Conformity Index CI Research Articles

A New Treatment Planning Method for Efficient Proton ARC Therapy with Direct Minimization of Number of Energy Jumps

The optimization of energy layer distributions is crucial for efficient proton ARC therapy: on one hand, a sufficient number of energy layers is needed to ensure the plan quality; on the other hand, an excess number of energy jumps can substantially slow down the treatment delivery. This work will develop a new treatment plan optimization method with direct minimization of number of energy jumps (NEJ), which will be shown to outperform state-of-the-art methods in both plan quality and delivery efficiency. The proposed method jointly optimizes the plan quality and minimizes the NEJ. To minimize NEJ, (1) the proton spots x is summed per energy layer to form the energy vector y; (2) y is binarized via sigmoid transform into y1; (3) y1 is multiplied with a predefined energy order vector via dot product into y2; (4) y2 is filtered through the finite-differencing kernel into y3 in order to identify NEJ; (5) only the NEJ of y3 is penalized, while x is optimized for plan quality. The solution algorithm to this new method is based on iterative convex relaxation. The new method is validated in comparison with state-of-the-art methods called energy sequencing (ES) method and energy matrix (EM) method. In terms of delivery efficiency, the new method had fewer NEJ, less energy switching time, and generally less total delivery time. In terms of plan quality, the new method had smaller optimization objective values, lower normal tissue dose, and generally better target coverage. A head-and-neck case is provided in the table with the following dosimetric parameters: planning objective value F; conformity index CI; homogeneity index HI; mean dose of larynx DOAR; mean body dose Dbody; the unit of dose is Gy. We have developed a new treatment plan optimization method with direct minimization of NEJ, and demonstrated that this new method outperformed state-of-the-art methods (ES and EM) in both plan quality and delivery efficiency.

Use of dose-area product to assess plan quality in robotic radiosurgery

PurposeIn robotic stereotactic radiosurgery (SRS), optimal selection of collimators from a set of fixed cones must be determined manually by trial and error. A unique and uniformly scaled metric to characterize plan quality could help identify Pareto-efficient treatment plans. MethodsThe concept of dose-area product (DAP) was used to define a measure (DAPratio) of the targeting efficiency of a set of beams by relating the integral DAP of the beams to the mean dose achieved in the target volume. In a retrospective study of five clinical cases of brain metastases with representative target volumes (range: 0.5–5.68 ml) and 121 treatment plans with all possible collimator choices, the DAPratio was determined along with other plan metrics (conformity index CI, gradient index R50%, treatment time, total number of monitor units TotalMU, radiotoxicity index f12, and energy efficiency index η50%), and the respective Spearman's rank correlation coefficients were calculated. The ability of DAPratio to determine Pareto efficiency for collimator selection at DAPratio < 1 and DAPratio < 0.9 was tested using scatter plots. ResultsThe DAPratio for all plans was on average 0.95 ± 0.13 (range: 0.61–1.31). Only the variance of the DAPratio was strongly dependent on the number of collimators. For each target, there was a strong or very strong correlation of DAPratio with all other metrics of plan quality. Only for R50% and η50% was there a moderate correlation with DAPratio for the plans of all targets combined, as R50% and η50% strongly depended on target size. Optimal treatment plans with CI, R50%, f12, and η50% close to 1 were clearly associated with DAPratio < 1, and plans with DAPratio < 0.9 were even superior, but at the cost of longer treatment times and higher total monitor units. ConclusionsThe newly defined DAPratio has been demonstrated to be a metric that characterizes the target efficiency of a set of beams in robotic SRS in one single and uniformly scaled number. A DAPratio < 1 indicates Pareto efficiency. The trade-off between plan quality on the one hand and short treatment time or low total monitor units on the other hand is also represented by DAPratio.

