Critical Structure Sparing Research Articles

Conformal orbit sparing radiation therapy: a treatment option for advanced skin cancer of the parotid and ear region

IntroductionNew surgical methods have enabled resection of previously in‐operable tumours in the region of the parotid gland and ear. This has translated to deeper target volumes being treated with adjuvant radiotherapy. Due to the limitations of existing conformal techniques, alternative planning approaches are required to cover the target volume with appropriate sparing of adjacent critical structures. Although intensity modulated radiation therapy (IMRT) may be able to achieve these goals compared with the existing conformal method, a new orbital sparing radiation therapy (OSRaT) technique was evaluated as an alternative conformal planning process. The study objective was to evaluate the dosimetry of three planning methods: pre‐existing conformal, IMRT and OSRaT techniques.MethodsTen patients were planned retrospectively using the existing three‐dimensional conformal radiotherapy (3DCRT), IMRT and OSRaT techniques. Dosimetry was analysed using the homogeneity index (HI), conformity index (CI), the volume of planning target volumes (PTV) under and over treated by the 95% isodose and dose to critical structures.Results OSRaT achieved superior 95% coverage of the high‐dose PTV while delivering HI similar to IMRT for intermediate and high‐dose PTVs. The CI for the high‐dose PTV was comparable between the three techniques, however IMRT was statistically better for the low‐ and intermediate dose PTVs. All three techniques showed adequate orbital sparing, however OSRaT and IMRT achieved this with less under dosing of the PTVs.ConclusionFor the treatment of patients with advanced skin cancer of the parotid and ear, both IMRT and the OSRaT techniques are viable options.

A novel software and conceptual design of the hardware platform for intensity modulated radiation therapy.

To deliver high quality intensity modulated radiotherapy (IMRT) using a novel generalized sparse orthogonal collimators (SOCs), the authors introduce a novel direct aperture optimization (DAO) approach based on discrete rectangular representation. A total of seven patients-two glioblastoma multiforme, three head & neck (including one with three prescription doses), and two lung-were included. 20 noncoplanar beams were selected using a column generation and pricing optimization method. The SOC is a generalized conventional orthogonal collimators with N leaves in each collimator bank, where N = 1, 2, or 4. SOC degenerates to conventional jaws when N = 1. For SOC-based IMRT, rectangular aperture optimization (RAO) was performed to optimize the fluence maps using rectangular representation, producing fluence maps that can be directly converted into a set of deliverable rectangular apertures. In order to optimize the dose distribution and minimize the number of apertures used, the overall objective was formulated to incorporate an L2 penalty reflecting the difference between the prescription and the projected doses, and an L1 sparsity regularization term to encourage a low number of nonzero rectangular basis coefficients. The optimization problem was solved using the Chambolle-Pock algorithm, a first-order primal-dual algorithm. Performance of RAO was compared to conventional two-step IMRT optimization including fluence map optimization and direct stratification for multileaf collimator (MLC) segmentation (DMS) using the same number of segments. For the RAO plans, segment travel time for SOC delivery was evaluated for the N = 1, N = 2, and N = 4 SOC designs to characterize the improvement in delivery efficiency as a function of N. Comparable PTV dose homogeneity and coverage were observed between the RAO and the DMS plans. The RAO plans were slightly superior to the DMS plans in sparing critical structures. On average, the maximum and mean critical organ doses were reduced by 1.94% and 1.44% of the prescription dose. The average number of delivery segments was 12.68 segments per beam for both the RAO and DMS plans. The N = 2 and N = 4 SOC designs were, on average, 1.56 and 1.80 times more efficient than the N = 1 SOC design to deliver. The mean aperture size produced by the RAO plans was 3.9 times larger than that of the DMS plans. The DAO and dose domain optimization approach enabled high quality IMRT plans using a low-complexity collimator setup. The dosimetric quality is comparable or slightly superior to conventional MLC-based IMRT plans using the same number of delivery segments. The SOC IMRT delivery efficiency can be significantly improved by increasing the leaf numbers, but the number is still significantly lower than the number of leaves in a typical MLC.

FEASIBILITY FOR USING HYPOFRACTIONATED STEREOTACTIC VOLUMETRIC MODULATED ARC RADIOTHERAPY (VMAT) WITH ADAPTIVE PLANNING FOR TREATMENT OF THYMOMA IN RABBITS: 15 CASES.

