bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐77: IMRT of Superficial Tumors
Authors: Sethi A , Leybovich L , King S , Rusu I , Xu M
Published: June 2005

Purpose: Due to inadequate beam modeling in the buildup region, large differences exist between planned and delivered IMRT doses for superficial targets. Our measurements show that plans underestimate doses delivered to superficial target regions by upto 40%. This results in unexpected skin reactions in patients and overdosing of critical structures. In this work, surface dose for such cases was investigated. Method and Materials: Using Rando phantom, IMRT plans were developed for PTVs located at: (1) skin surface (PTV0); (2) 10mm from skin (PTV10), and (3) phantom center (PTVctr). Dose distributions in Rando phantom were measured with EDR films and verified by MOSFET detectors. QA for all IMRT plans was done in a cube phantom using EDR films and ion chamber. Results: For PTV0, measurements in Rando phantom showed surface receiving full dose. In contrast, planned surface dose was 40% lower. In fact, superficial regions (up to 7mm) of PTV were underdosed. In distal target regions, agreement between planned and measured doses was good. For PTV10, agreement between planned and delivered surface doses was significantly better (∼10%). MOSFET measurements indicated 42.5% and 8.5% surface overdose(average) for PTV0 and PTV10 respectively. For centrally located tumors, measured and planned doses were in excellent agreement. In QA phantom, dose uniformity of transferred targets depended on their location. For PTV0, the side of the tumor that was facing surface received 30% higher dose than prescription. For PTV10, dose increase was 10%. Conclusion: In IMRT plans, adequate target dose is delivered without bolus. Extreme care must be exercised in delineating superficial PTVs. Whenever possible, a minimum distance of 1cm between skin surface and PTV must be preserved. With smaller distances, the skin may appear to be erroneously spared. This phenomenon is not detected by commonly used IMRT QA procedures. Therefore, in‐vivo measurements are recommended.
SU‐FF‐T‐79: IMRT with MLC Rotation — Dose Volume Benefits of High Precision Treatment Planning
Authors: Otto K
Published: June 2005

Purpose: To evaluate a new method of treatment planning that incorporates collimator rotation into MLC based fluence segmentation. Method and Materials: A set of treatment plans were generated using a new technique for fluence segmentation where the collimator is rotated in between each MLC sub‐field. It was previously shown that this technique has improved spatial resolution, increased flexibility in generating aperture shapes and decreased systematic error due to interleaf effects. Unfortunately these studies were limited to single fields. In this study we evaluate the ability of the rotating collimator technique to generate conformal dose distributions by comparing to conventional IMRT plans. Plans for a simple geometric phantom as well as a prostate and nasopharynx carcinoma recurrence were generated. One set of plans was generated with a conventional step‐and‐shoot delivery method (no MLC rotation) while a second set was generated with the collimator rotation segmentation method. Results: 3D dose comparisons as well as DVH analyses show that target coverage is equivalent or better with collimator rotation. Healthy tissue sparing is improved with collimator rotation, although more so for the smaller, more complex, nasopharynx target. It is also shown that increasing radiation efficiency (decreasing monitor units) will increase healthy tissue sparing. A decrease in the number of monitor units is seen with collimator rotation for both clinical cases. Conclusion: By incorporating MLC rotation into IMRT planning it is possible to improve target conformity and healthy tissue sparing. These benefits are more significant for smaller targets of higher complexity. The number of monitor units will also decrease, providing less transmission and total body scatter dose. Conflict of Interest: This work was supported in part by Varian Medical Systems.
SU‐FF‐T‐80: Incorporation of Isodose‐Level Control to a Simplified Intensity‐Modulated Radiation Therapy (sIMRT) Technique for Whole Breast Radiotherapy
Authors: Chen Z , Deng J , Song H , Fan Y , Ahmad M , Bond J , Chui C , Haffty B , Nath R
Published: June 2005

Purpose: To incorporate isodose‐level control in a simplified intensity‐modulated radiation therapy (sIMRT) technique (Med. Phys. 29, 522–529, 2002) for radiotherapy of large intact breast. Method and Materials: The parallel‐opposed tangential beam configuration has been used in many implementations of intensity‐modulated radiation therapy (IMRT) for large breast because of the dose constraints of contralateral breast and lung and the geographical constraint of breast target volume. For this beam configuration, a simplified IMRT (sIMRT) technique (Med. Phys. 29, 522–529, 2002), based on delivering equal prescription dose to the mid‐point of breast tissue seen by each pair of beamlets of intensity‐modulated 6 MV beams, provides efficient and robust IMRT planning. However, hot spots greater than 10% still exist near the base in the medial and lateral aspects of breast when the irradiated tissue in the beamlet path is greater than 20 cm. These beamlet paths often include part of ipsilateral lung and rib cage, to which the dose should be reduced. Modifications to the sIMRT algorithm were made so that dose to lung and consequently the hot spots in the medial and lateral aspects of breast can be reduced. Results: An automated algorithm, which delivers prescription dose to tissues near the lung‐breast tissue interface and to the mid‐point of the breast tissue, and a manual user‐definable isodose‐level control algorithm were developed and tested. Incorporation of these algorithms in sIMRT yielded lower dose to lung with smaller medial and lateral hot spots, in addition to the reduction of hot spots in the superior and inferior aspect of breast. Conclusion: sIMRT with isodose‐level control at the lung‐breast tissue interface near the base of large breast can be used efficiently for whole breast radiotherapy using the parallel‐opposed tangential beam setup.
SU‐FF‐T‐83: Partial Breast Treatment Using Energy‐ and Intensity‐ Modulated Photon and Electron Beams
Authors: Li J , Xiong W , Fan J , Ma C
Published: June 2005

