bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐46: Performance of Xoft FlexiShieldTM Flexible X‐Ray Shielding in Laboratory Tests and in a Goat Mammary Model
Authors: Rusch T , Axelrod S , Smith P
Published: June 2005

Purpose: These studies measured the low energy x‐ray shielding effectiveness of Xoft FlexiShield™ flexible tungsten‐silicone sheets. The ability of the shielding material to lower the ambient radiation level in a treatment room was then evaluated during dose delivery to goats in the course of simulated accelerated partial breast irradiation (APBI). Method and Materials: X‐ray attenuation of 1 mm thick tungsten‐silicone flexible sheeting was measured using a collimated beam from a Xoft AXXENT™ X‐ray Source operated at 30 to 50 kV. X‐ray attenuation was calculated as the ratio of air kerma rate from the Source measured using an Exradin A600 Ionization Chamber with and without the shield in the beam path. To evaluate shielding effectiveness in a clinical setting, exposure rate was measured during simulated APBI of four Nubian milk goats with balloon applicators inserted percutaneously into simulated lumpectomy cavities in their udders. A Victoreen 451B Ion Chamber Survey Meter was used to measure exposure rates at twelve locations with nominal distances of 1 meter from the udder being treated. Results: X‐ray attenuation measured using the collimated beam was 104 to 106 at 30 to 50 kV operating voltage. A calculated lead‐equivalence of 0.45 mm at 50 kVp was based on the equivalent of 0.35 mm thick tungsten in the 1.0 mm thick composite sheet. During dose delivery to goat udders draped with FlexiShield™ the average ambient exposure rates at 1 m were 1.3 and 13 mR per hour at 40 and 50 kV operating voltages, respectively. The exposure rate at 50 kV was 170x lower than without shielding. Conclusion: Xoft FlexiShield™ flexible tungsten‐silicone sheet is a conformable low energy x‐ray shield that very effectively reduces the ambient exposure rate while performing APBI. Conflict of Interest: Research was supported by Xoft, Inc.
SU‐FF‐T‐52: Selecting Patients with a Pretreatment Post‐Void Residual Urine < 100 Cc May Favorably Influence Brachytherapy‐Related Urinary Morbidity
Authors: Galbreath R , Beekman M , Merrick G , Butler W , Allen Z , Gutman S , Usher J
Published: June 2005

Purpose: To evaluate the relationship between pretreatment post‐void residual urine (PVR) < 100 cc and brachytherapy‐related urinary morbidity. Method and Materials: 204 patients with a pretreatment PVR underwent permanent prostate brachytherapy with urethral sparing techniques (100–140% mPD). Patients were stratified into pretreatment PVR cohorts ⩽ 20 cc, 20–50 cc, and ⩾ 50 cc. In all patients, an alpha blocker was initiated prior to implantation and continued until the International Prostate Symptom Score (IPSS) returned to baseline. IPSS resolution was defined as a return to within 1 point of baseline. Clinical, treatment and dosimetric parameters evaluated included pretreatment PVR, patient age, PSA, Gleason score, clinical T‐stage, risk group, preimplant IPSS, ultrasound volume, planning volume, isotope, values of the minimum dose received by 90% of the prostate gland (D90), the percent of the prostate volume receiving 100%, 150% and 200% of the prescribed mPD (V100/150/200), urethral dose, pretreatment TURP, hypertension, diabetes and tobacco status. Catheter dependency and the need for postsurgical intervention were also evaluated. Results: In patients with a pretreatment PVR < 106 cc, stratification into PVR cohorts ⩽ 20 cc, 20–50 cc, and ⩾ 50 cc did not predict for meaningful differences in urinary morbidity. For the entire cohort, the mean time to IPSS resolution was 2.5 months. The urinary catheter was removed on the day of implantation in 171 patients (83.8%) with no patient remaining catheter dependent for > 3 days. In multivariate analysis, pretreatment PVR predicted for clinically irrelevant differences in IPSS resolution and did not influence catheter dependency. To date, no patient has required postimplant surgical intervention. Conclusion: The selection of patients with a pretreatment PVR urine < 100 cc was associated with rapid IPSS resolution, the absence of prolonged (> 3 day) catheter dependency, and the elimination of post‐brachytherapy surgical intervention for bladder outlet obstruction.
SU‐FF‐T‐55: Variability of Prostate Brachytherapy Preimplant Dosimetry: A Multi‐Institutional Analysis
Authors: Butler W , Merrick G , Wallner K , Blasko J , Michalski J , Aronowitz J , Grimm P , Moran B , McLaughlin P
Published: June 2005