COMPARISON OF COPLANAR AND NON- COPLANARVMAT FOR BRAIN CANCER BY USING THE DOSIMETRICAL AND RADIOBIOLOGICAL INDICES

Background: Recent techniques of radiotherapy such as volumetric modulated arc therapy (VMAT) that delivered in Coplanar technique or non-coplanar technique allows to deliver high doses to the brain tumors, at the same time reducing the risk of normal tissues as compared with intensity modulated radiotherapy (IMRT) and the three-dimensional conformal radiotherapy (3D-CRT). Aim: The aim of the current work is to compare dosimetrical and radiobiological indices of treatment plans for brain tumor using CO-VMAT and NC-VMAT techniques to choose the optimum technique for the treated cases. Patients and methods: Twenty-one cases with brain tumors were performed for the treatment planning study. The cases are planned by using the coplanar and non coplanar VMAT techniques and optimized to evaluate and compare dosimetrical and radiobiological parameters related to PTV dose coverage and sparing of organs at risk. The total dose of CO-VMAT and NC-VMAT plans is 60 Gray in 30 fractions during a single phase with a daily dose of 2 Gray. Results: In dosimetrical calculations, CO-VMAT and NC-VMAT techniques gave similar (homogeneity index HI, modified homogeneity index MHI, conformity index CI and quality factor QF) values for the PTV, while CO-VMAT was the higher in (target coverage index TCI, prescription isodose to target volume ratio PITV and conformity number CN) values and the lower in (gradient index GI, gradient measure GM) values, and NC-VMAT was the lower in Monitor units MUs values. In radiobiological calculations, equivalent uniform dose EUD values, tumor control probability TCP and complication free tumor control probability P+ were large in CO-VMAT and normal tissue complication probability NTCP was less in NC-VMAT. Conclusion: While the previous studies showed that CO-VMAT technique was used when the tumor is far from the organs at risk, the present work found CO-VMAT can be used when the tumor is near to or far from organs at risk (OARs) because it can achieve the target dose coverage and sparing of OARs together. We strongly recommend that NC-VMAT technique should be used when the OARs are located inside the tumor to be able to achieve more sparing of them.

Assessment of the Quality of the Combined Radiotherapy Plans for Patients with Advanced Cervical Cancer

Purpose: To assess the quality of planning for the combined radiotherapy of patients with advanced cervical cancer (cervical cancer). To assess the impact of taking into account not only the external, but also the internal wall of the organ of risk during contouring on the magnitude of the radiation load on the organ of risk (OR). Estimate the total dose to the OR from the remote and intracavitary components.Material and methods: The planning quality assessment was carried out for three technologies of remote exposure, namely: 3DCRT, IMRT and IMRT in the rotational RapidArc mode. Radiotherapy was performed on linac Clinac iX models (Varian, USA) with photon energy of 6 MeV, intracavitary RT (IRT) — on a MicroSelectron apparatus with a 192Ir source. The calculation was carried out on the treatment planning system Eclipse (Varian, USA) and Oncentra Brachy.Results: The evaluation of the quality of conformal radiotherapy planning was carried out in 69 patients with locally advanced cervical cancer 2b–3b stages according to homogeneity and dose conformance indexes in target volumes and OR in combined radiation therapy programs. The best values of the indices of homogeneity and conformity were obtained with the IMRT technology and 3DCRT. The values of the conformity index CI for PTV for the IMRT technology has advantages in terms of doses to OR. For a group of 94 patients, an analysis of dose loads on the rectum, bladder was carried out with only the external contour of the OR and taking into account the inner wall. For 37 patients, the total values of doses to OR for combined RT were evaluated.Conclusion: The quantitative advantages of homogeneity and conformance indices for the tumor and dose loads in OR of 3D CRT and IMRT technologies for patients with cervical cancer 2b–3b are revealed. We removed from the use of technology RapidArc for this group of patients. The advantages of contouring with or without thickness are not revealed. The contouring for all the mentioned cases of technology can be carried out along the outer contours of the OR.

