Rotational Intensity-modulated Radiotherapy Research Articles

Motion Analysis of Target during Stereotactic Radiotherapy of Lung Tumors Using Volumetric Modulated Arc Therapy

Volumetric modulated arc therapy (VMAT) is a rotational intensity-modulated radiotherapy (IMRT) technique capable of acquiring projection images for cone-beam computed tomography (CBCT). Respiratory-correlated cone-beam computed tomography, namely 4D-CBCT, serves to assess the displacement of a tumor position between planning and treatment due to organ motion and respiration, and is important for more accurate radiation therapy. On the other hand, recently, a 320-detector row CT scanner, namely 4D-CT, has become available that allows axial volumetric scanning of a 16-cm-long range in a patient without table movement. The goal of our research is to establish a new method of verification during treatment in stereotactic body radiotherapy. In this study, we compare the movement of the tumor between "before treatment" using 4D-CT and "in treatment" using 4D-CBCT. Three patients (55-68 years of age) with lung tumors underwent CT scans for radiotherapy planning using 4D-CT scans to analyze the movement of the tumor before treatment. The patients were treated by VMAT while acquiring projection images. 4D-CBCT datasets were reconstructed from the projection images using in-house programs. The tumor positions in 4D-CT and 4D-CBCT were detected and the movement of the tumor between "before treatment" and "in treatment" was similar. The movement of the tumors during treatment was predictable from 4D-CT before treatment. Furthermore, 4D-CBCT clarified the tumor position during treatment and could reevaluate the actual tumor position and dose distribution. We have successfully shown the movement of the tumor between "before treatment" using 4D-CT and "in treatment" using 4D-CBCT.

Effect of Photon-Beam Energy on VMAT and IMRT Treatment Plan Quality and Dosimetric Accuracy for Advanced Prostate Cancer

The goal of the research was to evaluate treatment plan quality and dosimetric accuracy of volumetric modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) plans using 6, 10, and 15 MV photon beams for prostate cancer including lymph nodes. In this retrospective study, VMAT and IMRT plans were generated with the Pinnacle© treatment planning system (TPS) (V9.0) for 10 prostate cancer cases. Each plan consisted of two target volumes: PTV(B) included the prostate bed, PTV(PC+LN) contained PTV(B) and lymph nodes. For plan evaluation statistics, the homogeneity index, conformity index, mean doses, and near-max doses to organs at risk (OAR) were analyzed. Treatment time and number of monitor units were assessed to compare delivery efficiency. Dosimetric plan verification was performed with a 2D ionization chamber array placed in a full scatter phantom. No differences were found for target and OAR parameters in low and high energy photon beam plans for both VMAT and IMRT. A slightly higher low dose volume was detected for 6 MV VMAT plans (normal tissue: D(mean) = 16.47 Gy) compared to 10 and 15 MV VMAT plans (D(mean) = 15.90 Gy and 15.74 Gy, respectively), similar to the findings in IMRT. In VMAT, > 96% of detector points passed the 3%/ 3 mm γ criterion; marginally better accuracy was found in IMRT (> 97%). For static and rotational IMRT, 15 MV photons did not show advantages over 6 and 10 MV high energy photon beams in large volume pelvic plans. For the investigated TPS and linac combination, 10 MV photon beams can be used as the general purpose energy for intensity modulation.

Poster — Thur Eve — 08: Dosimetric Comparison Study of Rotational and Static IMRT Treatment Plans for the Prostate

It has been reported that for certain clinical applications, Rotational Intensity Modulated Radiation Therapy (R‐IMRT) techniques such as Volumetric Modulated Arc Therapy are capable of improved target dose coverage and shorter delivery time when compared to static, step‐and‐shoot IMRT. Five similar early stage prostate cases were used to generate R‐IMRT and static IMRT plans. The R‐IMRT plans consisted of 72 single‐segmented 6MV beams, equally spaced with beam angle separations of 5 degrees. The static IMRT plans employed 7 multi‐segmented 6 MV beams. Both types of plans were optimized with the direct machine parameter optimization algorithm using the same set of optimization objectives. Dose volume histograms were obtained for both types of plans and were comparatively evaluated based on target coverage and dose to organs at risk. For 3 out of 5 cases, target coverage was found to be better with R‐IMRT: standard deviations were consistently lower, and V95 values were equal or better. Critical structure sparing was better for static IMRT with mean dose, V50 and V75 values for the rectum and bladder consistently lower for the majority of the 5 cases. In cases where target coverage was better for static IMRT, the critical structure sparing was better for R‐IMRT. Treatment time was approximately 3 times as long for R‐IMRT due to the lack of dynamic MLC's on the Primus linac. It was found that R‐IMRT can provide improved target coverage in certain applications when compared to static IMRT, however critical structures may receive a slightly higher dose with this technique.