Thymoma is a relatively common tumor in rabbits. Treatment with surgery, radiation therapy, and chemotherapy alone or in combination has been reported with varying outcomes. Stereotactic volumetric modulated arc radiotherapy delivered in a hypofractionated manner allows high doses of radiation to be delivered to the target volume while allowing sparing of adjacent critical structures. This therapy is ideally suited for thymomas in rabbits given their size, the difficulty of multiple anesthesia episodes and the complexity of the radiotherapy plans required due to the tumor's proximity to the heart, lungs, and mediastinal structures. Fifteen rabbits with thymoma were prospectively recruited for this observational, single institution, single arm clinical study. All rabbits were imaged with both computed tomography (CT) and magnetic resonance imaging (MRI). A total dose of 40 Gy in six fractions was delivered using a single arc over an 11-day period with repeat CT simulation done every other fraction for adaptive planning. Follow-up evaluation was done through repeat CT and MRI imaging and revealed complete responses using the Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Two rabbits had died at 618 and 718 days, 10 were alive and three were lost to follow-up. Observed acute and late effects were graded according to the Veterinary Radiation Therapy Oncology Group (VRTOG) criteria and were found to be minimal.

Treatment optimization with concurrent SBRT and intracavitary brachytherapy for locally advanced cervical cancer.

This work is aimed at investigating treatment planning strategies to optimally combine stereotactic body radiation therapy (SBRT) with intracavitary brachytherapy (ICBT) for the treatment of locally advanced cervical cancer. Forty patients (stage IIB – IIIB) previously treated with combined SBRT and ICBT were randomly selected for this retrospective study. All patients were CT‐ and MR‐scanned with a ring applicator in situ. HR‐CTV and OARs were contoured according to fused CT and MR images. Several ICBT plans were generated for each patient based on different dose prescription points, and then a matching SBRT plan was generated for each ICBT plan. The dose distribution of each composite plan was analyzed with a focus on the doses received by 90% and 100% of the target volume (D90 and D100), the target volume receiving 100% of the prescription dose (V100%), and the doses received by 2 cc and 40% of the OARs (D2cc and D40). As the distance, d, between the prescription point and the tandem varied within 1.0 and 1.9 cm, the D90, D100 and V100% for the target, as well as D2cc and D40 for the bladder and rectum approached their optimal values for d value between 1.0 and 1.4 cm. When designing a combined ICBT+SBRT plan, one should measure the size of the cervix and set the prescription isodose line 1.0 to 1.4 cm away from the tandem for the ICBT plan first and then optimize the SBRT plan based on the ICBT dose distribution to achieve the best target coverage and critical structures sparing.PACS number: 87.53.jw; 87.55.D‐

Toxicity Profile of IMRT Vs. 3D-CRT in Head and Neck Cancer: A Retrospective Study.

Role of radiotherapy in comprehensive management of head and neck cancer for achieving tumour control and organ preservation is now well established and radiotherapy is routinely used in adjuvant setting after surgery, concurrently with chemotherapy or targeted agents and for palliation. Development of linear accelerator with Multileaf Collimator (MLC) have revolutionized radiation delivery techniques, allowing conformal and Intensity Modulated Radiotherapy (IMRT) to deliver highly conformal sculpted radiation dose to a very complex structure with improved sparing of adjoining critical structures like salivary glands, spinal cord, eyes, brainstem and larynx amounting to better therapeutic gain. This retrospective study was to compare toxicity profile of IMRT with Three Dimensional Conformal Radiotherapy (3D CRT) in head and neck cancer. Total of 80 patients from January 2013 to July 2015 with proven head and neck cancer who underwent radiotherapy on linac 2300 C/D machine were included in the study, IMRT group and 3D-CRT group comprised of 40 patients each. We have searched patient's radiotherapy details in record section of our institute and observations were noted down. Patients received 70Gy/35 fractions, Monday to Friday as radical treatment and 60 Gy/30 fractions as adjuvant treatment were included. The 3D-CRT group demonstrated significantly more acute toxic effects compared with the IMRT group in our analysis. Acute Grade 3 or greater toxic effects to the skin occurred in 5 of 40 (12.5%), patients in the 3D-CRT group compared with 3 of 40 (7.5%) patients in the IMRT group. Acute Grade 3 or greater toxic effects to the mucous membranes occurred in 23 of 40 (57.5%) patients in the 3D-CRT group and only 16 of 40 (40%) patients in the IMRT group. Statistically significant dysphagia developed in 34 of 40 (85%), patients in 3D-CRT group compared with 23 of 40 (57.5%) patients in IMRT group, while statistically significant xerostomia developed in 29 of 40 patients in 3D-CRT group (72.5%), compared with18 of 40 (45%) patients in IMRT group. In our analysis, IMRT was associated with a significantly lower incidence of Grade 3 or greater xerostomia, acute toxic effects to skin and mucous membranes than 3D-CRT. In addition, compared to 3D-CRT, IMRT had lower rates of Grade 3 or greater mucositis and skin toxicity as well as less feeding tube use during radiotherapy. Our analysis showed potentially less toxicity in patients treated with IMRT in comparision to 3D-CRT.

