Non-coplanar Beam Arrangements Research Articles

Comparison between Volumetric Modulated arc Therapy based Coplanar and Noncoplanar Planning for Stereotactic Body Radiation Therapy of Liver

The study aims to investigate potential dosimetric benefits between non-coplanar and coplanar beam arrangements of Volumetric-Modulated Arc Therapy (VMAT) plans for liver stereotactic body radiotherapy (SBRT). Thirteen patients who had undergone liver SBRT treatment in our department were chosen retrospectively for the study. Two sets of SBRT-VMAT plans namely, non-coplanar (NC-VMAT) and Coplanar (C-VMAT) were generated in Monaco(v5.11) planning system for Elekta Versa HD Linac using unflatten 6MV photon. The NC-VMAT plans were created by two/three non-coplanar partial arcs with couch rotation of ±150 and had an arc span of 1300 to 1600 whereas the C-VMAT plans consisted of a full arc. Both plans were compared by statistically analyzing various dosimetric and technical parameters. There is no statistically significant difference observed between the C-VMAT and NC-VMAT plans for planning target volume (PTV) coverage. However, the spine dose (D1cc) was much less in the NC-VMAT plan compared to the C-VMAT plan, with mean values of 6.127 ± 3.08Gy and 9.058 ± 4.76Gy, respectively (p-value=0.002). The low dose spillage to the healthy tissue was compared by the volume receiving 5Gy (V5Gy) and 10Gy (V10Gy). V5Gy of the NC-VMAT plan was 2399.23±1870.76cc while that of C-VMAT plans was 2835.36±1930.20cc with the p-value <0.001. Moreover, the monitor units(MU) were less with NC-VMAT than with C-VMAT SBRT plans (p=0.015). The plan quality of NC-VMAT plans was favorable compared to C-VMAT plans for liver SBRT especially in reducing spine dose, low dose spillage to healthy tissue, and MU.

Dosimetric Comparision of Coplanar versus Noncoplanar Volumetric Modulated Arc Therapy for Treatment of Bilateral Breast Cancers.

The purpose of this study was to compare the dosimetric parameters of volumetric modulated arc therapy (VMAT) treatment plans using coplanar and noncoplanar beams in patients with bilateral breast cancer/s (BBCs) in terms of organ at risk sparing and target volume coverage. The hypothesis was to test whether VMAT with noncoplanar beams can result in lesser dose delivery to critical organs such as heart and lung, which will result in lesser overall toxicity. Data of nine BBC cases treated at our hospital were retrieved. Computed tomography simulation data of these cases was used to generate noncoplanar VMAT plans and the parameters were compared with standard VMAT coplanar plans. Contouring was done using radiation therapy oncology group guidelines. Forty-five Gray in 25 fractions was planned followed by 10 Gy in five fractions boost in breast conservation cases. No significant difference in planning target volume (PTV) coverage was found for the right breast/chestwall (P = 0.940), left breast/chestwall (P = 0.872), and in the total PTV (P = 0.929). Noncoplanar beams resulted in better cardiac sparing in terms of Dmean heart. The difference in mean dose was >1 Gy (8.80 ± 0.28 - 7.28 ± 0.33, P < 0.001). The Dmean, V20 and V30 values for total lung slightly favor noncoplanar beams, although there was no statistically significant difference. The average monitor units (MUs) were similar for coplanar plans (1515 MU) and noncoplanar plans (1455 MU), but the overall treatment time was higher in noncoplanar plans due to more complex setup and beam arrangement. For noncoplanar VMAT plans, the mean conformity index was slightly better although the homogeneity indices were similar. VMAT plans with noncoplanar beam arrangements had significant dosimetric advantages in terms of sparing of critical organs, that is Dmean of heart doses with almost equivalent lung doses and equally good target coverage. Larger studies with clinical implications need to be considered to validate this data.

Noncoplanar Volumetric Modulated Arc Therapy for Hepatocellular Carcinoma Based on a Cage-Like Radiotherapy System: A Simulation Study.

