Compensators: an alternative IMRT delivery technique

Cobalt compensator-based IMRT device: A treatment planning study of head and neck cases

PURPOSE

Our goal is to develop a novel cobalt-compensator-based IMRT device for low- and middle-income countries that is reliable and cost-effective while delivering treatment plans of equal quality to those from linac-MLC devices. The present study examines the quality of treatment plans using this device.

METHODS

A commercial treatment planning system (TPS; RayStation v.8B) was commissioned for this device using Monte Carlo simulations from the Geant4 toolkit. Patient-specific compensators were created as regions-of-interest. Thirty clinical head & neck cases were planned and compared to clinical plans with a 6MV linac using IMRT. The mock head and neck plan from TG-119 was used for further validation.

RESULTS

PTV objectives were achieved in all 30 plans with PTV V95% >95 %. OAR sparing was similar to clinical plans. There were 14 cases where OAR dose limits exceeded the recommended QUANTEC limits in the clinical plan in order to achieve target coverage. OAR sparing was better in the cobalt compensator plan in 8 cases and worse in 3 cases, in the latter cases exceeding the clinical plan doses by an average of 8.22 % (0.0 %-13.5 %). Average field-by-field gamma pass-rate were 93.7 % (2 %/2mm). Estimated treatment times using the Co-60 compensator device were 1 min 27 s vs 1 min 2 s for the clinical system.

CONCLUSION

This system is the first of its kind to allow for IMRT with a Co-60 device. Data here suggests that the delivery meets plan quality criteria while maintaining short treatment times which may offer a sustainable and cost-low option for IMRT on the global scale.

Affordable compensator for IMRT delivery designed for low-and-middle income countries, a Monte Carlo study

We propose a novel cost-effective compensator that can be used to facilitate access to IMRT in low-and-middle income countries. The compensator has the advantages of simplicity, less downtime, increased reliability and less impact on treatment quality from patient motion during treatment. Moreover, the system can be used with either a cobalt-60 unit or linear accelerator. In this Monte Carlo study, the dosimetric properties of the new compensator design have been evaluated. Results were obtained for different field sizes of cobalt teletherapy machine, and the dose was scored at 0.5 cm depth in a water phantom. The effects of compensator thickness, filling material type and shape, and field size were identified. Furthermore, the percentage depth dose and beam profiles for various field sizes and at different depths were obtained. Beam profiles show no significant signature of the beads relative to a solid compensator; in addition, they exhibit a better flatness while preserving symmetry for all field sizes. A reusable bead-based compensator appears to be feasible, and provides dose distribution similar to a solid compensator with low cost and no hazards. Our results avail the ongoing efforts to expand the reach to IMRT in low- and middle-income countries.

Small Animal IMRT Using 3D-Printed Compensators

PURPOSE

Preclinical radiation replicating clinical intensity modulated radiation therapy (IMRT) techniques can provide data translatable to clinical practice. For this work, treatment plans were created for oxygen-guided dose-painting in small animals using inverse-planned IMRT. Spatially varying beam intensities were achieved using 3-dimensional (3D)-printed compensators.

METHODS AND MATERIALS

Optimized beam fluence from arbitrary gantry angles was determined using a verified model of the XRAD225Cx treatment beam. Compensators were 3D-printed with varied thickness to provide desired attenuation using copper/polylactic-acid. Spatial resolution capabilities were investigated using printed test-patterns. Following American Association of Physicists in Medicine TG119, a 5-beam IMRT plan was created for a miniaturized (∼1/8th scale) C-shape target. Electron paramagnetic resonance imaging of murine tumor oxygenation guided simultaneous integrated boost (SIB) plans conformally treating tumor to a base dose (Rx₁) with boost (Rx₂) based on tumor oxygenation. The 3D-printed compensator intensity modulation accuracy and precision was evaluated by individually delivering each field to a phantom containing radiochromic film and subsequent per-field gamma analysis. The methodology was validated end-to-end with composite delivery (incorporating 3D-printed tungsten/polylactic-acid beam trimmers to reduce out-of-field leakage) of the oxygen-guided SIB plan to a phantom containing film and subsequent gamma analysis.

RESULTS

Resolution test-patterns demonstrate practical printer resolution of ∼0.7 mm, corresponding to 1.0 mm bixels at the isocenter. The miniaturized C-shape plan provides planning target volume coverage (V_95% = 95%) with organ sparing (organs at risk D_max < 50%). The SIB plan to hypoxic tumor demonstrates the utility of this approach (hypoxic tumor V_95%,Rx2 = 91.6%, normoxic tumor V_95%,Rx1 = 95.7%, normal tissue V_100%,Rx1 = 7.1%). The more challenging SIB plan to boost the normoxic tumor rim achieved normoxic tumor V_95%,Rx2 = 90.9%, hypoxic tumor V_95%,Rx1 = 62.7%, and normal tissue V_100%,Rx2 = 5.3%. Average per-field gamma passing rates using 3%/1.0 mm, 3%/0.7 mm, and 3%/0.5 mm criteria were 98.8% ± 2.8%, 96.6% ± 4.1%, and 90.6% ± 5.9%, respectively. Composite delivery of the hypoxia boost plan and gamma analysis (3%/1 mm) gave passing results of 95.3% and 98.1% for the 2 measured orthogonal dose planes.