29 Dosimetric comparaison of the anisotropic analytical algorithm and acuros XB algorithm in dose calculation for lung cancer heterogeneity

Introduction The aim of this study was to investigate the impact of heterogeneity on the dose calculation for two algorithms implemented in the TPS “Analytical Anisotropic Algorithm (AAA) and Acuros XB” and validated the use of Acuros XB algorithm in clinical routine. Methods First, we compare the dose calculated by these algorithms and the dose measured at the given point P, which is found after heterogeneity insert. Second, we extend our work on clinical cases that present a complex heterogeneity. By evaluating the impact of the choice of the algorithm on the dose coverage of the tumor, and the dose received by the organs at risk for 20 patients affected by lung cancer. Results The result of our phantom study showed a good agreement with several studies that showed the superiority of the Acuros XB over the AAA in predicting dose when it concerns heterogeneous media. The treatment plans for 20 lung cancers were calculated by two algorithms AAA and Acuros XB, the results show a statistical significant difference between algorithms for Homogeneity Index and the maximum dose of planning target volume (HI: 0.11 ± 0.01 vs 0.05 ± 0.01 p = 0.04; Dmax: 69.30 ± 3.12 vs 68.51 ± 2.64 p = 0.02). Instead, no statistically significant difference was observed for conformity index CI and mean dose (CI: 0.98 ± 0.18 vs 0.99 ± 0.14 p = 0.33; Dmean: 66.3 ± 0.65 vs 66.10 ± 0.61 p = 0.54). For organs at risk, the maximum dose for spinal cord, mean dose and D37 % of lung minus GTV (dose receiving 37% of lung volume) were found to be lower for AAA plans than Acuros XB and the differences were statistically significant (p 0.05). For the heart D33% and D67% were found to be higher for AAA plans than Acuros XB and the differences were statistically significant (p 0.05), but No difference was observed for D100% of the heart. Conclusions The use of the AXB algorithm is suitable in the case of presence of heterogeneity, because it allows to have a better accuracy close to the Monte Carlo calculation.

Comparative study between Acuros XB algorithm and Anisotropic Analytical Algorithm in the case of heterogeneity for the treatment of lung cancer

Abstract The aim of this study was to investigate the impact of heterogeneity on the dose calculation for two algorithms implemented in the TPS “Analytical Anisotropic Algorithm (AAA) and Acuros XB” and validated the use of Acuros XB algorithm in clinical routine. First, we compare the dose calculated by these algorithms and the dose measured at the given point P, which is found after heterogeneity insert. Second, we extend our work on clinical cases that present a complex heterogeneity. By evaluating the impact of the choice of the algorithm on the dose coverage of the tumor, and the dose received by the organs at risk for 20 patients affected by lung cancer. The result of our phantom study showed a good agreement with several studies that showed the superiority of the Acuros XB over the AAA in predicting dose when it concerns heterogeneous media. The treatment plans for 20 lung cancers were calculated by two algorithms AAA and Acuros XB, the results show a statistical significant difference between algorithms for Homogeneity Index and the maximum dose of planning target volume (HI: 0.11±0.01 vs 0.05±0.01 p = 0.04; Dmax: 69.30±3.12 vs 68.51±2.64 p = 0.02). Instead, no statistically significant difference was observed for conformity index CI and mean dose (CI: 0.98±0.18 vs 0.99±0.14 p = 0.33; Dmean: 66.3±0.65 vs 66.10 ±0.61 p = 0.54). For organs at risk, the maximum dose for spinal cord, mean dose and D37 % of lung minus GTV (dose receiving 37% of lung volume) were found to be lower for AAA plans than Acuros XB and the differences were statistically significant (p&lt;0.05). For the heart D33% and D67% were found to be higher for AAA plans than Acuros XB and the differences were statistically significant (p&lt;0.05), but No difference was observed for D100% of the heart. The use of the AXB algorithm is suitable in the case of presence of heterogeneity, because it allows to have a better accuracy close to the Monte Carlo calculation.

P203] WEPL-corrected margins for moving targets in carbon-ion gated treatments: Preliminary results