Purpose: A new treatment technique has been developed for partial breast treatment using energy‐ and intensity‐modulated photon and electron beams. Treatment plans for partial breast irradiation using this technique are investigated in this work. Method and Materials: Two tangential photon beams with several electron beams of different energies and field shapes were used for partial breast irradiation. The patient setup and the orientations of the two opposed tangential photon beams were the same as conventional treatment for the whole breast. The electron beams were perpendicular to the patient skin surface. The dose‐distributions for the photon beamlets and electron apertures were calculated by a Monte Carlo code, EGS4/MCSIM. The optimization was performed based on the Monte Carlo dose distributions employing a gradient search method. The photon beams were delivered using a multi‐leaf collimator with a step‐and‐shot technique and the electron beams were delivered using cutouts. The effects of bremsstrahlung leakage and electron scattering by the cutout were taken into account in the Monte Carlo simulations. Results: Treatment plans using this new technique for five breast patients were generated for partial breast treatment. Compared with multi‐field photon IMRT, this new technique required no change for patient setup from the conventional technique and resulted in less dose to the ipsilateral lung and other normal tissues and less dose to the contralateral breast. Our results showed that the dose to the contralateral breast was reduced by about 50%. The volumes of normal tissues exposed to high and moderate radiation dose were reduced up to 50%. The doses to critical structures, such as the lung and the heart, were also reduced significantly. Conclusion: The new treatment technique integrates intensity‐ and energy‐modulated photon and electron beams and combines the advantages from both treatment modalities for partial breast treatment.
SU‐FF‐T‐84: Preferential Non‐Uniform Target Dose Distribution Using IMRT and PET/CT Imaging‐ a Feasibility Study
Authors: Sarfaraz M , Zhang B
Published: June 2005

Purpose: To investigate the feasibility of planning non‐uniform dose distribution based on FDG PET uptake within target and its impact on normal tissue structures using IMRT. Method and Materials: An IMRT treatment planning study was performed on a patient with colon metastasized liver cancer to investigate the effect of the non‐uniform target dose on the normal structures. Tumor and normal tissue such as liver, right kidney and cord were outlined on the CT scans registered with PET images. Two IMRT plans were generated for evaluation. In plan A, tumor was prescribed uniform dose of 50 Gy. In plan B, dose to the tumor periphery that showed mean SUV uptake four times the tumor core was increased to 100 Gy. Normal liver dose constrain was set such that 30% of the liver receives less than 30 Gy. The maximum spinal cord and right kidney doses were constrained to 45, and 30 Gy respectively, in both plans. Results: Plan A met the prescribed dose constrains with six beams. Dose to tumor was within 5% of the prescribed dose. However, the number of beams had to be increased to nine to meet the dose constrains in plan B. In this plan while 30% of the normal liver was still receiving less than 30 Gy, dose to 50% of liver was increase from 10 Gy to 20 Gy. Also, maximum spinal cord that was 20 Gy in plan B, was increased to 40 Gy in plan B. The right kidney doses were virtually unchanged in both plans. Although partial doses to liver and spinal cord were increased in plan B but they were still below the tolerances for these organs. Conclusion: This study showed that in this particular case dose to biologically active tumor subregion could be increased considerably without exceeding normal tissue tolerances.
SU‐FF‐T‐86: Rapid (<15 Minutes) Automated Tangential Breast IMRT Treatment Planning With Cutplane Evaluation
Authors: Hanley J , Barbiere J , Ndlovu A , Kapulsky A , Rebo I , Napoli J , Gejerman G
Published: June 2005

Purpose: To develop an automated, dose‐based IMRT planning technique for tangential breast irradiation and to introduce a novel technique for efficient plan evaluation in three‐dimensions. Method and Materials: Patients were CT‐simulated using an isocentric SAD technique with the medial and lateral tangents designed to encompass the breast tissue. The CT data and the beam information were transferred to the treatment planning system (TPS). The dose was then calculated for standard open equally weighted split beams. The planar dose rate matrix for a beams‐eye‐view plane was calculated and exported to a PC. An in‐house program was developed to rescale this matrix and automatically create a series of dose‐based MLC segments. Creation of the MLC segments took less than a minute. The MLC segments were imported back into the TPS where utilities were used to determine the optimal segment weights. An efficient plan evaluation tool was developed that presented the dose distributions in only three or less cutplanes parallel to the plane formed by the posterior beam border. The mathematical formalism for determining the cutplane orientation will be demonstrated. The planning and evaluation tools were tested on three representative case studies and compared to wedged tangent plans. To ensure unbiased comparison all plans were normalized to identical ICRU Reference Points. Results: The entire planning process, from CT import to plan completion, took approximately 15 minutes. For all cases studied the IMRT plans demonstrated superior coverage and dose homogeneity as determined by both isodose coverage and the ICRU Conformity Index. Cutplanes through the lung, mid breast, and apex were found to be the most clinically useful. Conclusion: The automated procedure is able to consistently produce optimized IMRT breast treatment plans within a few minutes. Isodose distributions in three cutplanes parallel to the posterior border are an effective tool for rapid plan evaluation.
SU‐FF‐T‐88: Study of Merits On IMRT with Gating Technique for Treatment of Intrahepatic Cancer
Authors: Kuo H , Liu W , Chang S , Wu A , Chuang K , Lalonde R
Published: June 2005

