Conformity Index Of Planning Target Volume Research Articles

A modified U-net with graph representation for dose prediction in esophageal cancer radiotherapy plans

The manual design of esophageal cancer radiotherapy plan is time-consuming and labor-intensive. Automatic planning (AP) is prevalent nowadays to increase physicists’ work efficiency. Because of the intuitiveness of dose distribution in AP evaluation, obtaining reasonable dose prediction provides effective guarantees to generate a satisfactory AP. Existing fully convolutional network-based methods for predicting dose distribution in esophageal cancer radiotherapy plans often capture features in a limited receptive field. Additionally, the correlations between voxel pairs are often ignored. This work modifies the U-net architecture and exploits graph convolution to capture long-range information for dose prediction in esophageal cancer plans. Meanwhile, attention mechanism gets correlations between planning target volume (PTV) and organs at risk, and adaptively learns their feature weights. Finally, a novel loss function that considers features between voxel pairs is used to highlight the predictions. 152 subjects with prescription doses of 50 Gy or 60 Gy are collected in this study. The mean absolute error and standard deviation of conformity index, homogeneity index, and max dose for PTV achieved by the proposed method are 0.036 ± 0.030, 0.036 ± 0.027, and 0.930 ± 1.162, respectively, which outperform other state-of-the-art models. The superior performance demonstrates that our proposed method has great potential for AP generation.

Planning study: prone versus supine position for stereotactic body radiotherapy in prostate by CyberKnife.

This study aimed to clarify the differences in radiotherapy dose characteristics and delivery efficiency between the supine and prone positions in patients with prostate cancer using the CyberKnife. The planning computed tomography (CT) and delineations of the prone position were obtained by rotating the supine CT images with delineations of 180° using image processing software. The optimization parameters for planning target volume (PTV) and organs at risk (OARs) were based on the prone position. The optimization parameters determined for the prone position were applied to the supine position for optimization and dose calculation. The dosimetric characteristics of the PTV and OARs, and delivery efficiency were compared between the two different patient positions. The plans in the prone position resulted in better PTV conformity index (nCI), rectum V90%, V80%, V75%, V50% and bladder V50%. A significant difference was observed in treatment time and depth along the central axis (dCAX) between the two plans. The mean treatment time per fraction and dCAX for the supine and prone positions were 20.9 ± 1.7min versus 19.8 ± 1.3min (P = 0.019) and 151.1 ± 33.6mm versus 233.2 ± 8.8mm (P < 0.001), respectively. In this study the prone position was found to improve dosimetric characteristics and delivery efficiency compared with the supine position during prostate cancer treatment with the CyberKnife.

Three-dimensional conformal radiotherapy (3D-CRT) vs. volumetric modulated arc therapy (VMAT) in deep inspiration breath-hold (DIBH) technique in left-sided breast cancer patients-comparative analysis of dose distribution and estimation of projected secondary cancer risk.

The purpose of this study was to compare two techniques of irradiation of left-sided breast cancer patients who underwent breast-conserving surgery, three-dimensional conformal radiotherapy technique (3D-CRT) and volumetric modulated arc therapy (VMAT), in terms of dose distribution in the planning target volume (PTV) and organs at risk (OARs). The second aim of the study was estimation of the projected risk of radiation-induced secondary cancer for both radiotherapy techniques. For 25patients who underwent CT simulation in deep inspiration breath-hold (DIBH), three treatment plans were generated: one using athree-dimensional conformal radiotherapy technique and two using volumetric modulated arc therapy. First VMAT-DIBH geometry consisted of three partial arcs (ARC-DIBH 3A) and second consisted of four partial arcs (ARC-DIBH 4A). Cumulative dose-volume histograms (DVHs) were used to compare dose distributions within the PTV and OARs (heart, left anterior descending coronary artery [LAD], ipsilateral and contralateral lung [IL, CL], and contralateral breast [CB]). Normal tissue complication probabilities (NTCPs) and organ equivalent doses (OEDs) were calculated using the differential DVHs. Excess absolute risks (EARs) for second cancers were estimated using Schneider's full mechanistic dose-response model. All plans fulfilled the criterium for PTV V95% ≥ 95%. The PTV coverage, homogeneity, and conformity indices were significantly better for VMAT-DIBH. VMAT showed asignificantly increased mean dose and V5Gy for all OARs, but reduced LAD Dmax by 15 Gy. For IL, CL, and CB, the 3D-CRT DIBH method achieved the lowest values of EAR: 28.38 per 10,000PYs, 2.55 per 10,000PYs, and 4.48 per 10,000PYs (p < 0.001), compared to 40.29 per 10,000PYs, 15.62 per 10,000PYs, and 23.44 per 10,000PYs for ARC-DIBH 3A plans and 41.12 per 10,000PYs, 15.59 per 10,000PYs, and 22.73 per 10,000PYs for ARC-DIBH 4A plans. Both techniques provided negligibly low NTCPs for all OARs. The study shows that VMAT-DIBH provides better OAR sparing against high doses. However, the large low-dose-bath (≤ 5 Gy) is still aconcern due to the fact that alarger volume of normal tissues exposed to lower doses may increase aradiation-induced risk of secondary cancer.