Use of Emission Guided Radiation Therapy Can Better Spare Critical Structures Compared With Intensity Modulated Radiation Therapy, Volumetric Modulated Arc Therapy, or Proton Therapy

Use of Emission Guided Radiation Therapy Can Better Spare Critical Structures Compared With Intensity Modulated Radiation Therapy, Volumetric Modulated Arc Therapy, or Proton Therapy

A Monte Carlo model for independent dose verification in IMRT and VMAT for the Varian Novalis TX with high definition MLC

Purpose: With intensity modulated radiation therapy (IMRT), the physician can prescribe, design and deliver optimized treatment plans that target the tumor and spare adjacent critical structures. The increased conformity of such plans often comes at the expenses of adding significant complexity to the delivery of the treatment. With volumetrically modulated arc therapy (VMAT), in addition to the modulation of the intensity of the radiation beam, other mechanical parameters such as gantry speed and dose rate are varied during treatment delivery. It is therefore imperative that we develop comprehensive and accurate methods to validate such complex delivery techniques prior to the commencement of the patient’s treatment. Methods : In this study, a Monte Carlo simulation was performed for the high definition multileaf collimator (HD-MLC) of a Varian Novalis TX linac. Our simulation is based on the MCSIM code and provides a comprehensive model of the linac head. After validating the model in reference geometries, treatment plans for different anatomical sites were simulated and compared against the treatment planning system (TPS) dose calculations. All simulations were performed in a cylindrical water phantom as opposed to the patient anatomy, to remove any complexities associated with density effects. Finally, a comparison through gamma analysis of dose plane between the simulation, the TPS and the measurements from the Matrixx array (IBA) was conducted to verify the accuracy of our model against both the measurements and the TPS. Results : Gamma analysis of ten IMRT and ten VMAT cases for different anatomical sites was performed, using a 3%/3 mm passing criterion. The average passing rates were 97.5% and 94.3% for the IMRT and the VMAT plans respectively when comparing the MCSIM and TPS dose calculations. Conclusion : In the present work a Monte Carlo model of a Novalis TX linac which has been tested and benchmarked to produce phase-space files for the treatment head of the linac was used to produce a input phase-space to calculated dose deposition phenomena in different geometries for IMRT and VMAT treatment modalities. The control points defined for the MLC were replaced by blocks with the same characteristics and materials of the linac MLC to speed up the simulation time. With this technique a simulation of a typical IMRT case can be performed with a 10 computer cluster in about 1.02 hours in average. If the number of computer used is increased the computing time can be reduced even more which make our model suitable for clinical use as a second check method to compare the TPS dose calculated. Our results showed that for IMRT and VMAT deliveries with a HD-MLC, there is an average of 95.9% of the points have a gamma index less than 1 with our chosen criterion between our Monte Carlo simulations and the corresponding measurements and TPS calculations in a cylindrical water equivalent phantom. This Monte Carlo code can be used as pre-treatment, independent dose calculation verification for IMRT and VMAT deliveries.