Purpose: To investigate and compare the benefits in terms of dose escalation, increases of TCP, decreases of NTCP and increases of dose conformity of four various techniques, i.e., 3DCRT and IMRT with and without gating techniques respectively, for treatments of intrahepatic malignancies. Method and Materials: Total 9 patients with liver cancer went through this study. Every patient has four set of plans, i.e., 3D conformal with and without gating, and IMRT with and without gating. Plans of different techniques were then evaluated with the following elaborated approaches: Kutcher's effective volume, Lyman's NTCP for the normal liver, and the Ten Haken's TCP for the intrahepatic cancer. Results: To keep a maximum of 5% NTCP of the normal liver for each technique and assume 3DCRT technique without gating as our norm for comparison. The 3DCRT combined with gating technique would increase 4.5Gy more in total target dose and 4.5% higher in TCP of liver lesion. Furthermore, 12 Gy and 6 Gy more in the total target dose and 13% and 8% higher in TCP of liver lesion were found with the IMRT technique with and without gating respectively. Conclusion: After reviewing and analyzing these various techniques, the combined technology of IMRT and gating seems to be right approach for treatment of liver lesion. IMRT with gating technique may have the most advantages in reducing the treatment margins, increasing the dose conformity, allowing us to escalate total target dose and improving tumor control probability simultaneously.
SU‐FF‐T‐89: The Effect of Breast Motion On Dose Distribution in Tangential IMRT with Direct Aperture Optimization
Authors: Zhang G , Jiang Z , Shepard D , Zhang B , Yu C
Published: June 2005

Purpose: Inverse planning for breast IMRT can be accomplished with Direct Aperture Optimization (DAO) where the user defines the traditional tangential fields within which the shapes and weights of multiple segments, including an open segment as ‘flash’, are optimized. The purpose of this study is to evaluate the technique and examine the effects of relative weights of the open segments to other segments on the impact of breathing motion. Method and Materials: IMRT plans were generated using DAO inverse planning with manually fixed relative weights of the open segments. To simulate breathing motion, the same set of optimized segments was reapplied with the isocenter shifted up and down by up to 1cm and the resulting dose distributions were added. The quality of both the static plans and the composite plans were compared among different open segment weights. Results: Five cases with varying breast sizes were examined. For the static plans, we found that uniform dose distributions could be generated with relative weights of the open segments in the range of 45–85%. Unacceptable hot spots were produced when the weights of the open segments were larger than 85%. For composite plans simulating breathing motion, the most uniform dose distribution happens when the open segment weights were in the range of 75–85%, below which the dose uniformity degrades. We observed that in the composite plans, for each 1% decrease in the open segment weighting, the volume covered by the 98% dose decreased by approximately 0.06%, independent of breast size. Conclusion: DAO provides a robust and efficient technique for breast IMRT planning. The proper open segment weight was found to be between 75% and 85%, and is not significantly dependent of breast size and laterality. Within this range, high‐quality IMRT plans were produced for each case.
SU‐FF‐T‐90: The Use of Bluetooth Technology in Multiple Monitoring of Vital Signs ‐ ECG and Pulse
Authors: Cosma V , Ciorba D , Cosma C
Published: June 2005

Purpose: The aim of this work is to evaluate the strengths and demonstrate the benefit of using Bluetooth wireless technology in medical equipment, for close monitoring of multiple patients with heart problems (ECG and vital signs) in recuperatory stage. The main issue discussed and tested is whether Bluetooth technology is ready to be implemented and used to realize an environment were computers would assist the medical staff in offering solutions and real time response, based on the data received from patients, in critical situations. Method and Materials: Our preliminary work consists in a simulation soft (VC++) for monitoring two patients (ECG and pulse), with two Bluetooth equipped devices. Bluetooth is a rather new low‐cost radio technology which operates in the free, unlicensed 2.4 – 2.4835 GHz ISM (Industrial, Scientific, Medical) band, and is designed to unite or connect all different types of devices to effectively work as one. The soft created is a server‐client based program that interact through information messages interpreted as graphs and action instructions. Results: The server receives instantaneously the data sent by the patients and after a process of comparing, in which if the data for Pulse or ECG is higher or lower than some user‐specified values, it builds dynamic charts for the two patients. We assumed the following limits both for ECG and Pulse: lower=50 and upper=130. A counter holds the number of times these values were crossed (under/over), and at 5 or more the application is stopped, and pre‐defined messages will appear on the server and patient interfaces. Conclusion: The tests performed presented a very good response of the system — real time — thus giving us a very good reason to think that this technology could be used to perform hard tasks and intervene much more accurate than human personnel can.
SU‐FF‐T‐91: Unusually Large Fluctuation of Monitor Unit (MU) Efficiency in Large‐Field IMRT Plans Using Sliding‐Window Technique
Authors: Song H , Chen Z , Fan Y , Deng J , Ahmad M , Nath R
Published: June 2005

