Field Size Limitations Research Articles

An IMRT planning technique for treating whole breast or chest wall with regional lymph nodes on Halcyon and Ethos.

Field size limitations on Halcyon and Ethos treatment machines largely preclude use of the conventional monoisocentric three-field technique for breast/chest wall and regional lymph nodes. We present an alternative, IMRT-based planning approach that facilitates treatment on Halcyon and Ethos while preserving plan quality. Eight breast and regional node cases (four left-sided, four right-sided) were planned for an Ethos machine using a 15-17 field IMRT technique. Institutional plan quality metrics for CTV and PTV coverage and OAR sparing were assessed. Five plans (four right-sided, one left-sided) were also planned using a hybrid 3D multisocenter technique. CTV coverage and OAR sparing were compared to the IMRT plans. Eclipse scripting tools were developed to aid in beam placement and plan evaluation through a set of dosimetric scorecards, and both are shared publicly. On average, the IMRT plans achieved breast CTV and PTV coverage at 50Gy of 97.9% and 95.7%, respectively. Supraclavicular CTV and PTV coverages at 45Gy were 100% and 95.5%. Axillary lymph node CTV and PTV coverages at 45Gy were 100% and 97.1%, and IMN CTV coverage at 45Gy was 99.2%. Mean ipsilateral lung V20 Gy was 19.3%, and average mean heart dose was 1.6Gy for right-sided cases and 3.0Gy for left-sided. In comparison to the hybrid 3D plans, IMRT plans achieved higher breast and supraclavicular CTV coverage (99.9%vs. 98.6% and 99.9%vs. 93.4%), higher IMN coverage (99.6%vs. 78.2%), and lower ipsilateral lung V20 Gy (19.6%vs. 28.2%). Institutional plan quality benchmarks were achieved for all eight cases using the IMRT-based planning approach. The IMRT-based planning approach offered superior conformity and OAR sparing than a competing hybrid 3D approach.

Clinical Experience with Commissioning a GRID Collimator for Spatially Fractionated Radiation Therapy

In this study, we report our physics commissioning experience for clinically implementing a GRID collimator based Spatially Fractionated Radiation Therapy (SFRT). For SFRT commissioning, we utilized a commercially available brass GRID collimator (DotDecimal, Sanford, FL) that is mounted on a blocking tray for positioning into the accessory mount of the Linac. The brass GRID is 7.62 cm thick and weighs 15.8 kg. The GRID has a total of 149 divergent holes arranged in a hexagonal pattern (1.43 cm in diameter and hole-centers spaced at 2.11 cm when projected at the isocenter distance). The GRID encompasses a maximum field size of 25 × 25 cm at the isocenter. A commercial 48 × 48 × 48 cm water phantom scanning system was used to collect GRID output factors (GRID field to open field ratio), depth dose and beam profile data. Output measurements were performed using a 0.13 cm3 active volume ion-chamber and beam scans were obtained with a diode detector. Data was collected for both flattening and flattening-free beams of nominal energies 6 MV and 10 MV photons. The measurement depths were at dmax (1.5 cm for 6 MV and 2.5 cm for 10 MV), 5-cm and 10-cm respectively. For each energy and depth of measurement, collimator settings were varied from 5 × 5 cm to 28 × 28 cm. From scan profiles at different depths, the valley (lowest) to peak (highest) dose ratios (VPDR) were calculated. A commercial treatment planning system (TPS) was used to test the accuracy of dose calculations with GRID. This was accomplished by importing vendor generated DICOM RT file into the TPS. A block transmission factor of 7% for 6 MV and 10.2% for 10 MV energy beams were applied. All measured data were compared with corresponding TPS calculated data. Test patient treatment plans with GRID were created in TPS and planned distributions were verified using a commercial detector array with 2.5 mm detector spacing. The VPDR, expressed as %, are presented in Table 1. Measured and TPS calculated output factors agreed within 2% for 6 MV and within 3% for 10 MV photon beams. Percent depth doses were lower in magnitude for GRID field compared to open field for all energies studied (for e.g., 6 MV depth dose at 10 cm depth for 10 cm x10cm field 62% for GRID field vs. 66% for open field). Measured and calculated GRID beam profiles agreed within 5% dose difference and 1 mm distance-to agreement. For all test cases, the planned vs. measured dose distributions passed at an average gamma passing rate of 96.5% using a 3% dose difference/ 3 mm distance to agreement and 10% threshold criteria. The Dot Decimal GRID collimator provides a simple way of achieving SFRT in the clinic, albeit heavy to use and has an irradiation field size limitation of 25 cm × 25 cm.