Biological effect of different IMRT delivery techniques: SMLC, DMLC, and helical tomotherapy

Intensity-modulated radiotherapy (IMRT) is delivered using a variety of techniques with differing temporal dose characteristics. Spatial dose metrics are generally used to evaluate treatment plan quality. However, the use of this information alone neglects the effects of the significant differences in dose delivery duration and dose accumulation patterns, both of which can impact cell survival. This study uses the linear-quadratic model with dose protraction corrections to evaluate the biological effectiveness of different IMRT delivery techniques, including fixed gantry IMRT in SMLC (step-and-shoot) and DMLC (sliding window) modes and a rotational IMRT technique (helical tomotherapy) for the treatment of prostate and head/neck sites. The temporal dose pattern was measured using a small volume ion chamber (A1SL--0.057 cm3) to calculate the protraction factor, and biological equivalent dose (BED) was calculated for a range of repair half-times and alpha/beta ratios. The treatment BED is compared to an ideal delivery of the target prescription dose, in which dose is delivered instantaneously (G(t0) = 1), to evaluate loss in biological effectiveness due to protraction in delivery. In the case of a conventional prescription, the loss in biological effectiveness was further evaluated using published tumor control probability (TCP) data. With SMLC and DMLC IMRT delivery, for both prostate and head/neck, the expected additional loss in BED is about 1% compared to 3D CRT, which corresponds to a predicted 2%-3% reduction in TCP. For tomotherapy, the prostate BED loss is smaller in comparison to 3D CRT; hence, the authors expect a TCP increase of the order of 2%-3%. The aforementioned differences are due to the dose accumulation time. While it is theoretically possible to compensate for changes in biologically effective dose, this would be hindered by large uncertainties in parameters used for such calculations; therefore, it is advantageous to irradiate target volume elements as rapidly as possible. The results of this study indicate that temporal dose delivery pattern is an important component in determining the biological effects of IMRT treatment.

The Intensity Modulated Multiple Arc (IMMA) Technique: Forward & Inverse Planned Procedures to Deliver Hypo- Fractionated IMAT Treatments

Intensity-modulated arc therapy (IMAT) is an arc-based approach to intensity-modulated radiotherapy (IMRT) that can be delivered on a conventional linear accelerator using multiple overlapping arcs. As with other rotational IMRT techniques, IMAT is capable of producing highly conformal dose distributions that are able to compete with the simplest three-dimensional (3D) conformal radiotherapy (CRT) and the most effective static IMRT. Despite its potential, the clinical application of IMAT technique has gone largely unrealized due to the lack of robust and commercially available inverse- planning tools. To overcome this limitation, an in-house forward-planning strategy for planning the IMAT technique, called intensity modulated multiple arcs (IMMA) was developed. This planning approach was implemented for lung and cranial treatments with the aim of generating superior treatment plans compared to the 3D CRT technique. Two treatments plans, on a lung and a cranial tumour respectively, were analyzed as important examples of this new approach. The IMMA plans achieve a similar coverage of the target volumes as well as the 3D CRT technique (coplanar and noncoplanar), but with a better sparing of the critical organs and a delivery efficiency that is superior to that of 3D CRT. This result changes when the comparison is made with the fixed gantry IMRT technique, since the inverse-planned (IP) treatments have a greater flexibility in conforming the dose to the complex targets and in controlling the dose to the critical structures. To improve the IMMA forward-planned procedure, a new IMAT-IP approach has been proposed to obtain treatment plans that approach, in term of tumour dose coverage and dose sparing of the critical organs, the plan quality reached by the IMRT technique.

Arc-modulated radiation therapy (AMRT): a single-arc form of intensity-modulated arc therapy

Arc-modulated radiation therapy (AMRT) is a novel rotational intensity-modulated radiation therapy (IMRT) technique developed for a clinical linear accelerator that aims to deliver highly conformal radiation treatment using just one arc of gantry rotation. Compared to fixed-gantry IMRT and the multiple-arc intensity-modulated arc therapy (IMAT) techniques, AMRT promises the same treatment quality with a single-arc delivery. In this paper, we present a treatment planning scheme for AMRT, which addresses the challenges in inverse planning, leaf sequencing and dose calculation. The feasibility and performance of this AMRT treatment planning scheme have been verified with multiple clinical cases of various sites on Varian linear accelerators.