Purpose: To examine the Monitor Unit (MU) efficiency (prescription dose in cGy as a percentage of the total MU) of large‐field IMRT using sliding‐window and step‐and‐shoot techniques and their implication to patient whole‐body dose. Method and Materials: Seven large‐field IMRT plans for pelvic/abdominal cancers were generated on the Eclipse treatment planning system using 18 MV photon beams. The MU efficiency of each plan was calculated for both the sliding window and the step‐and‐shoot (with 15 intensity levels) techniques for a 120‐leaf multileaf collimator on a Varian 2100CD linear accelerator. The impact on patient whole‐body dose resulted from accelerator leakage was estimated from the ratio of total treatment MUs of the two techniques. Results: The MU efficiency for large‐field IMRT using either sliding‐window or step‐and‐shoot technique (< 20%) is lower than that of medium‐field‐size prostate plans (∼30%). As expected, the MU efficiency of step‐and‐shoot technique is better than that of sliding window for all seven plans. However, unusually large fluctuation of MU efficiency (to as low as 5.5%) was observed among the plans using sliding‐window technique. In the extreme case, the use of step‐and‐shoot resulted in a 130% increase in MU efficiency over sliding‐window as opposed to the typical increase of 10 – 30%. The implication on patient whole‐body dose is enormous as the total MU needed to deliver 70 Gy would be 127,000 for sliding‐window and only about 55,000 for step‐and‐shoot. In this case, the whole‐body dose due to leakage x‐ray and neutrons is reduced by a factor of 2.3 using step‐and‐shoot technique instead of sliding‐window. Conclusion: The MU efficiency for large‐field IMRT is low and large MU is usually required to deliver a given prescription dose. Since step‐and‐shoot technique yields consistently better MU efficiency than that of sliding‐window, it should be used for large‐field IMRT delivery.
SU‐FF‐T‐94: Clinical Evaluation of Direct Machine Parameter Optimization Algorithm for Head and Neck IMRT Treatment
Authors: Tung S , Lii M , Lai P , Wong P , Mason B , Garden A , Chao K , Zhu X
Published: June 2005

Purpose: Because many critical structures are in close proximity to target volumes, cancers of the head and neck (H&N) are often suited for treatment with IMRT. However, the time required to generate and deliver a clinically acceptable IMRT plan can be significantly longer than a conventional plan. This study evaluated a new inverse planning algorithm, DMPO (direct machine parameter optimization), with attention to parameter settings, plan quality and treatment efficiency for H&N cancers. Method and Materials: The Pinnacle treatment planning system version 7.4 was used. The DMPO allows users to limit the number of total MLC segments (N) for treatment. After a user‐defined number of iterations (n) for pencil beam optimization, the DMPO generates MLC segments for each field for dose calculations using a convolution algorithm. Both the MLC leaf positions and the weight of each segment are then optimized until cost tolerance or iteration number is reached. Treatment plans generated using DMPO were compared with H&N cases that were previously treated using an older version (6.2). The plan quality was compared using cost functions and DVHs of target volumes and critical structures. The total monitor units and MLC segments for treatment were compared for different combinations of n and N. Results: The DMPO provided plans of DVHs similar to clinical cases with significantly less planning time. More importantly, the total MU and MLC segments for treatment delivery were reduced by 40% to 50%. Cost functions changed only slightly on n and N and total MU increased as n increased, but was independent of N. Our preliminary data indicated a combination of n=10–15 with 10 segments per field appeared to be optimal for most H&N cases. Conclusion: The DMPO algorithm generated more efficient plans while providing equal or better quality than the previous plans for IMRT treatment.
SU‐FF‐T‐99: An Analysis of Intensity Discretization Errors in IMRT
Authors: Mackie T , Flynn R , Kissick M , Jeraj R
Published: June 2005

Purpose: A simple analysis is presented of the deviation in dose when intensity is converted from continuous to discrete levels. This analysis assumes that intensity levels are equally likely and beams are discretized independently. Method and Materials: If there are i non‐zero intensity levels possible then the standard error due to discretization for a single beam direction is just equal to:
urn:x-wiley:0094-2405:media:mp7770:mp7770-math-0001 Eqn. 1 There are situations in multiple beam delivery where one beam direction dominates the dose to a voxel. In this case, the accuracy that can be delivered to that voxel is going to be dependent only on the graininess of the discretization and Eqn. 1 also applies. For the case of beam direction independence, the standard error to be expected in the dose distribution for N beam directions is given by:
urn:x-wiley:0094-2405:media:mp7770:mp7770-math-0002 Eqn. 2 Results: For the case of 5 intensity levels, dominated by a single beam, the standard error to be expected is 5.8% according to Eqn. 1. For the case of 5 intensity levels and 5 roughly equally contributing beam directions, the standard error to be expected is about 2.2% from Eqn. 2. The standard error to be expected for a tomotherapy dose distribution with 50 intensity levels is about 0.58% when one beam direction from one rotation is dominant (using Eqn. 1). Using Eqn. 2 the standard error is 0.04% for 50 levels when beams from 51 directions and 4 rotations (i.e., pitch 0.25) contribute equally to each voxel. Conclusion: Equations 1 or 2 represent upper and lower limits, respectively, of the standard error due to discretization. Finer discretization and more beams reduce error, but there is more to be gained in increasing intensity levels than in increasing the number of beam directions. Conflict of Interest: This work was supported by TomoTherapy Inc.
SU‐FF‐T‐101: Clinical Feasibility of “jaws‐Only” IMRT Using Direct Aperture Optimization
Authors: Earl M , Yu C , Jiang Z , Shepard D
Published: June 2005

