Preliminary dosimetric comparison between fixed and rotating source stereotactic radiosurgery systems.

PURPOSE: The Akesis Galaxy RTi (AK) is a novel rotational 60 Co-based cranial stereotactic radiosurgery (SRS) system. While similar systems have been compared against the fixed-source Leksell Gamma Knife (GK) system using stylized phantoms, dosimetric plan quality with realistic anatomy has yet to be characterized for this or any other rotating system versus GK. This study aims to benchmark AK dosimetric performance against GK by retrospectively replanning previously-treated GK patients at our institution. METHODS: Thirteen patients, previously treated on a GK Icon, were re-planned on the AK treatment planning system using the same prescription doses and isodoses as the original GK plans. The cohort includes patients treated for brain metastases, schwannomas, pituitary adenomas, trigeminal neuralgias, and arteriovenous malformations. Plans are evaluated with target coverage metrics (Dmin , Dmean , D95% , V150% ) and dose conformality indices: Radiation Therapy Oncology Group conformity index (CI), selectivity, Paddick CI (PCI), gradient index (GI). RESULTS: AK plans use fewer shots and larger collimation compared to GK plans, resulting in statistically significant reductions in treatment time (p = 0.047) by as much as 88.4 minutes while maintaining comparable target V100% . For most metastatic cases, GK produces higher Dmin (16.0-25.9 vs. 12.5-24.3 Gy, p = 0.008) while AK produces higher V150% (0.03-14.92 vs. 0.02-11.59 cc, p = 0.028). For non-metastatic cases, GK provides superior CI (p = 0.025) and GI (p = 0.044). No statistically significant differences were found in the remaining metrics. CONCLUSION: This cohort demonstrates that the AK system is able to achieve largely comparable dosimetric results to GK, typically with shorter treatment times. Further investigation with a larger cohort is underway.

A small footprint couch-top support device for image-guided radiotherapy of heavy patients.

PURPOSE: Patients with body weights close to or above 400 lbs present unique challenges in radiation therapy since the weight limit of most treatment couches decreases as the couch-top extends toward the treatment gantry. The purpose of this work was to develop a small footprint couch-top support platform to safely perform image-guided radiotherapy (IGRT) for extremely heavy patients. METHODS: One way to protect the couch-top from damage and prevent a catastrophic breakdown is to provide additional support as the couch extends toward the treatment gantry. To allow a maximal range of gantry movement, a small-footprint adjustable jack stand, placed underneath the couch-top, was chosen and modified from a commercial jack stand (with 1100 lbs capacity). The couch could be easily extended longitudinally and laterally with a modified 8-ball-transfer plate mounted at the top. The operation of a couch-top support platform was used for two heavy patients after phantom testing. kV and MV imaging options and ranges were quantified. RESULTS: The custom-constructed couch-top support platform was found to provide stable support with smooth couch shifts. The small footprint allowed gantry rotation from 133° to 227°, which would allow both fixed beam radiotherapy and partial-arc volumetric modulated arc therapy (VMAT). For IGRT, orthogonal 2D kV-kV image pairs with source angles of 40o and 130o were acquired and tested successfully. With the support platform, two clinical cases with patient weights greater than 415 lbs were successfully treated with image-guided partial arc VMAT radiotherapy. The study demonstrated the safety and efficiency of using this new couch-top support platform to prevent couch failure from treating heavy patients. CONCLUSIONS: A new couch-top support platform has been designed, assembled, and tested for IGRT. The new support platform is easy to use, cost-effective, and allows extremely heavy patients to be treated safely and robustly with IGRT and VMAT.

Associating dose-volume characteristics with theoretical radiobiological metrics for rapid Gamma Knife stereotactic radiosurgery plan evaluation.