Purpose: To conduct a multi‐institutional comparison of prostate brachytherapy preimplant dosimetry for Pd‐103 and I‐125. Method and Materials: Eight experienced brachytherapists submitted Pd‐103 and I‐125 monotherapeutic and boost preimplant dosimetry plans for central review. All 32 plans were calculated using the same transrectal ultrasound volumetric study. Seeds of any strength were acceptable, but were restricted to two manufacturers. The dosimetric analysis included evaluation of target volume, target to prostate ratio, target length, number of needles, seed activity, number of seeds, total activity, total activity per treatment planning volume, use of extracapsular seeds, and average treatment margins (distance between the prostate capsule and the 100% isodose). Prostate coverage was defined in terms of V100/150/ 200/300 and D100/90/50 while urethral dosimetry consisted of UV100/150/200 and UD90/50. Results: The mean planning target volume to prostate volume ratio varied dramatically (mean 1.29, range 0.99 – 1.76) with the target length ranging from 3.5 to 4.5 cm. Although the prostate V100 was > 95% in all cases, the V150 ranged from 29.9 to 92.1% and the V200 from 6.72% to 52.5%. The urethra V100 was 100% in all cases, with 6 of the 8 brachytherapists limiting the UV150 to < 3%. However, the median urethral dose varied by up to 50%. Treatment margins also varied significantly (average 3.98 mm, range 0.32 – 7.68 mm). All brachytherapists used extracapsular seeds with five implanting > 25% of the seeds in extracapsular locations (range 6.4% – 58.2%). In addition, significant variability existed in the number of needles, the number of seeds, and seed strength. Conclusion: This study highlights that although prostate brachytherapy prescription doses are uniform, substantial variability exists regarding target volume, seed strength, dose homogeneity, treatment margins and extracapsular seed placement. The standardization of preimplant dosimetry is essential for meaningful multi‐institutional comparisons of biochemical outcomes and morbidity.
SU‐FF‐T‐63: Breast IMRT Based On Direct Aperture Optimization
Authors: Ahunbay E , Albano K , Jursinic P , Li XA
Published: June 2005

Purpose: To investigate the advantages of DAO (Direct Aperture Optimization) based IMRT planning for breast cancer. Method and Materials: The IMRT plans were generated, using a DAO algorithm implemented in a commercial planning system (Panther, PROWESS), for 5 breast cancer patients who were treated in our clinic with 3D conformal plans. These 3D plans were produced by the Xio planning system (CMS) employing 4–6 tangential beams of different energies and wedges. In addition, we generated another IMRT plan for each of these patients using a beamlet‐based conventional IMRT planning system (Xio, CMS). Both the DAO and beamlet‐based IMRT plans employed two opposite tangential beam directions with about 70% of the dose delivered with stationary fields covering whole breast with 2–3 cm of flash. The target volume for all plans was the whole breast tissue; boost PTVs were not considered. All plans were done with homogeneous density calculation. The three sets of plans, DAO IMRT, 3D and beamlet IMRT, were compared in terms of dose uniformity, hot spots, normal tissue sparing, number of segments and MUs. Results: The results show that: 1) dose uniformity and coverage of breast is improved significantly with either IMRT methods compared to 3D, DAO method providing slightly better coverage than conventional beamlet method, 2) hot spots typical for an IMRT plan with a large number of segments are significantly reduced, for the DAO plan, 3) number of segments and MUs with the DAO plan were significantly less than those for beamlet IMRT plan. Also, the planning time for a DAO plan is short compared to a 3D plan which involves a search of multiple wedges and energies by a trial‐and error method. Conclusion: DAO based IMRT planning is capable of generating superior IMRT plans for breast irradiation, in terms of dosimetric criteria and delivery efficiency.
SU‐FF‐T‐66: Comparison of IMRT Plans with Tissue Heterogeneity Corrections Using the Pinnacle3 and CORVUS Treatment Planning Systems
Authors: Pillai S , Ayyangar K , Li S , Nehru R , Zhen W , Enke C
Published: June 2005

Purpose: To study and compare the results of IMRT plans with tissue heterogeneity corrections using the Pinnacle3 and CORVUS treatment planning systems (TPS). Method and Materials: A 30 cm × 30 cm × 15 cm solid water phantom embedded with a 7.5 cm styrofoam slab simulating air was scanned on Siemens Emotion Duo and the image set was transferred to both TPS for dose computation and analysis. A few clinical cases planned on the CORVUS were also transferred to Pinnacle3 and computed with heterogeneity corrections. Results: The dose profile along the central axis of a 23MV beam inside phantom was generated on both TPS and was compared with the Battista et al1 data from TG65. It is noted that a gradual build‐up effect after the in homogeneity region in the case of Pinnacle3 but it is not noticed in the case of CORVUS. In the case of clinical lung plan, CORVUS overestimated the dose homogeneity and uniformity in the target when compared to Pinnacle3 TPS. When re‐optimized and re‐computed on Pinnacle3 TPS, the target coverage on DVH was better than CORVUS. Discrepancies of 33% (relative to the prescribed target dose) in the target region were found in CORVUS TPS calculations. Conclusion: The FSPB algorithm as implemented in CORVUS results in significant under‐ or over‐estimation of the dose in some cases involving heterogeneities such as the air‐tissue, and lung‐tissue. Comparatively, the convolution algorithm in Pinnacle3 is far superior and takes into account the electronic disequilibrium due to tissue heterogeneity. This is in agreement with earlier published literature using Monte Carlo computations.
SU‐FF‐T‐69: Determining Patient Eligibility for Prostate IMRT Dose Painting Protocols: The Role of Image Resolution, Margin Requirements, and Intraprostatic Tumor Volume
Authors: Osterman K , Liu T , Zhang P , Mullokandov E , Ennis R , Kutcher G
Published: June 2005