CONCLUSIONS

This simple and cost-effective approach using 3D-printed compensators for small-animal IMRT provides a methodology enabling preclinical studies that can be readily translated into the clinic. The presented oxygen-guided dose-painting demonstrates that this methodology will facilitate studies driving much needed biologic personalization of radiation therapy for improvements in patient outcomes.

Optimization of Clarkson's Method for Calculating Absorbed Dose under Compensator Filters used in Intensity-modulated Radiation Therapy

Background:

Intensity Modulated Radiation Therapy (IMRT) is extensively used in the treatment of malignancies. Clarkson’s method is one of the leading methods for dose calculation at open points present in irregular fields.

Objective:

The aim of this study is to generalize the Clarkson’s method for dose calculation at points under compensator filters in IMRT method and its application in IMRT quality control as well.

Material and Methods:

In this experimental study, compensator filters were designed in two forms: flat filter and block piled-up compensator. The measurements for the compensator filters and open fields in 5 and10 cm depths at energy levels (6, 10 and 18 MV) and in fields with different dimensions were performed using “Mapcheck2” dosimeter. The aim of performing calculations is to derive the theoretical dose by the generalized Clarkson’s equation and comparing it with data resulted from the measurement for confirming the Clarkson’s equation presented.

Results:

These results demonstrate the data derived from the generalized Clarkson’s method are in good agreement with the data resulted from measurement; the highest error of the proposed equation was 3% for flat filter, and less than 5% for block-piled-up filter. Higher error in the block-piled-up filter compared with the flat filter was due to the presence of leakage between these blocks.

Conclusion:

The results of this study demonstrated that the presented equation is reliable and valid, and the proposed equation can be applied for dose calculation at all points under the compensator filter or the shielded areas.

A method for generating intensity-modulated radiation therapy fields for small animal irradiators utilizing 3D-printed compensator molds

PURPOSE

The purpose of this study was to investigate the feasibility of using fused deposition modeling (FDM) three-dimensional (3D) printer to generate radiation compensators for high-resolution (~1 mm) intensity-modulated radiation therapy (IMRT) for small animal radiation treatment. We propose a novel method incorporating 3D-printed compensator molds filled with NaI powder.

METHODS

The inverse planning module of the computational environment for radiotherapy research (CERR) software was adapted to simulate the XRAD-225Cx irradiator, both geometry and kV beam quality (the latter using a phase space file provided for XRAD-225Cx). A nine-field IMRT treatment was created for a scaled-down version of the imaging and radiation oncology core (IROC) Head and Neck IMRT credentialing test, recreated on a 2.2-cm-diameter cylindrical phantom. Optimized fluence maps comprising nine fields and a total of 2564 beamlets were calculated at resolution of 1.25 × 1.25 mm² . A hollow compensator mold was created (using in-house software and algorithm) for each field using 3D printing with polylactic acid (PLA) filaments. The molds were then packed with sodium iodide powder (NaI, measured density ρ_NaI  = 2.062 g/cm³ ). The mounted compensator mold thickness was limited to 13.8 mm due to clearance issues with couch collision. At treatment delivery, each compensator was manually mounted to a customized block tray attached to the reference 40 × 40 mm² collimator. Compensator reproducibility among three repeated 3D-printed molds was measured with Radiochromic EBT2 film. The two-dimensional (2D) dose distributions of the nine fields were compared to calculated 2D doses from CERR using gamma comparisons with distance-to-agreement criteria of 0.5-0.25 mm and dose difference criteria of 3-5%.

RESULTS

Good reproducibility of 3D-printed compensator manufacture was observed with mean error of ±0.024 Gy and relative dose error of ±4.2% within the modulated part of the beam. Within the limit of 13.8 mm compensator height, a maximum radiation blocking efficiency of 91.5% was achieved. Per field, about 45.5 g of NaI powder was used. Gamma analysis on each of the nine delivered IMRT fields using radiochromic films resulted in eight of nine treatment fields with >90% pass rate with 5%/0.5 mm tolerances. However, low gamma passing rate of 49-66% (3%/0.25 mm to 5%/0.5 mm) was noted in one field, attributed to fabrication errors resulting in over-filling the mold. The nine-field treatment plan was delivered in under 30 min with no mechanical or collisional issues.

