Arc Plans Research Articles

SU-G-BRC-14: Multi-Lesion, Multi-Rx, Brain Radiosurgery with Novel Single Isocenter Technique

Purpose: There is a strong trend to treat multiple brain metastases with radiosurgery rather than whole brain irradiation. This feasibility study investigates a novel planning technique for radio-surgical treatment of multiple brain lesions with differing dose prescriptions, a single isocenter, and dynamic conformal arcs. The novel technique will be compared to the well-established single-isocenter volumetric modulated arc therapy (VMAT) technique commonly used for treating brain lesions. Methods: Six patients with metastatic brain lesions were selected for a prospective treatment planning study to evaluate Interdigitating MLC Dynamic Conformal Arc (IMDCA) technique. Arcs were planned for simultaneous irradiation to maximize beam delivery efficiency. To accommodate varying PTV dose prescriptions, selected arcs were re-irradiated in reverse. Beam weights were adjusted until all prescription constraints were met. The number of lesions ranged between 2 to 4 (mode = 3). For comparison, SRS VMAT plans were generated utilizing an established single-isocenter, 3 arc planning template. All plans were compared by means of Paddick conformity index (PCI), RTOG Conformity Index (RCI), gradient index (GI), and the normal brain volume receiving 10% (V10) of the highest prescription dose. The monitor units and delivery time were tabulated for each plan. Results: IMDCA achieved conformal plans (PCI = 0.72±0.03, RCI = 1.33±0.03) with steep dose fall-off (GI = 3.79±0.03) on average for all of the plans evaluated. The VMAT plans had slightly better conformity (PCI = 0.85 ± 0.03, RCI = 1.13 ± 0.03) than IMDCA, but overall worse GI (4.29 ± 0.06). IMDCA plans had lower V10% values, required 50% fewer MUs, and had 34% shorter beam delivery time on average compared to VMAT plans. Conclusion: IMDCA plans with varying dose prescriptions for multiple lesions, had comparable dosimetric coverage as VMAT plans, but were obtained with significantly lower integral dose, fewer monitor units, and quicker delivery time.

SU‐F‐T‐284: The Effect of Linear Accelerator Output Variation On the Quality of Patient Specific Rapid Arc Verification Plans

Purpose:The aim of the current study is to investigate the effect of machine output variation on the delivery of the RapidArc verification plans.Methods:Three verification plans were generated using Eclipse™ treatment planning system (V11.031) with plan normalization value 100.0%. These plans were delivered on the linear accelerators using ArcCHECK− device, with machine output 1.000 cGy/MU at calibration point. These planned and delivered dose distributions were used as reference plans. Additional plans were created in Eclipse− with normalization values ranging 92.80%–102% to mimic the machine output ranging 1.072cGy/MU‐0.980cGy/MU, at the calibration point. These plans were compared against the reference plans using gamma indices (3%, 3mm) and (2%, 2mm). Calculated gammas were studied for its dependence on machine output. Plans were considered passed if 90% of the points satisfy the defined gamma criteria.Results:The gamma index (3%, 3mm) was insensitive to output fluctuation within the output tolerance level (2% of calibration), and showed failures, when the machine output exceeds ≥3%. Gamma (2%, 2mm) was found to be more sensitive to the output variation compared to the gamma (3%, 3mm), and showed failures, when output exceeds ≥1.7%. The variation of the gamma indices with output variability also showed dependence upon the plan parameters (e.g. MLC movement and gantry rotation). The variation of the percentage points passing gamma criteria with output variation followed a non‐linear decrease beyond the output tolerance level.Conclusion:Data from the limited plans and output conditions showed that gamma (2%, 2mm) is more sensitive to the output fluctuations compared to Gamma (3%,3mm). Work under progress, including detail data from a large number of plans and a wide range of output conditions, may be able to conclude the quantitative dependence of gammas on machine output, and hence the effect on the quality of delivered rapid arc plans.