High-accuracy calibration of cameras without depth of field and target size limitations.

During camera calibration, targets need to be placed in the depth of field of the lens to ensure clear imaging, and they should take up proper proportions in the image. These requirements cause difficulty in many calibration scenarios, such as those involving large-field-of-view, shallow-depth-of-field, or online operation cameras. In view of the above-mentioned problems, this study proposes a high-accuracy camera calibration method, which can overcome the influence of image blur and noise and is not limited by depth of field and target size. First, a high-accuracy light-spot small target is placed closely in front of the camera, so that the target image can take up a large proportion in the whole image. In case of defocus blur, the adaptive multi-scale method is used to extract feature point coordinates at first to ensure accuracy, and the location variance of each feature point is estimated concurrently. Finally, the high-accuracy intrinsic and extrinsic parameters of the camera under test are obtained by nonlinear optimization where re-projection errors are normalized by location variances according to the Gauss-Markov theorem. Simulation and physical experiments validate the effectiveness of the proposed method.

Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method.

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.

Remote dosimetric auditing for intensity modulated radiotherapy: A pilot study

Abstract Background and Purpose Electronic portal imaging devices (EPIDs) can be used to reconstruct dose inside a virtual phantom. This work aims to study the feasibility of using this method for remote dosimetry auditing of clinical trials. Materials and Methods Six centres participated in an intensity modulated radiotherapy (IMRT) pilot study of this new audit approach. Each centre produced a head and neck (HN) and post-prostatectomy (PP) trial plan and transferred the plans to virtual phantoms to calculate a reference dose distribution. They acquired in-air images of the treatment fields along with calibration images using their EPID. These data were sent to the central site where the images were converted to 2D field-by-field doses in a flat virtual water phantom and to 3D combined field doses in a cylindrical virtual phantom for comparison with corresponding reference dose distributions. Additional test images were used to assess the accuracy of the method when using different EPIDs. Results Field-by-field 2D analysis yielded mean gamma pass-rates of 99.6% (±0.3%) and 99.6% (±0.6%) for HN and PP plans respectively (3%/3 mm, doses greater than 10% global max). 3D combined field analysis gave mean pass-rates of 97.9% (±2.6%) and 97.9% (±1.8%) for the HN and PP plans. Dosimetry tests revealed some field size limitations of the EPIDs. Conclusions The remote auditing methodology using EPIDs is feasible and potentially an inexpensive method.

Technical Note: Investigating the impact of field size on patient selection for the 1.5T MR-Linac.

The 1.5 T Elekta MR-Linac, due to the construction of the system will have a maximum radiation field size in the superior-inferior patient direction of 22 cm at isocentre. The field size may impact on the patient groups which can be treated on the system. This technical note aims to address the question of which treatment sites will be affected by field size limitations on the MR-Linac. Using historical data for 11 595 cases over 2 yr treated at the authors' institution, the proportion of plans that would fit the MR-Linac's field size was determined for eleven patient groups. In addition, cervix plans were analyzed to determine the length of the two Clinical Target Volumes (CTVs) and any overlap between them. With a 1 cm margin to allow for online plan adaption, 80% of all plans would be suitable for the MR-Linac due to the field size. This percentage increases to 100% for smaller tumor volumes such as prostate and brain. However, for cervix and three dose-level head and neck plans the percentage becomes 61% and 66%, respectively. The maximum radiation field size of the MR-Linac in the superior-inferior patient direction is 22 cm. With a 1 cm margin approximately 80% of all plans would be suitable for the MR-Linac with the available field size, decreasing to 61% for larger tumor volumes. For cervix patients this may motivate investigations into treating each CTV with a separate isocentre, allowing for careful control of matching fields.

A Robust and Affordable Table Indexing Approach for Multi-isocenter Dosimetrically Matched Fields.

