Single-arc Plans Research Articles

Dosimetric effects of small field size, dose grid size, and variable split-arc methods on gamma pass rates in radiation therapy.

This study investigates the influence of calculation accuracy in peripheral low-dose regions on the gamma pass rate (GPR), utilizing the Acuros XB (AXB) algorithm and ArcCHECK™ measurement. The effects of varying small field sizes, dose grid sizes, and split-arc techniques on GPR were analyzed. Various small field sizes were employed. Thirty-two single-arc plans with dose grid sizes of 2mm and 1mm and prescribed doses of 2, 5, 10, and 20Gy were calculated using the AXB algorithm. In total, 128 GPR plans were examined. These plans were categorized into three sub-fields (3SF), four sub-fields (4SF), and six sub-fields (6SF). The GPR results deteriorated with smaller target sizes and a 2mm dose grid size in a single arc. A similar degradation in GPR was observed with smaller target sizes and a 1mm dose grid size. However, the 1mm dose grid size generally resulted in better GPR compared with the 2mm dose grid size for the same target sizes. The GPR improved with finer split angles and a 2mm dose grid size in the split-arc method. However, no statistically significant improvement was observed with finer split angles and a 1mm dose grid size. This study demonstrates that coarser dose grid sizes result in lower GPRs in peripheral low-dose regions as calculated by AXB with ArcCHECK™ measurement. To enhance GPR, employing split-arc methods and finer dose grid sizes could be beneficial.

Stereotactic body radiation therapy (SBRT) for prostate cancer: Improving treatment delivery efficiency and accuracy

PurposeIn stereotactic body radiation therapy (SBRT) for prostate cancer, intrafraction motion is an important source of treatment uncertainty as it could not be completely smoothed through fractionation. Herein, we compared different arrangements and beam qualities for extreme hypofractionated treatments to minimize beam delivery time and so intrafractional errors. MethodsA retrospective dataset of 11 patients was used. Three volumetric modulated arc therapy (VMAT) beam arrangements were compared for a prescription dose of 40 Gy/5 fractions: two full arcs, 6 MV flattening filter free (FFF); one full arc, 6 MV FFF; one full arc, 10 MV FFF. A plan quality index was defined to compare achievement of the planning goals. Plan complexity was evaluated with the modulation factor. Dose delivery accuracy and efficiency were measured with patient-specific quality assurance plans. ResultsAll treatment plans fulfilled all dose objectives. No statistical differences were found both in plan quality and complexity. Very accurate dose delivery was achieved with the three arrangements, with mean γ passing rates >96.5 % (2 %/2 mm criteria). Slightly but significantly higher γ passing rates were observed with single-arc 6 MV FFF. Contrariwise, statistically significant reductions of the delivery time were obtained with single-arc geometries: the average delivery times were 1.6 min (−46.1 %) and 1.3 min (−56.2 %) for 6 and 10 MV FFF respectively. ConclusionsThe high-quality, very fast and accurate dose delivery of single-arc plans confirmed the suitability of this arrangement for prostate SBRT. In particular, the significant reduction of delivery time would improve treatment robustness against intrafraction prostate motion.

P086 - Dosimetric comparison of imrt vs. vmat - Single arc, dual arc and combination of dual and single arc plans for high risk prostate cancers

P086 - Dosimetric comparison of imrt vs. vmat - Single arc, dual arc and combination of dual and single arc plans for high risk prostate cancers

A Novel Method for Improving VMAT Plan Quality and Deliverability.

