example problem to study the feasibility of using a multi-GPU platform to solve large-scale problems in medical physics. The column-generation approach generates VMAT apertures sequentially by solving a pricing problem (PP) and a master problem (MP) iteratively. In the authors' method, the sparse DDC matrix is first stored on a CPU in coordinate list format (COO). On the GPU side, this matrix is split into four submatrices according to beam angles, which are stored on four GPUs in compressed sparse row format. Computation of beamlet price, the first step in PP, is accomplished using multi-GPUs. A fast inter-GPU data transfer scheme is accomplished using peer-to-peer access. The remaining steps of PP and MP problems are implemented on CPU or a single GPU due to their modest problem scale and computational loads. Barzilai and Borwein algorithm with a subspace step scheme is adopted here to solve the MP problem. A head and neck (H&N) cancer case is then used to validate the authors' method. The authors also compare their multi-GPU implementation with three different single GPU implementation strategies, i.e., truncating DDC matrix (S1), repeatedly transferring DDC matrix between CPU and GPU (S2), and porting computations involving DDC matrix to CPU (S3), in terms of both plan quality and computational efficiency. Two more H&N patient cases and three prostate cases are used to demonstrate the advantages of the authors' method. The authors' multi-GPU implementation can finish the optimization process within ∼ 1 min for the H&N patient case. S1 leads to an inferior plan quality although its total time was 10 s shorter than the multi-GPU implementation due to the reduced matrix size. S2 and S3 yield the same plan quality as the multi-GPU implementation but take ∼4 and ∼6 min, respectively. High computational efficiency was consistently achieved for the other five patient cases tested, with VMAT plans of clinically acceptable quality obtained within 23-46 s. Conversely, to obtain clinically comparable or acceptable plans for all six of these VMAT cases that the authors have tested in this paper, the optimization time needed in a commercial TPS system on CPU was found to be in an order of several minutes. The results demonstrate that the multi-GPU implementation of the authors' column-generation-based VMAT optimization can handle the large-scale VMAT optimization problem efficiently without sacrificing plan quality. The authors' study may serve as an example to shed some light on other large-scale medical physics problems that require multi-GPU techniques.

Dynamic Collimator Angle Adjustments During Volumetric Modulated Arc Therapy to Account for Prostate Rotations

Rotations of the prostate gland induce considerable geometric uncertainties in prostate cancer radiation therapy. Collimator and gantry angle adjustments can correct these rotations in intensity modulated radiation therapy. Modern volumetric modulated arc therapy (VMAT) treatments, however, include a wide range of beam orientations that differ in modulation, and corrections require dynamic collimator rotations. The aim of this study was to implement a rotation correction strategy for VMAT dose delivery and validate it for left-right prostate rotations. Clinical VMAT treatment plans of 5 prostate cancer patients were used. Simulated left-right prostate rotations between +15° and -15° were corrected by collimator rotations. We compared corrected and uncorrected plans by dose volume histograms, minimum dose (Dmin) to the prostate, bladder surface receiving ≥78Gy (S78) and rectum equivalent uniform dose (EUD; n=0.13). Each corrected plan was delivered to a phantom, and its deliverability was evaluated by γ-evaluation between planned and delivered dose, which was reconstructed from portal images acquired during delivery. On average, clinical target volume minimum dose (Dmin) decreased up to 10% without corrections. Negative left-right rotations were corrected almost perfectly, whereas Dmin remained within 4% for positive rotations. Bladder S78 and rectum EUD of the corrected plans matched those of the original plans. The average pass rate for the corrected plans delivered to the phantom was 98.9% at 3% per 3mm gamma criteria. The measured dose in the planning target volume approximated the original dose, rotated around the simulated left-right angle, well. It is feasible to dynamically adjust the collimator angle during VMAT treatment delivery to correct for prostate rotations. This technique can safely correct for left-right prostate rotations up to 15°.