SU-F-T-596: Single Isocenter SRS Plan QA for Multiple Brain Lesion Cases

Purpose: Linac-based single isocenter SRS treatments for multiple brain lesions have been developed in recent years. However, it is a challenge to perform patient specific plan QA in cases where a large number of small and widely spread-out lesions are involved. We have developed a method to measure all target dose distributions in a single planar measurement. Methods: Varian Eclipse TPS was used to create dynamic conformal arc or RapidArc© plans. Verification plans are created with a fixed gantry angle, and planar projections of 3D dose distributions are created which show high dose regions correlated to each individual lesion. Two techniques were employed for dose measurements: an actual treatment plan delivered with a phantom mounted on the gantry; and/or a verification QA plan with a phantom placed on the couch. Gafchromic films and RIT© software were used for dosimetric analysis. A high dose region was selected for absolute dose measurements. Results: Test plans with up to 10 targets were performed with satisfactory results in the commissioning of this method. For clinical cases, we performed QA on plans with lesion number range from 2 to 7 and size from 0.5 to 3 cm. The absolute dose measurement for dynamic conformal arc plans is within 5% for target > 1 cm and 7% for target ≤1.0 cm. Gamma analyses were performed for whole area covered as well as each focused high-dose region corresponding to individual lesions. The pass criteria is 90% or higher for 3% and 1 mm. Each plan QA took about 1–2 hrs to complete. Conclusion: Our single isocenter SRS QA method has proven to be practical. The process is capable of capturing and analyzing multiple high dose regions in a single planar measurement. It takes reasonable time to verify each individual target without sacrificing accuracy and clinical relevance.

SU‐F‐T‐260: Using Portal Image Device for Pre‐Treatment QA in Volumetric Modulated Arc Plans with Flattening Filter Free (FFF) Beams

Purpose:To implement and validate a method of using electronic portal image device (EPID) for pre‐treatment quality assurance (QA) of volumetric modulated arc therapy (VMAT) plans using flattering filter free (FFF) beams for stereotactic body radiotherapy (SBRT).Methods:On Varian Edge with 6MV FFF beam, open field (from 2×2 cm to 20×20 cm) EPID images were acquired with 200 monitor unit (MU) at the image device to radiation source distance of 150cm. With 10×10 open field and calibration unit (CU) provided by vendor to EPID image pixel, a dose conversion factor was determined by dividing the center dose calculated from the treatment planning system (TPS) to the corresponding CU readout on the image. Water phantom measured beam profile and the output factors for various field sizes were further correlated to those of EPID images. The dose conversion factor and correction factors were then used for converting the portal images to the planner dose distributions of clinical fields. A total of 28 VMAT fields of 14 SBRT plans (8 lung, 2 prostate, 2 liver and 2 spine) were measured. With 10% low threshold cutoff, the delivered dose distributions were compared to the reference doses calculated in water phantom from the TPS. A gamma index analysis was performed for the comparison in percentage dose difference/distance‐to‐agreement specifications.Results:The EPID device has a linear response to the open fields with increasing MU. For the clinical fields, the gamma indices between the converted EPID dose distributions and the TPS calculated 2D dose distributions were 98.7%±1.1%, 94.0%±3.4% and 70.3%±7.7% for the criteria of 3%/3mm, 2%/2mm and 1%/1mm, respectively.Conclusion:Using a portal image device, a high resolution and high accuracy portal dosimerty was achieved for pre‐treatment QA verification for SBRT VMAT plans with FFF beams.

SU‐F‐T‐628: An Evaluation of Grid Size in Eclipse AcurosXB Dose Calculation Algorithm for SBRT Lung

Purpose:purpose of this study is to evaluate the effect of grid size in Eclipse AcurosXB dose calculation algorithm for SBRT lung.Methods:Five cases of SBRT lung previously treated have been chosen for present study. Four of the plans were 5 fields conventional IMRT and one was Rapid Arc plan. All five cases have been calculated with five grid sizes (1, 1.5, 2, 2.5 and 3mm) available for AXB algorithm with same plan normalization. Dosimetric indices relevant to SBRT along with MUs and time have been recorded for different grid sizes. The maximum difference was calculated as a percentage of mean of all five values. All the plans were IMRT QAed with portal dosimetry.Results:The maximum difference of MUs was within 2%. The time increased was as high as 7 times from highest 3mm to lowest 1mm grid size. The largest difference of PTV minimum, maximum and mean dose were 7.7%, 1.5% and 1.6% respectively. The highest D2‐Max difference was 6.1%. The highest difference in ipsilateral lung mean, V5Gy, V10Gy and V20Gy were 2.6%, 2.4%, 1.9% and 3.8% respectively. The maximum difference of heart, cord and esophagus dose were 6.5%, 7.8% and 4.02% respectively. The IMRT Gamma passing rate at 2%/2mm remains within 1.5% with at least 98% points passing with all grid sizes.Conclusion:This work indicates the lowest grid size of 1mm available in AXB is not necessarily required for accurate dose calculation. The IMRT passing rate was insignificant or not observed with the reduction of grid size less than 2mm. Although the maximum percentage difference of some of the dosimetric indices appear large, most of them are clinically insignificant in absolute dose values. So we conclude that 2mm grid size calculation is best compromise in light of dose calculation accuracy and time it takes to calculate dose.