Background:A new VMAT method called “Augmented Arc” (or simply Aug Arc) is proposed. The aim of this study is to demonstrate the validity of the proposed method in different clinical cases. Basically, Aug Arc refers to the portion(s) of the VMAT arc that is augmented with an additional arc to improve the plan quality. The Aug Arc portions in the Arc is determined using an objective function-based scoring method called “ψ – score”. Methods:To validate our approach, we have applied it in four clinical cases: Lung, Abdomen, Gynecologic (Gyn) and Pancreas. Basically, for Lung and Pancreas cases, four sets of plans were created, which are: (1) Single arc plan (S Arc), (2) Double arc plan (D Arc), (3) Partial Arc plan without Aug Arc (P Arc) and (4) Partial plan with Aug Arc (P+Aug Arc). For Abdomen and Gyn cases, three sets of plans were created, which are: (1) Single arc plan (S Arc), (2) Double arc plan (D Arc) and (3) Single Arc with Aug Arc (S+Aug Arc). To compute the “ψ – score”, an initial optimization was performed by using full Arc with 4-degree gantry spacing. Subsequently, Aug Arc portions were identified using the ψ – score plot in the single arc and partial arc scenarios. Results:The study finds that the proposed method is useful to improve the plan quality and plan deliverability for both centrically and non-centrically located tumors in terms of reducing the OAR dose, monitor units, beam on time and low dose volume without compromising the target coverage. Conclusion:The results indicate that the proposed approach could strike a balance between full double arc and single arc or partial arc in such a way that the planner can find a sweet spot of delivery parameters that result in optimal plan quality.

Comparison of O-ring and General Linacs for Treatment Planning of Volumetric Modulated Arc Therapy

Novel linac improvements in speed of gantry, collimator, leaf and dose rate may increase the time-efficiency of volumetric modulated arc therapy (VMAT) delivery, however remains to be investigated. In this study, a fast-rotating O-ring linac (Halcyon) with fast moving leaves is compared with a general linac (TrueBeam: TB) in terms of plan quality for VMAT of C-shape, prostate, multi target and, head and neck (H&N) cases from AAPM TG-119. For the four test cases, VMAT planning was performed using single to four-arc VMAT on a Halcyon and using single to three-arc VMAT on a TrueBeam. Same conditions for optimization were used in each test case. Target coverage metrics and organ at risks (OAR) dose were compared. Monitor unit (MU) and irradiation time in each plan were also compared. In all cases, single-arc plans of Halcyon were inferior to TB plans on dose objectives. Conformity index (CI) to outer target of C-shape case was better for Halcyon (1-arc: 1.242, 2-arc: 1.202, 3-arc: 1.198, 4-arc: 1.181) than for TB (1-arc: 1.247, 2-arc: 1.211, 3-arc: 1.211) except to single arc. D5 (Gy) of core for C-shape case was better for halcyon (1-arc: 23.29, 2-arc: 21.01, 3-arc: 20.64, 4-arc: 20.47) than for TB (1-arc: 24.04, 2-arc: 22.94, 3-arc: 23.04). Calculated MU was smaller for Halcyon than for TB. In addition, Halcyon is more faster than TB because mechanical movements were improved. For VMAT plan in each case, Halcyon as well or better at the plan quality of two or three arcs on TB while reducing the delivery time.

MO-FG-CAMPUS-TeP2-05: Optimizing Stereotactic Radiosurgery Treatment of Multiple Brain Metastasis Lesions with Individualized Rotational Arc Trajectories

Purpose: Radiosurgery of multiple (n>4) brain metastasis lesions requires 3–4 noncoplanar VMAT arcs with excessively high monitor units and long delivery time. We investigated whether an improved optimization technique would decrease the needed arc numbers and increase the delivery efficiency, while improving or maintaining the plan quality. Methods: The proposed 4pi arc space optimization algorithm consists of two steps: automatic couch angle selection followed by aperture generation for each arc with optimized control points distribution. We use a greedy algorithm to select the couch angles. Starting from a single coplanar arc plan we search through the candidate noncoplanar arcs to pick a single noncoplanar arc that will bring the best plan quality when added into the existing treatment plan. Each time, only one additional noncoplanar arc is considered making the calculation time tractable. This process repeats itself until desired number of arc is reached. The technique is first evaluated in coplanar arc delivery scheme with testing cases and then applied to noncoplanar treatments of a case with 12 brain metastasis lesions. Results: Clinically acceptable plans are created within minutes. For the coplanar testing cases the algorithm yields singlearc plans with better dose distributions than that of two-arc VMAT, simultaneously with a 12–17% reduction in the delivery time and a 14–21% reduction in MUs. For the treatment of 12 brain mets while Paddick conformity indexes of the two plans were comparable the SCG-optimization with 2 arcs (1 noncoplanar and 1 coplanar) significantly improved the conventional VMAT with 3 arcs (2 noncoplanar and 1 coplanar). Specifically V16 V10 and V5 of the brain were reduced by 11%, 11% and 12% respectively. The beam delivery time was shortened by approximately 30%. Conclusion: The proposed 4pi arc space optimization technique promises to significantly reduce the brain toxicity while greatly improving the treatment efficiency.