The incorporation of noncoplanar beam arrangements has been proposed in liver radiotherapy modalities, which can reduce the dose in normal tissues compared to coplanar techniques. Noncoplanar radiotherapy techniques for hepatocellular carcinoma treatment based on the Linac design have a limited effective arc angle to avoid collisions. To propose a novel noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system and investigate its performance in hepatocellular carcinoma patients. The computed tomography was deflected 90° to meet the structure of a cage-like radiotherapy system and design the noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system plan in the Pinnacle3 planning system. An noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system plan was customized for each of 10 included hepatocellular carcinoma patients, with 6 dual arcs ranging from -30° to 30°. Six couch angles were set with an interval of 36° and distributed along with the longest diameter of planning target volume. The dosimetric parameters of noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system plan were compared with the noncoplanar volumetric modulated arc therapy and volumetric modulated arc therapy plan. The 3 radiotherapy techniques regarding planning target volume were statistically different for D98%, D2%, conformity index, and homogeneity index with χ2 = 9.692, 14.600, 8.600, and 12.600, and P = .008, .001, .014, and .002, respectively. Further multiple comparisons revealed that noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system significantly reduced the mean dose (P = .005) and V5 (P = .005) of the normal liver, the mean dose (P = .005) of the stomach, and V30 (P = .028) of the lung compared to noncoplanar volumetric modulated arc therapy. Noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system significantly reduced the mean dose (P = .005) and V5 (P = .005) of the normal liver, the mean dose (P = .017) of the spinal cord, V50 (P = .043) of the duodenum, the maximum dose (P = .007) of the esophagus, and V30 (P = .047) of the whole lung compared to volumetric modulated arc therapy. The results indicate that noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system protects the normal liver, stomach, and lung better than noncoplanar volumetric modulated arc therapy and protects the normal liver, spinal cord, duodenum, esophagus, and lung better than volumetric modulated arc therapy. The noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system technique with the arrangement of noncoplanar arcs provided optimal dosimetric gains compared with noncoplanar volumetric modulated arc therapy and volumetric modulated arc therapy, except for the heart. Noncoplanar volumetric modulated arc therapy technique based on a cage-like radiotherapy system should be considered in more clinically challenging cases.

The Optimal Choice of Technique for Stereotactic Radiosurgery—A LINAC-Based Dosimetric Study between DCA, DCA-SSO, DCA-SSO-VDR, and VMAT

Abstract Introduction Advanced radiation therapy delivery techniques require greater understanding of various planning sequences and methods. The aim of this study is to determine a class solution that finds the best possible technique to deliver for stereotactic radiosurgery between dynamic conformal arc (DCA) techniques using various options such as DCA, DCA + SSO (segment shape optimization), and DCA + SSO + VDR (variable dose rate) using noncoplanar beam arrangement and volumetric modulated arc therapy (VMAT) using coplanar beams. Materials and Methods In this dosimetric study, 11 brain cases were retrospectively planned for various techniques and analyzed for the Paddick conformity index (CI), Radiation Therapy Oncology Group homogeneity index (HI), Paddick gradient index (GI), treatment time in terms of monitor units (MU) and normal brain dose (V12Gy). The paired t-test was performed to know the statistical significance between the techniques. Results In terms of CI, GI, and control of the normal brain dose, the VMAT plan was superior to other techniques. But, HI was found to be better with DCA. Above all, VMAT delivered higher MU than any other technique. The p-values between DCA + SSO and DCA, DCA + SSO + VDR and DCA + SSO, and VMAT and DCA + SSO + VDR are as follows: CI: 0.0004, 0.015, and 0.03; GI: 0.03, 0.33, and 0.29; HI: 0.008, 0.04, and 0.06; V12 Gy of normal brain: 0.1, 0.01, 0.38. VMAT requires approximately 41 ± 17% more MU than DCA + SSO + VDR. Conclusion VMAT using coplanar beams is preferable among all the techniques, considering the dosimetric parameters studied. If VMAT is not available in the facility, DCA + SSO + VDR technique using non coplanar beams can be used to deliver SRS treatment.

Dosimetric comparison of coplanar and noncoplanar beam arrangements for radiotherapy of patients with lung cancer: A meta-analysis.

PurposeRadiotherapy plays an important role in the treatment of lung cancer, and both coplanar beam arrangements (CBA) and noncoplanar beam arrangements (NCBA) are adopted in clinic practice. The aim of this study is to answer the question whether NCBA are dosimetrically superior to CBA.MethodsSearch of publications were performed in PubMed, Web of Science, and the Cochran Library till March 2020. The searching terms were as following: ((noncoplanar) or ("non coplanar") or ("4pi") or ("4π")) AND (("lung cancer") or ("lung tumor") or ("lung carcinoma")) AND ((radiotherapy) or ("radiation therapy")). The included studies and extracted data were manually screened. All forest and funnel plots were carried out with RevMan software, and the Egger’s regression asymmetry tests were conducted with STATA software.ResultsNine studies were included and evaluated in the meta‐analysis and treatment plans were designed with both CBA and NCBA. For the planning target volumes (PTV), D98%, D2%, the conformity index (CI), and the gradient index (GI) had no statistically significant difference. For organs‐at‐risk (OAR), V20 of the whole lung and the maximum dose of the spinal cord were significantly reduced in NCBA plans compared with CBA ones. But V10, V5, and mean dose of the whole lung, the maximum dose of the heart, and the maximum dose of the esophagus exhibited no significant difference when the two types of beam arrangements were compared.ConclusionAfter combining multicenter results, NCBA plans have significant advantages in reducing V20 of the whole lung and max dose of spinal cord.