Purpose: To systematically examine the impact of margin and imaging resolution on dose painting protocols with regards to target definition and patient eligibility. Method and Materials: The prostate PTV was formed by expanding the clinical target volume by 1‐cm. The intraprostatic tumor nodules (IPTNs) were digitized using data from Chen et al.(2000) corresponding to a >50% probability of occurrence. The IPTNs were then discretized on three different grids: 1mm2 (our treatment planning pixel size), 2mm2, and 6.6mm2; grid sizes reflect imaging modality resolution. 75.6Gy was prescribed to the PTV and the target dose for the IPTNs was 90Gy. Our standard rectal and bladder constraints were used with maximum dose of 91Gy to rectum and urethra. The margin on the tumor nodules was varied from 0.2 to 1.5cm. Results: The volumes for the IPTNs were 3.2, 4.9 and 10.2cc for the three grid sizes; CTV and PTV volumes for the whole gland were 44.2cc and 137.2cc. A typical distribution of IPTNs (4 foci), expanded with a 6mm symmetrical margin to 33cc was boosted to 90 cGy without exceeding critical organ constraints. Margins were 5mm and 5.5mm with grids applied. Once the absolute volume of the targets (IPTNs + margin) was accounted for, however, neither grid size nor margin size had an impact on the ability to escalate dose. The ratio of the boost region to the PTV volume was the determining factor. The limiting IPTNs+margin volume was 25% of the PTV volume. By relaxing the requirements to accept >90% coverage with 90Gy, acceptable plans could be achieved with IPTNs+margin volumes occupying ⩽ 36% of the PTV volume. Conclusion: A simple volume‐based screening method may be used to determine patient eligibility for inclusion in a clinical IPTN dose‐escalation study, irrespective of the cancer‐specific imaging system's resolution and required margins.
SU‐FF‐T‐71: Effect of Daily Shifts of IMRT Prostate Patients On Dose to Pelvic Nodes
Authors: Halabi T , Bortfeld T , Craft D , Wolfgang J , Gierga D , Zietman A , Shih H
Published: June 2005

Purpose: Pelvic lymph nodes are often incorporated en bloc with the prostate as targets in radiation treatment. It is common practice to realign the fields daily before each treatment to account for prostate motion; however, pelvic nodes are relatively immobile such that adjusting the radiation fields to track the prostate may lead to a geographic miss of the nodes. Here, we explore the magnitude of this problem. Method and Materials: Information from two patients was used in this analysis. IMRT plans were created using the NOMOS/Corvus system and PTVs extending 1.0 cm about the nodes CTV in all directions were planned to 45 Gy in only 25 of the approximately 40 total fractions. Daily field shifts were made by pretreatment ultrasounds of the prostate using the B‐mode acquisition and targeting system. Dose of each shift was recalculated using Corvus and the results analyzed using Matlab and CERR. In addition, a random number generator used a clustered probability distribution derived from the total 40 or so shifts to produce alternative scenarios to the 25 shifts. This allowed evaluation of multiple scenarios without need for further timely dose calculation. Results: In all simulations, the cumulative dose over all shifts showed little under‐dosage, most of which was at the histogram's tail. For the 90% CTV volume there was a reduction of around 0.7% of prescribed dose. In shifted plans the maximums of each fraction no longer overlap in the same tissue so that most under dosage is expected to be at the histogram's tail. Conclusion: These results suggest that current PTV expansions are adequate to provide prescribed dose coverage of CTVs. It may be possible to further refine PTV expansion definitions to reduce radiation to normal tissues while maintaining treatment delivery to target tissues without causing a geographic miss.
SU‐FF‐T‐75: IG IMRT of Tumors On Convex Surfaces with Grazing Incidence Photon Therapy
Authors: Zacarias A , Mills M
Published: June 2005

Purpose: The treatment of tumors on convex surfaces with a uniform dose is a challenging problem in radiotherapy. Traditionally, surface tumors have been treated with electron beams or a combination of electron and photon beams with consequent problems of uniformity at regions of abutment. Grazing Incidence Photon Therapy (GIPT) can potentially provide better dose uniformity and tissue sparring than current standards of care but is typically more complex and requires exacting set‐up conditions. The advent of image guided (IG) intensity modulated radiation therapy (IMRT) provides a new opportunity to explore potential of this method for the treatment of tumors on convex surfaces. The treatment of the total scalp with GIPT is an ideal test of the potential of IGIMRT because the beams are grazing incidence throughout the treatment volume and a sensitive structure is in close and constant proximity. Method and Materials: Helical tomotherapy plans using GIPT were developed for the Rando phantom and a patient for the treatment of the total scalp. A patient plan using the Lateral Photon Electron (LPE) technique was developed for comparison. TLD measurements were made with Rando for dose verification in addition to dosimetry quality assurance. Results: The Rando plan and dosimerty demonstrate that the IMRT method using GIPT provides a clinically acceptable treatment. The patient plan shows a reduction in scalp dose non‐uniformity by 34% and improved sparing of peripheral brain by a reduction of the volume exposed to ⩾40 Gy to 17% of the LPE method. Conclusion: GIBT of the total scalp is clinically feasible when implemented with IGIMRT. It substantially improves the quality of patient care when compared to current standards of care by resolving the issue of abutting fields. This has broad implications for the treatment of tumors on convex surfaces by IGIMRT.
SU‐FF‐T‐76: Implementation and Initial Testing of a Monte Carlo Based Algorithm for IMRT Inverse Treatment Planning
Authors: Rosu M , Coselmon M , Acosta E , Fraass B , McShan D , Chetty I
Published: June 2005