SU-F-T-611: Critical Analysis and Efficacy of Linac Based (Beam Modulator) and Cyberknife Treatment Plans for Acoustic Neuroma/schwannoma

Purpose: To study the critical analysis and efficacy of Linac and Cyberknife (CK) treatment plans for acoustic neuroma/schwannoma. Methods: Twelve of acoustic neuroma/schwannoma patients were taken for these study that. Treatment plans were generated in Multiplan treatment planning system (TPS) for CK using 5,7.5 and 10mm diameter collimators. Target volumes were in the range of 0.280 cc to 9.256 cc. Prescription dose (Rx) ranges from 1150cGy to 1950cGy delivered over 1 to 3 Fractions. For same patients stereotactic Volumetric modulated arc plans were generated using Elekta Linac with MLC thickness of 4mm in Monaco TPS. Appropriate calculation algorithms and grid size were used with same Rx and organ at risk (OAR) constrains for both Linac and CK plans. Treatment plans were developed to achieve at least 95% of the target volume to receive the Rx. The dosimetric indices such as conformity index (CI), coverage, OAR dose and volume receiving 50% of Rx (V50%) were used to evaluate the plans. Results: Target volumes ranges from 0.280 cc to 3.5cc shows the CI of 1.16±0.109 and 1.53±0.360 for cyberknife and Linac plans respectively. For small volume targets, the OARs were well spared in CK plans. There are no significant differences in CI and OAR doses were observed between CK and Linac plans that have the target volume >3.5 cc. Perhaps the V50% were lesser in CK plans, and found to be 12.8± 8.4 and 22.8 ± 15.0 for CK and Linac respectively. Conclusion: The analysis shows the importance of collimator size for small volume targets. The target volumes >3.5 cc can be treated in Linac as comparable with CK. For targets <3.5cc CK plans showed superior plan quality with better CI and OAR sparing than the Linac based plans. Further studies may require evaluating the clinical advantage of CK robotic system.

SU-G-BRB-16: Vulnerabilities in the Gamma Metric

Purpose: To explore vulnerabilities in the gamma index metric that undermine its wide use as a radiation therapy quality assurance tool. Methods: 2D test field pairs (images) are created specifically to achieve high gamma passing rates, but to also include gross errors by exploiting the distance-to-agreement and percent-passing components of the metric. The first set has no requirement of clinical practicality, but is intended to expose vulnerabilities. The second set exposes clinically realistic vulnerabilities. To circumvent limitations inherent to user-specific tuning of prediction algorithms to match measurements, digital test cases are manually constructed, thereby mimicking high-quality image prediction. Results: With a 3 mm distance-to-agreement metric, changing field size by ±6 mm results in a gamma passing rate over 99%. For a uniform field, a lattice of passing points spaced 5 mm apart results in a passing rate of 100%. Exploiting the percent-passing component, a 10×10 cm2 field can have a 95% passing rate when an 8 cm2=2.8×2.8 cm2 highly out-of-tolerance (e.g. zero dose) square is missing from the comparison image. For clinically realistic vulnerabilities, an arc plan for which a 2D image is created can have a >95% passing rate solely due to agreement in the lateral spillage, with the failing 5% in the critical target region. A field with an integrated boost (e.g whole brain plus small metastases) could neglect the metastases entirely, yet still pass with a 95% threshold. All the failure modes described would be visually apparent on a gamma-map image. Conclusion: The %gamma<1 metric has significant vulnerabilities. High passing rates can obscure critical faults in hypothetical and delivered radiation doses. Great caution should be used with gamma as a QA metric; users should inspect the gamma-map. Visual analysis of gamma-maps may be impractical for cine acquisition.

SU-F-T-571: Objective Assessment of 3D Dosimetry for Flattened and Flattened Filter Free Stereotactic Rotational Delivery Using 729-Array Detector with Octavius 4D Phantom

Purpose: The purpose of this study was to assess the potential of 3D dosimetry for flattened and flattened filter free stereotactic rotational delivery in high definition MLC using 729-detector array with Octavius 4D phantom Methods: Twenty rapid arc plans were assessed for this study. For each patient two plans for 6X and 6FFF photon beams were generated with same prescription and critical organ constraints in Eclipse TPS version 13.0 using high definition MLC. Verification plans were generated in scanned Octavius 4D phantom in TPS. 3D dose measurements were collected from 729-ion chamber detector array in Octavius 4D phantom using verisoft software v 6.0. TPS calculated dose was compared with measured 3D dose in verisoft using the following gamma analysis parameters such as 3D volumetric, 3D planar and 2D global gamma in transverse, sagittal and coronal planes for 3mm/3% and 2mm/2% distance to agreement criteria.Passing rate and arithmetic mean of global gamma were analysed for 2D and 3D global gamma in all planes. Results: The average number of dose points passing rate for 2D global gamma with 3mm/3% criteria in transverse, sagittal and coronal planes was 99.06%±2.89%, 98.8%±0.88% and 99.06%±91%, respectively. For 2mm/2% criteria 97.86%±2.26%, 94.49± 2.64% and 94.34%±2.9% was observed. In 3D planar global gamma with 3mm/3% was 99.53%±0.49%, 98.93%±1.03% and 99.29%±1.29%, for 2mm 2% criteria was 97.50%±2.24%, 94.5%±2.5% and 95.38%±4.5%. The maximum arithmetic mean gamma deviation of 0.505%±0.13% was observed in coronal plane for 2D global gamma with 2mm/2% criteria. The 3D volumetric gamma passing rate was observed as 99.61%±0.433% for 3mm /3% and 95.91%±2.51% for 2mm/2%. Conclusion: The objective assessment of 3D dosimetry have demonstrated that the rotational delivery accuracy for flattened and flattened filter free stereotactic plans can be verified by using Octavius system comprising with 729 ion chamber array and Octavius 4D phantom.