A New Proton Therapy Solution Provides Superior Cardiac Sparing Compared With Photon Therapy in Whole Lung Irradiation for Pediatric Tumor Patients.

ObjectiveWhole lung irradiation (WLI) plays a crucial role in local control in pediatric patients with lung metastases and improves patient survival. The intention of this research was to explore the advantage of cardiac sparing between photons and protons during WLI. We also propose a new solution for cardiac sparing with proton techniques.MethodsEleven patients with pediatric tumors and pulmonary metastasis treated with 12 Gy WLI (all received volumetric-modulated arc therapy (VMAT)) in our institute between 2010 and 2019 were retrospectively selected. Each patient was replanned with intensity-modulated radiation therapy (IMRT), helical tomotherapy (HT), and two intensity-modulated proton radiotherapy (IMPT) plans (IMPT-1 and IMPT-2). IMPT-1 considered the whole lung as the planning target volume (PTV), utilizing the anteroposterior technique (0/180°). IMPT-2 was a new proton solution that we proposed in this research. This approach considered the unilateral lung as the PTV, and 3 ipsilateral fields were designed for each lung. Then, IMPT-2 was generated by summing two unilateral lung plans. The primary objective was to obtain adequate coverage (95% of the prescription dose to the PTV) while maximally sparing the dose to the heart. The PTV coverage, conformity index (CI), homogeneity index (HI), and dose–volume statistics of the heart and substructures were assessed by means of the averages of each comparison parameter.ResultsAll treatment techniques achieved the target volume coverage required by clinical practice. HT yielded the best coverage and homogeneity for the target structure compared with other techniques. The CI from IMRT was excellent. For photon radiation therapy, the HT plan afforded superior dose sparing for the V5, V6, V7, V8, and Dmean of the heart and Dmean of the right ventricle (RV). IMRT displayed the most notable dose reductions in the V9, V10, V11, and V12 of the heart and Dmean of the right atrium (RA). The VMAT plan was the least effective on the heart and substructures. However, compared with photon radiation therapy, IMPT-1 did not show an advantage for heart protection. Interestingly, IMPT-2 provided significant superiority in cardiac sparing, including maximum dose sparing for the V5, V6, V7, V8, V9 and Dmean of the heart and Dmean of the RA, RV, left atrium (LA) and left ventricle (LV) compared to all other techniques.ConclusionsConsidering the complex anatomical relation between target volumes and organs at risk (OARs), IMPT can provide a dose advantage for organs located outside of the target area rather than within or surrounding the area. It is hoped that advances in proton therapy (PT) plan design will lead to further improvements in radiotherapy approaches and provide the best treatment choice for individual patients.

Dosimetric Analysis of DVO and PO Algorithm in Pediatric Craniospinal Irradiation With Intensity-Modulated Radiotherapy

To investigate the dosimetric characteristics between dose volume optimization (DVO) algorithm and photon optimization (PO) algorithm in postoperative intensity-modulated radiotherapy (IMRT) of pediatric craniospinal irradiation, especially comparing the volume of cervical planning target volume (PTV) receiving lower than 95% of the prescription dose. Fifteen patients who received postoperative craniospinal irradiation with IMRT were enrolled in this study. After target areas and organs-at-risk were delineated, two kinds of IMRT plans, namely DVO plan and PO plan, were designed based on DVO algorithm and PO algorithm, separately. The dosimetric characteristics and optimization efficiency of the two kinds of treatment plans were evaluated. No significant difference was found in the D2%, D98%, homogeneity index and gradient index (GI) of PTV. However, compared with PO plan, the conformity index (CI) of PTV was better in DVO plan (p = 0.015, t = -2.442),the volume of cervical PTV receiving lower than 95% of the prescription dose is lower in DVO plan (p = 0.001, t = -3.28).The Dmax, V5 and V10 of left lung,Dmax,V5 and V10 of right lung, Dmax of right kidney and heart in DVO plan were lower than those in PO plan (p = 0.02,0.002,0.002,0.005,0.002,0.013,0.02,0.002),while Dmax of left kidney, liver, eyes and lens in PO plan were no significant difference. Compared with those of PO plan, the monitor units in DVO plan was increased by an average of 4.3%, and the optimization time was extended by an average of 248.6s. The application of DVO algorithm in pediatric craniospinal irradiation with IMRT can improve the target conformity, only increased few monitor units, but can increase the optimization time. Above all, the DVO algorithm can reduce the volume of cervical PTV receiving lower than 95% of the prescription dose, which can reduce the opportunity of tumor recurrence. So, we suggest the DVO algorithm in pediatric craniospinal irradiation with IMRT.