Dosimetric comparison of coplanar and non-coplanar volumetric-modulated arc therapy in head and neck cancer treated with radiotherapy.

PurposeTo evaluate the dosimetric variations in patients of head and neck cancer treated with definitive or adjuvant radiotherapy using optimized non-coplanar (ncVMAT) beams with coplanar (cVMAT) beams using volumetric arc therapy.Materials and MethodsTwenty-two patients of head and neck cancer that had received radiotherapy using VMAT in our department were retrospectively analyzed. Each of the patients was planned using coplanar and non-coplanar orientations using an optimized couch angle and fluences. We analyzed the Conformity Index (CIRTOG), Dose Homogeneity Index (DHI), Heterogeneity Index (HIRTOG), low dose volume, target and organs-at-risk coverage in both the plans without changing planning optimization parameters. ResultsThe prescription dose ranged from 60 Gy to 70 Gy. Using ncVMAT, CIRTOG, DHI and HIRTOG, and tumor coverage (ID95%) had improved, low dose spillage volume in the body V5Gy was increased and V10Gy was reduced. Integral dose and intensity-modulated radiation therapy factor had increased in ncVMAT. In the case of non-coplanar beam arrangements, maximum dose (Dmax) of right and left humeral head were reduced significantly whereas apex of the right and left lung mean dose were increased.ConclusionThe use of ncVMAT produced better target coverage and sparing of the shoulder and soft tissue of the neck as well as the critical organ compared with the cVMAT in patients of head and neck malignancy.

Optimizing highly noncoplanar VMAT trajectories: the NoVo method

We introduce a new method called NoVo (Noncoplanar VMAT Optimization) to produce volumetric modulated arc therapy (VMAT) treatment plans with noncoplanar trajectories. While the use of noncoplanar beam arrangements for intensity modulated radiation therapy (IMRT), and in particular high fraction stereotactic radiosurgery (SRS), is common, noncoplanar beam trajectories for VMAT are less common as the availability of treatment machines handling these is limited. For both IMRT and VMAT, the beam angle selection problem is highly nonconvex in nature, which is why automated beam angle selection procedures have not entered mainstream clinical usage. NoVo determines a noncoplanar VMAT solution (i.e. the simultaneous trajectories of the gantry and the couch) by first computing a solution (beams from every possible direction, suitably discretized) and then eliminating beams by examing fluence contributions. Also all beam angles are scored via geometrical considerations only to find out the usefulness of the whole beam space in a very short time. A custom path finding algorithm is applied to find an optimized, continuous trajectory through the most promising beam angles using the calculated score of the beam space. Finally, using this trajectory a VMAT plan is optimized. For three clinical cases, a lung, brain, and liver case, we compare NoVo to the ideal solution, nine beam noncoplanar IMRT, coplanar VMAT, and a recently published noncoplanar VMAT algorithm. NoVo comes closest to the solution considering the lung case (brain and liver case: second), as well as improving the solution time by using geometrical considerations, followed by a time effective iterative process reducing the solution. Compared to a recently published noncoplanar VMAT algorithm, using NoVo the computation time is reduced by a factor of 2–3 (depending on the case). Compared to coplanar VMAT, NoVo reduces the objective function value by 24%, 49% and 6% for the lung, brain and liver cases, respectively.

A Comparison between Three-Dimensional Conformal Radiotherapy, Intensity-Modulated Radiotherapy, and Volumetric-Modulated Arc Therapy Techniques for Stereotactic Body Radiotherapy of Lung Tumors

Introduction: The aim of this study is to dosimetrically compare 3D CRT, IMRT, and VMAT techniques that employ coplanar and non-coplanar beams for the SBRT of lung tumors. Methods and Materials: Nine (n = 9) consecutive SBRT lung patients with ten tumor sites who were previously treated at our institution were selected for this study. Six (n = 6) treatment plans were created for each PTV: 1 coplanar and 1 non-coplanar 3D CRT, IMRT, and VMAT such that 98% of PTV received 100% of prescription dose, Rx of 50 Gy in 5 fractions. The data collected from each plan included the conformity index, R50%, and homogeneity index of the target as well as the volume of normal tissue irradiated by 5 Gy, 25 Gy isodose lines, V5 and V20 of total lung, and maximum dose to organs at risk. Beam angles and arc lengths were chosen in order to achieve the lowest total lung V5, V20 and R50% values. Results: According to the observed data, the mean total lung V5 and V20 values were lowest for the non-coplanar VMAT plans, but were not statistically different from the other planning techniques. Conformity values were similar for the IMRT and VMAT plans, and significantly lower than the 3D CRT plans. R50% values were lowest for the VMAT plans and significantly lower than both IMRT and 3D CRT plans in both coplanar and non-coplanar beam arrangements. However, dose homogeneity in the PTV is significantly higher in the IMRT coplanar plans than the corresponding 3D CRT and SmartArc (SA) coplanar plans. Coplanar VMAT plans were able to produce a significant 35.5% reduction in the maximum cord dose than coplanar 3D CRT plans (p-value < 0.03). VMAT plans also reduce the volume of the normal tissue irradiated to high doses, but the reduction is insignificant. Conclusion: Overall, the VMAT plans were able to produce highly conformal plans. No single planning technique can outclass the others in all of the SBRT planning indices significantly and a thorough review of each patient plan is needed.