CONCLUSIONS

We show the feasibility of high spatial resolution IMRT treatment on a small animal irradiator utilizing 3D-printed compensator shells packed with NaI powder. Using the PLA mold with NaI powder was attractive due to the ease of 3D printing a PLA mold at high geometric resolution and the well-balanced attenuation properties of NaI powders that prevented the mold from becoming too bulky. IMRT fields with 1.25-mm resolution are capable with significant fluence modulation with relative dose accuracy of ±4.2%.

Three-Dimensionally-Precise Breast Conformal Device for IMRT in Breast Cancer Patients Treated With Breast-Conserving Surgery-A Pilot Randomized Controlled Trial

OBJECTIVE

To examine the accuracy and efficiency of breast radiotherapy after breast-conserving surgery of a novel 3-dimensional (3D) printing tissue compensator technology, the 3D-precise breast conformer, compared with a usual compensator and an unstructured compensator.

METHODS

This novel device is patented in China (patent No.: ZL2015 2 0259472.9). Thirty patients with breast cancer after breast-conserving surgery were randomly divided into 2 control groups (no compensator, NST group, and usual compensator, ST group) and 1 study group (3D-precise breast conformer, 3D-BCT group) (n = 10/group). Before radiotherapy, all patients were scanned in the same CT positioning conditions to prepare the treatment plans.

RESULTS

The 3D-BCT showed the best homogeneity index (HI) (0.08 ± 0.03) and conformity index (CI) (0.95 ± 0.03), while the NST group showed the worst HI (0.34 ± 0.07) and CI (0.78 ± 0.06), with the ST group between the 2 (HI: 0.15 ± 0.05; CI: 0.87 ± 0.04) (all P < 0.01). The common tissue compensation membrane could lead to 95-100% of the prescription dose covering 85-95% of the target volume, and the uniformity and conformability of the target dose were improved overall compared with the NST group. In the 3D-BCT group, 100% of the prescription dose covered the target volume of 95-100%.

CONCLUSION

The 3D-precision breast conformal device had the highest individualization, uniformity, and conformity. The V95, V98, CI, and HI of PTV were optimal in the 3D-BCT group, and an ideal isodose curve distribution of the breast and clavicle upper and lower target areas was achieved. This device could improve the surface dose and the efficacy of radiotherapy after breast-conserving surgery.

Phase II study of compensator-based non-coplanar intensity-modulated radiotherapy for Stage I non-small-cell lung cancer

We conducted a Phase II study to evaluate the usefulness of compensator-based non-coplanar intensity-modulated radiotherapy (ncIMRT) for patients with surgically inaccessible Stage I non-small-cell lung cancer (NSCLC). Patients with pathologically proven or clinically diagnosed surgically inaccessible Stage I NSCLC were enrolled in this study from May 2011 to April 2014. These patients underwent ncIMRT of 75 Gy in 30 fractions regardless of the tumor location. The primary end point was 3-year overall survival, and the secondary end points were local control rate and treatment-related toxicities. A total of 48 patients (50 tumors) were enrolled in this study. Of the 50 tumors, the Stage T1 to T2 ratio was 31 to 19, and the ratio of tumors located in the central to peripheral areas was 11 to 39. During the median follow-up time of 35.9 months, the 3-year actuarial local progression-free and overall survival rates were 82.6% and 87.1%, respectively. No patients experienced toxicities of Grade 3 or greater. Standard-fractionated ncIMRT was effective and safe for patients with surgically inaccessible stage I NSCLC, regardless of the tumor location.

Design, fabrication, and validation of patient-specific electron tissue compensators for postmastectomy radiation therapy

Background and purpose

Postmastectomy radiotherapy (PMRT) is complex to plan and deliver, but could be improved with 3D-printed, patient-specific electron tissue compensators. The purposes of this study were to develop an algorithm to design patient-specific compensators that achieve clinical goals, to 3D-print the planned compensators, and validate calculated dose distributions with film and thermoluminescent dosimeter (TLD) measurements in 3D-printed phantoms of PMRT patients.

Materials and methods

An iterative algorithm was developed to design compensators corresponding to single-field, single-energy electron plans for PMRT patients. The 3D-printable compensators were designed to fit into the electron aperture, with cerrobend poured around it. For a sample of eight patients, calculated dose distributions for compensator plans were compared with patients’ (multi-field, multi-energy) clinical treatment plans. For all patients, dosimetric parameters were compared including clinical target volume (CTV), lung, and heart metrics. For validation, compensators were fabricated and irradiated for a set of six 3D-printed patient-specific phantoms. Dose distributions in the phantoms were measured with TLD and film. These measurements were compared with the treatment planning system calculated dose distributions.

Results

The compensator treatment plans achieved superior CTV coverage (97% vs 89% of the CTV receiving the prescription dose, p < 0.0025), and similar heart and lung doses (p > 0.35) to the conventional treatment plans. Average differences between calculated and measured TLD values were 2%, and average film profile differences were <2 mm.