Purpose: To report on the implementation of a Monte Carlo (MC) based algorithm and to compare this system with a convolution/superposition‐based algorithm (CS) for IMRT inverse planning. Method and Materials: The DPM MC code was modified using a fluence matrix approach to perform beamlet calculations for IMRT planning. The code was integrated within our in‐house inverse treatment planning system and compared with the TPS (CS) algorithm. Initial testing involved the computation of 6 MV beamlet depth doses for 1×1, 2×2 and 10×10 (100, 1 cm beamlets) in a water phantom. MC and CS calculations were then performed for an example lung treatment plan to examine dosimetric differences between these algorithms. MC statistical uncertainties were on average less than 2% (in the depth doses) for all beamlet calculations. Optimization of beamlet doses is carried out using simulated annealing with quadratic cost functions derived from our clinical protocols. Results: Beamlet depth doses calculated with MC and CS are in good absolute agreement for field sizes larger than 2×2 cm2. Significant differences exist for 1×1 beamlets because CS is unable to accurately model lateral electron transport. For the example lung plan, much smaller differences were found. This is likely due to the fact that with larger field sizes (∼10×10 cm in the example), effects of lateral electron scattering are much less pronounced. Conclusion: We have implemented a fluence matrix method to perform MC‐based beamlet calculations for IMRT planning. Initial testing for an example lung plan and field sizes larger than 2×2, revealed good agreement between MC and CS. However, larger differences were found for 1×1 beamlets due to lateral electron transport issues. Testing is currently being performed for a variety of treatment plans, spanning a range of field sizes to thoroughly investigate dosimetric differences between MC and CS in IMRT planning. Supported by NIH P01‐CA59827
SU‐FF‐T‐78: IMRT Treatment Planning for Mantle Fields
Authors: Ndlovu A , Barbiere J , Ingenito A , Hanley J
Published: June 2005

Purpose: To develop a method for Mantle field IMRT treatment planning using commercially available equipment. Method and Materials: In this study all plans were created for a Siemens Primus linear accelerator on a Philips Pinnacle Treatment Planning System. A crucial feature of the treatment planning system was the capability to create dose‐based regions of interest (ROIs). The Siemens accelerator was chosen for its ability to deliver treatments using magna fields without field‐splitting. Patients were CT scanned and the data transferred to the planning system. Traditional mantle fields, including 50% transmission lung blocks, were drawn on the AP/PA digitally reconstructed radiographs. An initial plan with equally weighted AP/PA beams was set up with a homogeneous calculation to a point at mid separation. The 50% isodose curve of the resultant distribution was contoured and converted to a physical target volume. The isodose representing the involved lungs under the partial transmission blocks was also contoured and converted to a second ROI target. The lungs were manually outlined as critical structures. An IMRT plan with DVH‐based target objectives was performed and compared with a protocol‐based 3D conformal plan. Results: Creation of ROIs using dose contouring was determined to be straightforward and reliable. IMRT optimization using the ensuing ROIs was found to be reproducible and robust. For the case study presented, the 3D plan showed a 37.0% decrease in the ICRU defined conformity index for 100% of the prescribed target dose and an undesirable 63.5% increase in the volume of lung receiving 80% of the prescribed dose. Conclusion: IMRT mantle treatment planning can easily be performed in under an hour using commercially available equipment. Unlike the traditional approach with no defined target, this procedure allows for quantitative plan evaluation using both isodose distributions and dose volume histograms.
SU‐FF‐T‐82: Non‐Tumor Integral Dose in Conformal, External Beam Radiation Therapy
Authors: Westerly D , Aoyama H , Patel R , Jaradat H , Olivera G , Tome W , Ritter M , Mehta M , Mackie T
Published: June 2005

Purpose: To investigate the computed non‐tumor integral dose (NTID) delivered during treatment of prostate cancer with intensity modulated radiotherapy (IMRT) compared to three‐dimensional conformal radiotherapy (3DCRT). Low and high‐energy photon treatments were also evaluated in this study, as was helical tomotherapy for IMRT delivery versus a conventional linac. Method and Materials: Five patients with localized prostate cancer were selected. Five treatment plans were generated for each, including: IMRT with both 6 and 20 MV photons using a conventional linac (6MV‐IMRT, 20MV‐IMRT respectively), 3DCRT with both 6 and 20 MV photons (6MV‐3DCRT, 20MV‐3DCRT respectively), and IMRT with 6 MV photons delivered using helical tomotherapy (Tomo‐IMRT). For each plan, a total of 70 Gy was prescribed to 95% of the PTV and the integral dose was calculated from dose‐volume histograms for non‐tumor tissue as well as surrounding critical structures. Results: The NTID with conventional IMRT was 3.9–5.2% less than with 3DCRT, and the use of 20 MV photons resulted in 6.7–8.0% less NTID than treatments using 6 MV. Tomo‐IMRT treatments were comparable to those delivered with a conventional linac. Examination of the integral dose given to surrounding critical structures showed that, compared with 6MV‐3DCRT, 6MV‐IMRT reduced the integral doses to the rectal wall and penile bulb by 2.8% and 6.3% respectively. Tomo‐IMRT further reduced the integral doses to these structures by 12.6% and 18.0% respectively. No reductions were seen using 20 MV. Conclusion: The differences in NTID calculated from different treatment plans are relatively small and might be negligible after accounting for leakage and neutron production at higher energies. The advantage of helical tomotherapy in the treatment of localized prostate cancer was demonstrated through the greater sparing of critical structures with no significant increase in NTID. Conflict of Interest: This work was partially supported by TomoTherapy Inc. and PO1 Grant CA88960‐01‐05.
SU‐FF‐T‐87: Software Tools for Transferring Treatment Plans Between Two Planning Systems
Authors: Zhang X , Dong L , Zhu X , Zein R , Lim J , Wang X , Lii M , Xu J , Kang Y , Gillin M , Mohan R
Published: June 2005