PURPOSE: To examine general dose-volume characteristics in Gamma Knife (GK) plans which may be associated with higher tumor control probability (TCP) and equivalent uniform dose (EUD) using characteristic curve sets. METHODS: Two sets of dose-volume histograms (DVHs) were exported alongside an analytical purpose-generated DVH: (a) single-shot large collimator (8 or 16 mm) emulated with multiple shots of 4 mm collimator. (b) shot-within-shot (SWS) technique with isodose lines (IDLs) of 40-75%. TCP, average dose, EUD in single-fraction (EUDT ) and 2 Gy fractionated regimens (EUDR ) were examined for trends with cumulative DVH (cDVH) shape as calculated using a linear-quadratic cell survival model (α/ß = 10.0 Gy, N0  = 1 × 106 ) with both α = 0.20 Gy-1 and α = 0.23 Gy-1 . RESULTS: Using α = 0.20 Gy-1 (α = 0.23 Gy-1 ), plans in the analytical set with higher shoulder regions had TCP, EUDT , EUDR increased by 180%, 5.9%, 10.7% (11.2%, 6.3%, 10.0%), respectively. With α = 0.20 Gy-1 (α = 0.23 Gy-1 ), plans with higher heels had TCP, EUDT , EUDR increased by 4.0%, <1%, <1% (0.6%, <1%, <1%), respectively. In emulating a 16 (8) mm collimator, 64 (12) shots of the small collimators were used. Plans based on small collimators had higher shoulder regions and, with α = 0.20 Gy-1 (α = 0.23 Gy-1 ), TCP, EUDT , EUDR was increased up to 351.4%, 5.0%, 8.8% (270.4%, 5.0%, 6.8%) compared with the single-shot large collimator. Delivery times ranged from 10.2 to 130.3 min. The SWS technique used 16:8 mm collimator weightings ranging from 1:2 to 9.2:1 for 40-75% IDL. With α = 0.20 Gy-1 (α = 0.23 Gy-1 ), the 40% IDL plan had the highest shoulder with increased TCP, EUDT , EUDR by 130.7%, 9.6%, 17.1% (12.9%, 9.1%, 16.4%) over the 75% IDL plan. Delivery times ranged 6.9-13.8 min. CONCLUSIONS: The magnitude of the shoulder region characteristic to GK cDVHs may be used to rapidly identify superior plan among candidates. Practical issues such as delivery time may require further consideration.

A scattering-foil free electron beam to increase dose rate for total skin electron therapy (TSET).

BACKGROUND: Electron beams are used at extended distances ranging between 300 to 700 cm to uniformly cover the entirety of the patient's skin for total skin electron therapy (TSET). Even with electron beams utilizing the high dose rate total skin electron (HDTSe) mode from the Varian 23iX or TrueBeam accelerators, the dose rate is only 2500 cGy/min at source-to-surface distance (SSD) = 100 cm. At extended distances, the decrease in dose rate leads to long beam delivery times that can limit or even prevent the use of the treatment for patients who, in their weakened condition, may be unable to stand on their own for extended periods of time. Previously, to increase dose rate, a customized 6 MeV electron beam was created by removing the x-ray target, flattening filter, beam monitor chamber, and so forth. from the beam path (Chen, et at IJROBP 59, 2004) for TSET. Using this scattering-foil free (SFF) electron beam requires the treatment distance be extended to 700 cm to achieve dose uniformity from the single beam. This room size requirement has limited the widespread use of the 6 MeV-SFF beam. PURPOSE: This study explores an application of a dual-field technique with a 6 MeV-SFF beam to provide broad and uniform electron fields to reduce the treatment distances in order to overcome treatment room size limitations. METHODS: The EGSnrc system was used to generate incident beams. Gantry angles between 6 MeV-SFF dual-fields were optimized to achieve the similar patient skin dose distribution resulting from a standard 6 MeV-HDTSe dual-field configuration. The patient skin dose comparisons were performed based on the patient treatment setup geometries using dose-volume-histograms. RESULTS: Similar dose coverage can be achieved between 6 MeV-SFF and 6 MeV-HDTSe beams by reducing gantry angles between dual-field geometries by 8° and 7° at treatment distances of 400 and 500 cm, respectively. To achieve 95% mean dose to the first 5 mm of skin depth in the torso area, the mean dose to depths of 5-10 mm and 10-15 mm below the skin surface was 74% (74%) and 49% (50%) of the prescribed dose when using 6 MeV-SFF (6 MeV-HDTSe) beam, respectively. CONCLUSIONS: The 6 MeV-SFF electron beam is feasible to provide similar TSET skin dose coverage at SSD ≥ 400 cm using a dual-field technique. The dose rate of the 6 MeV-SFF beam is about 4 times that of current available 6 MeV-HDTSe beams at treatment distances of 400-500 cm, which significantly shortens the treatment beam-on time and makes TSET available to patients in weakened conditions.