Purpose Radiotherapy treatment planning of extended volume typically necessitates the utilization of multiple field isocenters and abutting dosimetrically matched fields in order to enable coverage beyond the field size limits. A common example includes total lymphoid irradiation (TLI) treatments, which are conventionally planned using dosimetric matching of the mantle, para-aortic/spleen, and pelvic fields. Due to the large irradiated volume and system limitations, such as field size and couch extension, a combination of couch shifts and sliding of patients are necessary to be correctly executed for accurate delivery of the plan. However, shifting of patients presents a substantial safety issue and has been shown to be prone to errors ranging from minor deviations to geometrical misses warranting a medical event. To address this complex setup and mitigate the safety issues relating to delivery, a practical technique for couch indexing of TLI treatments has been developed and evaluated through a retrospective analysis of couch position.MethodsThe indexing technique is based on the modification of the commonly available slide board to enable indexing of the patient position. Modifications include notching to enable coupling with indexing bars, and the addition of a headrest used to fixate the head of the patient relative to the slide board. For the clinical setup, a Varian Exact CouchTM (Varian Medical Systems, Inc, Palo Alto, CA) was utilized. Two groups of patients were treated: 20 patients with table indexing and 10 patients without. The standard deviations (SDs) of the couch positions in longitudinal, lateral, and vertical directions through the entire treatment cycle for each patient were calculated and differences in both groups were analyzed with Student's t-test.ResultsThe longitudinal direction showed the largest improvement. In the non-indexed group, the positioning SD ranged from 2.0 to 7.9 cm. With the indexing device, the positioning SD was reduced to a range of 0.4 to 1.3 cm (p < 0.05 with 95% confidence level). The lateral positioning was slightly improved (p < 0.05 with 95% confidence level), while no improvement was observed in the vertical direction.ConclusionsThe conventional matched field TLI treatment is error-prone to geometrical setup error. The feasibility of full indexing TLI treatments was validated and shown to result in a significant reduction of positioning and shifting errors.

SU‐E‐T‐748: Theoretical Investigation On Using High Energy Proton Beam for Total‐Body‐Irradiation

Purpose:The broad‐slow‐rising entrance dose region proximal to the Bragg peak made by a mono‐energetic proton beam could potentially be used for total body irradiation (TBI). Due to the quasi‐uniform dose deposition, customized thickness compensation may not be required to deliver a uniform dose to patients with varied thickness. We investigated the possibility, efficacy, and hardware requirement to use such proton beam for TBI.Methods:A wedge shaped water phantom with thickness varying from 2 cm to 40 cm was designed to mimic a patient. Geant4 based Monte Carlo code was used to simulate broad mono‐energetic proton beams with energy ranging from 250 MeV to 300 MeV radiating the phantom. A 6 MV photon with 1 cm water equivalent build‐up used for conventional TBI was also calculated. A paired‐opposing beam arrangement with no thickness compensation was used to generate TBI plans for all beam energies. Dose from all particles were scored on a grid size of 2 mm3. Dose uniformity across the phantom was calculated to evaluate the plan. The field size limit and the dose uniformity of Mevion S250 proton system was examined by using radiochromic films placed at extended treatment distance with the open large applicator and 90° gantry angle.Results:To achieve a maximum ± 7.5% dose variation, the largest patient thickness variation allowed for 250 MeV, 275 MeV, and 300 MeV proton beams were 27.0 cm, 34.9 cm and 36.7 cm. The value for 6 MV photon beam was only 8.0 cm to achieve the same dose variation. With open gantry, Mevion S250 system allows 5 m source‐to‐surface distance producing an expected 70 cm2 field size.Conclusion:Energetic proton beam can potentially be used to deliver TBI. Treatment planning and delivery would be much simple since no thickness compensation is required to achieve a uniform dose distribution.