Purpose To evaluate intra- and inter-fractional robustness of carbon-ion treatment plans based on water-equivalent path length (WEPL) corrected ITV margins for moving targets irradiated with gated pencil-beam scanning. Methods As a preliminary study, three patients with thoraco-abdominal targets were considered. Clinical plans were optimized with an Intensity Modulated Particle Therapy (IMPT) algorithm and the use of case-specific avoidance structures to prevent radiation pathways on regions subject to intra- and inter-fractional variations. Plans were optimized on the end-exhale CT of a 4DCT dataset using 2 fields and tested for consistency within the gating window by recalculation on the 30%-exhale and 30%-inhale CTs. Single Field Uniform Dose (SFUD) plans were optimized on beam-specific WEPL-corrected ITV ( ITV WEPL ), which comprised the envelope of water-equivalent target positions within the ideal gating window. Differently from clinical plans, avoidance structures were not used in this approach. Clinical plans were taken as reference for comparison. Dose recalculations on the planning 4DCT and on synthetic 4DCT simulating irregular breathing allowed to evaluate intra-fractional robustness of the two approaches, whereas inter-fractional robustness was estimated on re-evaluation CTs (one per patient). Differences in the conformity index CI = V 95 % / V target , target coverage D 95 % and homogeneity index HI = ( D 2 % − D 98 % )/ D 50 % between the end-exhale plan and the recalculated dose distributions were computed. Doses to the involved organs at risk (OARs) were evaluated as well. Results The CI presented slightly higher median intra-/inter-fractional variations and absolute values in the ITV WEPL approach (13%–1.47) with respect to reference plans (6%–1.36). D 95 % and HI showed lower median intra-fractional variations for the ITV WEPL approach (0.1% and 35%) compared to reference plans (3.4% and 46%). In case of inter-fractional changes, the use of ITV WEPL led to higher median variations but comparable absolute values for D 95 % (3.8% vs 2.2%) and for HI (144% vs 20%, absolute median values: 0.11 vs 0.14). The two methods performed similarly in OARs. Conclusions The use of WEPL-corrected ITV permits adequate tumor coverage even in presence of irregular breathing motion, for the small analyzed sample. Although uncertainties due only to intra-fractional organ motion were considered in the ITV WEPL approach, comparable inter-fractional robustness was found among the two strategies.

Tool Assessment of Quality of Dosimetric Plans of Remote Irradiation of Cancer Patients

The results of a quantitative assessment of the plans for remote irradiation of oropharyngeal cancer patients by means of the “dose–volume histogram” method (tool) are presented. The homogeneity index HI of the dose in the planned target volume PTV, conformity index CI, and other indices are calculated for 37 patients, The results of the calculations for various irradiation method are compared. It is shown that the homogeneity index HI for 80% of the patients and all irradiation technologies is within 0.05–0.12. The conformity index CI for patients does not depend on the irradiation technology.

A Randomized Singular Value Decomposition Based Linear Programming Technique for Robotic Radiation Therapy Treatment Plan Optimization

With highly reliable control of linac positioning, the robotic radiotherapy by CyberKnife significantly increases the freedom in radiation beam placement, but also imposes more challenges on treatment plan optimization. The resampling mechanism in vendor supplied treatment planning system (MultiPlan) could not fully explore the increased beam direction search space. This study proposes a randomized singular value decomposition (RSVD) based linear programming (LP) technique for circular collimator robotic radiotherapy treatment plan optimization (CCRTPO). The RSVDLP algorithm initializes beam angle of each available node is to cover the entire target with equivalent beam taper, which fully explores the complete beam space without redundancy. Beam weight is optimized by LP, which minimizes the deviation of relax constraints (RCs) subjected to hard constraints. Based on the degeneracy of dose influence matrix (DIM), of which the column vector is the dose delivered by one beam to all voxels, a RSVD approach is developed to compress the weight variables, and back project the compressed variables to construct beam weights after optimization. The beams with lower weight are removed, and the weight of remaining beams is further optimized using the same method to reduce number of beams. The technique was demonstrated and compared with the MultiPlan on a lung case with a PTV of 15.5cm3. Both algorithms used the same set of 94 nodes, and circular collimators of 15mm & 20mm diameter. The objectives and constraints used in MultiPlan are switched in RSVDLP model. The switched model is more suitable for SBRT, as it guarantees sufficient dose for PTV and avoids cold spot. For spinal cord (SC) and trachea (Tr), RSVDLP model uses stricter RCs to achieve better protection without triggering infeasibility. RSVD is performed to preserve 90% feature of DIM, the compression rate is 24.4% (1670 compressed variables over 6832 initial beams), and the optimization is sped up 4.8 times (1785s over 370s). The table indicates that RSVDLP plan achieves better homogeneity, conformity and OAR protection with lower MU and fewer beams. Similar results were observed on liver and brain cases.Abstract 3639; Table Comparison of optimization models and results (unit in cGy)Objective (O) & constraint (C)DmaxDmeanCI2HIMUBeam #PTVSCEsTrSCEsTrLLRLRSVDLP>4500 C<4650 O<1200 O<2200 O<1200 O1613304913561173491.281.1735745134MultiPlan>4500 O<4650 C<1500 C<4150 C<4050 O<2200 C1681318016591453621.581.2837820136Abbr: esophagus-Es, left lung-LL, right lung-RL, conformity index-CI, homogeneity index-HI. Open table in a new tab Abbr: esophagus-Es, left lung-LL, right lung-RL, conformity index-CI, homogeneity index-HI. We have developed a RSVDLP technique for CCRTPO. The results demonstrate that RSVD is effective in accelerating the optimization. Due to the use of complete beam space and flexible model, the plan generated by RSVDLP achieves better dose distribution than MultiPlan.