AAPM Task Group Report 267: A joint AAPM GEC-ESTRO report on biophysical models and tools for the planning and evaluation of brachytherapy.

Brachytherapy utilizes a multitude of radioactive sources and treatment techniques that often exhibit widely different spatial and temporal dose delivery patterns. Biophysical models, capable of modeling the key interacting effects of dose delivery patterns with the underlying cellular processes of the irradiated tissues, can be a potentially useful tool for elucidating the radiobiological effects of complex brachytherapy dose delivery patterns and for comparing their relative clinical effectiveness. While the biophysical models have been used largely in research settings by experts, it has also been used increasingly by clinical medical physicists over the last two decades. A good understanding of the potentials and limitations of the biophysical models and their intended use is critically important in the widespread use of these models. To facilitate meaningful and consistent use of biophysical models in brachytherapy, Task Group 267 (TG-267) was formed jointly with the American Association of Physics in Medicine (AAPM) and The Groupe Européen de Curiethérapie and the European Society for Radiotherapy & Oncology (GEC-ESTRO) to review the existing biophysical models, model parameters, and their use in selected brachytherapy modalities and to develop practice guidelines for clinical medical physicists regarding the selection, use, and interpretation of biophysical models. The report provides an overview of the clinical background and the rationale for the development of biophysical models in radiation oncology and, particularly, in brachytherapy; a summary of the results of literature review of the existing biophysical models that have been used in brachytherapy; a focused discussion of the applications of relevant biophysical models for five selected brachytherapy modalities; and the task group recommendations on the use, reporting, and implementation of biophysical models for brachytherapy treatment planning and evaluation. The report concludes with discussions on the challenges and opportunities in using biophysical models for brachytherapy and with an outlook for future developments.

Dose prescription and reporting in stereotactic body radiotherapy: A multi-institutional study.

BACKGROUND AND PURPOSE: Radiation dose prescriptions are foundational for optimizing treatment efficacy and limiting treatment-related toxicity. We sought to assess the lack of standardization of SBRT dose prescriptions across institutions. MATERIALS & METHODS: Dosimetric data from 1298 patients from 9 academic institutions treated with IMRT and VMAT were collected. Dose parameters D100, D98, D95, D50, and D2 were used to assess dosimetric variability. RESULTS: Disease sites included lung (48.3 %) followed by liver (29.7 %), prostate (7.5 %), spine (6.8 %), brain (4.1 %), and pancreas (2.5 %). The PTV volume in lung varied widely with bimodality into two main groups (22.0-28.7 cm3) and (48.0-67.1 cm3). A hot spot ranging from 120-150 % was noted in nearly half of the patients, with significant variation across institutions. A D50 ≥ 110 % was found in nearly half of the institutions. There was significant dosimetric variation across institutions. CONCLUSIONS: The SBRT prescriptions in the literature or in treatment guidelines currently lack nuance and hence there is significant variation in dose prescriptions across academic institutions. These findings add greater importance to the identification of dose parameters associated with improved clinical outcome comparisons as we move towards more hypofractionated treatments. There is a need for standardized reporting to help institutions in adapting treatment protocols based on the outcome of clinical trials. Dosimetric parameters are subsequently needed for uniformity and thereby standardizing planning guidelines to maximize efficacy, mitigate toxicity, and reduce treatment disparities are urgently needed.

Dosimetric study of Varian Universal Multi-Channel Cylinder System for High-Dose-Rate 192Ir Brachytherapy.

