Equidistant Beams Research Articles

SU-FF-T-240: Fast Efficient Global Fluence Map Optimization Using a Parallelized Objective Function for IMRT Treatment Planning

Purpose: Adaptive intensity modulated radiation therapy(IMRT) with image guidance requires frequent re‐optimization of the dose distribution for treatment plan verification. We present an efficient and robust algorithm that solves fluence map optimizations (FMO) for IMRT and provides good target coverage of homogeneous targets (R95% ⩾ RRx), while at the same time maintaining dose to critical structures within specified tolerances. Method and Materials: An analytic non‐linear convex model was developed that uses a projected gradient algorithm with Armijo line search to solve fluence map optimizations for IMRTtreatment planning. Voxel based penalty functions and a fluence non‐negativity constraint were applied for the iterative minimization of a parallelized convex objective function on a dual node processor. Model parameters were tuned for three treatment sites (H&N, CNS, and prostate) and results assessed in‐terms of algorithm speed, fluence maps and dose volume histograms. All cases were investigated for 5, 7, 9, and 11 equidistant beam angles and a generic set of parameters that provide good results obtained for each site. Improvements in plan quality are achieved on a case‐by‐case basis through dynamic parameter weighting. Results: Our implemented projected gradient algorithm model solved FMO's, on a dual node processor, for the three sites in 0.34–8.55 seconds corresponding to 10–75 iterations. Dynamic weighting produced tighter target coverage, while still maintaining critical organs within acceptable tolerances with only a small increase in FMO calculation times. Conclusion: The FMO optimization algorithm described shows that voxel based fluence map optimizations are feasible down to <10 second time scales while still achieving good target coverage and critical structure sparing for IMRTtreatment verification and planning. A Further 25% improvement in FMO calculation time is expected using up to 6 node parallelization. This work supported in part by NSF grant DMI‐0457394 and the State of Florida DOH Grant 04‐NIR03.

TU‐C‐224A‐08: A Scientific Comparison of Inverse Treatment Plan Quality Using a Convex Non‐Linear Programming Model as a Function of Beam Quality and Beam Number

Purpose: Recent advances in large scale fluence map optimizations for IMRT allow the use of large beam numbers that conform to the target to generate the desired target coverage while at the same time maintaining dose to critical organs below tolerance limits. Additionally, IMRT has effectively removed the need for high energy accelerator beams due to the excellent plan quality achievable with low beam quality. We investigate the diminishing returns in plan quality with increasing beam numbers and compare IMRT treatment planning of 6MV and 60Co therapy dose models. Method and Materials: A convex non‐linear model was used to compare the plan quality, from dose volume histograms and fluence maps, for three treatment sites (H &amp; N, CNS and prostate) for a 6MV and 60Co dose model. Plans were calculated for 5, 7, 9, 11, 17, 35 and 71 equidistant beam angles and quality assessed on target coverage (R95% &gt; RRx) and organ sparing for each case. Results: Similar target coverage was achieved for 60Co as with 6MV and equivalent organ sparing was also observed for all three sites. Increasing the number of beams provided some improvement in organ sparing while maintaining target coverage conditions. Dose calculation times increased linearly with beam number and FMO calculations increased by up to 900% between 5 and 71 beams. Conclusion: We have demonstrated that IMRT plan quality using a 60Co dose model produces similar dose distributions to 6MV. We also show that plan quality does not show considerable improvement above 11 beams for IMRT and significant increases in the treatment planning times are observed extending the number of treatment beams to 71 beams.This work supported in part by NSF grant DMI‐0457394 and the State of Florida DOH Grant 04‐NIR03.

Intensity modulated radiotherapy (IMRT) for recurrent, residual, or untreated skull-base meningiomas: preliminary clinical experience

Objective: To investigate the feasibility of using intensity modulated radiotherapy (IMRT) for complex-shaped benign meningiomas of the skull base and report clinical experience. Methods: Twenty patients with benign skull-base meningiomas WHO°I (histopathologically proven in 16/20) were treated with IMRT between June 1998 and August 1999. Each tumor was complex in shape and adherent to, or encompassed, organs at risk (cranial nerves, optic apparatus, and brainstem). All patients, immobilized in a customized head mask integrated into a stereotactic system, were planned on an inverse treatment planning system using 5 or 7 coplanar, equidistant beams and 5 intensity steps. Each treatment plan was verified extensively before treatment. Follow-up with MRI and clinical examination was performed at 6 and 18 weeks and every 6 months thereafter. Results: Target volumes ranged from 27 to 278 cc (median: 108 cc). Mean dose in 32 fractions ranged between 55.8 and 58.2 Gy. At median follow-up of 36 months (range: 31–43 months), pre-existing neurologic symptoms improved in 12/20 (60%), remained stable in 7/20 (35%), and worsened in 1 (5%) patient. Radiographic follow-up revealed significant tumor shrinkage 6 weeks post-IMRT in 2 patients and partial remission in 3 more patients at 9–17 months; other tumor volumes remained stable. There was no radiation-induced peritumoral edema, increase in tumor size, or new onset of neurologic deficits. Transient acute treatment side effects included nausea and vomiting and single occurrences of conjunctivitis/increased tearing and serous tympanitis. Conclusion: IMRT in the treatment of central nervous system meningiomas is feasible and safe, offering highly conformal irradiation for complex-shaped skull-base tumors while sparing adjacent critical structures. If the tumor remissions seen here are found in the ongoing treatments, IMRT may be considered the treatment of choice for inoperable or subtotally resected meningiomas and for otherwise difficult-to-treat, complex-shaped tumors of the central nervous system adjacent to critical structures, with the potential of dose escalation for malignant tumors.

Determination of particle diameter and lumped density and shape factor using a three-beam time-of-flight (TOF) instrument

A mathematical scheme was developed to calculate particle diameter as well as particle lumped density and shape factor using the information obtained from particle trajectory calculations using a 3-beam aerosizer (a time-of-flight particle size determination instrument). A suitable drag coefficient for the flow regime present in the aerosizer was selected by comparison of the calculated trajectories with experimental data (Numerical study on the simultaneous determination of particle physical diameter, lumped density and shape factor using time-of-flight sizers, Illinois Institute of Technology, Chicago, 1996, Aerosol Sci. Technol 29 (1998) 433). A model for particle trajectory in a one-dimensional supersonic compressible flow (Oskouie et al., 1998) was extended to provide the density and shape factor for the particles. The velocity of a particle was calculated along the centerline of three equidistant laser beams downstream of a sonic nozzle, and the physical diameter of this particle, and its lumped density and shape factor were theoretically calculated and compared to experimental results.

Tracking Simulation of the Ions Trapped in the Nonlinear Field of Electron Beams in Storage Rings

Tracking simulation was performed on the ions trapped in linear and nonlinear fields of an electron beam in storage rings. The stability of the ions was investigated with regard to the homogeneous partial-fill mode and the equidistant-bunch and -burst modes of the electron beam. It was found that the oscillation amplitude of the ions trapped in the beam grows very rapidly to 100–1000 times larger than the electron beam size, even in the nonlinear field, under some ion-beam conditions. The results are expressed as a stability diagram of the ions, and the diagrams for the linear and nonlinear fields are basically similar to each other, both representing a periodic structure of the parametric resonance. The critical mass in the equidistant bunched beam is the same for the linear and nonlinear fields. The present results are consistent with the experimental results in the storage rings of European Synchrotron Radiation Facility (ESRF) and Photon Factory of National Laboratory for High Energy Physics (KEK-PF).