Purpose: To demonstrate the clinical feasibility of delivering IMRT treatment plans using only independent collimators. Method and Materials: The Direct Aperture Optimization (DAO) technique is used to optimize the jaw positions and the relative weights assigned to each aperture. Since all of the delivery constraints imposed by the jaws are incorporated into the optimization, the need for leaf sequencing is eliminated. This allows for “jaws‐only” IMRT plans with a significantly reduced number of segments as compared with “jaws‐only” plans produced with the traditional two‐step IMRT approach. We applied the DAO “jaws‐only” technique to three clinical cases: an abdomen, a prostate, and a head and neck. For each case, “jaws‐only” DAO (JODAO) plans were produced with 5, 10, 15, 20, and 25 apertures. For comparison, a DAO plan was created that utilized an MLC (MLCDAO). The resulting JODAO and MLCDAO plans were delivered to a phantom using an Elekta Precise linear accelerator. Results: The results demonstrate that between 15 and 25 “jaws‐only” apertures are required per beam direction to obtain conformal IMRT treatment plans that are comparable to the MLCDAO plans. The delivery times for the JODAO plans were between 15 and 20 minutes. This compares to the delivery times of 7 to 12 minutes for the MLCDAO plans. Conclusion: Using DAO, it is possible to create IMRT treatment plans that utilize only independent collimators. In addition, these “jaws‐only” plans can be delivered in a reasonable amount of time. This can make IMRT feasible in clinics which have linear accelerators not equipped with an MLC.
SU‐FF‐T‐102: Dosimetric Properties of Respiratory‐Gated Intensity Modulated Arc Treatments
Authors: Wong E , Allanson S , Berryhill G , Chen J , Gaede S
Published: June 2005

Purpose: We investigated the dosimetric effects of gating on Intensity Modulated Arc Treatments. Method and Materials: One method to address respiratory motion during radiotherapy is 4DCT in conjunction with gated treatment. In the treatment of a solitary lung nodule or liver metastasis, intensity modulated arc therapy (IMAT) provides desired dose distributions. Here, we determined the dosimetric errors from delivering gated IMAT under a variety of conditions, including monitor units (MU), dose rate (MU/min), and direction of gantry rotation. We employed Varian Real Time Monitoring system for the gated delivery on a 2100C/D accelerator. To test the accuracy of the radiation delivery system, we used a static solid water phantom (IMRT phantom, MedTec). We chose a lateral 90° arc to stress test the gantry rotation delivery against gravity. Film dosimetry and point dose were performed. We delivered 30MU and 270MU treatments and used Kodak XV and EDR2 films respectively. RIT software was used to perform film dosimetry and analyses. Point doses at the isocentre were measured using a Scanditronix RK ion chamber (0.12cc). Dose distributions from gated deliveries are compared to non‐gated deliveries, normalized by the dose at isocenter. Amplitude gating was used with a 25% duty cycle. Results: With gating, we measured ≤1.5% change in dose at the isocentre. From film dosimetry, we observed beam transverse asymmetry in the gated delivery. Dose error < 3% was measured away from the isocentre using gated IMAT when the gantry is rotating at its maximum speed (295°/min). Errors less than 3% were measured when gantry rotates at or slower than 200°/min under clinical conditions. Conclusion: Gate IMAT treatments yield consistent output at the isocentre but accumulate beam symmetry errors. These errors are proportional to the gantry rotational speed which can be reduced to below 3% by lowering the delivery dose rate.
SU‐FF‐T‐104: Rotating Aperture Optimization — Planning and Delivery Characteristics
Authors: Otto K , Milette M , Schmuland M
Published: June 2005

Purpose: To describe and evaluate the dosimetric characteristics of a new method for directly optimizing dose distributions with MLC shaped apertures that fully exploit collimator rotation. Method and Materials: In direct aperture optimization only the leaf positions and segment weights are used to optimize the three dimensional dose distribution. It has been previously shown that equivalent dose distributions may be generated that have significantly fewer MU and number of segments when compared to fluence based optimization methods. Here we introduce an expansion of direct aperture optimization that includes collimator rotation. Collimator rotation allows for the generation of high spatial resolution dose distributions that are more efficient and have less interleaf errors. IMRT plans for a prostate as well as a nasopharynx target were generated to evaluate the benefits of combining both techniques. Results: Dose volume analyses showed that conformal dose distributions may be generated with only 6 segments per beam and with an average 29% reduction in MU when compared to our fluence based leaf sequencing with collimator rotation. Collimator angle was found to be accurate and reproducible to within 0.5 degrees and was independent of gantry angle on our Varian cl21ex linac. The accuracy of the MLC fluence model was also validated using film based verification methods. Conclusion: Our results show that rotating aperture optimization is capable of producing high quality dose distributions with a small number of segments and significantly few monitor units than a fluence based IMRT technique. Also, quality assurance tests of collimator rotation IMRT delivery show that current MLC/collimator rotation control systems are capable of delivering the dose distributions accurately and reproducibly. Conflict of Interest: This work was supported in part by Varian Medical Systems.
SU‐FF‐T‐106: A Greedy Segment Generating Algorithm for IMRT
Authors: Li K
Published: June 2005