Does Immunotherapy Influence the Risk of Developing Radiation Necrosis After Radiosurgery of Brain Metastases?

The risk of radiation necrosis (RN) in patients with brain metastases (BM) treated with stereotactic radiosurgery (SRS) can be estimated on the volume of normal brain receiving 12 Gy (V12). Reference RN risks at 1 year for V12 <3.3 cm3 and 3.3-5.9 cm3 are 0% and 16%, respectively. It is unclear if immunotherapy (IT), now commonly used in the treatment of various malignancies, modifies the risk of RN with long term follow-up. We have correlated V12 with radiographic appearance of RN for BMs treated with SRS +/- IT. We reviewed 71 patients harboring 112 brain lesions from breast cancer (n = 9), non-small cell lung cancer (n = 28) and melanoma (n = 34) primaries. In 63 of the lesions (56%), immunotherapy was administered during the course of the patient’s treatment, while 49 lesions (44%) only received SRS. The median SRS dose was 21 Gy (range 15-24 Gy) delivered on a Linac-based platform with a 6D robotic table. The median dosimetric data (and range) for all lesions were as follows: V12 2.0 cc (0.45-18.64), GTV 0.25 cc (0.01-4.20 cc), PTV 0.51 cc (0.07-8.55 cc), conformity index CI 1.56 (1.0-2.16), prescription IDL 79.4% (69.4-99.2%) with 99.3% (88.4-100%) coverage of the PTV. Patients were followed for a median follow-up of 13.1 months with serial MRIs and RN assessments were rendered based on multiparametric MRIs with perfusion techniques; in 6/16 cases RN was pathologically proven with surgical biopsy. The radiographic incidence of RN was then correlated with dosimetric factors and the infusion (or lack thereof) of IT. Statistical analysis was performed using Kaplan Meier analysis and Fischer’s exact chi-square test. The overall incidence of radiographic RN was 14% (16/112), peaking at a median time of 14.9 month post SRS (range 8.1-71.7 months). However only 5/16 patients had symptomatic RN necessitating treatment with bevacizumab, surgery and/or pentoxifylline/vitamin E. The 1-, 2-, and 3-year rates of RN were 2.7%, 11.6%, and 13.4%, respectively, for all patients. The risk of RN correlated with V12; lesions with V12 <5.09 cc had a significantly lower risk of RN at 3 years compared to V12 ≥5.09 cc (10% vs 32%, respectively, P = 0.044). The percentage of patients that developed RN in the presence of IT was higher (12/63, 19%) than in the control group (4/49, 8%) but this did not reach statistical significance. Finally, the V12 threshold for patients receiving IT was 5.39 cc; a V12 <5.39 cc was associated with 12.5% risk of RN at 3 years vs 46% for V12 ≥ 5.39 cc. Based on this retrospective analysis, the overall incidence of RN appears to be higher in the IT group (19% vs 8%), but this could be due to longer survival secondary to systemic response to immunotherapy. As a results, the rate of RN based on Kaplan-Meier estimates continues to increase at 3-years post SRS. This information may be useful to clinicians when trying to distinguish late recurrence vs late radiation necrosis in long-term survivors receiving SRS and immunotherapy.