EP-1653: Radiosurgery of brain metastases. A dosimetric comparison beetween VMAT and Dynamic arc plans

EP-1653: Radiosurgery of brain metastases. A dosimetric comparison beetween VMAT and Dynamic arc plans

Optimal technique of linear accelerator–based stereotactic radiosurgery for tumors adjacent to brainstem

Stereotactic radiosurgery (SRS) is a well-established technique that is replacing whole-brain irradiation in the treatment of intracranial lesions, which leads to better preservation of brain functions, and therefore a better quality of life for the patient. There are several available forms of linear accelerator (LINAC)–based SRS, and the goal of the present study is to identify which of these techniques is best (as evaluated by dosimetric outcomes statistically) when the target is located adjacent to brainstem. We collected the records of 17 patients with lesions close to the brainstem who had previously been treated with single-fraction radiosurgery. In all, 5 different lesion catalogs were collected, and the patients were divided into 2 distance groups—1 consisting of 7 patients with a target-to-brainstem distance of less than 0.5cm, and the other of 10 patients with a target-to-brainstem distance of ≥ 0.5 and < 1cm. Comparison was then made among the following 3 types of LINAC-based radiosurgery: dynamic conformal arcs (DCA), intensity-modulated radiosurgery (IMRS), and volumetric modulated arc radiotherapy (VMAT). All techniques included multiple noncoplanar beams or arcs with or without intensity-modulated delivery. The volume of gross tumor volume (GTV) ranged from 0.2cm3 to 21.9cm3. Regarding the dose homogeneity index (HIICRU) and conformity index (CIICRU) were without significant difference between techniques statistically. However, the average CIICRU = 1.09 ± 0.56 achieved by VMAT was the best of the 3 techniques. Moreover, notable improvement in gradient index (GI) was observed when VMAT was used (0.74 ± 0.13), and this result was significantly better than those achieved by the 2 other techniques (p < 0.05). For V4Gy of brainstem, both VMAT (2.5%) and IMRS (2.7%) were significantly lower than DCA (4.9%), both at the p < 0.05 level. Regarding V2Gy of normal brain, VMAT plans had attained 6.4 ± 5%; this was significantly better (p < 0.05) than either DCA or IMRS plans, at 9.2 ± 7% and 8.2 ± 6%, respectively. Owing to the multiple arc or beam planning designs of IMRS and VMAT, both of these techniques required higher MU delivery than DCA, with the averages being twice as high (p < 0.05). If linear accelerator is only 1 modality can to establish for SRS treatment. Based on statistical evidence retrospectively, we recommend VMAT as the optimal technique for delivering treatment to tumors adjacent to brainstem.

Cranio Spinal Irradiation of Medulloblastoma Using High Precision Techniques - A Dosimetric Comparison.