SU‐E‐T‐406: Evaluation of the Stealth Chamber by IBA Under Large Field Conditions

Purpose:To evaluate the performance of the stealth chamber by IBA under large field conditions.Methods:Measurement of PDDs and profiles for fields of 10×10, 15×15, 20×20 and 25×25 cm2 were acquired in a blue phantom2 scanning system for 6 and 15 MV beams in a Varian TrueBeam linear accelerator. The stealth chamber has a field size limit of 25×25 cm2. Measurements with two different detector set were performed. First a cc13 chamber as field and reference chamber was acquired. And with a cc13 chamber as field detector and the stealth chamber as the reference chamber. PDDs for the fields mentioned above were conducted and profiles with the same fields at depth of dmax, 5, 10 and 20 cm were acquired for comparison.Results:Examination of the PDDs and profile comparison with a criterion of 1% and 1 mm agreement was performed. Very good agreement between the cc13 and stealth chamber set was observed within the chosen criterion.Conclusion:This work indicates the comparable results obtained with the use of the stealth chamber and cc13 as the reference chamber in relative dosimetry measurements.

SU-E-T-226: Junction Free Craniospinal Irradiation in Linear Accelerator Using Volumetric Modulated Arc Therapy : A Novel Technique Using Dose Tapering

Purpose: Spatially separated fields are required for craniospinal irradiation due to field size limitation in linear accelerator. Field junction shits are conventionally done to avoid hot or cold spots. Our study was aimed to demonstrate the feasibility of junction free irradiation plan of craniospinal irradiation (CSI) for Meduloblastoma cases treated in linear accelerator using Volumetric modulated arc therapy (VMAT) technique. Methods: VMAT was planned using multiple isocenters in Monaco V 3.3.0 and delivered in Elekta Synergy linear accelerator. A full arc brain and 40° posterior arc spine fields were planned using two isocentre for short ( 10 cm) at the conventional field junction. Results: In this primary study five patients were planned and three patients were delivered using this novel technique. As the dose contribution from the adjacent beams were varied (gradient) to create a complete dose distribution, therefore there is no specific junction exists in the plan. The junction were extended from 10–14 cm dependingmore » on treatment plan. Dose gradient were 9.6±2.3% per cm for brain and 7.9±1.7 % per cm for spine field respectively. Dose delivery error due to positional inaccuracy was calculated for brain and spine field for ±1mm, ±2mm, ±3mm and ±5 mm were 1%–0.8%, 2%–1.6%, 2.8%–2.4% and 4.3%–4% respectively. Conclusion: Dose tapering in junction free CSI do not require a junction shift. Therefore daily imaging for all the field is also not essential. Due to inverse planning dose to organ at risk like thyroid kidney, heart and testis can be reduced significantly. VMAT gives a quicker delivery than Step and shoot or dynamic IMRT.« less

SU‐E‐T‐262: Planning for Proton Pencil Beam Scanning (PBS): Applications of Gradient Optimization for Field Matching

Purpose:To propose the gradient optimization(GO) approach in planning for matching proton PBS fields and present two commonly used applications in our institution.Methods:GO is employed for PBS field matching in the scenarios that when the size of the target is beyond the field size limit of the beam delivery system or matching is required for beams from different angles to either improve the sparing of important organs or to pass through a short and simple beam path. Overlap is designed between adjacent fields and in the overlapped junction, the dose was optimized such that it gradually decreases in one field and the decrease is compensated by increase from another field. Clinical applications of this approach on craniospinal irradiation(CSI) and whole pelvis treatment were presented. Mathematical model was developed to study the relationships between dose errors, setup errors and junction lengths.Results:Uniform and conformal dose coverage to the entire target volumes was achieved for both applications using GO approach. For CSI, the gradient matching (6.7cm junction) between fields overcame the complexity of planning associated with feathering match lines. A slow dose gradient in the junction area significantly reduced the sensitivity of the treatment to setup errors. For whole pelvis, gradient matching (4cm junction) between posterior fields for superior target and bilateral fields for inferior target provided dose sparing to organs such as bowel, bladder and rectum. For a setup error of 3 mm in longitudinal direction from one field, mathematical model predicted dose errors of 10%, 6% and 4.3% for junction length of 3, 5 and 7cm.Conclusion:This GO approach improves the quality of the PBS treatment plan with matching fields while maintaining the safety of treatment delivery relative to potential misalignments.