Evaluation of geometrical uncertainties on localized prostate radiotherapy of patients with bilateral metallic hip prostheses using 3D-CRT, IMRT and VMAT: A planning study.

Since most radiation treatment plans are based on computed tomography (CT) images, which makes it difficult to define the targeted tumor volume located near a metal implant, this study aims to evaluate and compare three treatment plans in order to optimally reduce geometrical uncertainty in external radiation treatment of localized prostate cancer. Experimental subjects were three prostate patients with bilateral hip prosthesis who had undergone radical radiotherapy. The treatment plans were five-field three-dimensional conformal radiation therapy (3D-CRT), fixed 5-field intensity-modulated radiation therapy (IMRT) using similar gantry angles, and single-arc volumetric modulated arc therapy (VMAT). The monitor units (MUs), dose volume histograms (DVHs), the dose indices of planning target volume (PTV), clinical target volume (CTV) and rectum were compared among the three techniques. The geometrical uncertainties were evaluated by shifting the iso-center (2- 10 mm in the anterior, posterior, left, right, superior, and inferior directions). The CTV and rectum dose indexes with and without the iso-center shifts were compared in each plan. The Conformity Index of PTV were 1.35 in 3D-CRT, 1.12 in IMRT, and 1.04 in VMAT, respectively. The rectum doses in 3D-CRT are also higher than those in IMRT and VMAT. The iso-center shift little affected the CTV dose when smaller than the margin size. The rectum dose increased especially after a posterior shift. Additionally, this dose increase was larger in the VMAT plan than in the 3D- CRT plan. However, the VMAT achieved a superior rectum DVH to that of 3D- CRT, and this effect clearly exceeded the rectum-dose increase elicited by the iso-center shift. For radiotherapy treatment of localized prostate cancer in patients with hip prosthesis, the dose distribution was better in the VMAT and Metal Artifact Reduction (MAR)-CT image methods than the conventional methods. Because the anatomical structure of the male pelvic region is relatively constant among individuals, we consider that VMAT is a valid treatment plan despite analyzing just three cases.

A retrospective evaluation of mixed energy volumetric modulated arc therapy for anal cancers with lymph node involvement

Applying dual, or mixed photon energies during radiation therapy is a common practice in 3-dimensional conformal radiation therapy (3D-CRT). Mixed photon energies are used to provide uniform dose coverage to a planning target volume (PTV) that ranges in depth from the skin surface. Though the application of mixed photon energies in 3D-CRT was once the convention for treating anal cancers with lymph node involvement (AC-LNI), the advantages offered by volumetric modulated arc therapy (VMAT) prove to be the optimal form of therapy for AC-LNI. Recently, multiple researchers have uncovered benefits in employing multiple photon energies in VMAT planning for prostate cancer. A retrospective study was completed to assess the impact of implementing mixed energy VMAT planning in comparison to conventional single energy VMAT planning for AC-LNI. Data from 20 patients with AC-LNI was collected to analyze the dosimetric effects of mixed energy VMAT treatments in terms of PTV conformity index, PTV homogeneity index, monitor unit usage, and organs at risk sparing. For each patient 3 treatment plans were created: a single energy 6 MV plan, a single energy 10 MV plan, and a mixed 6 MV and 10 MV energy plan. Analysis of the resulting dosimetric outcomes showed statistical significance. The current study concluded that mixed energy VMAT plans have some effect on treating AC-LNI when compared to single energy VMAT plans.

Dose distribution comparison in volumetric-modulated arc therapy plans for head and neck cancers with and without an external body contour extended technique.

This study compared volumetric-modulated arc therapy (VMAT) plans for head and neck cancers with and without an external body contour extended technique (EBCT). Dose calculation algorisms for VMAT have limitations in the buildup region. Three VMAT plans were enrolled, with one case having a metal artifact from an artificial tooth. The proper dose was calculated using Eclipse version 11.0. The body contours were extended 2 cm outward from the skin surface in three-dimensional space, and the dose was recalculated with an anisotropic analytical algorithm (AAA) and Acuros XB (AXB). Monitor units (MUs) were set, and the dose distributions in the planning target volume (PTV), clinical target volume, and organ at risk (OAR) and conformity index (CI) with and without an EBCT were compared. The influence of a metal artifact outside of the thermoplastic head mask was also compared. The coverage of PTV by the 95% dose line near the patient's skin was increased drastically by using an EBCT. Plan renormalization had a negligible impact on MUs and doses delivered to OARs. CI of PTV with a 6-MV photon beam was closer to 1 than that with a 10-MV photon beam when both AAA and AXB were used in all cases. Metal artifacts outside the head mask had no effect on dose distribution. An EBCT is needed to estimate the proper dose at object volumes near the patient's skin and can improve the accuracy of the calculated dose at target volumes.