Conclusions

We developed a new compensator based treatment methodology for PMRT and demonstrated its validity and superiority to conventional multi-field plans through end-to-end testing.

The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18MV Elekta SL 75/25 Medical Linear Accelerator with and without Flattening Filter: A Monte Carlo Study

BACKGROUND

One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk.

OBJECTIVE

We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method.

MATERIAL AND METHODS

MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac's manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam.

RESULTS AND CONCLUSION

Based on the Friedman and Wilcoxon nonparametric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss.

A ring-based compensator IMRT system optimized for low- and middle-income countries: Design and treatment planning study

PURPOSE

We propose a novel compensator-based IMRT system designed to provide a simple, reliable, and cost-effective adjunct technology, with the goal of expanding global access to advanced radiotherapy techniques. The system would employ easily reusable tungsten bead compensators that operate independent of a gantry (e.g., mounted in a ring around the patient). Thereby the system can be retrofitted to existing linac and cobalt teletherapy units. This study explores the quality of treatment plans from the proposed system and the dependence on associated design parameters.

METHODS

We considered 60 Co-based plans as the most challenging scenario for dosimetry and benchmarked them against clinical MLC-based plans delivered on a linac. Treatment planning was performed in the Pinnacle treatment planning system with commissioning based on Monte Carlo simulations of compensated beams. 60 Co-compensator IMRT plans were generated for five patients with head-and-neck cancer and five with gynecological cancer and compared to respective IMRT plans using a 6 MV linac beam with an MLC. The dependence of dosimetric endpoints on compensator resolution, thickness, position, and number of beams was assessed. Dosimetric accuracy was validated by Monte Carlo simulations of dose distribution in a water phantom from beams with the IMRT plan compensators.

RESULTS

The 60 Co-compensator plans had on average equivalent PTV coverage and somewhat inferior OAR sparing compared to the 6 MV-MLC plans, but the differences in dosimetric endpoints were clinically acceptable. Calculated treatment times for head-and-neck plans were 7.6 ± 2.0 min vs 3.9 ± 0.8 min (6 MV-MLC vs 60 Co-compensator) and for gynecological plans were 8.7 ± 3.1 min vs 4.3 ± 0.4 min. Plan quality was insensitive to most design parameters over much of the ranges studied, with no degradation found when the compensator resolution was finer than 6 mm, maximum thickness at least 2 tenth-value-layers, and more than five beams were used. Source-to-compensator distances of 53 and 63 cm resulted in very similar plan quality. Monte Carlo simulations suggest no increase in surface dose for the geometries considered here. Simulated dosimetric validation tests had median gamma pass rates of 97.6% for criteria of 3% (global)/3 mm with a 10% threshold.

CONCLUSIONS

The novel ring-compensator IMRT system can produce plans of comparable quality to standard 6 MV-MLC systems. Even when 60 Co beams are used the plan quality is acceptable and treatment times are substantially reduced. 60 Co-compensator IMRT plans are adequately modeled in an existing commercial treatment planning system. These results motivate further development of this low-cost adaptable technology with translation through clinical trials and deployment to expand the reach of IMRT in low- and middle-income countries.

Introduction to passive electron intensity modulation

This work introduces a new technology for electron intensity modulation, which uses small area island blocks within the collimating aperture and small area island apertures in the collimating insert. Due to multiple Coulomb scattering, electrons contribute dose under island blocks and lateral to island apertures. By selecting appropriate lateral positions and diameters of a set of island blocks and island apertures, for example, a hexagonal grid with variable diameter circular island blocks, intensity modulated beams can be produced for appropriate air gaps between the intensity modulator (position of collimating insert) and the patient. Such a passive radiotherapy intensity modulator for electrons (PRIME) is analogous to using physical attenuators (metal compensators) for intensity modulated x‐ray therapy (IMXT). For hexagonal spacing, the relationship between block (aperture) separation (r) and diameter (d) and the local intensity reduction factor (IRF) is discussed. The PRIME principle is illustrated using pencil beam calculations for select beam geometries in water with half beams modulated by 70%–95% and for one head and neck field of a patient treated with bolus electron conformal therapy. Proof of principle is further illustrated by showing agreement between measurement and calculation for a prototype PRIME. Potential utilization of PRIME for bolus electron conformal therapy, segmented‐field electron conformal therapy, modulated electron radiation therapy, and variable surface geometries is discussed. Further research and development of technology for the various applications is discussed. In summary, this paper introduces a practical, new technology for electron intensity modulation in the clinic, demonstrates proof of principle, discusses potential clinical applications, and suggests areas of further research and development.