SU‐E‐T‐477: Study of MLC Apertures and Their Significance for Volumetric Arc Therapy in the Management of Non‐Small Cell Lung Cancer Using Stereotactic Body Radiotherapy

Purpose: Electronic disequilibrium concerns limit the minimum field sizes to 3.5 × 3.5 cm2 for NSCLC patients per various RTOG protocols which is easily achievable using 3DCRT. However, the inverse planning nature of VMAT does not provide any control on the field‐size or MLC shapes and may often violate it. This study aims to analyze the difference between VMAT and 3DCRT planning techniques by evaluating aperture size &amp; hence conformity with RTOG protocols. Methods: 25 patients with NSCLC [5 tumors per lobe] who were treated using 3DCRT were retrospectively inverse planned for VMAT. Dosimetrically, VMAT plans were almost identical with that of clinical 3DCRT plans. DICOM plans were imported into MATLAB to determine the actual MLC area of the active leaves per control point and its relative weight in the overall dose delivery of that VMAT arc. The effective‐weight of all the MLC areas which were less than 3.5 × 3.5 cm2 was summated (VMAT3.5fs). We also studied the conditions where the total MLC area did satisfy the minimum requirement of 3.5 × 3.5 cm2 but where any two opposite MLC leaf positions were less than 3.5 cm apart (VMAT3.5MLCwt). These evaluations are contained in VMAT3.5MLCwt which gives the weighted sum of all small MLC segments in the entire VMAT arc. A zero value of both ensures that the VMAT arc satisfies the RTOG requirement of minimal jaws size in its entirety. Results: Respective values of VMAT3.5fs &amp; VMAT3.5MLCwt were found to be 0.88 ± 0.23 &amp; 0.9 ± 0.21 for 63 VMAT arcs. Only 2 patients had a value of 0 for both indices implying compliance to RTOG field size limits. Conclusion: Proposed indices provide an objective method to evaluate the suitability of VMAT plans in satisfying RTOG constraints for NSCLC treatment using SBRT.

Dosimetric effect of multileaf collimator leaf width in intensity-modulated radiotherapy delivery techniques for small- and large-volume targets

The purpose of this study was to evaluate the dosimetric effect of the leaf width of a multileaf collimator (MLC) in intensity-modulated radiotherapy (IMRT) delivery techniques for small- and large-volume targets. We retrospectively selected previously treated 5 intracranial and 5 head-neck patients for this study to represent small- (range, 18.37-72.75 cc; mean, 42.99 cc) and large-volume (range, 312.31-472.84 cc; mean, 361.14 cc) targets. A 6-MV photon beam data was configured for Brianlab m3 (3 mm), Varian Millennium 120 (5 mm) and Millennium 80 (10 mm) MLCs in the Eclipse treatment-planning system. Sliding window and step-shoot IMRT plans were generated for intracranial patients using all the above-mentioned MLCs; but due to the field size limitation of Brainlab MLC, we used only 5-mm and 10-mm MLCs in the head-and-neck patients. Target conformity, dose to the critical organs and dose to normal tissues were recorded and evaluated. Although the 3-mm MLC resulted in better target conformity (mean difference of 7.7% over 5-mm MLC and 12.7% over 10-mm MLC) over other MLCs for small-volume targets, it increased the total monitor units of the plans. No appreciable differences in terms of target conformity, organ at risk and normal-tissue sparing were observed between the 5-mm and 10-mm MLCs for large-volume targets. The effect of MLC leaf width was not quantifiably different in sliding window and step and shoot techniques. In addition, we observed that there was no additional benefit to the sliding-window (SW) technique when compared to the step-shoot (SS) technique as a result of reduction of MLC leaf width.