Dosimetric comparison between intensity-modulated radiotherapy and volumetric-modulated arc therapy in hippocampus sparing in brain metastasis treated by whole-brain irradiation and simultaneous integrated boost

AbstractBackground:While treating brain metastasis with whole-brain radiotherapy incorporating a simultaneous integrated boost (WBRT-SIB), the risk of hippocampus injury is high. The aim of this study is to compare dosimetrically between intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) in sparing of hippocampus and organs at risk (OARs) and planning target volume (PTV) coverage.Methods:In total, 16 patients presenting with more than one brain metastases were previously treated and then retrospectively planned using VMAT and IMRT techniques. For each patient, a dual-arc VMAT and another IMRT (five beams) plans were created. For both techniques, 30 Gy in 10 fractions was prescribed to the whole brain (WB) minus the hippocampi and 45 Gy in 10 fractions to the tumour with 0·5 cm margin. Dose–volume histogram (DVH), conformity index (CI) and homogeneity index (HI) of PTV, hippocampus mean and maximum dose and other OARs for both techniques were calculated and compared.Results:A statistically significant advantage was found in WB-PTV CI and HI with VMAT, compared to IMRT. There were lower hippocampus mean and maximum doses in VMAT than IMRT. The maximum hippocampus dose ranged between 15·5 and 19·2 Gy and between 18·4 and 20·6 Gy in VMAT and IMRT, respectively. The mean dose of the hippocampus ranged between 11·5 and 17·7 Gy and between 13·2 and 18·3 Gy in VMAT and IMRT, respectively.Conclusion:Using WBRT-SIB technique, VMAT showed better PTV coverage with less mean and maximum doses to the hippocampus than IMRT. Clinical randomised studies are needed to confirm safety and clinical benefit of WBRT-SIB.

Dose optimization strategy of sacrum limitation in cervical cancer intensity modulation radiation therapy planning.

The aim of this study was to investigate the dose optimization strategy for the sacrum to reduce the risk of pelvic insufficiency fracture (PIF).Using a retrospective study design, we analyzed data from 28 patients with cervical cancer who underwent postoperative adjuvant radiotherapy in our department from June 2017 to January 2018. Among these patients, 20 (71.4%) underwent external beam radiation therapy in the pelvic lymphatic drainage area (node-negative patients). Overall, 8 patients (28.6%) underwent radiotherapy in the pelvic lymphatic drainage area with a simultaneous integrated boost (node-positive patients). Furthermore, 20 patients were assigned to 2 groups of plans according to the prescribed doses of 5000 and 4500 cGy/25. Each group had 3 plans according to 3 different dose limit conditions: “pelvic bones and sacrum unlimited,” “pelvic bones limited,” and “pelvic bones + sacrum limited.” The irradiation dose of the sacrum and pelvis was analyzed in three limited optimization models.The planning target volume conformity index and homogeneity index, based on different optimization modes in the 4500 and 5000 cGy plans, showed no significant differences. The D50% and Dmean of the pelvis + sacrum limited mode were significantly lower than those of the pelvic limited mode (P < .001). The dose of the sacrum and pelvis in the 4500 cGy plan in the lymphatic drainage area was significantly lower than that of the 5000 cGy plan (P < .001). In the lymph node boost group, the irradiation dose of the sacrum and pelvis was significantly increased (P ≤ .001).Increasing the limitation of the sacrum, on the basis of pelvic bone limitation, in cervical cancer intensity-modulated radiation therapy can significantly reduce the dose to the sacrum. Compared with the dose of 5000 cGy to the lymphatic drainage area, the dose of 4500 cGy was the largest influencing factor to reduce the dose to the sacrum.