Stereotactic body radiation therapy in lung and hepatic cancer (SBRT) with helical tomotherapy (HT): Description of our method

Introduction We chose to implement the technique of SBRT in lung and liver lesions with Helical Tomotherapy (HT) for its high accuracy and dosimetric quality obtained, so as to deliver at least 1 Gy per minute for PTV less than 5 cm in optimal conditions for the machine (Gantry rotation, couch motion, leaf open time). Materials and methods From June 2012 until March 2013, 6 patients with 10 lung lesions and 2 patients with liver lesions were treated with HT-SBRT. Internal tumor volume (ITV) is defined by the sum of the GTV from 4 sets of kV-CT (The dose calculation using a density averaged ITV for lung lesions) and a series PET SCAN in free-breathing, patient immobilized with diaphragmatic compression Body ProLock commercialized by Civco ® . Planned treatment volume (PTV) consisted of 5-mm radial and a 10-mm craniocaudal expansion of ITV. The prescribed doses were: 3 × 20Gy, 3 × 18Gy, 8 × 7,5 Gy. Dose constraints to normal tissue were defined accorded to RTOG 0236 and 0438 protocols. The characteristics of the treatment plan (Pitch = 0.86/ n (0.123), modulation factor (1.5), field size (2.5 or 1 cm)) were chosen to achieve a compromise dosimetric quality (homogeneity index HI and conformity index CI)/ processing time. Results The average homogeneity and conformity index that we obtained are 1.04 and 0.96. Accurate positioning of the patient and control intra and post fraction of it through MVCT scan and adaptive planning tool allows us to assess for central pulmonary and hepatics lesions displacements in x, y and z Conclusions HT-SBRT is feasible, accurate and well tolerated for lung and liver lesions. The improvement of our methodology will be to introduce the 4D PET to define a more precise ITV and to make the calculation on a weighted averaged density scan.

Comparative dosimetric study of three-dimensional conformal (3DCRT), intensity modulated radiotherapy (IMRT), and volumetric modulated arc therapy (VMAT) for treatment in pituitary adenomas.

e13043 Background: Radiotherapy is usually delivered after surgery in pituitary adenomas. Usually stereotactic radiotherapy is used in their treatment. The studies evaluating the recent advances like IMRT, VMAT for pituitary adenomas are rare. Present study evaluates 3DCRT, IMRT, and VMAT for the treatment of pituitary adenomas. Methods: Ten patients of pituitary adenomas who were planned for post operative adjuvant radiotherapy were included. Planning CT was done with the patient supine with a customized thermoplastic cast of head on Philips wide bore CT (for 3 mm). The CT images were transferred online to the Pinnacle TPS V.8.0M. Tumor and the critical structures like brain stem and optic nerves were contoured on pinnacle. 3DCRT was planned on pinnacle. The CT images with the contoured structures were sent to Monaco TPS V.3.10.02. IMRT and VMAT were planned on Monaco. Dose: 50 Gy in 25 fractions. PTV indices analyzed: D mean, D2 (dose received by the hottest 2 percent volume), D98 (dose received by 98 percent), homogeneity index HI [(D2-D98)/D50)], and conformity index CI (PTV volume/volume of PTV covered by 95 percent isodose). Dmax and D2 (dose received by hottest 2cc) for the brain stem, right and left optic nerve PRV were evaluated. The average were calculated and compared. Statistical analysis was done by SPSS Version 14. Results: The Table shows that VMAT is better in terms of PTV indices. For the normal structures also VMAT provides the least dose. Conclusions: VMAT is superior to IMRT and both are better than 3DCRT for the radiotherapeutic treatment of pituitary adenoma. [Table: see text]

A comparative analysis of 3DCRT, IMRT, and VMAT in the palliative treatment of spinal metastases.