A dosimetric comparison of IMRT versus helical tomotherapy for brain tumors

Background and purposeHelical tomotherapy (HT) can deliver highly conformal, uniform doses to the target volume. However, HT can only be delivered in a coplanar mode.The purpose of this study was to perform a dosimetric comparison of HT versus coplanar (cIMRT) and non-coplanar (n-cIMRT) beam arrangements on a conventional linear accelerator in a diverse group of brain tumors. Materials and methodsA total of 45 treatment plans were calculated retrospectively for 15 cases. For each case, 3 different delivery techniques (n-cIMRT, cIMRT and HT) were used. The treatment plans were compared using the parameters of the target coverage (conformity index; CI) and homogeneity (HI) for the planning target volume (PTV) and the maximum and mean doses for organs at risk (OARs). ResultsMedian HI and CI were the best for HT plans and the worst for cIMRT. The largest reduction of maximum dose for lenses and mean dose for both eyes was achieved for n-cIMRT plans. Mean dose for chiasm and the ipsilateral optic nerve were the lowest for HT. The contralateral optic nerve was most spared with n-cIMRT. For D1% in the brain stem, there was no significant difference between HT and the IMRT plans. ConclusionsBoth HT and n-cIMRT are capable of producing conformal and homogeneous treatment plans with a good sparing of OARs. However, due to the non-coplanar capabilities of IMRT, n-cIMRT led to a superior dose reduction to the lenses.

Commissioning and initial stereotactic ablative radiotherapy experience with Vero

The purpose of this study is to describe the comprehensive commissioning process and initial clinical performance of the Vero linear accelerator, a new radiotherapy device recently installed at UT Southwestern Medical Center specifically developed for delivery of image‐guided stereotactic ablative radiotherapy (SABR). The Vero system utilizes a ring gantry to integrate a beam delivery platform with image guidance systems. The ring is capable of rotating ± 60° about the vertical axis to facilitate noncoplanar beam arrangements ideal for SABR delivery. The beam delivery platform consists of a 6 MV C‐band linac with a 60 leaf MLC projecting a maximum field size of 15×15 cm2 at isocenter. The Vero planning and delivery systems support a range of treatment techniques, including fixed beam conformal, dynamic conformal arcs, fixed gantry IMRT in either SMLC (step‐and‐shoot) or DMLC (dynamic) delivery, and hybrid arcs, which combines dynamic conformal arcs and fixed beam IMRT delivery. The accelerator and treatment head are mounted on a gimbal mechanism that allows the linac and MLC to pivot in two dimensions for tumor tracking. Two orthogonal kV imaging subsystems built into the ring facilitate both stereoscopic and volumetric (CBCT) image guidance. The system is also equipped with an always‐active electronic portal imaging device (EPID). We present our commissioning process and initial clinical experience focusing on SABR applications with the Vero, including: (1) beam data acquisition; (2) dosimetric commissioning of the treatment planning system, including evaluation of a Monte Carlo algorithm in a specially‐designed anthropomorphic thorax phantom; (3) validation using the Radiological Physics Center thorax, head and neck (IMRT), and spine credentialing phantoms; (4) end‐to‐end evaluation of IGRT localization accuracy; (5) ongoing system performance, including isocenter stability; and (6) clinical SABR applications.PACS number: 87.53.Ly

A comparative planning study of step-and-shoot IMRT versus helical tomotherapy in a model patient

In order to assess the potential benefit of being able to use non-coplanar beams in a step-andshoot intensity-modulated radiotherapy (ssIMRT) differently from helical tomotherapy (HT), we performed a comparative planning evaluation in a model patient whose clinical target volumes enclosed critical normal tissue lying in the same plane. A patient with extensively relapsed malignant lymphoma in the circumferential chest wall was selected for comparison. Multiple, separated gross tumors were delineated, and the clinical target volume included the circumferential chest wall containing gross tumor volumes so that the critical organ, the right lung was fully enclosed. Three types of plans were analyzed; the first one was ssIMRT using only coplanar beams, the second was ssIMRT using non-coplanar beams, and the third was HT. The obtained results showed that dose homogeneity inside the gross tumor volume was better in the case of HT than noncoplanar ssIMRT and coplanar ssIMRT. However, the volume fraction of the right lung receiving a high dose was much smaller in the noncoplanar ssIMRT. As far as the dose to the clinical target volume (CTV) fully enclosing the right lung is concerned, not only the dose coverage of the CTV but also the dose homogeneity and conformity in the noncoplanar ssIMRT was better than in both of HT and the coplanar ssIMRT. We can conclude that appropriate isodose coverage for a complex target, which fully encloses a critical organ lying in the same plane, can be achieved by using a non-coplanar beam arrangement with no enforced penalty of higher dose to a critical organ.