Purpose: Time is one of the factors that affect IMRT. Decreasing the number of segment is necessary. Knowing the fact that the most optimal number of segments can be reached is a help to estimate the plan's optimization level on time factor. Also an ideal optimal number of segments can help to overcome MLC delivery constraint without increasing the segment number too much. Therefore, greedy algorithm is used to find an optimal number of unconstraint segments. Method and Materials: For each IMRT intensity map, an objective function is defined as intensity level multiplied by the area which is the number of grids. Then two‐ dimension search problem turns into one dimension search problem so that greedy strategy can be used. The search space turns into O(N), which N is the number of intensity level. By sorting the objective function values, the intensity level satisfying that volume with maximum dose is delivered first (MDDF). Iteratively, an optimal segments sequence is generated. Given maximum step size, with the intensity map from IMRT plan being generated by CMS Xio plan system, this number of segments is compared with that generated by slide window algorithm. Results: To deliver the same intensity map dose, MDDF algorithm gives the smallest number of segments compared to slide window algorithm from CMS Xio plan system given maximum step size. Conclusion: The optimal delivery segments sequence generated by using MDDF algorithm can be an estimate of the IMRT delivery optimization level and the sequence can be a start for embedding MLC hardware constraint by divide‐and‐conquer strategy.
SU‐FF‐T‐108: Computing the Regularization Parameter for Inverse Treatment Planning Using the L‐Curve Method
Authors: Chvetsov A
Published: June 2005

Purpose: Regularization of inverse treatment planning can improve dose uniformity and decrease delivery time of intensity‐modulated radiotherapy (IMRT) thus improving the radiobiological effect. We attempt to select an optimal value of the regularization parameter which improves the smoothness of beam intensity functions without deterioration of accuracy of fit in the IMRT optimization problem. Method and Materials: We apply to inverse treatment planning the L‐curve method which was developed to determine an optimal value of regularization parameter in the discrete ill‐posed problems. The method is based on finding the regularization parameter which minimizes the residual norm which is a measure of accuracy of fit and the solution norm which is a measure of smoothness of solution. The main idea of the L‐curve method is to plot the smoothing norm as a function of the residual norm for all values of the regularization parameter. Remarkably, this characteristic curve has an L‐shaped dependence. The optimal value of regularization parameter can be found at the “corner” of the L‐curve. Results: We apply the L‐curve criterion to the optimization problem of the prostate radiotherapy cancer treatment with intensity‐modulated beams. We show the distinctive L‐shaped dependence of the smoothing norm as a function of the residual norm. Of great practical importance is the vertical part of the L‐curve because the smoothing norm is decreasing and the residual norm remains approximately of the same value. The corner of the L‐curve is located using criteria of 1) maximum curvature and 2) minimum total of the residual and smoothing norm. Conclusion: The L‐curve method can be an important graphical tool for selection of the regularization parameter in inverse treatment planning. The regularization parameter determined using the L‐curve provides the optimal trade‐off between the smoothness of beam intensity functions and the accuracy of fit.
SU‐FF‐T‐109: Fast Integer‐Valued Smoothing of Beam Profiles in Intensity Modulated Radiation Therapy
Authors: Lee E , Lewis D
Published: June 2005

Purpose: The delivery time for an intensity‐modulated radiation therapy plan using the step‐and‐shoot method may be impractical for complex beam profiles that require a large number of segments. We propose a fast smoothing algorithm which smooths a beam profile to integer‐valued intensities. Method and Materials: An integer program was formulated that smooths a beam profile to integer‐valued intensities. The user specifies the permitted intensity level values, the maximum number of intensity levels, and the percentage of total under‐/over‐dosage permitted. The IP minimizes the absolute difference between each beamlet intensity and a weighted average of the intensities of the beamlet's nearest neighbors (and itself) in the smoothed plan. The method is tested on two optimal head‐and‐neck plans, each with seven beams. Both plans were designed so that no pixel is permitted to have an intensity greater than 20. The number of intensity levels in each beam ranges from 71 to 124. Results: For all beams, a feasible integer solution was obtained within 15 seconds. This held true even after the total intensity delivered by the smoothed beam profile was constrained to be either the floor or ceiling of the total intensity of the original beam profile. The smoothed profiles were permitted to use up to ten distinct integer values between 1 and 20. Conclusion: This work indicates the potential of a quick heuristic for smoothing complex intensity profiles. The resulting beam complexity reduction improves deliverability of the leaf sequence of each beam. Further research is necessary to determine the effects of local changes in beamlet intensity on the dose received by the planning target volume and organs‐at‐risk.
SU‐FF‐T‐110: Fast Inverse Dose Optimization (FIDO) for IMRT Via Matrix Inversion Without Negative Beamlet Intensities
Authors: Goldman P , Chen J , Battista J
Published: June 2005