e20679 Background: Radiotherapy is very important in the palliation of spinal metastases. Traditionally, a simple two-dimensional single direct posterior spinal field giving unnecessary exit dose to the normal structures is used. The studies evaluating the recent advances like 3DCRT, IMRT, VMAT for spinal metastases are rare. Present study evaluates these three modalities for the treatment of spinal mets. Methods: Ten patients with metastases to the body of lumbar spine were included. Planning CT was done with the patient supine on Philips wide bore CT (for 3 mm). The CT images were transferred online to the Pinnacle TPS V.8.0M. Tumor and the critical structures like spinal cord, kidneys, liver and bowels were contoured on pinnacle. 3DCRT was planned on pinnacle with seven equidistant beams. The CT images with the contoured structures were sent to Monaco TPS V.3.10.02. IMRT (seven equidistant fields) and VMAT (a single 360 degree arc in 25 degree increments) were planned on Monaco. Dose: 30 Gy in 10 fractions. PTV indices analyzed: D mean, D2 (dose received by the hottest 2 percent volume), D98 (dose received by 98 percent), homogeneity index HI [(D2-D98)/D50)], and conformity index CI (PTV volume/volume of PTV covered by 95 percent isodose). Dmax and D2 (dose received by hottest 2cc) for the spine, V12, V18 and V28 (percent volume receiving the respective dose) for the kidneys, D50 (dose received by the 50 percent volume) of liver, V15 (percentage volume of bowel receiving 15 Gy) were evaluated. The average were calculated and compared. Statistical analysis was done by SPSS Version 14. Results: The Table shows that there is no much significant difference between the three plans for the PTV indices. But for the normal structures, VMAT provides the least dose followed by IMRT. Conclusions: VMAT is superior to IMRT and both are better than 3DCRT for normal tissue sparing in the palliative RT for spinal metastases. [Table: see text]

Cranio-spinal irradiation with volumetric modulated arc therapy: A multi-institutional treatment experience

Purpose To report the treatment of cranio-spinal irradiation (CSI) with volumetric modulated arc therapy (RapidArc) in adults and a child. Materials and methods Five patients from 5 institutions were treated with CSI using RapidArc technology. Patient age varied between 7 and 45 years. The lengths of the planning target volumes (PTV) and dose prescriptions ranged from 54.6 to 78.4 cm, and from 12 to 36 Gy, respectively. Different arc arrangements were used, with two or three isocentres, from two to six modulated arcs, for patient positioned either in prone ( n = 3) or supine position ( n = 2). Doses to PTV, organs at risk and non-target tissue are reported for each of the five patients. Data concerning imaging strategy and pre-treatment quality assurance are also reported. Results The mean conformity index CI 95% was 1.05 ± 0.05 (range: 1.00, 1.13). Mean doses to the lenses, lungs, heart, and kidneys were, respectively, 7.6 ± 1.6, 6.6 ± 3.0, 5.7 ± 2.3, and 6.1 ± 2.1 Gy, keeping those value acceptably low. Mean non-target tissue dose was 21.0 ± 3.8% (5.6 ± 1.8 Gy, range: 3.0–8.1 Gy). All patients were successfully planned and treated with RapidArc. Conclusions RapidArc techniques achieved highly conformal treatment plans for 5 adults or paediatric patients. Beam-on time was short, and RapidArc plans were satisfactorily delivered to all presented patients.

SU-GG-T-68: The Impact of Intensity Levels and Number of Segments On Skullbase IMRT Plan Quality and Efficiency