PURPOSE: Recently, the Varian multichannel vaginal cylinder (MCVC) set for high-dose-rate 192Ir brachytherapy was commercially released. This MCVC was distinct from our existing MCVC in its peripheral channel layout and tip design. This investigation sought to assess the dosimetric impact of these changes. METHODS AND MATERIALS: The dimensions of the virtual model for each applicator were compared against both physical and radiographic measurements. Volumetric dose distributions were generated in silico using a model-based dose calculation algorithm (MBDCA). To characterize the effects of the new peripheral channel layout on dose to adjacent areas ("dose-spill"), point doses were compared using two sets of applicator-based reference points: at surface or 5 mm radially from surface. To evaluate the dose-shaping capabilities, a dose distribution was generated for the new applicator and assessed against a representative dose distribution for a patient previously treated with existing equipment. RESULTS: Based on both physical and radiographic measurements, virtual models were representative of each applicator within ±1 mm. Commissioning of the MBDCA was benchmarked based on AAPM Working Group on Dose Calculation Algorithms in Brachytherapy. The layout of the new applicator reduced dose-spill to other reference points significantly, as much as a factor of 16.3, compared with the existing equipment. The rounded tip shape and curve of the peripheral channels in the new applicator produced more conformity to its HR-CTV than existing equipment. CONCLUSIONS: Compared with our existing equipment, the design changes in the new Varian MCVC set offered improved control of dose spill and better conformality to HR-CTV.

Clinically-implementable template plans for multidwell treatments using Leipzig-style applicators in 192Ir surface brachytherapy.

PURPOSE: Multiple dwell positions ("multidwell") within a Leipzig-style applicator can be used to increase dose uniformity and treatment area. Model-based dose calculation algorithms (MBDCAs) are necessary for accurate calculations involving these applicators because of their nonwater equivalency and complex geometry. The purpose of this work was to create template plans from MBDCA calculations and present their dwell times and positions for users of these applicators without access to MBDCAs. METHODS AND MATERIALS: The Leipzig-style solid applicator model within our treatment planning system was used to design template plans. Five template plans, normalized to 0.3 cm depth within a water phantom, were optimized using the treatment planning system MBDCA. Each template plan contained unique dwell positions, times, and active lengths (0.5-1.5 cm). A single-dwell distribution was optimized for comparison. The stem of this applicator stops within the shell; therefore, one template plan contained an intrafraction rotation to determine the largest dose distribution achievable. Effects of imperfect applicator rotation were quantified by inserting rotational offsets and comparing the V100%, D95%, and minimum dose coverage for planning target volumes created from 80%/90% isodose lines. RESULTS: The 90% (80%) isodose line dimensions at 0.3 cm depth for single-dwell increased from 0.94 × 0.97 (1.53 × 1.57) cm2 to 2.09 × 1.24 (2.75 × 1.88) cm2 in the largest template plan. Manually inserted angular offsets up to ±10° for the template plan requiring rotation preserved V100%, D95%, and minimum dose within 2.0%, 1.9%, and 8.0%, respectively. CONCLUSION: A set of template plans was created to provide accessibility to the multidwell methodology, even for users without access to MBDCAs. Each template plan should be reviewed before clinical implementation.

A photon spectrometric dose-rate constant determination for the Advantage Pd-103 brachytherapy source.