Comparison of planning parameter selection for volumetric modulated arc therapy of nasopharyngeal carcinoma in two different treatment planning systems

Objective To investigate the impact of planning parameter settings on plan quality and delivery efficiency of VMAT for nasopharyngeal carcinoma with two treatment planning systems (TPS), as references for clinic plan optimization. Methods 25 patients with nasopharyngeal carcinoma were selected and planned for SIB-VMAT treatment. The same planning aims were used in the two kinds of TPS (TPS-1 and TPS-2). Multiple planning parameters were set for plan optimization. Dose distribution to the target volumes and organs at risk, monitor unit (MU) and delivery time were compared. Paired t-test or one-way ANOVA was used for the data which was in accordance to normal distribution; otherwise, nonparametric Wilcoxon signed rank test or nonparametric Friedman test was used. Results More segments lead to better plan quality and less MU but longer delivery time (Minor impact was observed when segment number was larger than 120) in TPS-1, while it had little impact on both plan quality and delivery efficiency in TPS-2.Comparing to single-arc plans, dual-arc VMAT achieved no significant benefit in plan quality but had more MU and longer delivery time in TPS-1(P=0.000). However, dual-arc VMAT plans had better dose distribution in TPS-2, decreased the maximum and mean dose for spinal cord in 3.9% and 13.7% respectively (P=0.000, 0.000). Changing the settings of maximum or minimum dose rate did not affect the plan quality in both of the tested TPSs. Increasing the maximum or minimum dose rate reduced the delivery time but the latter increased the number of MU (P=0.000, 0.000). Conclusions VMAT plan quality and delivery efficiency is affected significantly and differently by planning parameter settings for two TPSs. Trial test should be conducted for different TPS to determine the optimal parameter settings. Key words: Nasopharyngeal neoplasms/volumetric modulated arc therapy; Operation parameter; Plan quality; Delivery efficiency

A dosimetric retrospective planning study comparing volumetric arc therapy (VMAT) and stereotactic body radiotherapy (SBRT) treatment plans for non-small cell lung cancer (NSCLC)

Purpose A retrospective planning study comparing volumetric arc therapy (VMAT) and stereotactic body radiotherapy (SBRT) treatment plans for non-small cell lung cancer (NSCLC). Methods and materials Five randomly selected early stage lung cancer patients were included in the study. For each patient, four plans were created: the SBRT plan and three VMAT plans using different optimisation methodologies. A total of 20 different plans were evaluated. The dose parameters of dose conformity results and the target dose constraints results were compared for these plans. Results The mean planning target volume (PTV) for all the plans (SBRT and VMAT) was 18·3 cm3, with a range from 15·6 to 20·1 cm3. The maximum dose tolerance to 1 cc of all the plans was within 140% (84 Gy) of the prescribed dose, and 95% of the PTV of all the plans received 100% of the prescribed dose (60 Gy). In all the plans, 99% of the PTV received a dose >90% of the prescribed dose, and the mean dose in all the plans ranged from 67 to 72 Gy. The planning target dose conformity for the SBRT and the VMAT (0°, 15° collimator single arc plans and dual arc) plans showed the tightness of the prescription isodose conformity to the target. Conclusions SBRT and VMAT are radiotherapy approaches that increase doses to small tumour targets without increasing doses to the organs at risk. Although VMAT offers an alternative to SBRT for NSCLC and the potential advantage of VMAT is the reduced treatment times over SBRT, the statistical results show that there was no significant difference between the SBRT and VMAT optimised plans in terms of dose conformity and organ-at-risk sparing.