Purpose: There is often a need to transfer treatment plan data from one system to another. However, even when both systems are claimed to be compliant with a standard (e.g., DICOM‐RT), the system‐specific implementations may be incompatible . The purpose of this work is to develop and evaluate tools to seamlessly transfer plans designed on one commercial treatment planning system (TPS) to another TPS and vice versa. Method and materials: Pinnacle and Eclipse are the two TPS used in this study. From Pinnacle to Eclipse, a filter to make the Pinnacle DICOM‐RT plans conform to Eclipse implementation was developed. From Eclipse to Pinnacle, a tool to covert a DICOM‐RT plan file to Pinnacle Script file was developed. To evaluate these tools, ten prostate patients planned and treated using IMRT at our institution were used. The 10 IMRT plans were first transferred from Pinnacle to Eclipse, and the doses of the 10 plans were recalculated on Eclipse. New IMRT plans were designed on Eclipse also. The latter were transferred back to Pinnacle and doses were re‐calculated there. Also, optimal fluence distributions generated on Eclipse were transferred to Pinnacle, and the Pinnacle's leaf sequencer was used to generate new leaf sequences. To evaluate the differences between Pinnacle and Eclipse plans, dose and dose‐volume indices were used. Results: The dosimetric data for the plans transferred to Eclipse from Pinnacle do not differ significantly from the original plans, and vice versa. For plans of similar quality, the ones designed on Eclipse had 56% fewer segments than the plans designed on Pinnacle. The Pinnacle generated 23% more segments than the Eclipse using the same optimized fluence distributions imported from Eclipse. Conclusions: DICOM‐RT implementations are often not complete and, therefore, compatible among different commercial planning systems. Special tools are needed to make the plans interchangeable.
SU‐FF‐T‐92: Volumetric Breast IMRT Planning Using ICRU‐50 Concepts
Authors: McNeeley S , Freedman G , Li J , Anderson P , Price R , Ma C
Published: June 2005

Purpose: In this study we present a method of performing volumetric breast IMRT using ICRU 50 concepts of CTV and PTV while respecting the irradiated volumes of conventional breast tangents. Method and Materials: The technique uses standard tangential beam directions. Conventional tangents are placed on the patient's planning data and are used as a guide. Breast tissue, defined by clinical palpation and CT, is contoured as the CTV. The PTV is created by expanding the CTV 0.7 cm in the medial and posterior direction to account for setup error and breathing motion. The PTV and CTV are kept to within 0.5 cm of the skin surface, and 0.6 cm of the conventional beam edges to allow for penumbra and buildup. In the superior and inferior directions the PTV may be extended to cover traditional margins. Anteriorly, the IMRT field pattern is extended 1.5 cm from the PTV into air for setup error and breathing motion. Results: Twenty patient plans were reviewed comparing conventional and IMRT CTV doses. The plans used both 6 and 10 MV. Analysis showed comparable coverage (V95) to the CTV with a reduction in the volume receiving higher dose using IMRT. The average percent difference to V95 was 0.6%. With IMRT the average reduction to V105 was 19.5%, and there was an average reduction of 1.72 Gy to the hottest 1% of breast tissue. Conclusion: The margins used in conventional breast treatment have been the standard of care for many years. Volumetric IMRT breast planning offers superior dosimetric results but defining the volumes can be a challenge. This technique allows the creation of the IMRT targets while preserving coverage of similar margins used in conventional treatment. This technique is currently used clinically at our institution.
SU‐FF‐T‐97: A Generalized MLC Segmentation Algorithm for Step‐And‐Shoot IMRT with No Tongue‐And‐Groove Error
Authors: Wang C , Luan S , Chen DZ , Hu X , Yu C
Published: June 2005

Purpose: To develop an MLC segmentation algorithm for step‐and‐shoot IMRT that can segment intensity maps with large maximum intensity levels (e.g., ranging from 10 to 100) into the minimum number of MLC‐segments in a fast computation time. Method and Materials: We improved a previous MLC segmentation algorithm, called SLSNOTG, for step‐and‐shoot IMRT by Luan et al in AAPM'2004. Our new segmentation algorithm, called GSLSNOTG, has two improvements. (1) It can segment intensity maps with large maximum intensity levels into the minimum number of MLC‐segments without tongue‐and‐groove error. The key to this improvement is a new level reduction technique that can partition an intensity map into an optimal set of sub‐intensity‐maps without introducing any additional tongue‐and‐groove error. In contrast, the previous SLSNOTG algorithm was mainly designed to segment intensity maps with small maximum intensity levels, and may introduce more tongue‐and‐groove error when segmenting intensity maps of large maximum intensity levels (due to the level reduction schemes that it uses). (2) Our new segmentation algorithm runs much faster than the previous SLSNOTG algorithm by pruning the sizes of the graphs used to model the intensity maps. This avoids using the slower k‐link shortest path routine as in the SLSNOTG algorithm, and thus significantly reduces the computation time. Results: Comparisons of our GSLSNOTG algorithm with the previous SLSNOTG algorithm and CORVUS 5.0 planning system demonstrated the advantages of the GSLSNOTG algorithm/software. For example, for an intensity map set of maximum intensity level 20, the GSLSNOTG algorithm computes 176 segments in less than 1.5 minutes. In comparison, CORVUS needs 382 segments, and the SLSNOTG algorithm runs in 10 minutes to compute a plan with the same number of segments with 2% tongue‐and‐groove error. Conclusion: The new MLC segmentation algorithm GSLSNOTG can segment intensity maps of large maximum intensity levels in a fast computation time.
SU‐FF‐T‐98: A Shuttling Window Method of Improving Intrafractional Errors for Intensity Modulated Stereotactic Body Radiation Therapy
Authors: Sun M , D'Souza W , Ma L
Published: June 2005