SU‐E‐T‐587: Optimal Volumetric Modulated Arc Radiotherapy Treatment Planning Technique for Multiple Brain Metastases with Increasing Number of Arcs

Purpose:To show improvements in dose conformity and normal brain tissue sparing using an optimal planning technique (OPT) against clinically acceptable planning technique (CAP) in the treatment of multiple brain metastases.Methods:A standardized international benchmark case with12 intracranial tumors was planned using two different VMAT optimization methods. Plans were split into four groups with 3, 6, 9, and 12 targets each planned with 3, 5, and 7 arcs using Eclipse TPS. The beam geometries were 1 full coplanar and half non‐coplanar arcs. A prescription dose of 20Gy was used for all targets. The following optimization criteria was used (OPT vs. CAP): (No upper limit vs.108% upper limit for target volume), (priority 140–150 vs. 75–85 for normal‐brain‐tissue), and (selection of automatic sparing Normal‐Tissue‐Objective (NTO) vs. Manual NTO). Both had priority 50 to critical structures such as brainstem and optic‐chiasm, and both had an NTO priority 150. Normal‐brain‐tissue doses along with Paddick Conformity Index (PCI) were evaluated.Results:In all cases PCI was higher for OPT plans. The average PCI (OPT,CAP) for all targets was (0.81,0.64), (0.81,0.63), (0.79,0.57), and (0.72,0.55) for 3, 6, 9, and 12 target plans respectively. The percent decrease in normal brain tissue volume (OPT/CAP*100) achieved by OPT plans was (reported as follows: V4, V8, V12, V16, V20) (184, 343, 350, 294, 371%), (192, 417, 380, 299, 360%), and (235, 390, 299, 281, 502%) for the 3, 5, 7 arc 12 target plans, respectively. The maximum brainstem dose decreased for the OPT plan by 4.93, 4.89, and 5.30 Gy for 3, 5, 7 arc 12 target plans, respectively.Conclusion:Substantial increases in PCI, critical structure sparing, and decreases in normal brain tissue dose were achieved by eliminating upper limits from optimization, using automatic sparing of normal tissue function with high priority, and a high priority to normal brain tissue.

SU-E-T-256: Development of a Monte Carlo-Based Dose-Calculation System in a Cloud Environment for IMRT and VMAT Dosimetric Verification

Purpose: Intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) are techniques that are widely used for treating cancer due to better target coverage and critical structure sparing. The increasing complexity of IMRT and VMAT plans leads to decreases in dose calculation accuracy. Monte Carlo simulations are the most accurate method for the determination of dose distributions in patients. However, the simulation settings for modeling an accurate treatment head are very complex and time consuming. The purpose of this work is to report our implementation of a simple Monte Carlo simulation system in a cloud-computing environment for dosimetric verification of IMRT and VMAT plans. Methods: Monte Carlo simulations of a Varian Clinac linear accelerator were performed using the BEAMnrc code, and dose distributions were calculated using the DOSXYZnrc code. Input files for the simulations were automatically generated from DICOM RT files by the developed web application. We therefore must only upload the DICOM RT files through the web interface, and the simulations are run in the cloud. The calculated dose distributions were exported to RT Dose files that can be downloaded through the web interface. The accuracy of the calculated dose distribution was verified by dose measurements. Results: IMRT and VMAT simulations were performed and good agreement results were observed for measured and MC dose comparison. Gamma analysis with a 3% dose and 3 mm DTA criteria shows a mean gamma index value of 95% for the studied cases. Conclusion: A Monte Carlo-based dose calculation system has been successfully implemented in a cloud environment. The developed system can be used for independent dose verification of IMRT and VMAT plans in routine clinical practice. The system will also be helpful for improving accuracy in beam modeling and dose calculation in treatment planning systems. This work was supported by JSPS KAKENHI Grant Number 25861057.

SU‐E‐T‐170: Characterization of the Location, Extent, and Proximity to Critical Structures of Target Volumes Provides Detail for Improved Outcome Predictions Among Pancreatic Cancer Patients

Purpose:In radiotherapy, size, location and proximity of the target to critical structures influence treatment decisions. It has been shown that proximity of the target predicts dosimetric sparing of critical structures. In addition to dosimetry, precise location of disease has further implications such as tumor invasion, or proximity to major arteries that inhibit surgery. Knowledge of which patients can be converted to surgical candidates by radiation may have high impact on future treat/no‐treat decisions. We propose a method to improve our characterization of the location of pancreatic cancer and treatment volume extent with respect to nearby arteries with the goal of developing features to improve clinical predictions and decisions.Methods:Oncospace is a local learning health system that systematically captures clinical outcomes and all aspects of radiotherapy treatment plans, including overlap volume histograms (OVH) – a measure of spatial relationships between two structures. Minimum and maximum distances of PTV and OARs based on OVH, PTV volume, anatomic location by ICD‐9 code, and surgical outcome were queried. Normalized distance to center from the left and right kidney was calculated to indicate tumor location and laterality. Distance to critical arteries (celiac, superior mesenteric, common hepatic) is validated by surgical status (borderline resectable, locally advanced converted to resectable).Results:There were 205 pancreas stereotactic body radiotherapy patients treated from 2009–2015 queried. Location/laterality of tumor based on kidney OVH show strong trends between location by OVH and by ICD‐9. Compared to the locally advanced group, the borderline resectable group showed larger geometrical distance from critical arteries (p=0.03).Conclusion:Our platform enabled analysis of shape/size‐location relationships. These data suggest that PTV volume and attention to distance between PTVs and surrounding OARs and major arteries may be promising for improving characterization of treatment anatomy that can refine our ability for outcome predictions and decision making.Elekta, Toshiba