Superficial and peripheral dose in compensator-based FFF beam IMRT

Flattening filter-free (FFF) beams produce higher dose rates. Combined with compensator-based intensity modulated radiotherapy (IMRT) techniques, the dose delivery for each beam can be much shorter compared to the flattened beam MLC-based or flattened beam compensator-based IMRT. This 'snap shot' IMRT delivery is beneficial to patients for tumor motion management. Due to softer energy, superficial doses in FFF beam treatment are usually higher than those from flattened beams. Due to no flattening filter, thus less photon scattering, peripheral doses are usually lower in FFF beam treatment. However, in compensator-based IMRT using FFF beams, the compensator is in the beam pathway. Does it introduce beam hardening effects and scattering such that the superficial dose is lower and peripheral dose is higher compared to FFF beam MLC-based IMRT? This study applied Monte Carlo techniques to investigate the superficial and peripheral doses in compensator-based IMRT using FFF beams and compared it to the MLC-based IMRT using FFF beams and flattened beams. Besides varying thicknesses of brass slabs to simulate varying thicknesses of compensators, a simple cone-shaped compensator was simulated to mimic a clinical application. The dose distribution in water phantom by the cone-shaped compensator was then simulated by multiple MLC-defined FFF and flattened beams with varying apertures. After normalization to the maximum dose, D_max, the superficial and peripheral doses were compared between the FFF beam compensator-based IMRT and FFF/flattened beam MLC-based IMRT. The superficial dose at the central 0.5 mm depth was about 1% (of D_max) lower in the compensator-based 6 MV FFF (6FFF) IMRT compared to the MLC-based 6FFF IMRT, and about 8% higher than the flattened 6 MV MLC-based IMRT dose. At 8 cm off-axis at depth of central maximum dose, d_max, the peripheral dose between the 6FFF and flattened 6 MV MLC demonstrated similar doses, while the compensator dose was about 1% (of D_max) higher. Compensators reduce the superficial doses slightly compared to open FFF beams, but increases the peripheral doses due to scatter in the compensator.

A novel software and conceptual design of the hardware platform for intensity modulated radiation therapy

PURPOSE

To deliver high quality intensity modulated radiotherapy (IMRT) using a novel generalized sparse orthogonal collimators (SOCs), the authors introduce a novel direct aperture optimization (DAO) approach based on discrete rectangular representation.

METHODS

A total of seven patients-two glioblastoma multiforme, three head & neck (including one with three prescription doses), and two lung-were included. 20 noncoplanar beams were selected using a column generation and pricing optimization method. The SOC is a generalized conventional orthogonal collimators with N leaves in each collimator bank, where N = 1, 2, or 4. SOC degenerates to conventional jaws when N = 1. For SOC-based IMRT, rectangular aperture optimization (RAO) was performed to optimize the fluence maps using rectangular representation, producing fluence maps that can be directly converted into a set of deliverable rectangular apertures. In order to optimize the dose distribution and minimize the number of apertures used, the overall objective was formulated to incorporate an L2 penalty reflecting the difference between the prescription and the projected doses, and an L1 sparsity regularization term to encourage a low number of nonzero rectangular basis coefficients. The optimization problem was solved using the Chambolle-Pock algorithm, a first-order primal-dual algorithm. Performance of RAO was compared to conventional two-step IMRT optimization including fluence map optimization and direct stratification for multileaf collimator (MLC) segmentation (DMS) using the same number of segments. For the RAO plans, segment travel time for SOC delivery was evaluated for the N = 1, N = 2, and N = 4 SOC designs to characterize the improvement in delivery efficiency as a function of N.

RESULTS

Comparable PTV dose homogeneity and coverage were observed between the RAO and the DMS plans. The RAO plans were slightly superior to the DMS plans in sparing critical structures. On average, the maximum and mean critical organ doses were reduced by 1.94% and 1.44% of the prescription dose. The average number of delivery segments was 12.68 segments per beam for both the RAO and DMS plans. The N = 2 and N = 4 SOC designs were, on average, 1.56 and 1.80 times more efficient than the N = 1 SOC design to deliver. The mean aperture size produced by the RAO plans was 3.9 times larger than that of the DMS plans.

CONCLUSIONS

The DAO and dose domain optimization approach enabled high quality IMRT plans using a low-complexity collimator setup. The dosimetric quality is comparable or slightly superior to conventional MLC-based IMRT plans using the same number of delivery segments. The SOC IMRT delivery efficiency can be significantly improved by increasing the leaf numbers, but the number is still significantly lower than the number of leaves in a typical MLC.