Automated generation of IMRT treatment plans for prostate cancer patients with metal hip prostheses: Comparison of different planning strategies

To compare IMRT planning strategies for prostate cancer patients with metal hip prostheses. All plans were generated fully automatically (i.e., no human trial-and-error interactions) using iCycle, the authors' in-house developed algorithm for multicriterial selection of beam angles and optimization of fluence profiles, allowing objective comparison of planning strategies. For 18 prostate cancer patients (eight with bilateral hip prostheses, ten with a right-sided unilateral prosthesis), two planning strategies were evaluated: (i) full exclusion of beams containing beamlets that would deliver dose to the target after passing a prosthesis (IMRT remove) and (ii) exclusion of those beamlets only (IMRT cut). Plans with optimized coplanar and noncoplanar beam arrangements were generated. Differences in PTV coverage and sparing of organs at risk (OARs) were quantified. The impact of beam number on plan quality was evaluated. Especially for patients with bilateral hip prostheses, IMRT cut significantly improved rectum and bladder sparing compared to IMRT remove. For 9-beam coplanar plans, rectum V60 Gy reduced by 17.5% ± 15.0% (maximum 37.4%, p = 0.036) and rectum D mean by 9.4% ± 7.8% (maximum 19.8%, p = 0.036). Further improvements in OAR sparing were achievable by using noncoplanar beam setups, reducing rectum V 60Gy by another 4.6% ± 4.9% (p = 0.012) for noncoplanar 9-beam IMRT cut plans. Large reductions in rectum dose delivery were also observed when increasing the number of beam directions in the plans. For bilateral implants, the rectum V 60Gy was 37.3% ± 12.1% for coplanar 7-beam plans and reduced on average by 13.5% (maximum 30.1%, p = 0.012) for 15 directions. iCycle was able to automatically generate high quality plans for prostate cancer patients with prostheses. Excluding only beamlets that passed through the prostheses (IMRTcut strategy) significantly improved OAR sparing. Noncoplanar beam arrangements and, to a larger extent, increasing the number of treatment beams further improved plan quality.

SU-E-CAMPUS-T-03: Automated Multicriterial Plan Generation for Prostate Cancer Patients with Metal Hip Prostheses: Comparison of Planning Strategies

Purpose: To compare different IMRT planning strategies for patients with metal hip prostheses and to determine the impact of increasing the number of beam directions. Methods: All plans were generated fully automated (i.e., no human trial-and-error interaction) using iCycle, our in-house developed algorithm for multicriterial optimization of beam angles and fluence profiles, allowing objective comparisons of planning strategies. For 18 prostate cancer patients (eight with bilateral hip prostheses, ten with a right-sided unilateral prosthesis), two planning strategies were evaluated: i) full exclusion of beams containing beamlets that would deliver dose to the target after passing through a prosthesis (IMRTremove, as recommended by AAPM taskgroup 63), and ii) exclusion of those beamlets plus a 5 mm margin only (IMRTcut). Plans with optimized coplanar and non-coplanar beam arrangements were generated. Differences in PTV coverage and sparing of organs at risk (OARs) were quantified. The impact of beam number on plan quality was evaluated. Results: Especially for patients with bilateral hip prostheses, IMRTcut significantly improved rectum and bladder sparing compared to IMRTremove, while adequate PTV coverage was maintained. For 9-beam coplanar plans, rectum V60Gy reduced by 17.5% ± 15.0% (maximum 37.4%, p=0.036) and rectum Dmean by 9.4% ± 7.8% (maximum 19.8%, p=0.036). Further improvements in OAR sparing were achievable by using non-coplanar beam set-ups, reducing rectum V60Gy by another 4.6% ± 4.9% (p=0.012) for non-coplanar 9-beam IMRTcut plans. Larger reductions in rectum V60Gy, by on average 13.5% ± 7.9% (p=0.012), were observed when comparing 7-and 15-beam coplanar plans. Conclusion: Excluding only beamlets that pass through a prosthesis prior to delivering dose to the PTV (IMRTcut strategy) significantly improved OAR sparing as opposed to full exclusion of beams containing such beamlets. Application of non-coplanar beam arrangements and, to a larger extent, an increased number of beams further improved plan quality.

