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Potential for reduced radiation‐induced toxicity using intensity‐modulated arc therapy for whole‐brain radiotherapy with hippocampal sparing

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      Abstract

      The purpose of this study was to retrospectively investigate the accuracy, plan quality, and efficiency of using intensity‐modulated arc therapy (IMAT) for whole brain radiotherapy (WBRT) patients with sparing not only the hippocampus (following RTOG 0933 compliance criteria) but also other organs at risk (OARs). A total of 10 patients previously treated with nonconformal opposed laterals whole‐brain radiotherapy (NC‐WBRT) were retrospectively replanned for hippocampal sparing using IMAT treatment planning. The hippocampus was volumetrically contoured on fused diagnostic T1‐weighted MRI with planning CT images and hippocampus avoidance zone (HAZ) was generated using a 5 mm uniform margin around the hippocampus. Both hippocampi were defined as one paired organ. Whole brain tissue minus HAZ was defined as the whole‐brain planning target volume (WB‐PTV). Highly conformal IMAT plans were generated in the Eclipse treatment planning system for Novalis TX linear accelerator consisting of high‐definition multileaf collimators (HD‐MLCs: 2.5 mm leaf width at isocenter) and 6 MV beam for a prescription dose of 30 Gy in 10 fractions following RTOG 0933 dosimetric criteria. Two full coplanar arcs with orbits avoidance sectors were used. In addition to RTOG criteria, doses to other organs at risk (OARs), such as parotid glands, cochlea, external/middle ear canals, skin, scalp, optic pathways, brainstem, and eyes/lens, were also evaluated. Subsequently, dose delivery efficiency and accuracy of each IMAT plan was assessed by delivering quality assurance (QA) plans with a MapCHECK device, recording actual beam‐on time and measuring planed vs. measured dose agreement using a gamma index. On IMAT plans, following RTOG 0933 dosimetric criteria, the maximum dose to WB‐PTV, mean WB‐PTV D2%, and mean WB‐PTV D98% were 34.9 ± 0.3   Gy , 33.2 ± 0.4   Gy , and 26.0 ± 0.4   Gy , respectively. Accordingly, WB‐PTV received the prescription dose of 30 Gy and mean V30 was 90.5 % ± 0.5 % . The D100%, and mean and maximum doses to hippocampus were 8.4 ± 0.3   Gy , 11.2 ± 0.3   Gy , and 15.6 ± 0.4   Gy , on average, respectively. The mean values of homogeneity index (HI) and conformity index (CI) were 0.23 × 0.02 and 0.96 × 0.02 , respectively. The maximum point dose to WB‐PTV was 35.3 Gy, well below the optic pathway tolerance of 37.5 Gy. In addition, compared to NC‐WBRT, dose reduction of mean and maximum of parotid glands from IMAT were 65% and 50%, respectively. Ear canals mean and maximum doses were reduced by 26% and 12%, and mean and maximum scalp doses were reduced by 9 Gy (32%) and 2 Gy (6%), on average, respectively. The mean dose to skin was 9.7 Gy with IMAT plans compared to 16 Gy with conventional NC‐WBRT, demonstrating that absolute reduction of skin dose by a factor of 2. The mean values of the total number of monitor units (MUs) and actual beam on time were 719 × 44 and 2.34 × 0.14 min, respectively. The accuracy of IMAT QA plan delivery was ( 98.1 ± 0.8 ) %, on average, with a 3 % / 3   mm gamma index passing rate criteria. All of these plans were considered clinically acceptable per RTOG 0933 criteria. IMAT planning provided highly conformal and homogenous plan with a fast and effective treatment option for WBRT patients, sparing not only hippocampi but also other OARs, which could potentially result in an additional improvement of the quality life (QoL). In the future, we plan to evaluate the clinical potential of IMAT planning and treatment option with hippocampal and other OARs avoidance in our patient's cohort and asses the QoL of the WBRT patients, as well as simultaneous integrated boost (SIB) for the brain metastases diseases.

      PACS number: 87

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      Most cited references 29

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      Tolerance of normal tissue to therapeutic irradiation.

      The importance of knowledge on tolerance of normal tissue organs to irradiation by radiation oncologists cannot be overemphasized. Unfortunately, current knowledge is less than adequate. With the increasing use of 3-D treatment planning and dose delivery, this issue, particularly volumetric information, will become even more critical. As a part of the NCI contract N01 CM-47316, a task force, chaired by the primary author, was formed and an extensive literature search was carried out to address this issue. In this issue. In this manuscript we present the updated information on tolerance of normal tissues of concern in the protocols of this contract, based on available data, with a special emphasis on partial volume effects. Due to a lack of precise and comprehensive data base, opinions and experience of the clinicians from four universities involved in the contract have also been contributory. Obviously, this is not and cannot be a comprehensive work, which is beyond the scope of this contract.
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        Volumetric modulated arc therapy: IMRT in a single gantry arc.