SU‐E‐T‐529: Study of Second Cancer Risk in the Treatment of Multiple Lung Lesion by SBRT Using Rapid Arc: Comparison Between Different Treatment Planning Parameters

Purpose:To compare the secondary cancer risk (SCR) between different plan settings (Collimator angle, number of Arcs and manual jaw opening) for Rapid Arc plan in the treatment of multiple lung lesion.Methods:Patient with multiple lung lesion was planned with single isocenter (i) single arc (CW:200‐160deg) and (ii) dual complementary arc. The plans were optimized to yield similar plan evaluation/acceptance criteria. The concept of equivalent OED (linear‐exponential, plateau, and the linear dose– response model) was used to evaluate the Second Cancer Risk (SCR). Then, the single arc plan was generated using five different collimator setting (0,30,45,90,120 deg) and the OED was analyzed. Also, plans were generated with automatic jaw setting and compared against the manual jaw setting.Results:The SCR values were normalized to the single arc plan. For dual complementary arc, the relative SCR for Rt.lung were (linear, plateau, linear‐exponential) (1.02,1.05,1.04), (0.99,1.02,1.02) for Lt.lung, (1.01,1.07,1.06) for Spinal cord, (1.18,1.1,1.09) for Esophagus, (1.3,1.19,1.16) for Heart, (1,1.01,1.02) for Lt.Breast and (1.05,1.05,1.05) for Rt.Breast. Since the single arc plan was superior in terms of SCR further analysis was carried out for various collimator angles. The low dose bath volumes 5Gy in cc for single and dual arc plans were 5468 and 5886 cc respectively. The computed OED values (linear‐exponential) for Rt.Lung were (1,0.94,0.96,1.06,0.97), (1,0.89,0.97,1.11,0.9) for Lt.Lung, (1,0.86,0.98,1.2,0.92) for Spinal cord, (1,0.87,0.99,1.15,0.91) for esophagus, (1,0.88,1.01,1.11,0.94) for heart, (1,0.83,0.97,1.20,0.86) for lt.breast and (1,0.94,0.95,1.03,1.11) for Rt.breast for collimator setting (0,30,45,90,120 deg) respectively.Conclusion:The Second cancer risk is considerably high for dual arc plan compared with single arc. In single arc plan the 30 degree collimator angle plan showed the least OED values for all critical structures, with the 90 degree collimator angle being the most. No significant difference between the manual and the automatic jaw setting plans were found.

SU-E-T-185: Feasibility Study of Dose Rate Modulated Arc Therapy (DrMAT) for Lung SBRT

Purpose: To show the feasibility of clinical application of DrMAT for SBRT in lung cancer patients. DrMAT is a form of dynamic conformal arc therapy where MLC segments and dose rates are controlled through simple field weight optimization. Methods: To show feasibility a new treatment plan was created based on the CT of SBRT lung cancer patients. Static plans with 33 fields are made, which have 11deg in between each field and are acquired rotating gantry angle from 180deg to 188deg in CCW direction, total 352deg is rotated. MLC maintained static aperture for each field. To optimize 33 individual fields, field weight was adjusted accordingly using weight optimization algorithm. Keeping weights and MU of static plan, static MLC aperture was converted to multiple arc segments. Arc plan could be created with the fields in the intervals of 11deg. Static MLC should be converted to arc segment MLC. Dynamic conformal arc therapy plan consists of 33 arc fields, is converted to one dose rate modulated arc therapy (DrMAT) plan. DrMAT plan consists of 166 control points which becomes a single arc plan that changes the shape of MLC for every 2.2deg. The resulting DrMAT plan is not an inverse plan it is a simple form of dynamic conformal arc plan using field weight obtained from static plan. This is compared and evaluated with the VMAT plan. Results: DrMAT and VMAT plans have been compared based on the RTOG1021. Both DrMAT and VMAT plans satisfy 100% irradiation to 95% of PTV and critical organs did not exceed dose limit suggested in RTOG1021. DrMAT plan is almost similar with VMAT plan in Result. Conclusion: Field weight optimization method did not show better Resultcompared to VMAT optimization. However, considering simplicity, DrMAT satisfies the condition in RTOG1021. Therefore clinical application of DrMAT is feasible.

Radiobiological impact of planning techniques for prostate cancer in terms of tumor control probability and normal tissue complication probability.