Dosimetric and geometric evaluation of a novel stereotactic radiotherapy device for breast cancer: The GammaPod™

A dedicated stereotactic gamma irradiation device, the GammaPod™ from Xcision Medical Systems, was developed specifically to treat small breast cancers. This study presents the first evaluation of dosimetric and geometric characteristics from the initial prototype installed at University of Maryland Radiation Oncology Department. The GammaPod™ stereotactic radiotherapy device is an assembly of a hemi-spherical source carrier containing 36 (60)Co sources, a tungsten collimator, a dynamically controlled patient support table, and the breast immobilization system which also functions as a stereotactic frame. The source carrier contains the sources in six columns spaced longitudinally at 60° intervals and it rotates together with the variable-size collimator to form 36 noncoplanar, concentric arcs focused at the isocenter. The patient support table enables motion in three dimensions to position the patient tumor at the focal point of the irradiation. The table moves continuously in three cardinal dimensions during treatment to provide dynamic shaping of the dose distribution. The breast is immobilized using a breast cup applying a small negative pressure, where the immobilization cup is embedded with fiducials also functioning as the stereotactic frame for the breast. Geometric and dosimetric evaluations of the system as well as a protocol for absorbed dose calibration are provided. Dosimetric verifications of dynamically delivered patient plans are performed for seven patients using radiochromic films in hypothetical preop, postop, and target-in-target treatment scenarios. Loaded with 36 (60)Co sources with cumulative activity of 4320 Ci, the prototype GammaPod™ unit delivers 5.31 Gy/min at the isocenter using the largest 2.5 cm diameter collimator. Due to the noncoplanar beam arrangement and dynamic dose shaping features, the GammaPod™ device is found to deliver uniform doses to targets with good conformity. The spatial accuracy of the device to locate the radiation isocenter is determined to be less than 1 mm. Single shot profiles with 2.5 cm collimator are measured with radiochromic film and found to be in good agreement with respect to the Monte Carlo based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all hypothetical treatment plans corresponding to three target scenarios for each of the seven patients demonstrated good agreement with gamma index pass rates of better than 97% (99.0% ± 0.7%). Dosimetric evaluation of the first GammaPod™ stereotactic breast radiotherapy unit was performed and the dosimetric and spatial accuracy of this novel technology is found to be feasible with respect to clinical radiotherapy standards. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy.

Considerations on IMRT for quasi-isotropic non-coplanar irradiation

The purpose of this study was the mathematical analysis of IMRT with many non-coplanar fields for planning target volumes (PTV) surrounding nearly spherical organs at risk (OAR). Our approach is partially analogous to the well known inverse planning for a cylindrically symmetric (CS) case (Brahme et al 1982 Phys. Med. Biol. 27 1221–9) and leads to a spherically symmetric (SS) solution. For the planning study we approximated isotropic 4 Pi irradiation by a quasi-isotropic non-coplanar IMRT technique with 16 fields which we compared to a coplanar IMRT technique with 15 equidistant fields. A virtual spherical phantom contained a spherical central organ at risk which was surrounded by a PTV shaped like a spherical shell with a gap towards the spherical OAR. We compared three types of plans: (1) non-segmented inversely planned fluence distributions prior to sequencing, (2) plans obtained by direct machine parameter optimization (DMPO) with up to 120 segments (good approximation of non-segmented fluence) and (3) more practical DMPO plans with up to 64 segments. In this study we sought an analytical SS solution for the non-segmented fluence distribution in 4 Pi-geometry. For the CS case Brahme et al found that a special narrow fluence peak (‘Brahme peak’) has to be applied to improve dose uniformity in PTV areas adjacent to the OAR. We showed that in the SS case the peak was steeper but the area under the peak was smaller. The relevance of the peak decreased for increasing gap between the OAR and the PTV. The plan quality of the non-segmented SS plans was higher albeit the fluence distributions were less uniform. The plan quality of the segmented plans degraded if the allowed number of segments was reduced; the degradation was quicker for the SS beam arrangement than for the CS beam arrangement. For 64 segments, the SS plans delivered less uniform and more conformal dose distributions than the CS plans, ensuring better sparing of the healthy tissue. Also, the SS plans always needed less monitor units than the CS plans. In conclusion, due to substructures or steeper fluence gradients, the improved potential of quasi-isotropic SS-plan quality can only be exploited, if many segments are allowed. SS plans seem to spare normal tissue better. Further analysis of non-coplanar beam arrangements with less degree of symmetry is planned, followed by a study on non-coplanar intensity modulated arc techniques.