        In this work a novel plan optimization platform is presented where treatment is delivered efficiently and accurately in a single dynamically modulated arc. Improvements in patient care achieved through image-guided positioning and plan adaptation have resulted in an increase in overall treatment times. Intensity-modulated radiation therapy (IMRT) has also increased treatment time by requiring a larger number of beam directions, increased monitor units (MU), and, in the case of tomotherapy, a slice-by-slice delivery. In order to maintain a similar level of patient throughput it will be necessary to increase the efficiency of treatment delivery. The solution proposed here is a novel aperture-based algorithm for treatment plan optimization where dose is delivered during a single gantry arc of up to 360 deg. The technique is similar to tomotherapy in that a full 360 deg of beam directions are available for optimization but is fundamentally different in that the entire dose volume is delivered in a single source rotation. The new technique is referred to as volumetric modulated arc therapy (VMAT). Multileaf collimator (MLC) leaf motion and number of MU per degree of gantry rotation is restricted during the optimization so that gantry rotation speed, leaf translation speed, and dose rate maxima do not excessively limit the delivery efficiency. During planning, investigators model continuous gantry motion by a coarse sampling of static gantry positions and fluence maps or MLC aperture shapes. The technique presented here is unique in that gantry and MLC position sampling is progressively increased throughout the optimization. Using the full gantry range will theoretically provide increased flexibility in generating highly conformal treatment plans. In practice, the additional flexibility is somewhat negated by the additional constraints placed on the amount of MLC leaf motion between gantry samples. A series of studies are performed that characterize the relationship between gantry and MLC sampling, dose modeling accuracy, and optimization time. Results show that gantry angle and MLC sample spacing as low as 1 deg and 0.5 cm, respectively, is desirable for accurate dose modeling. It is also shown that reducing the sample spacing dramatically reduces the ability of the optimization to arrive at a solution. The competing benefits of having small and large sample spacing are mutually realized using the progressive sampling technique described here. Preliminary results show that plans generated with VMAT optimization exhibit dose distributions equivalent or superior to static gantry IMRT. Timing studies have shown that the VMAT technique is well suited for on-line verification and adaptation with delivery times that are reduced to approximately 1.5-3 min for a 200 cGy fraction.
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          Postoperative radiotherapy in the treatment of single metastases to the brain: a randomized trial.

          For the treatment of a single metastasis to the brain, surgical resection combined with postoperative radiotherapy is more effective than treatment with radiotherapy alone. However, the efficacy of postoperative radiotherapy after complete surgical resection has not been established. To determine if postoperative radiotherapy resulted in improved neurologic control of disease and increased survival. Multicenter, randomized, parallel group trial. University-affiliated cancer treatment facilities. Ninety-five patients who had single metastases to the brain that were treated with complete surgical resections (as verified by postoperative magnetic resonance imaging) between September 1989 and November 1997 were entered into the study. Patients were randomly assigned to treatment with postoperative whole-brain radiotherapy (radiotherapy group, 49 patients) or no further treatment (observation group, 46 patients) for the brain metastasis, with median follow-up of 48 weeks and 43 weeks, respectively. The primary end point was recurrence of tumor in the brain; secondary end points were length of survival, cause of death, and preservation of ability to function independently. Recurrence of tumor anywhere in the brain was less frequent in the radiotherapy group than in the observation group (9 [18%] of 49 vs 32 [70%] of 46; P<.001). Postoperative radiotherapy prevented brain recurrence at the site of the original metastasis (5 [10%] of 49 vs 21 [46%] of 46; P<.001) and at other sites in the brain (7 [14%] of 49 vs 17 [37%] of 46; P<.01). Patients in the radiotherapy group were less likely to die of neurologic causes than patients in the observation group (6 [14%] of 43 who died vs 17 [44%] of 39; P=.003). There was no significant difference between the 2 groups in overall length of survival or the length of time that patients remained functionally independent. Patients with cancer and single metastases to the brain who receive treatment with surgical resection and postoperative radiotherapy have fewer recurrences of cancer in the brain and are less likely to die of neurologic causes than similar patients treated with surgical resection alone.
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            Author and article information

            Affiliations
            [ 1 ] Department of Radiation Oncology The University of Kansas Cancer Center Kansas City KS USA
            Author notes
            [* ] a Corresponding author: Damodar Pokhrel, Department of Radiation Oncology, The University of Kansas Cancer Center, 3901 Rainbow Boulevard, Kansas City, KS 66160, USA; phone: (913) 588 5310; fax: (913) 588 3663; email: dpokhrel@ 123456kumc.edu

            Contributors
            dpokhrel@kumc.edu
            Journal
            J Appl Clin Med Phys
            J Appl Clin Med Phys
            10.1002/(ISSN)1526-9914
            ACM2
            Journal of Applied Clinical Medical Physics
            John Wiley and Sons Inc. (Hoboken )
            1526-9914
            08 September 2015
            September 2015
            : 16
            : 5 ( doiID: 10.1002/acm2.2015.16.issue-5 )
            : 131-141
            26699321
            5690185
            10.1120/jacmp.v16i5.5587
            ACM20131
            © 2015 The Authors.

            This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

            Counts
            Figures: 4, Tables: 4, References: 33, Pages: 11, Words: 5784
            Product
            Categories
            Radiation Oncology Physics
            Radiation Oncology Physics
            Custom metadata
            2.0
            acm20131
            September 2015
            Converter:WILEY_ML3GV2_TO_NLMPMC version:5.2.5 mode:remove_FC converted:16.11.2017

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