Background:The radiobiological models describe the effects of the radiation treatment on cancer and healthy cells, and the radiobiological effects are generally characterized by the tumor control probability (TCP) and normal tissue complication probability (NTCP).Aim:The purpose of this study was to assess the radiobiological impact of RapidArc planning techniques for prostate cancer in terms of TCP and normal NTCP.Subjects and Methods:A computed tomography data set of ten cases involving low-risk prostate cancer was selected for this retrospective study. For each case, two RapidArc plans were created in Eclipse treatment planning system. The double arc (DA) plan was created using two full arcs and the single arc (SA) plan was created using one full arc. All treatment plans were calculated with anisotropic analytical algorithm. Radiobiological modeling response evaluation was performed by calculating Niemierko's equivalent uniform dose (EUD)-based Tumor TCP and NTCP values.Results:For prostate tumor, the average EUD in the SA plans was slightly higher than in the DA plans (78.10 Gy vs. 77.77 Gy; P = 0.01), but the average TCP was comparable (98.3% vs. 98.3%; P = 0.01). In comparison to the DA plans, the SA plans produced higher average EUD to bladder (40.71 Gy vs. 40.46 Gy; P = 0.03) and femoral heads (10.39 Gy vs. 9.40 Gy; P = 0.03), whereas both techniques produced NTCP well below 0.1% for bladder (P = 0.14) and femoral heads (P = 0.26). In contrast, the SA plans produced higher average NTCP compared to the DA plans (2.2% vs. 1.9%; P = 0.01). Furthermore, the EUD to rectum was slightly higher in the SA plans (62.88 Gy vs. 62.22 Gy; P = 0.01).Conclusion:The SA and DA techniques produced similar TCP for low-risk prostate cancer. The NTCP for femoral heads and bladder was comparable in the SA and DA plans; however, the SA technique resulted in higher NTCP for rectum in comparison with the DA technique.

Stereotactic Body Radiotherapy: Volumetric Modulated Arc Therapy versus 3D Non-Coplanar Conformal Radiotherapy for the Treatment of Early Stage Lung Cancer

Stereotactic body radiotherapy (SBRT) is a treatment option for patients with early stage lung cancer. Treatment duration can be >30 minutes per fraction with non-coplanar 3D-conformal radiotherapy (3D-CRT). Whilst this is generally well tolerated, faster delivery techniques are desirable. Volumetric modulated arc therapy (VMAT) allows for fast delivery of radiation treatment. The purpose of this planning study was to compare SBRT with 3D-CRT and VMAT, with VMAT plans generated using both single arc and 3 non-coplanar partial arcs. Ten patients who previously underwent SBRT (48 Gy in 4 fractions) with 3D-CRT were selected. VMAT plans were generated to treat the PTV while limiting doses to organs at risk. Cumulative dose volume histogram (DVH) parameters were compared between the 3 techniques using the Wilcoxon matched pairs test. Treatment delivery time was also assessed. Both VMAT techniques covered target volumes more conformally than 3D-CRT with a mean V48/VPTV of 1.21 for 3D-CRT, 1.03 for 3 arc plans and 1.01 for single arc plans (p = 0.005). Dose constraints to organs at risk were met using all three techniques. Mean lung doses were 2.93 Gy for 3D-CRT, 2.87 Gy for single arc and 2.73 Gy for the 3 arc technique (3-arc vs. 3D-CRT: p = 0.009). Lung V20 for 3D-CRT, 1 arc and 3 arcs were 3.24%, 2.89% and 2.73%, respectively (3 arc vs. 3D-CRT: p = 0.028). Mean time to deliver a single fraction was 13 minutes for 3D-CRT, 9.2 minutes for 3 arcs and 5.5 minutes for 1 arc. VMAT resulted in improved conformality compared to 3D-CRT. The 3 arc technique appears to have the lowest dose to lung although the magnitude is unlikely to be clinically significant. The main advantage of VMAT over 3D-CRT is faster treatment delivery time. Shortened treatment times are anticipated to improve tolerability of this treatment and reduce the chance of error due to intra-fraction motion.