Radiotherapy planning, delivery and junction dose verification remain exigent for Cranio Spinal Irradiation (CSI) in medulloblastoma patients. This study aims to evaluate high precision techniques such as Intensity Modulated Radiation Therapy (IMRT), Rapid Arc Therapy (RA) with and without flattening filter (FF) on the basis of dosimetric analysis. Five patients treated with jagged junction Intensity Modulated RadioTherapy (IMRT) using dynamic Multi Leaf Collimators (MLC) were randomly selected for this retrospective study. IMRT, Rapid Arc (RA) plans were simulated in the same CT data set with and without flattening filter. Total dose prescribed was 28.80 Gy in 16 fractions. An evaluation criterion of 98% of PTV receiving 100% of the prescription dose was followed in all plans. Twenty treatment plans with 260 Dose Volume Histograms (DVHs) was created. Dosimetric parameters such as Dmax, Dmin, Dmean, V95%, V107%, CI for PTV and Dmax, Dmean, V80%, V50%, V30%, V10% for Organs At Risk (OAR) were extracted from DVHs. Treatment delivery efficiency was also evaluated for total Beam On Time (BOT). FFF Rapid Arc therapy : 6F_RA) resulted in conformal doses throughout the cranio spinal axis. FF and FFF dynamic IMRT had minimal V107%, 1.23% and 2.88% compared to 49.15 and 66.36 of rapid arc therapy (with and without FF). 6F_IMRT resulted in lesser mean doses to eyes, liver, lungs and kidneys. Heart mean dose was less (3.08 Gy) with 6X_IMRT. Thyroid and esophagus doses could be reduced to about 41.2% and 10% respectively with 6F_RA. The BOT for the treatment techniques were 3.43 min (6X_IMRT), 1.59 min (6F_IMRT), 5min (6X_RA), 4.5 min (6F_RA). Removal of flattening filter in IMRT could improve dose coverage along the caniospinal axis and normal tissue sparing. A reduction of 46.3% BOT could increase treatment efficiency of 6F_IMRT compared to 6X_IMRT. CSI could be simpler since junction doses can be evaded in IMRT and RA techniques.

Evaluating the image quality of cone beam CT acquired during rotational delivery.

The aim of this work was to evaluate the quality of kilovoltage (kV) cone beam CT (CBCT) images acquired during arc delivery. Arc plans were delivered on a Catphan(®) 600 phantom (The Phantom Laboratory Inc., Salem, NY), and kV CBCT images were acquired during the treatment. The megavoltage (MV) scatter effect on kV CBCT image quality was evaluated using parameters such as Hounsfield unit (HU) accuracy, spatial resolution, contrast-to-noise ratio (CNR) and spatial non-uniformity (SNU). These CBCT images were compared with reference scans acquired with the same acquisition parameters without MV "beam on". This evaluation was carried out for different photon beams (6 and 15 MV), arc types (half vs full arc), static field sizes (10 × 10 and 25 × 25 cm(2)) and source-to-imager distances (SID) (150 and 170 cm). HU accuracy, CNR and SNU were considerably affected by MV scatter, and this effect was increased with increasing field size and decreasing photon energy, whereas the spatial resolution was almost unchanged. The MV scatter effect was observed to be more for full-rotation arc delivery than for half-arc delivery. In addition, increasing the SID resulted in decreased MV scatter effect and improved the image quality. Nowadays, volumetric modulated arc therapy (VMAT) is increasingly used in clinics, and this arc therapy enables us to acquire CBCT imaging simultaneously. But, the main issue of concurrent imaging is the "MV scatter" effect on CBCT imaging. This study aims to experimentally quantify the effect of MV scatter on CBCT image quality.

SU‐E‐T‐351: Effects of Isocenter Location, MLC Aperture Angle and Gantry Angle On Dose Distribution

Purpose:Single‐isocenter‐multiple‐target arc plans will have some of the targets displaced from the isocenter. This is a study of the effects of target size, the distance between the isocenter and a target, how many degrees of gantry angle per multileaf collimator (MLC) apertures, and how many degrees of gantry angle per calculation.Methods:Dynamic arc plans are generated with the isocenter placed at 0, 5 and 10 cm from the center of the targets with sizes of 1, 2 and 3 cm in Head/Neck phantom CT images respectively. Each arc plan has a 60 degree gantry rotation, calculated with gantry angle resolution of 1, 2 and 5 degrees, and MLC aperture resolution of 1, 2, 5 and 6 degrees. Some Static‐field plans are generated with static fields having gantry angle changes every 1 or 2 degrees and MLC aperture changing every 2 or 6 degrees to simulate actual LINAC delivery situation of arc plans of same gantry and MLC aperture resolutions.Results:When the isocenter is placed inside the target, all plans are identical regardless of target size, gantry resolution and MLC aperture resolution. When the isocenter is placed a certain distance outside the target center, the aforementioned parameters will affect the plan results. Decreasing target size and increasing distance from the isocenter will increase the effects. For a 1 cm the target with isocenter 10 cm away, the differences between the arc plan and the static‐field plan are: maximal dose 12.8%, minimal dose 24% and MU 14.9%.Conclusion:Arc plan calculation interpolates MLC apertures. This interpolation will cause dose discrepancy between the arc plan and actual delivery when the target size is small and is away from the isocenter. Single‐isocenter‐multiple‐target arc plan should be calculated with finest MLC aperture resolutions and finest gantry angle resolutions to optimize the plan.