SU‐GG‐T‐205: Limitations of Delta4 IMRT QA for Large Field Sizes

Purpose: Pre‐treatment QA of IMRT plans are critical to uphold patient safety. Given that IMRT plans are becoming more prevalent in radiation oncology it is important to exam the limitations of current IMRT QA devices. Current IMRT QA devices use a 2D array of detectors to examine IMRT QA plans, and 3D analysis is lacking. The Delta4 is a relatively new IMRT QA device, that can perform 3D plan analysis, however the field size limitations of the device are unknown. We herein examine the abilities of the Delta4 to perform IMRT QA on 80 IMRT plans on various treatment sites. Methods and Materials: Clinical IMRT plans from an in‐house TPS, PlanUNC, were retrospectively examined to determine the field size limitation on the Delta4. 20 IMRT plans for each of the following sites were examined: head and neck (H&amp;N), prostate, breast, and pelvis. We determined if the entire intensity map could be measured using the Delta4 based upon the treatment field size using the suggested isocentric set‐up. We also determined which treatment field intensity maps with large field sizes could be completely measured using isocenter shifts for the Delta4. Results: Using the Delta4, 6 H&amp;N, 20 prostate, and 3 pelvis IMRT QA plans were deliverable. Due to large field sizes 14 H&amp;N, and 17 Pelvis plans required a shift to complete measure the dose fluence. All 20 breast IMRT QA plans were deliverable on the Delta4, however all fields were prescribed a table kick out, which greatly increased the time required to measure the plans. Conclusions: The 20cm × 20cm field size of the Delta4 limits its ability to measure large treatment fields using the isocentric set‐up. However shifting the device can increase the detector size, but also decreases IMRT QA efficiency.

SU-GG-T-584: A Simple Mini-Total Body Irradiation Technique for Small Linac Room

Purpose: A single fraction of 2 Gy total body irradiation (mini‐TBI) has been used for immunosupression therapy for stem cells transplantation. Because of the low dose, blocks are not required. For patients in excessively ill heath, it can be very difficult to hold a standing or seated position for the length of time required to deliver a full mini‐TBI treatment. Small linac rooms can make it impossible for a patient to be treated lying down in the traditional manner both because of space and field size limitations. We have developed a simple technique to deliver a mini‐TBI treatment to a patient lying down in a room with minimal space. Method and Materials: Linac was setup with gantry and collimator rotated to 45 degree projecting a 210 cm long light field on the floor. Patients up to 190 cm height can be setup on the floor and treated with SAO, SPO, IPO, and IAO fields each giving 1/4 of the dose. The patient was repositioned exchanging first supine/prone and then head/feet to complete treatment.Dosimetry was evaluated with the Eclipse treatment planning system and verified with ion‐chamber measurement in acrylic phantom. Effects of energy and beam spoiler were also investigated. Results: Treatment setup was simple and showed a homogeneous dose distribution. The mid‐plane doses at 70 and 90 cm away the center were 110% and 116%f the prescription, respectively. The high dose area can easily be reduced by using 1–3 mm thick lead sheets mounted on a blocking tray as a partial transmission filter. Dose at 3 mm from surface was 99% of the prescription treated with 6 MVwithout spoiler. Conclusion: A single dose of TBI can be safely delivered with the method described in this study. Patient's comfort, setup complexity and treatment time were significantly improved using this method.

Using Intensity Modulated Fields to Treat Large Tumor Volume Exceeding the Maximum Field Length with Two Isocenters

It is a clinical challenge to treat a large target volume that exceeds the maximum field size limit. Conventionally, two isocenters, gapped fields with accepted under- and over- dose regions are used to treat volume beyond maximum field size. The potential under and over dose region could be 0% to 200% of the prescription dose. Using linear accelerator based IMRT, extended target volume can be treated with two isocenters without these under and over-dose regions. The purpose of this study is to evaluate dosimetric variations if the two isocenters of the IMRT fields are not perfectly aligned during treatment.

Navigating Through and Around the Field Size Limitation on Varian Linear Accelerators When Planning Large IMRT Cases Using the Philips Pinnacle Treatment Planning System

To determine the best method by which the result of the field width limitation on the Varian Linear Accelerator is minimized in the final treatment plan. There is a maximum field width on the Varian linear accelerators that is often smaller than the width of some PTV's in large IMRT cases. This often presents a difficult planning problem that must be navigated around during the planning process in order to achieve the best clinical result and outcome.

Technical and dosimetric considerations in IMRT treatment planning for large target volumes