PURPOSE: Although several dosimetric characterizations using Monte Carlo simulation and thermoluminescent dosimetry (TLD) have been reported for the new Advantage Pd-103 source (IsoAid, LLC, Port Richey, FL), no AAPM consensus value has been established for the dosimetric parameters of the source. The aim of this work was to perform an additional dose-rate constant (lamda) determination using a recently established photon spectrometry technique (PST) that is independent of the published TLD and Monte Carlo techniques. METHODS: Three Model IAPD-103A Advantage Pd-103 sources were used in this study. The relative photon energy spectrum emitted by each source along the transverse axis was measured using a high-resolution germanium spectrometer designed for low-energy photons. For each source, the dose-rate constant was determined from its emitted energy spectrum. The PST-determined dose-rate constant (PST lamda) was then compared to those determined by TLD (TLD lamda) and Monte Carlo (MC lamda) techniques. A likely consensus lamda value was estimated as the arithmetic mean of the average lamda values determined by each of three different techniques. RESULTS: The average PST lamda value for the three Advantage sources was found to be (0.676 +.- 0.026) cGyh(-1) U(-1). Intersource variation in PST lamda was less than 0.01%. The PST lamda was within 2% of the reported MC lamda values determined by PTRAN, EGSnrc, and MCNP5 codes. It was 3.4% lower than the reported TLD lamda. A likely consensus lamda value was estimated to be (0.688 +/- 0.026) cGyh(-1) U(-1), similar to the AAPM consensus values recommended currently for the Theragenics (Buford, GA) Model 200 (0.686 +/- 0.033) cGyh(-1) U(-1), the NASI (Chatsworth, CA) Model MED3633 (0.688 +/- 0.033) cGyh(-1) U(-1), and the Best Medical (Springfield, VA) Model 2335 (0.685 +/- 0.033) cGyh(-1) U(-1) 103Pd sources. CONCLUSIONS: An independent lamda determination has been performed for the Advantage Pd-103 source. The PST lamda obtained in this work provides additional information needed for establishing a more accurate consensus lamda value for the Advantage Pd-103 source.

A prototype open-ended multichannel intracavitary-interstitial hybrid applicator for gynecological high-dose-rate brachytherapy.

This manuscript introduces a novel open-ended, multichannel intracavitary-interstitial hybrid applicator for gynecological high-dose-rate brachytherapy. A prototype was three-dimensionally (3D) printed using polylactic acid. The device was 25 mm in diameter and 150 mm in length, with eight evenly spaced peripheral channels around a central tandem channel, each 2.7 mm in diameter and with 2 mm source-to-cylinder-surface-distance. In contrast to conventional multichannel applicators, the new hybrid applicator was designed with open distal ends. Interstitial needles utilized in peripheral channels provided a closed environment for sources. The applicator body served as a template to aid in the placement of central tandem and peripheral needles. The physical prototype appropriately accommodated needles, tandem, and locking devices and, thus, was deemed magnetic resonance (MR) conditional. In our retrospective in silico studies of two representative prior patients, we simultaneously increased the target coverage and decreased the organ-at-risk (OAR) dose to 2 cc (D2cc). Specifically, the minimum dose covering 90% of the volume (D90%) was improved by 2.1% (9.2%) minimum (maximum) of the prescription dose. Additionally, the OAR D2cc was decreased by 0.5% (4.5%), 8.2% (12.9%), 3.9% (9.2%), and 4.6% (19.8%) minimum (maximum) of the prescription dose to the sigmoid, bladder, rectum, and bowel, respectively. This prototype demonstrated significant potential for patients in whom it would be useful to provide multichannel capabilities beyond the applicator body.

Feasibility of using multiple-dwell positions in 192Ir Leipzig-style brachytherapy surface applicators to expand target coverage and clinical application.

PURPOSE: Leipzig-style applicators for surface brachytherapy are traditionally used with a single-source dwell position. This study explores the feasibility of using multiple-source dwell positions ("multidwell") to improve the dose coverage and applicability of Leipzig-style applicators. METHODS AND MATERIALS: A virtual model of the Leipzig-style applicator was commissioned for a model-based dose calculation algorithm (MBDCA) and compared against American Association of Physicists in Medicine working group 186 benchmarking data sets and ionization chamber point measurements. An absolute dosimetry technique based on radiochromic film was used to validate both single-dwell and multidwell plans. RESULTS: Dose distributions generated from the MBDCA-based virtual model were consistent with working group data sets, ion chamber measurements, and radiochromic film analysis. In one multidwell configuration, at 3 mm prescription depth, the 80% isodose width was increased to 25 mm, compared with 15 mm in the same dimension for a single-dwell delivery. In the same multidwell configuration, the flatness, measured as >98% isodose line, was more than doubled to 8 mm, compared with 3 mm in the same dimension. For multidwell plans, 2-D planar agreement between radiochromic film and MBDCA exceeded 93% in gamma analysis (3%/1 mm criteria). Submillimeter positional agreement was found, with a total dosimetric uncertainty of 4.5% estimated for the entire system. CONCLUSIONS: Leipzig-style surface applicators with multiple-source dwell positions have been benchmarked against radiochromic film dosimetry. Results show that the clinically viable coverage area can be increased significantly.