SU‐E‐T‐217: Comprehensive Dosimetric Evaluation On 3D‐CRT, IMRT and Non‐Coplanar Arc Treatment for Prone Accelerated Partial Breast Irradiation (APBI)

Purpose:Accelerated partial breast irradiation (APBI) is an effective treatment for early stage breast‐cancer. Irradiation in a prone position can mitigate breast motion and spare heart and lung. In this study, a comprehensive study is performed to evaluate various treatment techniques for prone APBI treatment including: 3D‐CRT, IMRT, co‐planar and non‐coplanar partial arcs treatment.Methods:In this treatment planning study, a left breast patient treated in prone position in our clinic was imported into Varian Eclipse TPS. Six beams tangential to chest wall were used in both 3D‐CRT and IMRT plans. These six beams were coplanar in a transactional plane achieved by both gantry and couch rotation. A 60‐beam IMRT plan was also created to explore the maximum benefit of co‐planar IMRT. Within deliverable couch rotation range (±30°), partial arc treatment plans with one and up to ten couch positions were generated for comparison. For each plan, 30Gy in 6 fractions was prescribed to 95% PTV volume. Critical dosimetric parameters, such as conformity index, mean, maximum, and volume dose of organ at risk, are evaluated.Results:The conformity indexes (CI) are 3.53, 3.17, 2.21 and 1.08 respectively to 3D‐CRT, 6‐beam IMRT, 60‐beam IMRT, and two‐partial‐arcs coplanar plans. However, arc plans increase heart dose. CI for non‐coplanar arc plans decreases from 1.19 to 1.10 when increases couch positions. Maximum dose in ipsilateral lung (1.98 to 1.13 Gy), and heart (0.62 to 0.43 Gy) are steadily decreased with the increased number of non‐coplanar arcs.Conclusions:The dosimetric evaluation results show that partial arc plans have improved CIs compared to conventional 3D‐CRT and IMRT plans. Increasing number of partial arcs decreases lung and heart dose. The dosimetric benefit obtained from non‐coplanar arcs should be considered with treatment delivery time.

SU‐E‐T‐389: Evaluation of Flattening‐Filter‐Free Arcs for Lung SBRT

Purpose:To evaluate dynamic conformal arc therapy (DCAT) and volumetric‐modulated arc therapy (VMAT) using flattening‐filter‐free (FFF) beams for treating non‐small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT).Methods:Five clinical patients, previously treated with SBRT using non‐coplanar 3D conformal radiation therapy (3DCRT), were selected and re‐planned with DCAT and VMAT with both FFF beam (6xFFF with a dose rate of 1200MU/min) and flattened beams (6x with 600 MU/min). All the arc plans were planned with one 360° arc and normalized to the same PTV coverage (100% prescription cover 95% PTV volume) for comparison. Treatment planning metrics such as R100%, R50%, D2cm and lung V20 were compared to the original plans. To evaluate the treatment efficiency differences, all the arc plans were delivered and plan delivery time was compared to that of the clinical treatment as recorded in Mosaiq.Results:All plans meet RTOG conformality constraints and normal tissue tolerances. Average R100% was similar for FFFDCAT (1.07±0.05), FFFVMAT (1.03±0.08) and flattened VMAT (fVMAT) (1.03±0.09) while flattened DCAT (fDCAT) (1.09±0.07) and 3DCRT (1.11±0.06) were significantly inferior (p&lt;0.05, t test). FFFDCAT produced the best average intermediate dose conformality as indicated by R50% (3.86±0.44) and D2cm (43.7±5.3%) when compared to all the other techniques. Significant improvement (p&lt;0.05) in lung V20 was also found with FFFDCAT (2.33±2.06%) when compared to FFFVMAT (2.48±2.03%), fVMAT (2.52±2.07%) and fDCAT (2.64±2.11%) and was slightly better than 3DCRT (2.43±2.04%), though not significant. The FFFDCAT delivery significantly improves the treatment efficiency with an average plan delivery time of 2.70±1.57 min (p&lt;0.05), as compared to fDCAT (5.98±3.45min), fVMAT (6.51±2.94) and 3DCRT (25.14±5.67), but is not significantly better than the FFFVMAT (3.18±1.04).Conclusion:Combining FFF beams and DCAT provide promising improvements in NSCLC SBRT treatment in both plan quality and treatment efficiency and unlike VMAT, tumor coverage is not affected by MLC interplay effects.