Purpose: To optimize an IMRT treatment plan by a matrix inversion method that is very fast, robust and yields a global optimization minimum without negative intensities. Given the availability of online imaging tools, the conformal dose distributions obtained through IMRT and its dynamic delivery features, adaptive radiotherapy becomes an important factor to be considered. A fast and reliable optimization algorithm is crucial not only for designing good radiation treatment plans but also for the successful implementation of future interactive adaptive treatment techniques. Method and Materials: The objective function for the optimization of a large number of beamlets is reformulated such that the optimization problem is reduced to a linear set of equations. The optimal set of intensities is found through a matrix inversion, and negative beamlet intensities are avoided without the need for externally imposed ad‐hoc constraints. The objective function remains quadratic and the optimal result obtained corresponds to a single global minimum. Results: The method has been demonstrated with a test phantom and a few clinical radiotherapy cases using primary photons in the dose calculation only. We have showed that the new method is not only faster but also gives better optimization results. Typical optimization times for a single anatomical slice (2D) (head and neck) using a standard LAPack matrix inversion routine in a single processor desktop computer, are: 0.03 sec. for 500 beamlets; 0.28 sec. for 1,000 beamlets; 3.1 sec. for 2,000 beamlets and 12 sec for 3,000 beamlets. Conclusion: We have developed a fast and robust technique to find a global minimum of an objective function that yields excellent results for the inverse optimization problem for the radiation treatment of tumours with IMRT.
SU‐FF‐T‐114: Local Minima in Anatomic Aperture‐Based IMRT Optimization
Authors: Aubry J , Beaulieu F , Beaulieu L , Tremblay D
Published: June 2005

Purpose: An anatomic aperture‐based IMRT optimization program, named Ballista, was recently developed at our institution. Even though studies previously published concluded local minima in full‐IMRT optimization were not problematic, early observations with Ballista revealed their nuisance in the case of simplified IMRT. The purpose of this study was to evaluate the extent of local minima and their impact on the optimization process.Method and Materials: In Ballista beam weights are optimized by a bound‐constrained quasi‐Newton algorithm, which cannot escape local minima, even with a quadratic dose‐based objective function. Therefore, a high number (20 000) of descents were launched with random initial weights to explore the solution space for a varying number of beams. Actual treatment plan DVHs corresponding to different local minima were analyzed, yielding information on the nature of those minima. Results: When only four beam weights were optimized, only a few but very distinctive local minima were found. For a more realistic case of 20 beam weights, the optimization revealed an astonishing number of local minima, almost forming a continuum in the objective function value space. DVH analysis showed local minima generally favor one or more organs‐at‐risk (OARs) while the other objectives, especially those concerning the target volume, are less than optimal compared with the global minimum. Also, all minima lie on the boundary of the solution space. It was found that limiting the initial beam weights to small values eliminates the vast majority of the solution space containing local minima. Conclusion: With Ballista local minima proved to be a major problem. Plans corresponding to different minima differed drastically. In order to give the optimization a “clear shot” at the global minimum, initial beam weights must be limited to small values. This focuses the optimization on improving the target volume objectives since all OARs objectives are initially met.
SU‐FF‐T‐115: Magnitude of Dose Optimization with 2‐Field Left Breast IMRT
Authors: Shang C , Girouard R
Published: June 2005

Purpose: To quantify the magnitude of dose optimization in 2 opposed tangential‐field IMRT for left whole breast irradiation. Method and Materials: 6 left breast IMRT cases were randomly selected from our recent IMRT group for analysis. Each had 3 plan trials using 2 opposed tangential fields of 6 MV photons with heterogeneity corrections comprise a wedge paired 3D plan, IMRT plan with lung and heart dose constraints, and IMRT plan without organ avoidance (IMRT0). In all trials the prescribed 50.40 Gy conceals 96% of PTV with clinically acceptable dose homogeneities. PTV was defined as entire involved breast with 5 mm margins from skin surface, lung and heart. A smaller volume was used for IMRT optimization due to electron buildups. The global maximum doses, doses at the cardiac surrogate (1 cm cardiac peripheral ring of in the upper‐left quadrant), and volume of the left lung receiving 25.00 Gy dose (V25Gy) were compared. Results: Although global maximum doses were improved from 60.99 ±1.62 Gy in 3D plans to 59.96 ±0.97 Gy in IMRT0 plans, no statistical differences (p ≥ 0.10) were indicated between groups, including IMRT (60.68 ±1.63 Gy) group. Mean doses to cardiac surrogate (12.61 ±6.45 Gy for 3D, 10.77 ±3.68 Gy for IMRT, and 11.83 ±3.08 Gy for IMRT0) were technically undistinguishable. Similar V25Gy for left lung was seen in all 3D, IMRT, and IMRT0 groups (12.8%, 12.5%, and 14.8% respectively). Conclusion: Observed 1.02 Gy in prescribed 50.40 Gy or 1.5% reduction in global maximal dose by IMRT 0 from 3D plans did not reach statistical significance due to limited sample size. With 96% PTV dose coverage and acceptable dose homogeneities, IMRT or IMRT 0 plans did not show meaningful dose sparing for cardiac surrogate or left lung in this investigation.
SU‐FF‐T‐116: Multicriteria IMRT Planning with Equivalent Uniform Dose (EUD) Objectives: Tumor Dose Homogeneity vs. Critical Structure Sparing
Authors: Craft D , Bortfeld T , Halabi T , Martin B , Uhrig M
Published: June 2005