TU‐D‐AUD B‐01: Volumetric Modulated Arc Therapy for Head & Neck Cancer

Purpose: Volumetric Modulated Arc Therapy (VMAT) is an arc‐based technique that utilizes dynamically modulated arcs to deliver intensity‐modulated radiation therapy (IMRT) treatments on a conventional linear accelerator. In this work, we evaluate VMAT as a treatment technique for patients with carcinomas of the head‐and‐neck. Method and Materials: Five complex head and neck cancer patients, with multiple prescription levels, were selected for this study. Fully inverse planned VMAT plans were optimized for these patients using our homegrown arc‐sequencing software. The software uses simulated annealing to optimize the aperture shapes and weights while minimizing the differences between the optimized (ideal) and sequenced intensities. The optimized plans were compared with step‐and‐shoot IMRT plans generated using the Pinnacle3 treatment planning system. VMAT plan verifications have also been performed using Elekta's Precise Beam Infinity™ control system which has been installed on an Elekta Precise linear accelerator in our clinic. Results: Using our arc‐sequencing tool, VMAT can be used to create highly conformal head‐and‐neck treatment plans. As compared with traditional fixed‐field plans, VMAT was able to reduce the average parotid dose from 85.3 cGy to 73.6 cGy per fraction. Additionally, the average number of monitor units was reduced from 1058.3 to 502.3 per fraction. Initial delivery tests demonstrate that using VMAT complex head‐and‐neck deliveries can be completed in under 6 minutes. Conclusion: VMAT should serve as an important tool in the delivery of radiation therapy for head‐and‐neck carcinomas. By utilizing the dosimetric advantages of rotational IMRT, VMAT can provide more uniform target doses and reduced critical structure doses as compared with fixed field IMRT. Conflict of Interest: Research partially sponsored by Elekta Corporation.

Sweeping-window arc therapy: an implementation of rotational IMRT with automatic beam-weight calculation

Sweeping-window arc therapy (SWAT) is a variation of intensity-modulated radiation therapy (IMRT) with direct aperture optimization (DAO) that is initialized with a leaf sequence of sweeping windows that move back and forth periodically across the target as the gantry rotates. This initial sequence induces modulation in the dose and is assumed to be near enough to a minimum to allow successful optimization, done with simulated annealing, without requiring excessive leaf speeds. Optimal beam weights are calculated analytically, with easy extension to allow for variable beam weights. In this paper SWAT is tested on a phantom model and clinical prostate case. For the phantom, constant and variable beam weights are used. Although further work (in particular, improving the dose model) is required, the results show SWAT to be a feasible approach to generating deliverable dynamic arc treatments that are optimized.

A quality assurance method for analyzing and verifying intensity modulated fields

A quality assurance method is developed for measuring, verifying and analyzing intensity modulated radiation fields. It is applicable for rotational and fixed-beam intensity modulated radiation therapy (IMRT) treatments. A gantry-mount device was constructed to measure the transmission dose of an IMRT field using radiographic films. A double-exposure technique with optimal kernel estimate method was developed to minimize the errors from measurements. A chi2 confidence level test method was developed to detect the discrepancies between measured and prescribed IMRT fluence distributions. Our method was tested for rotational and fixed-beam IMRT treatment verifications. The method was found insensitive to the hardware-related parameters for rotational and fixed-beam IMRT deliveries. The chi2 confidence level test was found to be more sensitive than linear correlation method in detecting relative small errors for cases with a few segments or narrow regions of interest. In conclusion, we demonstrated a quantitative method for verifying and analyzing IMRT treatment deliveries.

On the verification of the incident energy fluence in tomotherapy IMRT

For any radiotherapy verification technique, it is desirable that issues with the accelerator, multileaf collimator and patient position be detected. In previous works, an effective method for this level of verification was presented. This paper identifies second-order issues affecting the part of the process in which the incident energy fluence is verified. These problems will affect any rotational intensity-modulated radiotherapy delivery that divides each rotation or arc into projections; however the solutions offered in this paper are specific to the method previously developed.The issues affecting the energy fluence verification method include leaf bouncing, delivery implementation and leaf latency. All three matters were found to introduce small errors in the verified energy fluence values for a small fraction of leaf states. The overall effect on the deposited dose over the course of a rotational delivery involving thousands of beam pulses per rotation is negligible. Regardless, effective correction strategies are presented; these are utilized in order to characterize both the delivered energy fluence and deposited dose as accurately as possible.