Tangent-based volumetric modulated arc therapy for advanced left breast cancer

PurposeTo introduce the benefits of tangent-based volumetric modulated arc therapy (TVMAT), an innovative radiotherapy planning technique, compared with traditional volumetric modulated arc therapy (VMAT) for advanced left breast cancer needing nodal irradiation.Materials and methodsTwenty-three patients with advanced left breast cancer who had received modified radical mastectomy (MRM) and needed adjuvant radiotherapy including nodal irradiation were assessed. Among 23 radiotherapy treatment plans, 17 plans were designed by using TVMAT technique and 6 plans were designed by using traditional VMAT. The main difference of TVMAT from VMAT was that the area of avoidance sector within specific degrees of angle that had no monitor unit (MU) delivery was used in the arc planning, including a total of 5 sectors in 5 partial arcs. The dosimetries of planning target volume (PTV), right breast, bilateral lungs, and heart between TVMAT and VMAT were compared.ResultsThe conformity index (CI) and homogeneity index (HI) of PTV between two groups were statistically equivalent (CI: 0.98 ± 0.02 and 0.98 ± 0.03, P = 0.431; HI: 0.12 ± 0.03 and 0.11 ± 0.05, P = 0.177), which indicated that the treatment efficacy of the plans regarding TVMAT was compatible with VMAT. However, all neighboring organs at risk (OAR) showed a great percentage of reduction in mean doses (right breast: 53.1%, right lung: 37.7%, left lung: 8.8%, heart: 21.2%) and low dose parameters (V10: right breast: 72.3%, right lung: 86.1%, left lung: 12.5%, heart: 25.1%; V5: right breast: 56.5%, right lung: 28.3%, left lung: 12.7%, heart: 18.2%) by using TVMAT.ConclusionTVMAT greatly decreases the radiation doses delivered to the OAR with maintained therapeutic efficacy. It is highly recommended for treating breast cancer, especially for difficult cases with left side disease needing nodal irradiation.

Demonstration of simulated annealing optimization for permanent breast seed implant treatment planning

PurposePermanent breast seed implant (PBSI) is a developing brachytherapy technique for the treatment of early-stage breast cancer. In current practice, PBSI uses manual planning strategies to generate clinical treatment plans. In this work, a simulated annealing-based algorithm is developed to demonstrate the first application of inverse optimization for PBSI. Methods and MaterialsTarget, skin, and chest wall muscle contours, exported from a treatment planning system in digital imaging and communications in medicine format, are used as inputs. To optimize, the user defines the dose–volume histogram objectives for the target and specifies a relative weighting for target and skin constraints. A 10-patient cohort of previously treated patients was planned by using the inverse optimization algorithm. Plan quality was compared to the clinically treated manually generated plans using the V90%, V100%, V150%, and V200% for the planning target volume (PTV), V90% and D0.2 cc for skin dose, and PTV conformity indices. ResultsFor each of the 10 patients, patient-wise paired differences between inverse and manual plans were analyzed and presented in box plots. Comparing inverse and manual planning techniques, a statistical difference was not observed (p > 0.05) in PTV coverage criteria (V90%, V100%) and dose to skin2mm. A statistical difference was observed in the inverse plans as a reduction of the V150% (mean of 6.2%) and increase in conformity index of the 20%, 50%, 90%, and 100% isodose lines. ConclusionsThis work presents the first application of inverse optimization used to generate PBSI treatment plans. A 10-patient cohort previously treated with PBSI was retrospectively planned for comparison with the clinically treated manually generated plans.

Dosimetric Comparison of Treatment Plans Using Physical Wedge and Enhanced Dynamic Wedge for the Planning of Breast Radiotherapy.

The aim of this study is to compare the physical wedge (PW) with enhanced dynamic wedge (EDW) to determine the difference in the dose distribution affecting the treated breast and the contralateral breast, lungs, heart, esophagus, spine, and surrounding skin in the radiotherapy of breast cancer. Computed tomography (CT) data sets of 30 breast cancer patients were selected from the database for the study. The treatment plans which were executed with PW were re-planned with EDW without changing the beam parameters. Keeping the wedge angles same, the analytic anisotropic algorithm (AAA) with heterogeneity correction was used for dose calculation in all plans. The prescription was 50 Gy in 25 fractions. The dose- volume histogram (DVH) of the planning target volume (PTV) and critical structures of both PW and EDW plans were analyzed. The analysis showed that the maximum dose within the target volume is higher in EDW plan compared to PW plan. However the PTV conformity index (CI) remained the same in both plans. For all the critical structures, the EDW technique offered less dose compared to PW technique. The effect of volume of the contralateral breast on the dose to contralateral breast and the effect of volume of PTV breast for patients with carcinoma left breast on the dose to heart were studied and analyzed for the two wedges. No correlation between volumes and dose parameters was found for the two techniques. The number of monitor units to deliver a particular dose with EDW field is less than that of PW field due to change in wedge factor. As EDW produces less scattered dose to structures outside the treatment field, the risk of a second malignancy can be reduced with this technique.