SU-E-T-262: Development of An In-House Forward Planning System for Conformal Arc Therapy

Purpose: Conformal radiotherapy with forward planning has many practical advantages (e.g., more uniform target dose) and has been used routinely for stereotactic body radiation therapy (SBRT) and other conventional treatment. This study investigates a forward-planning approach for conformal arc therapy and a dedicated treatment planning system (TPS) for a newly designed cobalt 60 machine with a ring type gantry and a multileaf collimator (MLC). Methods: Conformal arcs were modeled with continuous delivery of treatment fields conformal to the target cross-section. Treatment plans were further optimized by weighting or offsetting beams based on the target-critical structure relationship. CT and structure data of seven previous SBRT patients treated at our center were used for plan evaluation. An in-house forward planning tool was developed to aid the forward planning process. A graphical user interface (GUI) was designed with interactive and automatic adjustment between beam gantry angle and the beam weight. Dose distributions were calculated using our in-house Monte Carlo based TPS. Dose contributions from individual beam angles were summed together with different weight factors, which were continuously optimized until the pre-set dose conformity goal was met. Results: We were able to achieve target coverage and critical structure sparing to meet the required clinical criteria in all studied cases. It should be mentioned that those cases were selected having critical structures not proximal to the target. However, this was usually the case in the majority of SBRT cases especially those involving lung targets. The developed GUI tool was very useful in the forward treatment planning process. Conclusion: Conformal arcs can be used successfully on the new Cobalt 60 modality for the treatment of a sizeable fraction of cancer patients, which can be efficiently planned with our simple forward planning approach.

Influence of collimator rotation on dose distribution and delivery in intensity modulated radiation therapy for parotid cancer

Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed. Methods : The computed tomography (CT) datasets of 5 patients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I’mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation. Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles. Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT.

A comparative study on patient specific absolute dosimetry using slab phantom, acrylic body phantom and goat head phantom

Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed. Methods : The computed tomography (CT) datasets of 5 patients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I’mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation. Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles. Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT.

Pseudo-automatic beam orientations in multi-criteria intensity-modulated radiation therapy

Selecting a number of beams and their orientations in intensity-modulated radiation therapy (IMRT) is an important, but large-scale NP-hard optimization problem. Unfortunately, the quality of beam directions depends heavily on its corresponding beam intensity profiles. Usually, the number of beams is manually pre-determined and passed for a stochastic selector which optimizes beam orientations, and then a single objective inverse treatment planning algorithm is used for optimizing beam intensity profiles. The overall time needed to solve the inverse planning for every random selection of beam orientations becomes excessive. Therefore selecting an appropriate set of beam directions in IMRT is still a time-consuming manual trial-and-error search procedure that depends on intuition and empirical knowledge. During the last decade considerable progress has been made in optimizing beam intensity profiles by using multi-objective inverse treatment planning. Multi-criteria optimization results in a variety of beam intensity profiles for every selection of beam orientations. The planner then needs to search, manually, through this variety of solutions to decide upon an optimal plan. This work takes advantage of this human interaction to present a two-phase algorithm in which both processes of beam orientations and multi-criteria optimization with human guidelines cooperate together to determine a suitable number of beams and their orientations in a clinically practical time-slot. To illustrate the efficiency of the algorithm it has been applied to one artificial example and three real clinical cases. The results show that using the algorithm, both target coverage and sparing critical structures were significantly improved compared to the standard equally spaced beam plans. Moreover in some cases the algorithm suggested plans with one beam fewer than the equally spaced beam plans without compromising the quality of the treatment plan.

Simultaneous beam sampling and aperture shape optimization for SPORT.