Purpose: To quantify the tradeoff between target dose homogeneity and critical structure sparing in two typical IMRT situations (prostate, para‐spinal). Furthermore, to determine the sensitivity to the response model used for critical structures (maximum vs. mean dose). Method and Materials: An EUD‐based multicriteria linear programming environment has been developed. In this work, we enforce a tumor minimum dose and compute solutions which efficiently tradeoff the tumor maximum dose and organ‐at‐risk (OAR) EUD (α⋅max dose+(1 − α) ⋅mean dose). Pareto surfaces resulting from different OAR α values are compared. The technique is applied to the RTOG horseshoe target and circular OAR geometry (varying the OAR's size and location), and to two clinical cases. Results: Mathematically, if the maximum and mean doses of a structure are correlated then the choice of α does not affect the shape of the Pareto frontier. We demonstrate that this correlation is stronger for smaller OARs (a single voxel has a large impact on the mean), and also for symmetrically located OARs, which have a large set of outer ring voxels near the maximum level, as opposed to asymmetrically located OARs where the maximum dose is more localized. As the dose requirements in the tumor get more strict, we see less variance with α, since the feasible solution space is smaller. We consistently see little to no difference between Pareto surfaces for α from 0.5 to 1. Conclusion: By characterizing the conditions under which the Pareto frontier is insensitive to α, we highlight situations where it may not be necessary to know the best value of α, i.e., the exact tissue organization between purely serial and purely parallel. In general we see smooth Pareto surfaces but in some cases there were kinks pointing to outstanding treatment plans.
SU‐FF‐T‐117: On Probabilistic Treatment Planning: A Novel Concept for Including Organ Motion Into IMRT Optimization
Authors: Unkelbach J , Maleike D , Oelfke U
Published: June 2005

Purpose: We investigate an offline strategy to incorporate interfraction organ motion into IMRT treatment planning. In order to improve the sparing of healthy tissue it was suggested that regions where both, tumor and an organ at risk can be located is irradiated with a lower dose than the dose prescribed to the tumor. This has to be compensated for by irradiating neighboring regions with a higher dose. The approach potentially allows for better sparing of healthy tissue but on the other hand, delivering inhomogeneous dose distributions per fraction is associated with higher risks. In order to make such a treatment planning approach robust and safe, one has to quantify and minimize the associated risks. Method and Materials: Practically, such an inverse planning approach can be realized when the optimization of the dose distribution is based on the expectation value of the dose. As a surrogate for the associated risks, we calculate the variance distribution by applying Bayesian inference. The variance term is added to the objective function and is hence minimized in the treatment planning process. Treatment planning is demonstrated for a prostate patient. Results: The inverse planning method yields three 3D distributions: a) the expectation value of the dose, b) its standard deviation in each point and c) a dose distribution that maps the dose delivered in a single fraction. The latter shows characteristic dose inhomogeneities within the CTV which are likely to be leveled out when the prostate is in different positions from day to day. However, this may not always be the case. The standard deviation quantifies the average uncertainty of the dose prediction. The dose delivered in a single fraction represents to some extent a worst case szenario. Conclusion: Finally, the treatment planner has to find a reasonable tradeoff between potential benefit and risks.
SU‐FF‐T‐119: Optimization of Basis Function Sets to Represent IMRT Intensity Patterns in Inverse Planning
Authors: Coselmon M , Larsen E , McShan D , Fraass B
Published: June 2005

Purpose: To investigate the use of mathematical basis functions instead of discrete or smoothed beamlets to represent and optimize IMRT beams in inverse planning in order to reduce the degrees of freedom required for producing high quality IMRT plans. Method and Materials: Our in‐house beamlet‐based optimization system was extended to support the optimization of basis function coefficients in order to represent IMRT beams by mathematical surfaces. Comparison studies were performed using a phantom and a prostate case. Four methods were compared; (1) beamlet optimization, (2) beamlet optimization incorporating smoothing in the cost function, (3) Basis function optimization (BFO) using a radial basis function grid (optimization variables are individual function weights), and (4) BFO using polynomials (optimization variables are term coefficients). Results were compared using dose and dose‐volume metrics, beam modulation, MU, and robustness to geometric deviations. Results: In the phantom, BFO plans were comparable to beamlet plans in terms of dose and dose‐volume metrics and superior in terms of using 75–90% fewer optimization variables, requiring 26% fewer MU, containing 38% less plan modulation, and demonstrating improved target coverage when subjected to geometric shifts. Beamlet‐based plans that incorporated smoothing met the clinical objectives, with a 16% reduction in MU compared to beamlets. In the prostate, method (3) resulted in a 24% MU reduction compared to method (1) and was less sensitive to geometric changes. Method (2) also produced favorable results in the prostate with a 19% reduction in MU. Conclusion: BFO plans were clinically comparable to beamlet plans and superior to beamlet plans with smoothing in terms of MU reduction and lessened geometric sensitivity. The use of basis function sets to represent and optimize IMRT intensity patterns is a promising method to reduce IMRT beam complexity and its implications. Supported in part by NIH P01‐CA59827.