Dose-volume and radiobiological dependence on the calculation grid size in prostate VMAT planning

This study evaluated the effects of dose-volume and radiobiological dependence on the calculation grid size in prostate volumetric-modulated arc therapy (VMAT) planning. Ten patients with prostate cancer were selected for this retrospective treatment planning study. Prostate VMAT plans were created for the patients using the 6 MV photon beam produced by a Varian TrueBEAM linac with the calculation grid size equal to 1, 2, 2.5, 3, 4, and 5 mm. Dose-volume histograms (DVHs) of targets and organs at risk were generated for different grid sizes. We calculated the radiobiological parameters of the tumor control probability (TCP) of clinical target volume (CTV) and planning target volume (PTV), and the normal tissue complication probability (NTCP) of organs at risk (rectal wall, rectum, bladder wall, bladder, left femur, and right femur). The homogeneity, conformity, and gradient indexes of CTV and PTV were calculated for different grid sizes. The TCP of PTV was found decreasing with a rate of 0.06%/mm when the calculation grid size increased from 1 to 5 mm. On the other hand, both NTCPs of rectal wall and rectum were found decreasing with rates of 0.03%/mm and 0.05%/mm, respectively, with an increase of grid size. The homogeneity index of PTV increased with a rate of 0.57/mm of the calculation grid size, whereas the conformity index of PTV decreased with a rate of 0.0075/mm. The gradient index of PTV was found increasing with a rate equal to 0.05/mm. In prostate VMAT planning, variations of dose-volume and radiobiological parameters with calculation grid size on PTV, rectal wall, and rectum were more significant than those of CTV and other organs at risk such as bladder wall, bladder, and femurs. Results in this study are important in the treatment planning quality assurance when the calculation grid size is varied to compromise a shorter dose computing time.

Dosimetric comparison between automatic three-dimensional conformal radiotherapy, inverse three-dimensional conformal radiotherapy, and inverse intensity-modulated radiotherapy

Objective To compare dosimetric parameters between automatic three-dimensional conformal radiotherapy (a3DCRT), inverse three-dimensional conformal radiotherapy (i3DRT), inverse intensity-modulated radiotherapy (iIMRT). Methods Ten lung cancer patients with a single target and 10 intracranial tumor patients also with a single target, who were treated in our radiotherapy center from 2014 to 2015, were included in the study. Their image data were transferred to RayStation 4.5 via network, and then the treatment plans for a3DCRT, i3DCRT, and iIMRT were designed for the 20 patients. The dosimetric parameters for planning target volume (PTV) and organs at risk (OAR) were compared between the three plans using multiple dependent variables and two dependent variables. Results For lung cancer patients, iIMRT achieved better results in D98%, D50%, D2%, conformity index (CI), and homogeneity index (HI) than i3DCRT and a3DCRT (P=0.007, 0.001, 0.002, 0.000, and 0.000), and the CI of a3DCRT was superior to that of i3DCRT (P=0.000); there were no significant differences in heart D33, spinal cord Dmax and D1 cm3, and both lungs between the three plans (P=0.702, 0.237, 0.163, 0.739, 0.908, 0.832, 0.886, 0.722, 0.429, 0.840, and 0.702). For intracranial tumor patients, there were no significant differences in dosimetric parameters between the three plans, except that the CI of iIMRT and a3DCRT was superior to that of i3DCRT (P=0.648, 0.783, 0.256, 0.931, 0.002, and 0.034); there were no significant differences in whole brain irradiation dose between the three plans (P=0.446, 0.755, 0.772, 0.0266, 0.440, 0.290, and 0.939). Conclusions For the single target in patients with lung cancer and intracranial tumors, a3DCRT can improve the CI of PTV compared with i3DCRT, and shows no significant dosimetric disadvantage for OARs compared with iIMRT. Considering the simplicity and low cost of 3DCRT, a3DCRT holds promise as a novel radiotherapy technique. Key words: Beamset’s orientation auto-optimization; Inverse three-dementional conformal radiotherapy; Automatic three-dementional conformal radiotherapy; Inverse intensity-modulated radiotherapy

Immune priming by stereotactic body radiation: Dosimetric characteristics.