Dosimetric Impact of the Interplay Effect During Stereotactic Lung Radiation Therapy Delivery Using Flattening Filter-Free Beams and Volumetric Modulated Arc Therapy

We investigated the dosimetric impact of the interplay effect during RapidArc stereotactic body radiation therapy for lung tumors using flattening filter-free (FFF) beams with different dose rates. Seven tumors with motion ≤20 mm, treated with 10-MV FFF RapidArc, were analyzed. A programmable phantom with sinusoidal longitudinal motion (30-mm diameter "tumor" insert; period = 5 s; individualized amplitude from planning 4-dimensional computed tomography) was used for dynamic dose measurements. Measurements were made with GafChromic EBT III films. Plans delivered the prescribed dose to 95% of the planning target volume, created by a 5-mm expansion of the internal target volume. They comprised 2 arcs and maximum dose rates of 400 and 2400 MU/min. For 2400 MU/min plans, measurements were repeated at 3 different initial breathing phases to model interplay over 2 to 3 fractions. For 3 cases, 2 extra plans were created using 1 full rotational arc (with contralateral lung avoidance sector) and 1 partial arc of 224° to 244°. Dynamic and convolved static measurements were compared by use of gamma analysis of 3% dose difference and 1 mm distance-to-agreement. For 2-arc 2400 MU/min plans, maximum dose deviation of 9.4% was found in a single arc; 7.4% for 2 arcs (single fraction) and <5% and 3% when measurements made at 2 and 3 different initial breathing phases were combined, simulating 2 or 3 fractions. For all 7 cases, >99% of the area within the region of interest passed the gamma criteria when all 3 measurements with different initial phases were combined. Single-fraction single-arc plans showed higher dose deviations, which diminished when dose distributions were summed over 2 fractions. All 400 MU/min plans showed good agreement in a single fraction measurement. Under phantom conditions, single-arc and single-fraction 2400 MU/min FFF RapidArc lung stereotactic body radiation therapy is susceptible to interplay. Two arcs and ≥2 fractions reduced the effect to a level that appeared unlikely to be clinically significant.

Volumetric-modulated arc therapy in postprostatectomy radiotherapy patients: A planning comparison study

The purpose of this study was to compare postprostatectomy planning for volumetric-modulated arc therapy (VMAT) with both single arc (SA) and double arcs (DA) against dynamic sliding window intensity-modulated radiotherapy (IMRT). Ten cases were planned with IMRT, SA VMAT, and DA VMAT. All cases were planned to achieve a minimum dose of 68Gy to 95% of the planning target volume (PTV) and goals to limit rectal volume >40Gy to 35% and >65Gy to 17%, and bladder volumes >40Gy to 50% and >65Gy to 25%. Plans were averaged across the 10 patients and compared for mean dose, conformity, homogeneity, rectal and bladder doses, and monitor units. The mean dose to the clinical target volume and PTV was significantly higher (p<0.05) for SA compared with DA or IMRT. The homogeneity index was not significantly different: SA = 0.09; DA = 0.08; and IMRT = 0.07. The rectal V40 was lowest for the DA plan. The rectal V20 was significantly lower (p<0.05) for both the VMAT plans compared with IMRT. There were no significant differences for bladder V40 or rectal and bladder V65. The IMRT plans required 1400MU compared with 745 for DA and 708 for SA. This study shows that for equivalent dose coverage, SA and DA VMAT plans result in higher mean doses to the clinical target volume and PTV. This greater dose heterogeneity is balanced by improved low-range rectal doses and halving of the monitor units.

Poster - Thur Eve - 61: A dosimetric comparison between single-arc and double-arc prostate VMAT plans.

This study compared the dosimetry between prostate plans using the single-arc and double-arc technique in volumetric modulated arc therapy (VMAT). Single-arc and double-arc prostate VMAT plans were created in five patients with prostate volumes ranging from 39.0-68.1 cm3 . 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator, and prescription dose of 78 Gy with 39 fractions were used. Dose-volume histograms of the target and critical organs, dose criteria such as the planning target volume (PTV) D99%, rectum D30% and rectum D50%, and dose profiles along the anterior-posterior and lateral directions crossing the isocenter were determined using the Eclipse treatment planning system with RapidArc optimization. It is found that the double-arc technique provided a better PTV coverage and rectal sparing compared to the single-arc for all patients. For dose profiles crossing the isocenter, it is seen that the dose in the anterior-posterior direction was shifted to the lateral, when the single-arc technique was replaced by the double-arc. This dosimetric change made the dose in the rectal position shift laterally to the femoral head, resulting in a lower rectal dose. The calculated monitor units (MUs) were 471-600 for the single-arc plans and 587-730 for the double-arc. Although the double-arc plans on average required more MUs of about 20%, and almost twice the treatment time compared to the single-arc, it is worthwhile to be considered particularly in some cases, where the dose criteria are difficult to achieve using the single-arc technique.