Coplanar versus noncoplanar intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) treatment planning for fronto‐temporal high‐grade glioma

The purpose of this study was to compare dosimetric and radiobiological parameters of treatment plans using coplanar and noncoplanar beam arrangements in patients with fronto‐temporal high‐grade glioma (HGG) generated for intensity‐modulated radiotherapy (IMRT) or volumetric‐modulated arc therapy (VMAT). Ten cases of HGG overlapping the optic apparatus were selected. Four separate plans were created for each case: coplanar IMRT, noncoplanar IMRT (ncIMRT), VMAT, and noncoplanar VMAT (ncVMAT). The prescription dose was 60 Gy in 30 fractions. Dose‐volume histograms and equivalent uniform doses (EUD) for planning target volumes (PTVs) and organs at risk (OARs) were generated. The four techniques resulted in comparable mean, minimum, maximum PTV doses, and PTV EUDs (p≥0.33). The mean PTV dose and EUD averaged for all techniques were 59.98 Gy (Standard Deviation (SD)±0.15) and 59.86 Gy (SD±0.27). Noncoplanar IMRT significantly reduced contralateral anterior globe EUDs (6.7 Gy versus 8.2 Gy, p=0.05), while both ncIMRT and ncVMAT reduced contralateral retina EUDs (16 Gy versus 18.8 Gy, p=0.03). Noncoplanar techniques resulted in lower contralateral temporal lobe dose (22.2 Gy versus 24.7 Gy). Compared to IMRT, VMAT techniques required fewer monitor units (755 vs. 478, p≤0.001) but longer optimization times. Treatment delivery times were 6.1 and 10.5 minutes for coplanar and ncIMRT versus 2.9 and 5.0 minutes for coplanar and ncVMAT. In this study, all techniques achieved comparable target coverage. Superior sparing of contralateral optic structures was seen with ncIMRT. The VMAT techniques reduced treatment delivery duration but prolonged plan optimization times, compared to IMRT techniques. Technique selection should be individualized, based on patient‐specific clinical and dosimetric parameters.PACS number: 87

SU‐E‐T‐449: Dosimetric Comparison of Beam Arrangements for Stereotactic Body Radiotherapy of Lung Lesion

Dosimetric comparison of beam arrangements of coplanar, non- coplanar and conformal arc for Stereotactic body radiotherapy of lung lesion. Four spherical targets with diameters of 2, 4, 6 and 7cm are contoured in the geometric center of right lung of a selected SBRT CT scan. For each target, treatment plans of optimized 7, 9 and 11 coplanar, non- coplanar and conformal arc beam arrangements are generated in Pinnacle (Pinnacle Version 9.0, Philips Radiation Oncology Systems, WI). Prescription isodoses are chosen to cover at least 95% of target volumes. The volumes enclosed by the prescription and 50% of prescription isodose surfaces are computed. Conformality index (CI) and effective dose fall off gradient are computed for dosimetric comparison. All plans with optimized coplanar and conformal arc beam arrangements have similar CIs while optimized non-coplanar beam arrangements have the smallest CIs. For each target size, optimized 7 and 9 coplanar beam arrangements have the largest half prescription isodose volumes while optimized 11 coplanar beam arrangements have slightly less half prescription isodose volumes than conformal arc plans. All 3 optimized non-coplanar beam arrangements Result in faster dose gradient than conformal arcs except for the smallest target cases, for a 2cm diameter target, 9 or 11 non-coplanar beams are needed to achieve better dose gradient than their rival conformal arc plan. Treatment plans using optimized 9 and 11 non-coplanar beam arrangements are superior to conformal arc plans for SBRT of lung lesions based on dosimetric comparison. To achieve a superior plan for small lesions, 11 or more optimized coplanar beams may be required.

SU-E-T-399: Dosimetric and Geometric Evaluation of a Novel Stereotactic Radiotherapy Device for Breast Cancer: The GammaPod.

A dedicated stereotactic irradiation device, the GammaPodTM, was developed to treat early stage breast cancer. This study presents the first description of the dosimetric and geometric characteristics from the prototype unit. The GammaPod stereotactic radiotherapy device is an assembly of a hemi-spherical source carrier containing 36 Co-60 sources, a tungsten collimator, a dynamically controlled treatment table and a breast immobilization cup embedded with a stereotactic coordinate system. The source carrier and the variable-size collimator rotate synchronously to form 36 non-coplanar, concentric arcs focused at the isocenter. The treatment table enables motion in three dimensions facilitating continuous dose painting in comparison to a sphere packing approach. Geometric and dosimetric evaluations and a method for absorbed dose calibration are provided. Dosimetric verifications of the dynamically delivered plans are performed for eight patients in hypothetical pre-op, post-op and dose painting treatment scenarios. Loaded with a cumulative activity of 4320 Ci, the GammaPod unit delivers 5.31 Gy/min at the isocenter. Due to non-coplanar beam arrangement and dynamic dose shaping features, the GammaPod delivers uniform doses to the targets with excellent conformity. The spatial accuracy of the device is less than 1 mm. Single shot profiles with the 25 mm collimator are measured with radiochromic film and found to be in good agreement with respect to the MC based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all treatment plans corresponding to three target scenarios for each of the eight patients demonstrated Gamma index pass rates greater than 97%. The first description of the dosimetric and geometric evaluation of the GammaPod was performed. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy. C Yu and J Zhang have commercial affiliations with Xcision Medical Systems.