Purpose: Intensity Modulated Stereotactic body radiation therapy (SBRT) uses multiple intensity modulated beams to deliver a large dose in limited fractions with on‐board imaging/stereotactic guidance. Despite active organ motion managements such as abdomen depression or rigid frame alignment, random residual errors in the order of a few mm are still observed. In this study, we aim to develop a shuttling window delivery technique to minimize such errors. Method and Materials: Our method allows the MLC leaves to shuttle back and forth to accumulate the dose sequentially throughout the target volume instead of delivering a large dose to parts of the target volume. Our rationale is to avoid deleterious misfiring events from large dose depositions. A sequencing method is developed to minimize the beam‐on and leaf travel for implementing the shuttle delivery. Phantom measurements were performed for simulated and patient cases. A home‐made phantom was used to simulate random intrafractional errors of 0–5 mm with 0.5 mm resolution. Comparison between the prescribed dose distributions and the error‐intrinsic dose distributions were carried out using chi‐square confidence level analyses. Results: The shuttling window technique significantly reduced random dose errors and improved the agreements between the prescribed and the delivered dose distributions. As the number of the shutting cycles increases, the average dose variance decreases. The most gain for the technique was harvested when the shuttle number approaches five. Despite extra leaf motions, no significant increase in the treatment time was observed (e.g. < 1 minute for a 10‐minute beam). The increase in the leaf travel distance of the shuttling delivery was largely compensated by increased leaf speed during each cycle. Conclusion: We demonstrated an effective approach for reducing residual intrafraction errors for intensity modulated SBRT deliveries.
SU‐FF‐T‐100: Aperture Effects in Dynamic Fields
Authors: Higgins P , Alaei P
Published: June 2005

Purpose: We have measured collimator and phantom scatter factors (Sc, Sp) as a function of field size for a variety of moving slit widths and compared them with static field factors. Dynamic treatment plans were devised to compare the ability of two planning systems to model the aperture effect on dose delivery, especially for small targets. Method and Materials: An 80 leaf Varian 2300cd and a 120 leaf Varian 21ex accelerator were used with 6 MV photons. 0.6 cm3 ionization chambers were used, for both Sc and Scp measurements. 1–10mm leaf gaps were dynamically scanned across a range of field sizes (4×4–14×40). To mimic the small aperture effect in a treatment plan, we defined a series of cylindrical targets, 1–20 mm diameter by 3cm length and an IMRT plan using Eclipse or Pinnacle3 was developed to optimally treat them. EDR2 films were taken and used to compare delivered with planned doses. Results: The dynamic measurements of Sc and Scp were very similar on the two Varian machines and clearly demonstrated an aperture effect in the Sc measurements of as much as 30% (1mm gap, 4×4 field) which smoothly converged to the static field distribution as the gap was increased. Derived Sp values were approximately independent of gap width, essentially matching the static field cases. Planning system intercomparison of small target doses seem to indicate the Pinnacle3 system to be slightly better at correctly including aperture effect. Conclusions: Conventionally obtained output factors, using the secondary collimators, even extended down to 1–2 cm2 still do not describe the same head scatter contribution as is delivered by a dynamic aperture. Consequently, dose uncertainty may be amplified in the treatment of single or multiple small lesions using IMRT techniques.
SU‐FF‐T‐105: The Impact of Multileaf Collimator Rotation in IMRT Planning
Authors: Roedersheimer K , Chen DZ , Luan S , Xing L
Published: June 2005

Purpose: Collimator angle is a degree of freedom that has seldom been explored in IMRT inverse planning. This work investigates the impact of multi‐leaf collimator rotation (IMLCR) on a dosimetric scoring function and lays the groundwork for optimal collimator angle selection in IMRT planning. Method and Materials: The scoring function models the maximum deliverable dose at an arbitrary beam orientation and is calculated using a sampling method. To calculate the scoring function, the beam portal is divided into a grid of beamlets based on collimator movement constraints. The intensity of each beamlet is set at the maximum intensity that does not violate the tolerance of any structure intersected by that beamlet. The score is calculated from the fraction of the dose prescription deliverable by each beamlet. The IMLCR is measured as the increase in the scoring function over all collimator angles. Results: For large tumors (not fully coverable by the beam portal), the mean IMLCR is 22.2%. Collimator rotation enables more exposure of the tumor to the beam, and thus increases the irradiation of the tumor. For small tumors (fully coverable by the beam portal) with nearby critical structures, the mean IMLCR is 9.3%. Collimator rotation allows critical structures to be better isolated from tumor irradiation. When the tolerance of the nearby structures is high relative to the tumor's prescription, collimator rotation has little impact for small tumors. Conclusion: The variation observed in the dosimetric scoring function indicates that collimator rotation can significantly raise the maximum deliverable dose at a given gantry angle. When the tumor is large and/or has nearby critical structures (tolerance <=50% of the tumor's prescription), collimator rotation has the greatest impact and is worth considering. Collimator angle selection could play an important role in improving the quality of IMRT treatment plans.
SU‐FF‐T‐112: First Evaluation of a New Multicriteria Optimization Tool‐Investigation of Pareto‐Surfaces for IMRT Prostate Plans
Authors: Uhrig M , Thieke C , Alonso F , Küfer K , Monz M , Scherrer A , Oelfke U
Published: June 2005