The maximum width of an intensity-modulated radiotherapy (IMRT) treatment field is usually smaller than the conventional maximum collimator opening because of design limitations inherent in some multileaf collimators (MLCs). To increase the effective field width, IMRT fluences can be split and delivered with multiple carriage positions. However, not all treatment-planning systems and MLCs support this technique, and if they do, the maximum field width in multiple carriage position delivery is still significantly less than the maximum collimator opening. For target volumes with dimensions exceeding the field size limit for multiple carriage position delivery, such as liver tumors or other malignancies in the abdominal cavity, IMRT treatment can be accomplished with multiple isocenters or with an extended treatment distance. To study dosimetric statistics of large field IMRT planning, an elliptical volume was chosen as a target within a cubic phantom centered at a depth of 7.5 cm. Multiple three-field plans (one AP and two oblique beams with 160° between them to avoid parallel opposed geometry) with constraints designed to give 100% dose to the elliptical target were developed. Plans were designed with a single anterior field with dual carriage positions, or with the anterior field split into two fields with separate isocenters 8 cm apart with the beams being forcibly matched at the isocenter or with a 1 cm, 2 cm, 3 cm, and 4 cm overlap. The oblique beams were planned with a single carriage position in all cases. All beams had a nominal energy of 6 MV. In the dual isocenter plans, jaws were manually fixed and dose constraints remained unaltered. Dosimetric statistics were studied for plans developed for treatment delivery using both dynamic leaf motion (sliding window) and multiple static segments (step and shoot) with the number of segments varying from 5 to 30. All plans were analyzed based on the dose homogeneity in the isocenter plane, 2 cm anterior and 2 cm posterior to it, along with their corresponding dose-volume histograms (DVHs). All the dual isocenter plans had slight underdosage anterior to the match point and slight overdosage posterior to it, while the dual carriage plan had a nice blending of the dose distribution without the accompanying hot or cold spots. Based on the dose statistics, it was noted that the dual isocenter plans can be clinically acceptable if they have at least a 3-cm overlap. In the case of step and shoot IMRT, the number of segments used in a dual carriage plan was found to affect the overall plan dosimetric indices. PACS number: 87.53.Tf

Patterning large-area devices with a 5:1 reduction stepper

Contact or proximity aligners or 1:1 projection aligners are conventionally used for patterning large-area devices like silicon strip detectors for high-energy particle detection. Reduction steppers offer a far better pattern fidelity and an increased productivity but a very limited field size. This paper describes methods to overcome the field size limitations to pattern large-area position-sensitive particle detectors automatically and accurately with a 5:1 reduction stepper.

Technical and dosimetric considerations in IMRT treatment planning for large target volumes

The maximum width of an intensity‐modulated radiotherapy (IMRT) treatment field is usually smaller than the conventional maximum collimator opening because of design limitations inherent in some multileaf collimators (MLCs). To increase the effective field width, IMRT fluences can be split and delivered with multiple carriage positions. However, not all treatment‐planning systems and MLCs support this technique, and if they do, the maximum field width in multiple carriage position delivery is still significantly less than the maximum collimator opening. For target volumes with dimensions exceeding the field size limit for multiple carriage position delivery, such as liver tumors or other malignancies in the abdominal cavity, IMRT treatment can be accomplished with multiple isocenters or with an extended treatment distance. To study dosimetric statistics of large field IMRT planning, an elliptical volume was chosen as a target within a cubic phantom centered at a depth of 7.5 cm. Multiple three‐field plans (one AP and two oblique beams with 160° between them to avoid parallel opposed geometry) with constraints designed to give 100% dose to the elliptical target were developed. Plans were designed with a single anterior field with dual carriage positions, or with the anterior field split into two fields with separate isocenters 8 cm apart with the beams being forcibly matched at the isocenter or with a 1 cm, 2 cm, 3 cm, and 4 cm overlap. The oblique beams were planned with a single carriage position in all cases. All beams had a nominal energy of 6 MV. In the dual isocenter plans, jaws were manually fixed and dose constraints remained unaltered. Dosimetric statistics were studied for plans developed for treatment delivery using both dynamic leaf motion (sliding window) and multiple static segments (step and shoot) with the number of segments varying from 5 to 30. All plans were analyzed based on the dose homogeneity in the isocenter plane, 2 cm anterior and 2 cm posterior to it, along with their corresponding dose‐volume histograms (DVHs). All the dual isocenter plans had slight underdosage anterior to the match point and slight overdosage posterior to it, while the dual carriage plan had a nice blending of the dose distribution without the accompanying hot or cold spots. Based on the dose statistics, it was noted that the dual isocenter plans can be clinically acceptable if they have at least a 3‐cm overlap. In the case of step and shoot IMRT, the number of segments used in a dual carriage plan was found to affect the overall plan dosimetric indices.PACS number: 87.53.Tf