Station parameter optimized radiation therapy (SPORT) was recently proposed to fully utilize the technical capability of emerging digital linear accelerators, in which the station parameters of a delivery system, such as aperture shape and weight, couch position/angle, gantry/collimator angle, can be optimized simultaneously. SPORT promises to deliver remarkable radiation dose distributions in an efficient manner, yet there exists no optimization algorithm for its implementation. The purpose of this work is to develop an algorithm to simultaneously optimize the beam sampling and aperture shapes. The authors build a mathematical model with the fundamental station point parameters as the decision variables. To solve the resulting large-scale optimization problem, the authors devise an effective algorithm by integrating three advanced optimization techniques: column generation, subgradient method, and pattern search. Column generation adds the most beneficial stations sequentially until the plan quality improvement saturates and provides a good starting point for the subsequent optimization. It also adds the new stations during the algorithm if beneficial. For each update resulted from column generation, the subgradient method improves the selected stations locally by reshaping the apertures and updating the beam angles toward a descent subgradient direction. The algorithm continues to improve the selected stations locally and globally by a pattern search algorithm to explore the part of search space not reachable by the subgradient method. By combining these three techniques together, all plausible combinations of station parameters are searched efficiently to yield the optimal solution. A SPORT optimization framework with seamlessly integration of three complementary algorithms, column generation, subgradient method, and pattern search, was established. The proposed technique was applied to two previously treated clinical cases: a head and neck and a prostate case. It significantly improved the target conformality and at the same time critical structure sparing compared with conventional intensity modulated radiation therapy (IMRT). In the head and neck case, for example, the average PTV coverage D99% for two PTVs, cord and brainstem max doses, and right parotid gland mean dose were improved, respectively, by about 7%, 37%, 12%, and 16%. The proposed method automatically determines the number of the stations required to generate a satisfactory plan and optimizes simultaneously the involved station parameters, leading to improved quality of the resultant treatment plans as compared with the conventional IMRT plans.

A New Beam Selection Method for MLC-Based Robotic Radiotherapy

Purpose: The CyberKnife system equipped with multileaf collimator (MLC) has been shown promising in treatment-time reduction and plan-quality improvement, because of the enhanced coverage of larger lesions and the improved target conformity. In this study, we aim to develop an efficient non-coplanar beam selection program for CyberKnife-based IMRT. Method: The candidate beam set in this study consists of 94 non-coplanar beams, each defined by a vector connecting a CyberKnife node and a target point. Our goal is to choose an adequately small number of beams that will allow the generation of high quality IMRT plans. We use the beam coverage of patient-surface as a surrogate for the solution space of beamlet-based inverse planning. Based on body-surface coverage and beam-projection overlap on the surface, a beam-selection program was developed. To evaluate the effectiveness of the beam selection method, IMRT plans with the selected beams for different treatment sites were generated using the Varian Eclipse treatment planning system and compared with the IMRT plans with conventional coplanar beams. Results: Our program efficiently selected a subset of relatively small number of non-coplanar beams, while pre-serving the body-surface coverage and therefore the solution space for inverse planning optimization. For example, a set of 17 beams were selected for a pancreatic cancer case, covering 92.5% of the surface area which was covered by all the 94 candidate beams with the same field size. The IMRT plans with the selected beams show superior quality with dramatically improved critical structure sparing, as compared with the clinically approved IMRT plans. Conclusion: One can efficiently select effective sets of non-coplanar beams with our program, which allow the generation of high-quality plans for MLC-based robotic radiotherapy.

Is high–dose rate RapidArc-based radiosurgery dosimetrically advantageous for the treatment of intracranial tumors?