Purpose: The aim of this paper is a first evaluation of the performance of a new multi‐criteria optimization (MCO) tool developed for IMRT treatment planning Method and Materials: The new MCO‐tool computes a set of Pareto‐optimal plans by simultaneously minimizing dose indicators for tumor targets and considered organs at risk. For the tumor, the dose homogeneity in the target is maximized for a given, prescribed mean dose while relative deviations to upper equivalent uniform dose (EUD) limits are minimized for organs at risk. The exploration of the solution space is done with a visual navigation tool, which provides control bars for each defined dose indicator. The navigation tool uses real‐time interpolation to allow a smooth transition between the pre‐computed plans. As a clinical example we consider a simplified prostate case where the only structures to be optimized are the PTV, rectum and bladder. The complete database of plans is visualized using EUD‐values of the organs at risk and the standard deviation of the target dose as axes. This approximation of the 3D‐Pareto‐surface is then examined. The sensitivity of the navigation process on the individual dose indicators is analyzed in terms of the gradients on the Pareto‐surface. Moreover, effects of rescaling the target dose homogeneity in terms of TCP‐values are analyzed. Results: The database provided by the MCO optimization for the simplified prostate case can indeed be presented as a 3D‐Pareto‐surface. The sensitivity of the navigation process is well reflected by the respective gradient on the Pareto‐surface. The rescaling of the target dose homogeneity in terms of TCP values allows studying the sensitivity of the Pareto‐surface in terms of the assumed radio‐sensitivity of the tumor. Conclusion: A first evaluation of a new MCO‐tool has been successfully completed for a simplified prostate case with 3 mutually conflicting optimization criteria.
SU‐FF‐T‐123: Robust Optimization Including Effects of Systematic Treatment Errors
Authors: Lee E , Bortfeld T , Deasy J
Published: June 2005

Purpose: Intensity‐modulated treatment planning methods have not previously accounted for systematic errors in tumor and tissue locations. We introduce a novel method for including systematic errors during the optimization phase, based on estimated probabilities of systematic geometrical shifts or tissue misidentification(i.e.tumor location). Method and Materials: Given a planning geometry, the users supply estimates of the systematic probability that tissues are actually displaced by a given vector(“ensemble”). Overall plan quality is based on probability‐weighted measures of actual plan realization. Ensembles are estimated here based on dose‐convolution with Gaussian probability distributions. Two different objective function metrics based on this paradigm are investigated:the maximum probability of tumor under‐dosage is computed using integer variables which divide dose values into under‐dose vs. adequate dose. Similarly, the probability that dose to a critical serial normal structure will exceed tolerance is formed into an objective function. A third metric is the uncertainty in the mean dose given to the tumor, averaged over ensembles. Both of these can be mixed with other standard metrics in the IMRT treatment planning optimization process. Results: Mathematical formulas for the metrics were derived. We demonstrate the use of ensemble averaging using large‐scale mixed integer programming. A systematic setup shift Gaussian probability distribution of 5 mm(half‐width) was assumed. A lung treatment plan was tested partly based on minimizing the highest probability of clinical target volume under‐dosage. The plan was more robust against systematic errors than the comparable plan without considering the robust metric. Conclusion: Systematic uncertainties can be considered directly within the optimization process by averaging over ensembles. The result is increased robustness of the treatment plans. The method of estimating dose to the ensemble treatment plans needs to be studied to potentially improve its accuracy. We have introduced novel metric functions based on explicitly considering the effects of systematic errors, thereby resulting in treatment plans of increased robustness.
SU‐FF‐T‐125: Use of Modified Fourier Series for Radiotherapy Optimization
Authors: Garnica‐Garza HM
Published: June 2005

Purpose: To introduce the use of modified Fourier series as a tool in the optimization of intensity profiles to be used in the calculation of absorbed dose in IMRT and to determine the number of terms in the series necessary to achieve a proper optimization. Method and Materials: An analytic kernel is used in conjunction with a modified Fourier series approximation to the unknown intensity profile that optimizes the dose prescribed to a certain region in a mathematical phantom. A computer program was written to perform the optimization process. The advantage of using a Fourier series is that by adjusting a few constants, the exact number depending on the number of terms in the series, the whole intensity profile is modified, instead of having to discretize such a profile and change each of the intensity levels as one searches for the optimum profile. The effect of the number of terms in the Fourier series on the quality of the optimization is also explored. Results: For the test case presented, it was determined that the number of terms in the series necessary to achieve a satisfactory solution, determined by a cost function, is 20. One problem, however, is that some of the resultant profiles show steep gradients over short distances, which poses a problem for the delivery process. Conclusion: A useful method of calculating intensity profiles for IMRT has been introduced. Further work is needed to compare the quality of the optimization achieved using this technique to that with more established methods. The technique offers a way to introduce changes in the intensity profile that allows for a rapid dose calculation, since the total intensity is a sum of sine and cosine terms for which the dose can be pre‐computed prior to the optimization process.
SU‐FF‐T‐129: A Practical Monte Carlo MU Verification Tool for IMRT Quality Assurance
Authors: Fan J , Li J , Chen L , Xiong W , Stathakis S , Luo W , du Plessis F , Ma C
Published: June 2005