Purpose: The aim of this study was to investigate the impact of the number of intensity levels and segments, on plan quality and efficiency, when planning treatment for skullbase lesions. Method and materials: Five previously treated cases of skullbase meningioma were re‐planned using inversed planned IMRT to an escalated dose of 60Gy/30♯. 3 IMRT plans were calculated for each case, using 5, 10 and 15 intensity levels. These plans were then compared, using physical (PTV Dmax, V100, V95, OAR Dmax, conformity index CI) and radiobiological parameters (Equivalent Uniform Dose EUD, Integral dose ID). Plan efficiency was measured as a function of ‘beam on’ time. Results: The optimal plan generated was a 7–9 coplanar beam arrangement in all cases. Doses to OAR were within clinical tolerance levels. There was no statistically significant difference between OAR sparing or PTV coverage based on the intensity level used to generate the plans. (p>0.05) The average V90%, V95% and V100% across all plans was 96.2%, 93.3% and 84.6% respectively. The average EUD and CI were 62Gy and 1.3 respectively. Brainstem Dmax was limited to an average of 47.4Gy (range 37.9Gy to 52.7Gy), Optics Dmax to an average of 48.9Gy (range 40Gy to 51.8Gy) and ID was limited to an average of 10.2Gy (range 3.4 to 27 Gy). On average, the numbers of segments for a 5, 10 and 15 level plan were 85, 171 and 238 respectively. Beam on time per fraction increased from an average of 17mins for a 5 intensity level plan to 50 minutes for a 15 intensity level plan. Discussion: When using IMRT for skullbase lesions, our department has adopted a standard intensity level of 5 for treatment planning. This will result in maximal plan efficiency without compromising plan quality. Conflict of interest: Research sponsored by Siemens Oncology Solutions.

Evaluating target coverage and normal tissue sparing in the adjuvant radiotherapy of malignant pleural mesothelioma: Helical tomotherapy compared with step-and-shoot IMRT

Purpose To evaluate the potential of helical tomotherapy in the adjuvant treatment of malignant pleural mesothelioma and compare target homogeneity, conformity and normal tissue dose with step-and-shoot intensity-modulated radiotherapy. Methods and materials Ten patients with malignant pleural mesothelioma who had undergone neoadjuvant chemotherapy with cisplatin and permetrexed followed by extrapleural pneumonectomy (EPP) were treated in our department with 54 Gy to the hemithorax delivered by step-and-shoot IMRT. A planning comparison was performed by creating radiation plans for helical tomotherapy. The different plans were compared by analysing target homogeneity using the homogeneity indices HI max and HI min and target conformity by using the conformity index CI 95. To assess target coverage and normal tissue sparing TV 90, TV 95 and mean and maximum doses were compared. Results Both modalities achieved excellent dose distributions while sparing organs at risk. Target coverage and homogeneity could be increased significantly with helical tomotherapy compared with step-and-shoot IMRT. Mean dose to the contralateral lung could be lowered beyond 5 Gy. Conclusions Our planning study showed that helical tomotherapy is an excellent option for the adjuvant intensity-modulated radiotherapy of MPM. It is capable of improving target coverage and homogeneity.

SU‐FF‐T‐74: Effects of Patient Positioning and Treatment Techniques On the Potential for Dose Escalation in Patients with Gynecological Malignancies with Para‐Aortic Lymph Node Involvement

Purpose: To investigate the effect of patient positioning and treatment technique on the potential for dose escalation in pelvic and para‐aortic irradiation. Method and Materials: 5 patients with gynecological malignancies were CT‐scanned in prone, with belly‐board, and supine positions. For each patient and position 3D and IMRT plans were generated to 45 Gy. The CTV encompassed the pelvis and para‐aortic nodes. PTV was a 3D, 0.6 cm expansion. We contoured kidneys, spine, small bowel, bladder and rectum, and compared the doses distributions. Dose conformity was assessed using the conformity index CI = VT, P / VP, where VT, P is the target volume receiving prescription dose or greater, and VP is the volume receiving the prescription dose. For organs at risk we compared mean (e.g. for kidneys) or maximum (e.g. for spine) doses. Results: Target conformity was significantly improved for IMRT plans as compared to 3D plans. IMRT plans, regardless of position, resulted in superior sparing of spine and bowel compared to 3D plans . For IMRT we found no significant difference in bowel doses between supine and prone belly‐board positions. The kidneys received lower doses in the 3D plans, but IMRT doses were much lower than tolerance. Contrary to small field irradiation, the integral dose in these large‐field IMRT plans was not more than in the 3D plans. Conclusion: IMRT has the potential to allow dose escalation in whole pelvic and para‐aortic radiation as compared to 3D plans, with increased sparing of critical organs. For IMRT, regardless of patient positioning, there is no significant difference in doses to either targets or critical structures. Thus, for reasons of setup reproducibility and patient comfort, supine IMRT would appear to be the better choice. The problem of organ motion in the irradiated area must be addressed before IMRT can be implemented.