Can volumetric modulated arc therapy with flattening filter free beams play a role in stereotactic body radiotherapy for liver lesions? A volume-based analysis

To compare volumetric modulated arc therapy with flattening filter free (FFF) and flattening filter (FF) beams in patients with hepatic metastases subject to hypofractionated radiotherapy (RT). A planning study on 13 virtual lesions of increasing volume was performed. Two single arc plans were optimized with the RapidArc technique using either FFF or FF beams. A second planning study was performed on ten patients treated for liver metastases to validate conclusions. In all cases, a dose of 75 Gy in 3 fractions was prescribed to the planning target volume (PTV) and plans were evaluated in terms of coverage, homogeneity, conformity, mean dose to healthy liver and to healthy tissue. For each parameter, results were expressed in relative terms as the percentage ratio between FFF and FF data. In terms of PTV coverage, conformity index favored FFF for targets of intermediate size while FF resulted more suitable for small (<100 cm(3)) and large (>300 cm(3)) targets. Plans optimized with FFF beams resulted in increased sparing of healthy tissue in ≈85% of cases. Despite the qualitative results, no statistically significant differences were found between FFF and FF results. Plans optimized with un-flattened beams resulted in higher average MU∕Gy than plans with FF beams. A remarkable and significant difference was observed in the beam-on time (BOT) needed to deliver plans. The BOT for FF plans was 8.2 ± 1.0 min; for FFF plans BOT was 2.2 ± 0.2 min. RapidArc plans optimized using FFF were dosimetrically equivalent to those optimized using FF beams, showing the feasibility of SBRT treatments with FFF beams. Some improvement in healthy tissue sparing was observed when using the FFF modality due to the different beam's profile. The main advantage was a considerable reduction of beam-on time, relevant for SBRT techniques.

RapidArc radiotherapy planning for prostate cancer: Single-arc and double-arc techniques vs. intensity-modulated radiotherapy

RapidArc is a novel technique using arc radiotherapy aiming to achieve intensity-modulated radiotherapy (IMRT)-quality radiotherapy plans with shorter treatment time. This study compared the dosimetric quality and treatment efficiency of single-arc (SA) vs. double-arc (DA) and IMRT in the treatment of prostate cancer. Fourteen patients were included in the analysis. The planning target volume (PTV), which contained the prostate gland and proximal seminal vesicles, received 76 Gy in 38 fractions. Seven-field IMRT, SA, and DA plans were generated for each patient. Dosimetric quality in terms of the minimum PTV dose, PTV hotspot, inhomogeneity, and conformity index; and sparing of rectum, bladder, and femoral heads as measured by V70, V-40, and V20 (% of volume receiving >70 Gy, 40 Gy, and 20 Gy, respectively), treatment efficiency as assessed by monitor units (MU) and treatment time were compared. All plan objectives were met satisfactorily by all techniques. DA achieved the best dosimetric quality with the highest minimum PTV dose, lowest hotspot, and the best homogeneity and conformity. It was also more efficient than IMRT. SA achieved the highest treatment efficiency with the lowest MU and shortest treatment time. The mean treatment time for a 2-Gy fraction was 4.80 min, 2.78 min, and 1.30 min for IMRT, DA, and SA, respectively. However, SA also resulted in the highest rectal dose. DA could improve target volume coverage and reduce treatment time and MU while maintaining equivalent normal tissue sparing when compared with IMRT. SA achieved the greatest treatment efficiency but with the highest rectal dose, which was nonetheless within tolerable limits. For busy units with high patient throughput, SA could be an acceptable option.