Quantitative analysis of brass compensators for commissioning of the Pinnacle planning system for IMRT

Brass compensators for beam modulation present an alternative method for IMRT treatment planning to traditional dynamic leaf modulation. In this work, we present a detailed method to commission the Pinnacle treatment planning system for IMRT using brass compensators. Beam attenuation from various brass thicknesses were measured using an ion chamber, as well as a MapCHECK device, for a representative seven-field IMRT plan. We show that Pinnacle's parameters for compensators can be optimized to match the measured results while still maintaining the original open-beam model. Only Pinnacle's modified scatter factor and brass density value were adjusted. Beam attenuation in several brass slabs were optimized by minimizing a χ(2) function of measured and modeled data for several depths in solid water. Using MapCHECK, Pinnacle's parameters were optimized by minimizing the number of points that fail during a gamma analysis of seven fields in a typical IMRT plan. Our results show that Pinnacle's 10X beam is best modeled using a brass density of 7.8 g/cm(3) and a modified scatter factor of 0.1 cm(-1).

Dosimetric evaluation of compensator intensity modulation-based stereotactic body radiotherapy for Stage I non-small-cell lung cancer

OBJECTIVE

To evaluate the dosimetry of compensator intensity modulation-based stereotactic body radiotherapy (SBRT) [non-coplanar intensity-modulated radiotherapy (ncIMRT)], its use was compared with that of three-dimensional conformation-based SBRT, for patients with Stage I non-small-cell lung cancer (NSCLC).

METHODS

21 consecutive patients with Stage I NSCLC were treated with ncIMRT or SBRT at Tokyo Medical University. To compare the two techniques, ncIMRT and SBRT plans for each patient were generated, where the planning target volume (PTV) coverages were adjusted to be equivalent to each other. The prescribed dose was set as 75 Gy in 30 fractions. PTV coverage, conformity index, conformation number (CN) and homogeneity index (HI) were used to compare the two strategies.

RESULTS

There was no statistically significant difference between PTV coverage for the 100%, 95% and 90% dose levels in the SBRT plan and those in the ncIMRT plan. The CN values were 0.53 ± 0.13 in the SBRT plan and 0.72 ± 0.10 in the ncIMRT plan. These values were significantly better than those of the SBRT plan (p < 0.001). The HI in the ncIMRT plan was 1.04 ± 0.03%, which was also significantly better than that of SBRT.

CONCLUSION

The ncIMRT plan provided superior conformity and reduced the doses to the lung for patients with Stage I NSCLC.

ADVANCES IN KNOWLEDGE

The delivery technique with compensator intensity modulation-based SBRT was evaluated. Concerning target motion, this is thought to be more robust and safer than SBRT for early-stage NSCLC.

American College of Radiology (ACR) and American Society for Radiation Oncology (ASTRO) Practice Guideline for Intensity-modulated Radiation Therapy (IMRT)

Intensity-modulated radiation therapy (IMRT) is a complex technique for the delivery of radiation therapy preferentially to target structures while minimizing doses to adjacent normal critical structures. It is widely utilized in the treatment of a variety of clinical indications in radiation oncology, including tumors of the central nervous system, head and neck, breast, prostate, gastrointestinal tract, and gynecologic organs, as well as in situations where previous radiation therapy has been delivered, and has allowed for significant therapeutic advances in many clinical areas. IMRT treatment planning and delivery is a complex process. Safe and reliable delivery of IMRT requires appropriate process design and adherence to quality assurance (QA) standards. A collaborative effort of the American College of Radiology and American Society for Therapeutic Radiation Oncology has produced a practice guideline for IMRT. The guideline defines the qualifications and responsibilities of all the involved personnel, including the radiation oncologist, physicist, dosimetrist, and radiation therapist. Factors with respect to the QA of the treatment planning system, treatment-planning process, and treatment-delivery process are discussed, as are issues related to the utilization of volumetric modulated arc therapy. Patient-specific QA procedures are presented. Successful IMRT programs involve integration of many processes: patient selection, patient positioning/immobilization, target definition, treatment plan development, and accurate treatment delivery. Appropriate QA procedures, including patient-specific QA procedures, are essential to ensure quality in an IMRT program and to assure patient safety.

Commissioning compensator-based IMRT on the Pinnacle treatment planning system

We present a systematic approach to commissioning of the compensator‐based IMRT in Pinnacle treatment planning system for commercially manufactured brass compensators. Some model parameters for the beams modulated by the variable‐thickness compensators can only be associated with a single compensator thickness. To intelligently choose that thickness for beam modeling, we empirically determined the most probable filter thickness occurring within the modulated portion of the compensators typically used in clinics. We demonstrated that a set of relative output factors measured with the brass slab of most probable thickness (2 cm) differs from the traditionally used open field set, and leads to improved agreement between measurements and calculations, particularly for the larger field sizes. By iteratively adjusting the modifier scatter factor and filter density, the calculated effective attenuation of the flat filters was brought to within 2% of the ion chamber measurement for the clinically‐relevant range of filter thicknesses, depths and filed sizes. Beam hardening representation in Pinnacle provides for adequate depth dose modeling beyond the depth of about 5 cm. Disagreement at shallower depth for the large field sizes is likely due to the algorithm's inability to account for the low‐energy scattered photons generated in the filter. The average ion chamber point dose error at isocenter for ten clinical compensator‐based IMRT plans was under 1%. A biplanar 3D diode dosimeter was calibrated and validated for use with the compensators. The average gamma analysis (3%/3 mm) passing rate for ten IMRT plans was 98.9%± 1.0%. The device is particularly attractive because it easily generates dose comparisons at both the fraction and beam levels. Overlaying dose profiles for individual beams would easily uncover any errors in compensator orientation.