Comparison of intensity-modulated radiotherapy with conventional conformal radiotherapy for complex-shaped tumors

Purpose: Conformal and intensity-modulated radiotherapy (IMRT) plans for 9 patients were compared based on characterization of plan quality and effects on the oncology department. Methods and Materials: These clinical cases, treated originally with conformal radiotherapy (CRT), required extraordinary effort to produce conformal treatment plans using nonmodulated, shaped noncoplanar fields with multileaf collimators (MLCs). IMRT plans created for comparison included rotational treatments with slit collimator, and fixed-field MLC treatments using equispaced coplanar, and noncoplanar fields. Plans were compared based upon target coverage, target conformality, dose homogeneity, monitor units (MU), user-interactive planning time, and treatment delivery time. The results were subjected to a statistical analysis. Results: IMRT increased target coverage an average of 36% and conformality by 10%. Where dose escalation was a goal, IMRT increased mean dose by 4–6 Gy and target coverage by 19% with the same degree of conformality. Rotational IMRT was slightly superior to fixed-field IMRT. All IMRT techniques increased integral dose and target dose heterogeneity. IMRT planning times were significantly less, whereas MU increased significantly; estimated delivery times were similar. Conclusion: IMRT techniques increase dose and target coverage while continuing to spare organs-at-risk, and can be delivered in a time frame comparable to other sophisticated techniques.

558. Simple gauge for obtaining preselected magnifications with the electron microscope: C. B. Reimer, Rev. Sci. Instrum., 33 (10), Oct. 1962, 1126–1127

Radiation oncologists were surveyed to assess practice patterns in the use of stereotactic body radiotherapy (SBRT) for lung cancer.A customized patterns-of-care survey, consisting of 18 questions and two clinical scenarios, was e-mailed to 136 academic radiation oncologists and 768 community practitioners to evaluate the technical basis and delivery parameters associated with SBRT.A total of 117 surveys were evaluable. The cited delivery techniques included: static noncoplanar beams (48%), intensity-modulated radiotherapy (41%), rotational intensity-modulated radiotherapy (47%), dynamic conformal arcs (7%), and small-beam delivery with fiducial tracking (24%), with 46% using multiple techniques. The immobilization methods included: stereotactic frame (10%), alpha cradle or vacuum-lock system (52%), wingboard (3%), stereotactic frame with an alpha cradle or vacuum-lock system (11%); combination of devices (14%), or no immobilization (9%). Abdominal compression was used by 51% and respiratory gating by 31%. For a peripheral T1N0 tumor, the preferred doses included: 25 to 34 Gy in one fraction (1%); 54 to 60 Gy in three fractions (56%), 48 to 50 Gy in four fractions (18%), and 50 to 60 Gy in five fractions (25%). For a centrally located T1N0 tumor, 58% recommended SBRT outside a clinical protocol, with recommended doses ranging from 40 to 60 Gy in three to 10 fractions. The recommended interval to first surveillance imaging ranged from 6 weeks or lesser (32%) to 25 weeks or more (2%).Considerable variation exists for thoracic SBRT with regard to dose selection, fractionation, immobilization, planning, management of central lesions, and surveillance. Ongoing prospective evaluation is recommended to identify best practices and provide continual process improvement.

561. Vacuum technical problems in the food industry: G. Nemitz, Vakuum-Technik, 11 (6), Sept. 1962, 172–173

Radiation oncologists were surveyed to assess practice patterns in the use of stereotactic body radiotherapy (SBRT) for lung cancer.A customized patterns-of-care survey, consisting of 18 questions and two clinical scenarios, was e-mailed to 136 academic radiation oncologists and 768 community practitioners to evaluate the technical basis and delivery parameters associated with SBRT.A total of 117 surveys were evaluable. The cited delivery techniques included: static noncoplanar beams (48%), intensity-modulated radiotherapy (41%), rotational intensity-modulated radiotherapy (47%), dynamic conformal arcs (7%), and small-beam delivery with fiducial tracking (24%), with 46% using multiple techniques. The immobilization methods included: stereotactic frame (10%), alpha cradle or vacuum-lock system (52%), wingboard (3%), stereotactic frame with an alpha cradle or vacuum-lock system (11%); combination of devices (14%), or no immobilization (9%). Abdominal compression was used by 51% and respiratory gating by 31%. For a peripheral T1N0 tumor, the preferred doses included: 25 to 34 Gy in one fraction (1%); 54 to 60 Gy in three fractions (56%), 48 to 50 Gy in four fractions (18%), and 50 to 60 Gy in five fractions (25%). For a centrally located T1N0 tumor, 58% recommended SBRT outside a clinical protocol, with recommended doses ranging from 40 to 60 Gy in three to 10 fractions. The recommended interval to first surveillance imaging ranged from 6 weeks or lesser (32%) to 25 weeks or more (2%).Considerable variation exists for thoracic SBRT with regard to dose selection, fractionation, immobilization, planning, management of central lesions, and surveillance. Ongoing prospective evaluation is recommended to identify best practices and provide continual process improvement.