Purpose: Quality assurance (QA) for intensity‐modulated radiation therapy (IMRT) treatment planning and beam delivery, using ionization chamber measurements and film dosimetry in phantom, is time consuming. The Monte Carlo method is the most accurate method for radiotherapy dose calculation. However, a major drawback of Monte Carlo dose calculation as it is currently implemented, is its slow speed. The goal of this work is to bring the efficiency of Monte Carlo into a practical range by developing a fast Monte Carlo monitor unit (MU) verification tool for IMRT. Method and Materials: A special estimator for dose at a point called the point detector has been used in this research. The point detector uses the next event estimation (NEE) method to calculate the photon energy fluence at a point of interest and then convert it into dose by mass energy absorption coefficient assuming the presence of quasi charged particle equilibrium. An improvement to this method is to initiate the electron transport at the surface of a sphere centered at the point of interest to account for the effect of electron disequilibrium and patient heterogeneous anatomy. The point detector method is implemented with the MCNP Monte Carlo code system. Results: The MU verification tool can be used for both patient dose verification and phantom QA calculation. Dose calculations in a water phantom have been performed using the point detector method. Results were compared with direct Monte Carlo simulations using EGS4/MCSIM, which is a well‐ benchmarked Monte Carlo code. The results between the point detector and MCSIM agreed within 1%. A factor of 10 speedup can be achieved with the point detector method compared with direct Monte Carlo simulations. Conclusion: An effective and efficient Monte Carlo MU verification method has been developed as a practical routine procedure for IMRT QA.
SU‐FF‐T‐137: Development of a Software Tool for Generating Predicted Dose Images From Pinnacle Dose Maps for the Purpose of IMRT Quality Assurance Using PortalVision
Authors: Norrlinger B , Smale J , Heaton R , Islam M
Published: June 2005

Purpose: To develop and evaluate a method of importing the predicted dose maps of patient treatment fields generated by the Pinnacle treatment planning system (TPS) into the dosimetry module of Varian's PortalVision electronic portal imaging system (EPID), for the purpose of verifying planned IMRT fields. Method and Materials: PortalVision, when equipped with the dosimetry module, is equipped with tools to perform relative dose comparisons of dose images acquired by the EPID vs. predicted dose images generated by a TPS. Treatment plans were created using the Pinnacle TPS. Dose maps of individual fields were saved as files using Pinnacle's Planar Dose Map function. The dose maps were calculated in a geometry equivalent to that of the detector panel. Using in‐house developed software, the dose map files were converted to the correct predicted dose format and imported into the PortalVision software. Predicted dose maps were generated for open square fields of several standard sizes, wedged fields, and IMRT fields. The IMRT fields were taken from typical prostate, and head and neck patients. Fields from ten IMRT patients were evaluated. The treatment fields were delivered using a Cl2100EX accelerator. Point dose measurements were obtained from the acquired dose image and compared with film measurements. The predicted vs. delivered dose distributions were evaluated using the available PortalVision tools. The results were compared against those of films analyzed using RIT. Results: Point dose measurements made with PortalVision in areas of high dose, low dose gradient agreed with film to an average of +/− 3%. Dose Difference and Gamma results agreed with film results to within +/− 3%. Conclusion: This work shows that a predicted dose map from a TPS can be modified and imported into Varian's PortalVision software in order to perform IMRT patient QA with accuracy comparable to traditional film methods.
SU‐FF‐T‐139: Dosimetric Effect of Jaws for Small MLC Fields in 6 and 15 MV Photon Beam
Authors: Chow J , Seguin M , Alexander A
Published: June 2005

Purpose: This study measures how the relative position of the jaws above the MLC could affect the dose characteristics of a small field. The study focused on the variations in the percentage depth dose (PDD), beam profile and output of the small field when the jaw‐ends were moved away from the leaf‐ends, that is, when the leaf leakage/transmission and jaw scattering were changed. Method and Materials: A big scanning water tank system (RFA 300, Scanditronix Medical AB) generally used in the commissioning was used to measure the beam characteristics. A photon diode (Scanditronix Medical AB, PDF‐3G) was used to measure both the PDD and beam profile for both 6 and 15 MV photon beam. To measure the output, a micro‐ionization chamber (Scanditronix Medical AB, RK8304) was used. Results: It is found that moving the jaw to different positions away from the leaf‐ends will increase the output and penumbra width for the small fields. Such increase is particularly more significant when the field size is small (0.5 × 0.5 cm2), and when the jaw‐end is at about 1 to 2 cm from the leaf‐end, when the degree of increase changes quickly. Conclusion: Measurement is important in Intensity Modulated Radiotherapy because the jaw cannot cover all the leaf‐ends in a segment of irregular field completely. This results in additional dose contributed by (1) the end surface of the jaw, (2) the leaf‐end and (3) the inter‐ and intra‐leaf leakage/transmissions during the dosimetric measurement. In addition, most of the conventional treatment planning systems ignore these effects in the dose calculation. It is suggested that similar measurements will be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system's accuracy with regard to the leaf leakage/transmission and the jaw effects.