1128 poster COLLIMATOR ANGLE AND LESION VOLUME RELATIONSHIP IN LIVER LESION HYPO-FRACTIONATED RADIOTHERAPY WITH FLATTENING FILTER AND FLATTENING FILTER FREE MODALITY

Materials/Methods: In phase 1 a theoretic study on 13 concentric volume-increasing lesions drawn on the same patient’s CTwas performed. For each patient, four 360 single arc plans were optimized: for both collimator positions (45 and 10 ) two FFF plans (2400MU/min) and two FF (600MU/min). In the second part a retrospective analysis on 13 liver metastatic patients was performed. For both phases PTV was defined as CTV+5mm, Healthy Tissue (HT) was PTV+5mm (inner wall) and PTV+15mm (outer wall). Plans were normalized to PTV mean dose of 75 Gy (25 Gy/3 fr) and were evaluated in terms of healthy liver mean dose (MD), HT MD and PTV Conformity Index (CI = V95%-body/V95%-PTV). For each parameter the % ratio (FF/FFF) *100 was calculated.

Volumetric Modulated Arc Therapy vs. IMRT for the Treatment of Distal Esophageal Cancer

Several studies have demonstrated that volumetric modulated arc therapy (VMAT) has the ability to reduce monitor units and treatment time when compared with intensity-modulated radiation therapy (IMRT). This study aims to demonstrate that VMAT is able to provide adequate organs at risk (OAR) sparing and planning target volume (PTV) coverage for adenocarcinoma of the distal esophagus while reducing monitor units and treatment time. Fourteen patients having been treated previously for esophageal cancer were planned using both VMAT and IMRT techniques. Dosimetric quality was evaluated based on doses to several OARs, as well as coverage of the PTV. Treatment times were assessed by recording the number of monitor units required for dose delivery. Body V 5 was also recorded to evaluate the increased volume of healthy tissue irradiated to low doses. Dosimetric differences in OAR sparing between VMAT and IMRT were comparable. PTV coverage was similar for the 2 techniques but it was found that IMRT was capable of delivering a slightly more homogenous dose distribution. Of the 14 patients, 12 were treated with a single arc and 2 were treated with a double arc. Single-arc plans reduced monitor units by 42% when compared with the IMRT plans. Double-arc plans reduced monitor units by 67% when compared with IMRT. The V 5 for the body was found to be 18% greater for VMAT than for IMRT. VMAT has the capability to decrease treatment times over IMRT while still providing similar OAR sparing and PTV coverage. Although there will be a smaller risk of patient movement during VMAT treatments, this advantage comes at the cost of delivering small doses to a greater volume of the patient.

Volumetric Modulated Arc Therapy Treatment Protocol for Hypo-fractionated Stereotactic Body Radiotherapy for Localized Prostate Cancer

table rotations, collimator angle and photon beam energy have been investigated. Results: VMAT plans with ±45 degree collimator rotations required on average 38% less monitor units compared to plans with no collimator rotations and 20% less monitor units than plans with ±22.5 degree rotations. Plans with ±45 degree collimator rotations provided more homogeneous dose distribution with an average of 6% less maximal dose. VMAT plans with two arcs provided improved conformality and homogeneity with an average of 3% lower maximal dose compared to single arc plans. However, increasingthe number of arcsto threedid not provideany significantimprovement. Introductionof±5 and±10degree table rotations between arcs did not result in any significant dosimetric improvements. The selection of photon beam energy between 6MV and 10MV did not provide any notable dosimetric differences either. Conclusions: Thefinalselectionoftheprotocolincludes twocoplanar10MV RapidArcs with±45degreecollimatorrotations.This prospective clinical trial is designed to deliver 5 fractions of 8 Gy with the prescription dose covering the 95% of the planning target volume. The beam-on time for eachof the coplanar arcs is approximately 2 minutes.For the purpose ofintrafraction motionmanagement, two Rapid Arcs are further split to 4 half arcs with approximately one minute treatment time for each segment. The patient is stereoscopicallyimagedandrepositionedifnecessaryusingExacTracpriortothedeliveryofeacharc.Additionally,CBCTisobtained prior to eachfractionforthe purposesof confirmationofExacTracpositioningand volumetric evaluation of bladderand rectalfilling.