PACS number: 87.55Qr

The inter- and intrafraction reproducibilities of three common IMRT delivery techniques

PURPOSE

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability.

METHODS

In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files.

RESULTS

Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both.

CONCLUSIONS

The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

Compensator-based intensity-modulated radiation therapy for malignant pleural mesothelioma post extrapleural pneumonectomy

The present work investigated the potential of compensator‐based intensity‐modulated radiation therapy (CB‐IMRT) as an alternative to multileaf collimator (MLC)–based intensity‐modulated radiation therapy (IMRT) to treat malignant pleural mesothelioma (MPM) post extrapleural pneumonectomy.

Treatment plans for 4 right‐sided and 1 left‐sided MPM post‐surgery cases were generated using a commercial treatment planning system, XIO/CMS (Computerized Medical Systems, St. Louis, MO). We used a 7‐gantry‐angle arrangement with 6 MV beams to generate these plans. The maximum required field size was 30×40 cm. We evaluated IMRT plans with brass compensators (•Decimal, Sanford, FL) by examining isodose distributions, dose–volume histograms, metrics to quantify conformal plan quality, and homogeneity. Quality assurance was performed for one of the compensator plans.

Conformal dose distributions were achieved with CB‐IMRT for all 5 cases, the average planning target volume (PTV) coverage being 95.1% of the PTV volume receiving the full prescription dose. The average lung V20 (volume of lung receiving 20 Gy) was 1.8%, the mean lung dose was 6.7 Gy, and the average contralateral kidney V15 was 0.6%. The average liver dose V30 was 34.0% for the right‐sided cases and 10% for the left‐sided case. The average monitor units (MUs) per fraction were 980 MUs for the 45‐Gy prescriptions (mean: 50 Gy) and 1083 MUs for the 50‐Gy prescriptions (mean: 54 Gy).

Post surgery, CB‐IMRT for MPM is a feasible IMRT technique for treatment with a single isocenter. Compensator plans achieved dose objectives and were safely delivered on a Siemens Oncor machine (Siemens Medical Solutions, Malvern, PA). These plans showed acceptably conformal dose distributions as confirmed by multiple measurement techniques. Not all linear accelerators can deliver large‐field MLC‐based IMRT, but most can deliver a maximum conformal field of 40×40 cm. It is possible and reasonable to deliver IMRT with compensators for fields this size with most conventional linear accelerators.

PACS numbers: 87.56.ng, 87.56.N, 87.55.D, 87.55.dk

Linear attenuation coefficients for compensator based IMRT

With rapid technological improvements in computer driven 3-D radiotherapy treatment planning systems (RTPS) the use of compensating filters for intensity modulated radiation therapy (IMRT) will dramatically increase the ease of treatment. The procedure for commissioning .decimal (Sanford, Florida) compensators involved the measurement of the effective linear attenuation coefficients for aluminium and brass. Field sizes to be measured vary from small square field size of 5 cm to the larger square field size of 25 cm with additional measurements at each 5 cm2 increments. The energies commissioned where 6 MV and 18 MV photons. The depth of measurements varied from 5 cm to 10 cm within phantom material and the source surface distance varied from 100 cm to 90 cm. The beam quality was measured by obtaining percentage depth dose (PDD) curves for the various field sizes with and without the compensating material. Results of the series of measurements showed no significant differences in the effective linear attenuation coefficients with respect to chamber depth and source surface distance with constant energy and field size. The main factor that was shown to influence the effective linear attenuation coefficient was field size variation. A correlation was shown between the effective linear attenuation coefficient and field size, up to a field size of 15 cm x 15 cm. Our results showed that for optimal patient treatments using IMRT compensating filters, there is a need for establishing two field size dependent linear attenuation coefficients.

Alternative Methods for Intensity-Modulated Radiation Therapy Inverse Planning and Dose Delivery

A large number of IMRT systems are currently being marketed. Many of these systems appear to be unique, and manufacturers often emphasize design differences as they argue the merits of their particular approach. This paper focuses on highlighting the underlying feature that is intrinsically part of all IMRT systems. On the other hand, major differences often appear at the implementation stage for dose delivery. Such variations are evident because each manufacturer has a unique approach to balancing the issues of treatment time, leakage radiation reaching the patient's total body, aperture approximation of the ideal intensity maps, increasing the angles of approach for the treatment fields, integration of on-line imaging, selection of treatment distance, availability of different photon energies, and overall system complexity (i.e., cost). How these different issues are handled in the process of system design affects the relative advantages and disadvantages that appear in the final product. This paper takes the approach of dividing the various IMRT methods into categories that are divided roughly along the lines of the technique used during dose delivery to approximate the intensity patterns. Other features of each system are included under these sub-sections.