85 Background: A systemic immune response to tumors after stereotactic body radiation therapy (SBRT) has been observed. The role of radiation dosimetry variables in priming an immune response is not well characterized. Methods: We analyzed radiation dosimetry variables in a patient with metastatic melanoma receiving pembrolizumab for about 1 year prior to concurrent use with Cyberknife SBRT for a symptomatic lesion in the right lower lobe of the lung. The patient received 50 Gy in 5 fractions prescribed to the 74% isodose line (using a Monte Carlo dose calculation) and developed a systemic immune response three months after this intervention. Results: We observed priming of the immune system as man ifested by radiological and clinical responses in non-irradiated cervical lymph nodes and a new persistent FDG-avid lesion on PET scan located in the base of the tongue. Given the concern for a new malignancy, the patient underwent hemi tongue base glossectomy; interestingly, this surgical specimen showed tonsillar mucosa with reactive lymphoid hyperplasia without evidence of tumor. Upon review of the CyberKnife SBRT plan, we identified that the planning target volume (PTV) was atypically under covered to 89.9% and also had a relatively high value of gross tumor volume (GTV) conformity index (CI) of 2.1. In our experience with CyberKnife plans for lung lesions, usually the PTV coverage is more than 95% and the GTV CI is less than 1.5. Other dosimetry variables had more typical values, including GTV coverage of 99.9%, PTV CI of 1.1, and both GTV and PTV homogeneity index of 1.4. Conclusions: Relatively low PTV coverage and high GTV CI were associated with immune system priming in a well-documented case of systemic responses in non-irradiated pathological and normal tissues, in the setting of concurrent immunotherapy and SBRT. Further studies are warranted to confirm this association.

A treatment planning study of proton arc therapy for para-aortic lymph node tumors: dosimetric evaluation of conventional proton therapy, proton arc therapy, and intensity modulated radiotherapy.

BackgroundThe purpose of this study is to evaluate the dosimetric benefits of a proton arc technique for treating tumors of the para-aortic lymph nodes (PALN).MethodIn nine patients, a proton arc therapy (PAT) technique was compared with intensity modulated radiation therapy (IMRT) and proton beam therapy (PBT) techniques with respect to the planning target volume (PTV) and organs at risk (OAR). PTV coverage, conformity index (CI), homogeneity index (HI) and OAR doses were compared. Organ-specific radiation induced cancer risks were estimated by applying organ equivalent dose (OED) and normal tissue complication probability (NTCP).ResultsThe PAT techniques showed better PTV coverage than IMRT and PBT plans. The CI obtained with PAT was 1.19 ± 0.02, which was significantly better than that for the IMRT techniques. The HI was lowest for the PAT plan and highest for IMRT. The dose to the OARs was always below the acceptable limits and comparable for all three techniques. OED results calculated based on a plateau dose–response model showed that the risk of secondary cancers in organs was much higher when IMRT or PBT were employed than when PAT was used. NTCPs of PAT to the stomach (0.29 %), small bowel (0.69 %) and liver (0.38 %) were substantially lower than those of IMRT and PBT.ConclusionThis study demonstrates that there is a potential role for PAT as a commercialized instrument in the future to proton therapy.

SU‐F‐T‐448: Use of Mixed Photon Energy Beam in Volumetric Modulated Arc Therapy (VMAT) Treatment Plan for Prostate Cancer

Purpose:To study the impact of different photon beam combination during VMAT planning and treatment delivery.Methods:Five prostate patients with no nodal involvement were chosen for the study and only prostate was considered as target (7920cGy/44fractions). In each case, three different VMAT plans were generated with two arcs (200°–160°&amp;160°–200°). First plan used only 6MV in both arcs (6X‐6X) and second utilized 6MV&amp;15MV (6X‐15X), whereas third one used 15MV&amp;15MV (15X‐15X). For consistency, all the plans were generated by the same planner using Monaco− treatment planning system (V5.1) for Elekta Synergy− linear accelerator with 1cm leaf‐width. For plan comparison, target mean dose, conformity index (CI)=Planning target volume (PTV) covered by 95% of prescription dose/PTV were analyzed. Mean doses of bladder, rectum, left femur and right femur were analyzed. Integral dose (liter‐Gray) to normal tissue (patient volume minus PTV), total monitor unit (MU) required to deliver a plan and gamma pass rate for each plan was analyzed.Results:The CI for PTV was 0.9937±0.0037, 0.9917±0.0033, and 0.9897±0.0048 for 6X‐6X, 6X‐15X and 15X‐15X, respectively. Mean dose to target slightly increases with the decrease of energy. Mean doses to bladder were 3546.23±692.13cGy, 3487.43±715.53cGy and 3504.40±683.1cGy for 6X‐6X, 6X‐15X and 15X‐15X, respectively. Mean doses to rectum were 4294.60±309.5cGy, 4277.07±279.93cGy and 4290.77±379.07cGy. Mean doses to left femur were 2737.13±545.93cGy, 2668.67±407.12cGy and 2416.77±300.73cGy and mean doses to the right femur were 2682.70±460.81cGy, 2722.58±541.92cGy and 2598.57±481.83cGy. Higher Integral doses to normal tissue observed for 6X‐6X (163.06±24.6 Litre‐Gray) followed by 6X‐15X (154.35±24.74 Litre‐Gray) and 15X‐15X (145.84±26.03 Litre‐Gray). Average MU required to deliver one fraction was 680.75±72.09, 634.81±95.07 and 605.06±114.65. Gamma pass rates were 99.83±0.21, 99.53±0.27 and 99.2±0.20.Conclusion:6X‐15X VMAT plan offer dosimetric advantage compared to 6X‐6X in terms of lesser MU and integral dose without significant compromise in plan quality, where as in 15X‐15X, neutron contamination risk is relatively higher.