In linac-based stereotactic radiosurgery (SRS) and radiotherapy (SRT), circular cone(s) or conformal arc(s) are conventionally used to treat intracranial lesions. However, when the target is in close proximity to critical structures, it is frequently quite challenging to generate a quality plan using these techniques. In this study, we investigated the dosimetric characteristics of using high–dose rate RapidArc (RA) technique for radiosurgical treatment of intracranial lesions. A total of 10 intracranial SRS/SRT cases previously planned using dynamic conformal arc (DCA) or cone-based techniques have been included in this study. For each case, 3 treatment plans were generated: (1) a DCA plan with multiple noncoplanar arcs, (2) a high–dose rate RA plan with arcs oriented the same as DCA (multiple-arc RA), and 3) a high–dose rate RA plan with a single coplanar arc (single-arc RA). All treatment plans were generated under the same prescription and similar critical structure dose limits. Plan quality for different plans was evaluated by comparing various dosimetric parameters such as target coverage, conformity index (CI), homogeneity index (HI), critical structures, and normal brain tissue doses as well as beam delivery time. With similar critical structure sparing, high–dose rate RA plans can achieve much better target coverage, dose conformity, and dose homogeneity than the DCA plans can. Plan quality indices CI and HI, for the DCA, multiple-arc RA, and single-arc RA techniques, were measured as 1.67 ± 0.39, 1.32 ± 0.28, and 1.38 ± 0.30 and 1.24 ± 0.11, 1.10 ± 0.04, and 1.12 ± 0.07, respectively. Normal brain tissue dose (V12Gy) was found to be similar for DCA and multiple-arc RA plans but much larger for the single-arc RA plans. Beam delivery was similar for DCA and multiple-arc RA plans but shorter with single-arc RA plans. Multiple-arc RA SRS/SRT can provide better treatment plans than conventional DCA plans, especially for complex cases.

Hybrid Modulated Arc Therapy (HMAT) on an Artiste Linac

Purpose/Objective(s): Pulsed low dose rate radiation therapy (PLDR) has been investigated in a number of clinical trials for the management of recurrent cancers and radiation resistant malignancies based on the in vitro and in vivo experimental findings indicating that many tumor cell lines exhibited hyper radio-sensitivities (HRS) at small doses and enhanced normal tissue repair at low dose rates. This retrospective study aimed to develop treatment planning strategies for PLDR treatment of recurrent cancers utilizing advanced delivery techniques including IMRT and VMAT. Materials/Methods: The current PLDR treatment protocol requires that the daily 2Gy dose be delivered in 10 x 0.2Gy sub-fractions (pulses) with a 3min interval. To take advantage of low-dose HRS the mean dose to the target volume is set at 0.2Gy and the maximum dose is limited to 0.4Gy per pulse. The 3min interval is selected for clinical practicality; PLDR is primarily for palliative treatment, which should be delivered within a reasonable time for recurrent patients. Sixty patients were randomly selected from our patient database for the following body sites: head and neck, breast, thorax, abdomen and pelvis. Practical planning strategies/ guidelines were developed for IMRT and VMAT delivery techniques: (1) use 10 beam angles for IMRT and 10 arcs for VMAT (2-arc plan to be delivered 5 times) with each angle/arc as a pulse; (2) set the mean dose (0.2Gy) and maximum dose (0.4Gy) to the target per pulse as hard constraints (no hard constraints to the OAR); (3) select optimal beam/arc angles to avoid OARs; and (4) add reference structures in or around target/ OARs to reduce maximum dose to the target and OARs. IMRT, VMAT and 3DCRT plans were generated for these patients and compared. Results: Target dose heterogeneity is intrinsic for PLDR with 3DCRT when each pulse is delivered using one single field due to difference in beam attenuation and scattering. Further dose variation is introduced with IMRT due to lateral intensity modulation. A single arc can minimize the difference in the target dose due to multi-angle incidence although the delivery time is longer than 3DCRT and IMRT. Following the planning guidelines developed in this work, all PLDR treatment plans using IMRT and VMAT met the dosimetry requirements of the PLDR protocol. The mean dose was 0.20Gy 0.01Gy and the maximum dose was <0.3Gy for 75% of fields/arcs, <0.35Gy for 98% of fields/arcs and all < 0.4Gy. In comparison with 3DCRT, IMRT, and VMAT exhibited significantly improved target dose conformity and OAR dose sparing. Conclusions: Both IMRT and VMAT can be used for PLDR treatment of recurrent cancers with superior target dose conformity and critical structure sparing. The planning strategies/guidelines developed in this work are practical for IMRT/VMAT treatment planning to meet the dosimetry requirements of the PLDR protocol. Author Disclosure: C. Ma: None. M. Lin: None. K. Wei: None. J. Li: None. P. Wang: None. J. Lang: None.