Inclusion of compensator-induced scatter and beam filtration in pencil beam dose calculations

Compensators can be used as beam intensity modulation devices for intensity-modulated radiation therapy applications. In contrast with multileaf collimators, compensators introduce scatter and beam hardening into the therapeutic x-ray beam. The degree of scatter and beam filtering depends on the compensator material and beam energy. Pencil beam dose calculation models can be used to derive the shape of the compensator. In this study a novel way of incorporating the effect of compensator-induced scatter and beam filtration is presented. The study was conducted using 6, 8, and 15 MV polyenergetic pencil beams (PBs). The compensator materials that were studied included wax, brass, copper, and lead. The perturbation effects of the compensators on the PB dose profiles were built in the PB dose profiles and tested for regular fields containing a step compensator and benchmarked against DOSXYZnrc Monte Carlo calculated dose profiles. These effects include compensator beam filtration and Compton-scattered photons generated in the compensator materials that influence the resulting PB dose profiles. These data were obtained from DOSXYZnrc simulations. A Gaussian function was used to model off-axis scatter and an exponential function was used to model beam hardening at any radius, r. Dose profiles were calculated under a step compensator using the method that can model beam hardening and off-axis scatter, as well as a conventional method where the PB profiles are not adjusted, but a single effective attenuation coefficient is used instead to best match the dose profiles. Both sets of data were compared to the DOSXYZnrc data. Depth and profile dose data for 10 x 10 cm2 and 20 x 20 cm2 fields indicated that at 2 cm depth in water the method that takes compensator scatter into account agrees more closely with the DOSXYZnrc data compared to the data using only an effective attenuation coefficient. Further, it was found that the effective attenuation method can only replicate the DOSXYZnrc data at 10 cm depth where it was chosen to do so. At shallower depths the effective attenuation method overestimates the dose and beyond 10 cm depth it causes an underestimation in the dose. The scatter and beam hardening inclusion method does not exhibit such properties. The exclusion of scatter can lead to dose errors of up to 4 percent with a copper compensator at 5 cm depth for a 10 X 10 cm2 field under a thickness of 5 cm at 6 MV. For materials such as lead this discrepancy could be as high as 7 to 8 percent at 6 MV. For larger fields (20 X 20 cm2) the effect of in-phantom scatter reduces the differences between the dose profiles calculated with the mentioned methods.

Dosimetric characteristics of a cubic-block-piled compensator for intensity-modulated radiation therapy in the Pinnacle radiotherapy treatment planning system

We examined the dose distributions generated by Pinnacle3 (Philips Radiation Oncology Systems, Milpitas, CA) for intensity‐modulated radiotherapy (IMRT) plans using a cubic‐block‐piled compensator as the intensity modulator for 4‐MV and 10‐MV photon beams. The Pinnacle treatment planning system (TPS) uses an algorithm in which only the physical density of the absorber is required for calculating the characteristics of the modulator. The intensity modulator consists of cubic blocks (attenuator) of a tungsten alloy, plus cubic blocks of polyethylene resin foam that fill the spaces between the attenuator blocks and polymethyl methacrylate (PMMA) boards that act as the platform for the modulator. By measuring the transmission for various thicknesses of attenuator and by deriving values for the total physical density of the modulator, we determined the optimal effective density by comparing the curves fitted for the actual transmission data with the transmission calculated by the TPS. Using these effective densities, we examined the accuracy of Pinnacle3 for dose profiles of specific geometric patterns. The levels of consistency between the measurements and the calculations were within a tolerance of 3% of the dose difference and had a 3‐mm distance to agreement for the ladder‐, stairstep‐, and pyramid‐shaped test patterns, except in the high dose gradient region. In this modulator assembly, leakage occurred from the slits between the cubic blocks. This leakage was about 1.6% at maximum, and its influence on dose distribution was not crucial. In the TPS, in which physical density was the only user‐controllable parameter, we used the effective density of the absorber deduced from the effective mass attenuation coefficient. We conclude that the intensity modulation compensator system, together with a piled cubic attenuator, is clinically applicable, with an acceptable tolerance level. For the intensity map of the IMRT plan, measurements in treatment fields met 3% and 3‐mm criteria, excluding some regions of high gradient, which had a discrepancy of less than 5% and 4 mm.

PACS numbers: 87.53.Mr, 87.53.Tf