SU‐E‐T‐202: Comparison of 4D‐Measurement‐Guided Dose Reconstructions (MGDR) with COMPASS and OCTAVIUS 4D System

Purpose:To cross‐validate the MGDR of COMPASS (IBA dosimetry, GmbH, Germany) and OCTAVIUS 4D system (PTW, Freiburg, Germany).Methods:Volumetric‐modulated arc plans (5 head‐and‐neck and 3 prostate) collapsed to 40° gantry on the OCTAVIUS 4D phantom in QA mode on Monaco v5.0 (Elekta, CMS, Maryland Heights, MO) were delivered on a Elekta Agility linac. This study was divided into two parts: (1) error‐free measurements by gantry‐mounted EvolutionXX 2D array were reconstructed in COMPASS (IBA dosimetry, GmbH, Germany), and by OCTAVIUS 1500 array in Versoft v6.1 (PTW, Freiburg, Germany) to obtain the 3D doses (COM4D and OCTA4D). COM4D and OCTA4D were compared to the raw measurement (OCTA3D) at the same detector plane for which OCTAVIUS 1500 was perpendicular to 0° gantry axis while the plans were delivered at gantry 40°; (2) beam steering errors of energy (Hump=‐2%) and symmetry (2T=+2%) were introduced during the delivery of 5 plans to compare the MGDR doses COM4D_Hump (COM4D_2T), OCTA4D_Hump (OCTA4D_2T), with raw doses OCTA3D_Hump (OCTA3D_2T) and with OCTA3D to assess the error reconstruction and detection ability of MGDR tools. All comparisons used Υ‐criteria of 2%(local dose)/2mm and 3%/3mm.Results:Averaged Υ passing rates were 85% and 96% for COM4D,and 94% and 99% for OCTA4D at 2%/2mm and 3%/3mm criteria respectively. For error reconstruction, COM4D_Hump (COM4D_2T) showed 81% (93%) at 2%/2mm and 94% (98%) at 3%/3mm, while OCTA4D_Hump (OCTA4D_2T) showed 96% (96%) at 2%/2mm and 99% (99%) at 3%/3mm. For error detection, OCTA3D doses were compared to COM4D_Hump (COM4D_2T) showing Υ passing rates of 93% (93%) at 2%/2mm and 98% (98%), and to OCTA4D_Hump (OCTA4D _2T) showing 94% (99%) at 2%/2mm and 81% (96%) at 3%/3mm, respectively.Conclusion:OCTAVIUS MGDR showed better agreement to raw measurements in both error‐ and error‐free comparisons. COMPASS MGDR deviated from the raw measurements possibly owing to beam modeling uncertainty.

SU‐C‐BRB‐05: Investigation of Conformal Arc Therapy Utilizing Newly Designed Cobalt 60 Machine

Purpose:A new modernized design for cobalt 60 (Co‐60) machines is being developed with a ring type gantry. In this study we investigate the beneficial outcome of the new design for conformal arc therapy for various clinical sites. The new modality was evaluated based on isodose distributions and dose volume histograms as compared to 6MV photon beams from conventional linear accelerators.Methods:Computed tomographic images of seven different SBRT patients were selected from our patient database. All of these cases were previously planned on the Eclipse treatment planning system. New plans for these patients were generated with a modified conformal arc technique using both 6MV and Co‐60 beams. The conformal arc was created by the delivery of treatment fields conformal to the target cross‐section at every 5 or 10 degrees. The field shape was modified or turned off when it initially passed through a critical structure. Monte Carlo codes, MCBEAM and MCPLAN, were used for the machine head simulation and phantom/patient dose calculation, respectively. In the new Co‐60 machine design, the source‐to‐isocenter distance was 60cm and the treatment head included the Co‐60 source, primary collimator, jaws and MLC.Results:For all cases investigated, conformal arc plans utilizing Co‐60 beams achieved similar conformity (mean conformity index=1.19) comparing to 6MV photon beams. Isodose distributions were tailored similarly around the PTV; both Co‐60 and 6MV plans met our clinical acceptance criteria for the target coverage, and the maximum and minimum target doses. The DVH for the Co‐60 plans showed slightly lower doses to the critical structures although the differences were small in most cases.Conclusion:There were no significant dosimetric differences between 6MV and Co‐60 plans. Our results confirmed that this new Co‐60 design could be a cost‐effective machine for advanced radiotherapy due to its low cost, low maintenance and high up time.

SU‐E‐T‐674: Relationship Between Conformity Indices and Treatment Characteristics in Cone Based Linac Stereotactic Radiosurgery