Preoperative Single Fraction Partial Breast Radiotherapy for Early-Stage Breast Cancer

Several recent series evaluating external beam accelerated partial breast irradiation (PBI) have reported adverse cosmetic outcomes, possibly related to large volumes of normal tissue receiving near-prescription doses. We hypothesized that delivery of external beam PBI in a single fraction to the preoperative tumor volume would be feasible and result in a decreased dose to the uninvolved breast compared with institutional postoperative PBI historical controls. A total of 17 patients with unifocal Stage T1 breast cancer were identified. Contrast-enhanced subtraction magnetic resonance images were loaded into an Eclipse treatment planning system and used to define the target volumes. A "virtual plan" was created using four photon beams in a noncoplanar beam arrangement and optimized to deliver 15 Gy to the planning target volume. The median breast volume was 1,713 cm(3) (range: 1,014-2,140), and the median clinical target volume was 44 cm(3) (range: 26-73). In all cases, 100% of the prescription dose covered 95% of the clinical target volume. The median conformity index was 0.86 (range: 0.70-1.12). The median percentage of the ipsilateral breast volume receiving 100% and 50% of the prescribed dose was 3.8% (range: 2.2-6.9) and 13.3% (range: 7.5-20.8) compared with 18% (range: 3-42) and 53% (range: 24-65) in the institutional historical controls treated with postoperative external beam PBI (p = .002). The median maximum skin dose was 9 Gy. The median dose to 1 and 10 cm(3) of skin was 6.7 and 4.9 Gy. The doses to the heart and ipsilateral lung were negligible. Preoperative PBI resulted in a substantial reduction in ipsilateral breast tissue dose compared with postoperative PBI. The skin dose appeared reasonable, given the small volumes. A prospective Phase I trial evaluating this technique is ongoing.

Feasibility of Preoperative Single Fraction Partial Breast Radiotherapy for Early Stage Breast Cancer.

Abstract Introduction: Preliminary data supports the efficacy of partial breast irradiation (PBI) in carefully selected patients. However, in 2008, several series evaluating external beam PBI reported negative toxicity outcomes, possibly related to large volumes of normal tissue receiving near-prescription doses.We hypothesized that delivery of external beam PBI in a single fraction to the preoperative tumor volume, as defined by magnetic resonance imaging (MRI), would be dosimetrically feasible and result in decreased dose to the ipsilateral breast as compared to institutional historical controls.Methods: Ten patients with unifocal T1 breast cancer who had undergone a preoperative MRI were identified. Contrast-enhanced subtraction MR images were loaded into Eclipse treatment planning software and used to define the gross tumor volume (GTV). A 1.5cm uniform expansion (minus first 5mm of skin) was used to generate the clinical target volume (CTV) for dose-volume analysis. An additional 2-3 mm margin was included for penumbra.The first 3mm of skin/subcutaneous tissue was defined as an avoidance structure. Bilateral lungs and heart were segmented. A prospective “virtual plan” was created using four photon beams in a non-coplanar beam arrangement and optimized to deliver 15Gray (Gy) to the CTV.Specific dosimetric data was extracted including dose conformity (breast volume receiving 100% of the dose/CTV) and dose to the ipsilateral breast, ipsilateral lung, and heart.Results: The median breast volume was 1713cc's (range 1014 – 2140cc's) and the median target volume only 41cc's (26-62cc's). In all cases, 95% of the prescription dose covered 100% of the CTV volume. Median dose conformity was 1.2 (range 1.1-1.6). Maximum dose for all plans was not greater than 110%. Doses to the heart and lung were negligible.Median maximum skin dose was 9.3Gy or 62% of the prescription dose. Median maximum dose to 1cc of skin was reduced to 7.0Gy and at 10cc's skin volume, the median maximum was only 4.9Gy.Median percent of the ipsilateral breast volume receiving 100% of the dose (V100) was only 3.5% (range 2.4-4.7%). The median percent volume receiving 50% of the dose (V50) was 13% (range 7.5-20.8%), substantially less than institutional experience with post-operative PBI. Pre-operative PBIPost-operative PBI n=10n=27 Median (range)Median (range)V1003.5% (2.4-4.7)20% (3-42)V5013% (7.5-20.8)43% (24-65) Conclusion: Pre-operative radiotherapy is dosimetrically feasible in women with small breast tumors. Excellent target coverage with negligible dose to normal tissue can be achieved. Skin dose appears reasonable given the small volumes.Median V100 and V50 compare favorably to institutional experience with post-operative PBI. A prospective Phase I trial evaluating this technique is undergoing institutional review. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4105.