Comparative dosimetric analysis of IMRT and VMAT (RapidArc) in brain, head and neck, breast and prostate malignancies

Purpose: Intensity modulated radiotherapy (IMRT) in the recent past has established itself as a gold standard for organs at risk (OAR) sparing, target coverage and dose conformity. With the advent of a rotational treatment technology such as volumetric modulated arc therapy (VMAT), an inter-comparison is warranted to address the advantages and disadvantages of each technique. Methods: Twenty patients were selected retrospectively from our patient database. Sites included were brain, head and neck, chest wall, and prostate, with five patients for each site. For all the selected patients, both the IMRT and VMAT treatment plans were generated. Plan comparison was done in terms of OAR dose, dose homogeneity index (HI), dose conformity index (CI), target coverage, low isodose volumes, monitor units (MUs), and treatment time. Results: The VMAT showed better sparing of “parotids minus planning target volume (PTV)”, spinal cord and head of femur as compared to the IMRT. The lung V 40 for VMAT was lower, whereas the lung V 10 , contralateral lung mean dose, contralateral breast mean dose and mean body dose were lower with IMRT for chest wall cases. Both the VMAT and IMRT achieved comparable HI except for the brain site, where IMRT scored over VMAT. The CI achieved by the IMRT and VMAT were similar except for chest wall cases, whereas the VMAT achieved better dose conformity. The target coverage was comparable with both the plans. The VMAT clearly scored over IMRT in terms of average MUs (486 versus 812 respectively) and average treatment time (2.54 minutes versus 5.54 minutes) per treatment session. Conclusion: The VMAT (RapidArc) has a potential to generate treatment plans for various anatomical sites which are comparable with the corresponding IMRT plans in terms of OAR sparing and plan quality parameters. The VMAT significantly reduces treatment time as compared to the IMRT, thus VMAT can increase the throughput of a busy radiotherapy department.

Radiotherapy for unresectable sinonasal cancers: Dosimetric comparison of intensity modulated radiation therapy with coplanar and non-coplanar volumetric modulated arc therapy

Background and purposeTo compare volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) plans for treatment of unresectable paranasal sinuses cancers (PNSCs) with different clinical presentations. Material and methodsFour patients treated for primary target volume only (group 1), four requiring elective nodal irradiation (group 2) and four with positive nodes in macroscopic disease (group 3) were selected. For each patient were generated 7 fields IMRT, coplanar VMAT (c-VMAT) and non-coplanar VMAT (nc-VMAT) treatment plans. Total doses were 70Gy and 54Gy to high dose planning target volume (HD-PTV) and low-dose-PTV, respectively. Dose–volume histogram, conformity and homogeneity index (CI and HI), and monitor units (MUs) per Gy were evaluated. ResultsVMAT provided significantly better target coverage, in terms of V100% (Volume encompassed by the isodose 100%), than IMRT, in particular when nc-VMAT was used. In general, organ at risk sparing is similar with the three approaches, although nc-VMAT can allow a statistically significant reduction of dose to contralateral parotid gland and cochlea for all three groups. ConclusionsVMAT can offer significant improvement of treatment for all unresectable PNSCs over existing IMRT techniques. In particular, nc-VMAT may be a further advantage for those patients with sinonasal cancers and involvement of the nodes in whom large volumes and complex/irregular shape have to be irradiated, even if clinical benefits should be established in the future.

VMAT technique enables concomitant radiotherapy of prostate cancer and pelvic bone metastases

Background. Prostate cancer (PCa) patients with metastatic disease often suffer from skeletal pain and urinary retention impairing their quality of life. Prophylactic radiotherapy to bone metastases planned concomitantly with primary PCa radiotherapy could enable more precise control of combined dose in healthy tissues when compared to sequential palliative treatment.Materials and methods. Volumetric modulated arc therapy (VMAT) was planned for 14 PCa patients with primary bone metastases. The bone planning target volume (PTVbone) was contoured together with the PTVs of prostate (pr), pelvic lymph nodes (ln) and seminal vesicles (sv). Another virtual plan was calculated excluding PTVbone for dose volume histogram (DVH) comparison. DVHs were additionally compared to a set of actual VMAT treatment plans of a control cohort of 13 high risk PCa patients treated with PTVpr, PTVsv and PTVln. The prescribed doses varied between 42 and 76 Gy for PTVbone.Results. Recommended healthy tissue tolerances (Quantec) were not exceeded except for one patient's rectum V50Gy value. Rectum doses did not increase significantly due to the inclusion of PTVbone. For bladder, there was a slight increase for V65Gy and V50Gy (2.7% and 7.4%). The DVHs of metastatic and non-metastatic patients were comparable. There were no differences in the PTVpr DVH parameters, while mean PTVln dose increased by 3.7 Gy–4.4 Gy due to the increased treatment volume related to PTVbone. All side effects were < grade 3 during the mean follow-up duration of 25 months.Conclusions. VMAT offers a good optimization tool for adding extra PTVs to the radiotherapy plan. Radiotherapy of bone metastases concomitantly with irradiation of the primary prostate tumor is a safe and well-tolerated approach and deserves to be studied in a randomized setting.