Purpose:To investigate the correlation between conformity indices and dose prescription methodologies for LINAC based stereotactic surgery (SRS) cone plans.Methods:A vestibular schwannoma Linac based SRS cone plan was selected for this study. The target volume was 0.49cc and its equivalent sphere diameter is 1.0cm. Single 360 degree ARC Plans with different sizes of cones were done in Eclipse Cone planning System from Varian Medical. The plans were prescribed to satisfy to be normalized to 50% isodose line, 80% isodose line, 100% isodose line covering 95% of the target volume, at target mean dose and target minimum dose. The cones were select to be 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, and 22mm in diameter. The indices for this study were conformity index(CI), which is defined to be the ratio between prescription isodose volume (PIV) and target volume (TV), and integrated conformity index (ICI), which is defined to be (TVpiv)2/(PIVxTV), where TVpiv is the intersection between PIV and TV.Results:When the plans were normalized to 50% of maximum dose, 80% of maximum dose, 100% isodose line covering 95% of the target volume, at target mean dose and target minimum dose, the corresponding values of ICI ranged from 0.45 to 0.04, 0.68 to 0.08, 0.58 to 0.54, 0.65 to 0.58, and 0.37 to 0.26; and the corresponding values of CI ranged from 2.2 to 22.8, 0.9 to 11.4, 1.5 to 1.7, 0.7 to 0.5, and 2.7 to 3.9.Conclusion:The values of indices used to describe SRS plans vary with different prescription conditions for single 360 degree ARC cone plans. For plans satisfied 100 % isodose covering 95% of target volume, the values of CI were between 1 and 2. A dynamic sensitive analysis system should be developed to correlate the plan evaluation index such as CI and ICI with clinical parameters.

SU‐E‐T‐685: Root Cause Analysis of in Vivo Dosimetry Controls for Dynamic Arc Treatments

Purpose:To produce an insight on the influence of selected parameters on the pass/fail rates of in‐vivo dosimetry (IVD) controls for dynamic arc treatments.Methods:A group of 25 patient treatment plans optimized with the treatment planning system Eclipse™ (Varian) for dual‐arc RapidArc treatment was analyzed. All plans were tracked with the IVD solution EPIgray (DOSIsoft S.A) over all sessions of the treatment course. Based on the acquired a‐Si EPID images, the dose differences to the TPS at given reference points and the corresponding gamma indices were computed. In addition, the dose difference variations over the course were determined post‐treatment. Five parameters — considered as possible fail or pass causes of IVD for dynamic arc plans — were chosen. Based on the treatment plan, two types of modulation indices qualifying the movement of the MLC were calculated. An “in‐field” coefficient and a “weighted‐in‐field” coefficient were established to quantify the position of the dose reference points in the planned volume. Finally, the dose rate variations for each arc were also determined. The described parameters were confronted to the results of the IVD controls.Results:The analyzed plans have a mean relative dose difference of 0.2% (±7.2%). No correlation could be established between the modulation indices and the dose differences, the gamma indices or the dose difference variation. Slightly better passing rates for individual sessions and total treatment plans were observed for “in‐field” coefficients superior to 50%. Similar passing rates were observed for arcs with below and above average dose rate deviations.Conclusion:Though all the analyzed parameters are cited as possible pass/fail criteria, only the position of the dose reference point can be linked to dose differences in IVD controls. Automatically generated dose measurement points could avoid false interpretations and an overflow of false negative alerts.The author's PhD research is funded by DOSIsoft S.A.

SU‐E‐T‐345: Effect of DLG and MLC Transmission Value Set in the Treatment Planning System (TPS) On Dosimetric Accuracy of True Beam Hypofractionated SRT/SBRT and 2Gy/fx Prostate Rapid Arc Plans

Purpose:Due to limited commissioning time, we previously only released our True beam non‐FFF mode for prostate treatment. Clinical demand now pushes us to release the non‐FFF mode for SRT/SBRT treatment. When re‐planning on True beam previously treated SRT/SBRT cases on iX machine we found the patient specific QA pass rate was worse than iX's, though the 2Gy/fx prostate Result had been as good. We hypothesize that in TPS the True beam DLG and MLC transmission values, of those measured during commissioning could not yet provide accurate SRS/SBRT dosimetry. Hence this work is to investigate how the TPS DLG and transmission value affects Rapid Arc plans’ dosimetric accuracy.Methods:We increased DLG and transmission value of True beam in TPS such that their percentage differences against the measured matched those of iX's. We re‐calculated 2 SRT, 1 SBRT and 2 prostate plans, performed patient specific QA on these new plans and compared the results to the previous.Results:With DLG and transmission value set respectively 40 and 8% higher than the measured, the patient specific QA pass rate (at 3%/3mm) improved from 95.0 to 97.6% vs previous iX's 97.8% in the case of SRT. In the case of SBRT, the pass rate improved from 75.2 to 93.9% vs previous iX's 92.5%. In the case of prostate, the pass rate improved from 99.3 to 100%. The maximum dose difference in plans before and after adjusting DLG and transmission was approximately 1% of the prescription dose among all plans.Conclusion:The impact of adjusting DLG and transmission value on dosimetry might be the same among all Rapid Arc plans regardless hypofractionated or not. The large variation observed in patient specific QA pass rate might be due to the data analysis method in the QA software being more sensitive to hypofractionated plans.