SU‐F‐T‐391: Comparative Study of Treatment Planning Between IMRT and IMAT for Malignant Pleural Mesothelioma

Purpose:The purpose of this study was to compare the dosimetric differences between intensitymodulated radiation therapy (IMRT) and intensity modulated arc therapy (IMAT) for malignant pleural mesothelioma (MPM) patients with regard to the sparing effect on organs at risk (OARs), plan quality, and delivery efficiency.Methods:Ten MPM patients were recruited in this study. To avoid the inter‐operator variability, IMRT and IMAT plans for each patient were performed by one experienced dosimetrist. The treatment planning optimization process was carried out using the Eclipse 13.0 software. For a fair comparison, the planning target volume (PTV) coverage of the two plans was normalized to the same level. The treatment plans were evaluated on the following dosimetric variables: conformity index (CI) and homogeneity index (HI) for PTV, OARs dose, and the delivery efficiency for each plan.Results:All plans satisfied clinical requirements. The IMAT plans gained better CI and HI. The IMRT plans performed better sparing for heart and lung. Less MUs and control points were found in the IMAT plans. IMAT shortened delivery time compared with IMRT.Conclusion:For MPM, IMAT gains better conformity and homogeneity for PTV with IMRT, but increases the irradiation dose for OARs. IMAT shows an advantage in delivery efficiency.

Implementation of Constant Dose Rate and Constant Angular Spacing Intensity-modulated Arc Therapy for Cervical Cancer by Using a Conventional Linear Accelerator.

Background:Volumetric-modulated arc therapy (VMAT) can only be implemented on the new generation linacs such as the Varian Trilogy® and Elekta Synergy®. This prevents most existing linacs from delivering VMAT. The purpose of this study was to investigate the feasibility of using a conventional linear accelerator delivering constant dose rate and constant angular spacing intensity-modulated arc therapy (CDR-CAS-IMAT) for treating cervical cancer.Methods:Twenty patients with cervical cancer previously treated with intensity-modulated radiation therapy (IMRT) using Varian Clinical 23EX were retreated using CDR-CAS-IMAT. The planning target volume (PTV) was set as 50.4 Gy in 28 fractions. Plans were evaluated based on the ability to meet the dose volume histogram. The homogeneity index (HI), target volume conformity index (CI), the dose to organs at risk, radiation delivery time, and monitor units (MUs) were also compared. The paired t-test was used to analyze the two data sets. All statistical analyses were performed using SPSS 19.0 software.Results:Compared to the IMRT group, the CDR-CAS-IMAT group showed better PTV CI (0.85 ± 0.03 vs. 0.81 ± 0.03, P = 0.001), clinical target volume CI (0.46 ± 0.05 vs. 0.43 ± 0.05, P = 0.001), HI (0.09±0.02 vs. 0.11 ± 0.02, P = 0.005) and D95 (5196.33 ± 28.24 cGy vs. 5162.63 ± 31.12 cGy, P = 0.000), and cord D2 (3743.8 ± 118.7 cGy vs. 3806.2 ± 98.7 cGy, P = 0.017) and rectum V40 (41.9 ± 6.1% vs. 44.2 ± 4.8%, P = 0.026). Treatment time (422.7 ± 46.7 s vs. 84.6 ± 7.8 s, P = 0.000) and the total plan Mus (927.4 ± 79.1 vs. 787.5 ± 78.5, P = 0.000) decreased by a factor of 0.8 and 0.15, respectively. The IMRT group plans were superior to the CDR-CAS-IMAT group plans considering decreasing bladder V50 (17.4 ± 4.5% vs. 16.6 ± 4.2%, P = 0.049), bowel V30 (39.6 ± 6.5% vs. 36.6 ± 7.5%, P = 0.008), and low-dose irradiation volume; there were no significant differences in other statistical indexes.Conclusions:Patients with cervical cancer treated with CDR-CAS-IMAT using Varian Clinical 23EX can get equivalent or superior dose distribution compared to those treated with IMRT. CDR-CAS-IMAT has a less treatment time and MU, which can reduce the uncertainty factor and patient discomfort in treatment.