Pulsed Low-Dose-Rate Radiation Therapy (PLDR) for Recurrent Cancers: Treatment Planning Strategies for IMRT and VMAT

Purpose/Objective(s): Pulsed low dose rate radiation therapy (PLDR) has been investigated in a number of clinical trials for the management of recurrent cancers and radiation resistant malignancies based on the in vitro and in vivo experimental findings indicating that many tumor cell lines exhibited hyper radio-sensitivities (HRS) at small doses and enhanced normal tissue repair at low dose rates. This retrospective study aimed to develop treatment planning strategies for PLDR treatment of recurrent cancers utilizing advanced delivery techniques including IMRT and VMAT. Materials/Methods: The current PLDR treatment protocol requires that the daily 2Gy dose be delivered in 10 x 0.2Gy sub-fractions (pulses) with a 3min interval. To take advantage of low-dose HRS the mean dose to the target volume is set at 0.2Gy and the maximum dose is limited to 0.4Gy per pulse. The 3min interval is selected for clinical practicality; PLDR is primarily for palliative treatment, which should be delivered within a reasonable time for recurrent patients. Sixty patients were randomly selected from our patient database for the following body sites: head and neck, breast, thorax, abdomen and pelvis. Practical planning strategies/ guidelines were developed for IMRT and VMAT delivery techniques: (1) use 10 beam angles for IMRT and 10 arcs for VMAT (2-arc plan to be delivered 5 times) with each angle/arc as a pulse; (2) set the mean dose (0.2Gy) and maximum dose (0.4Gy) to the target per pulse as hard constraints (no hard constraints to the OAR); (3) select optimal beam/arc angles to avoid OARs; and (4) add reference structures in or around target/ OARs to reduce maximum dose to the target and OARs. IMRT, VMAT and 3DCRT plans were generated for these patients and compared. Results: Target dose heterogeneity is intrinsic for PLDR with 3DCRT when each pulse is delivered using one single field due to difference in beam attenuation and scattering. Further dose variation is introduced with IMRT due to lateral intensity modulation. A single arc can minimize the difference in the target dose due to multi-angle incidence although the delivery time is longer than 3DCRT and IMRT. Following the planning guidelines developed in this work, all PLDR treatment plans using IMRT and VMAT met the dosimetry requirements of the PLDR protocol. The mean dose was 0.20Gy 0.01Gy and the maximum dose was <0.3Gy for 75% of fields/arcs, <0.35Gy for 98% of fields/arcs and all < 0.4Gy. In comparison with 3DCRT, IMRT, and VMAT exhibited significantly improved target dose conformity and OAR dose sparing. Conclusions: Both IMRT and VMAT can be used for PLDR treatment of recurrent cancers with superior target dose conformity and critical structure sparing. The planning strategies/guidelines developed in this work are practical for IMRT/VMAT treatment planning to meet the dosimetry requirements of the PLDR protocol. Author Disclosure: C. Ma: None. M. Lin: None. K. Wei: None. J. Li: None. P. Wang: None. J. Lang: None.

Static and rotational intensity modulated techniques for head-neck cancer radiotherapy: A planning comparison

PurposeTo compare helical Tomotherapy (HT), two volumetric-modulated arc techniques and conventional fixed-field intensity modulated techniques (S-IMRT) for head-neck (HN) cancers. Methods and materialsEighteen HN patients were considered. Four treatment plans were generated for each patient: HT, S-IMRT optimised with Eclipse treatment planning system and two volumetric techniques using Elekta–Oncentra approach (VMAT) and Varian-RapidArc (RA), using two full arcs. All techniques were optimised to simultaneously deliver 66Gy to PTV1 (GTV and enlarged nodes) and 54Gy to PTV2 (subclinical and electively treated nodes). Comparisons were assessed on several dosimetric parameters and, secondarily, on planned MUs and delivery time. ResultsConcerning PTV coverage, significantly better results were found for HT and RA. HT significantly improved the target coverage both compared to S-IMRT and VMAT. No significant differences were found between S-IMRT and volumetric techniques in terms of dose homogeneity. For OARs, all the techniques were able to satisfy all hard constraints; significantly better results were found for HT, especially in the intermediate dose range (15–30 Gy). S-IMRT reached a significantly better OARs sparing with respect to VMAT and RA. No significant differences were found for body mean dose, excepting higher values of V5–V10 for HT. A reduction of planned MUs and delivery treatment time was found with volumetric techniques. ConclusionsThe objectives of satisfying target coverage and sparing of critical structures were reached with all techniques. S-IMRT techniques were found more advantageous compared to RA and VMAT for OARs sparing. HT reached the best overall treatment plan quality.