Deployment and performance of model-based dose calculation algorithm in 192Ir shielded cylinder brachytherapy.

PURPOSE: The purpose of the study was to integrate model-based dose calculation algorithm (MBDCA) and 3-D planning into our institutional capabilities for clinical cases with inherent heterogeneities, namely shielded cylinder cases, which were previously performed using templates. METHODS AND MATERIALS: AcurosBV (Varian Medical Systems) was selected as MBDCA, and we compared results against the American Association of Physicists in Medicine working group (WG) reference Test Case #4, which contains a 36-mm-diameter 180-degree shielded cylinder. The last five clinically used template plans, as generated with ABACUS (Varian Medical Systems), were compared against MBDCA results. Clinical plans used 20-, 23-, and 26-mm-diameter cylinders, prescribed from 5 to 7 Gy, 50- to 110-mm active length, 7 to 21 dwell positions, with 5- or 10-mm spacing. RESULTS: Compared with the WG reference plan, AcurosBV produced excellent agreement, within 1% at reference points. Larger deviations arose only within the applicator itself. Historical plans generated using ABACUS had higher point dose than AcurosBV by 3-4% or 2-3% using transport within medium at prescription points, with dose to medium or water, respectively. CONCLUSIONS: To verify the accuracy of our MBDCA algorithm, we benchmarked against the WG data set available for shielded cylinders. We discovered a 3-4% difference in dose from our established historical templates, which is easily outweighed by daily positioning uncertainties. Dose distributions from MBDCA were used to assess and validate existing historical templates.

High-dose-rate brachytherapy as monotherapy for prostate cancer: The impact of cellular repair and source decay.

PURPOSE: This work quantifies the influence of intrafraction DNA damage repair and cellular repopulation on biologically effective dose (BED) in Ir-192 high-dose-rate brachytherapy for prostate cancer. In addition, it examines the effect of source-decay-induced BED variation for patients treated at different time points in a source exchange cycle. MATERIALS AND METHODS: Current fractionation schemes are based on simplified-form BED = nd(1 + d/(α/ß)), which assumes that intrafraction repair, interfraction repair, and repopulation are negligible. We took accepted radiobiological parameters of Tk, Tp, and α from the recommendations of the AAPM TG-137, and recalculated the full-form BED. Fraction times were normalized to require 15 min for 20 Gy at 10 Ci. Calculations were carried out for both α/ß = 1.5 and 3 Gy. RESULTS: After accounting for intrafraction repair, interfraction repair, and/or repopulation, full-form BED calculations showed significant values, as compared with simplified-form BED. For 1-fraction 20 Gy fractionation, the full-form BED was only 64-82% of the simplified-form BED. Dose protraction effects were milder for smaller prescriptions (6 Gy/Fx), where full form was 87-94%. With regard to source decay, BED varied >20% for patients treated at the beginning and the end of a source exchange cycle for 20 Gy single-fraction prescription. CONCLUSIONS: Repair and repopulation can be significant in monotherapy high-dose-rate for prostate cancer. As fractionation schemes are established, the simplified BED calculation may not be appropriate. Investigators should consider evaluating BED as a range rather than a discrete value when presenting results unless source activity is explicitly incorporated as well.

Photon energy spectrum emitted by a novel polymer-encapsulated 103Pd source and its effect on the dose rate constant.

Two independent groups have published intrinsic dosimetry parameters for the recently introduced OptiSeed103 interstitial brachytherapy source which contains 103Pd encapsulated by a novel polymer shell. The dose rate constant (Lambda) reported by the two groups, however, differed by more than 6% and there is currently no AAPM recommended consensus value for this source in clinical dosimetry. The aim of this work was to perform an independent determination of Lambda for the OptiSeed103 source using a recently developed photon spectrometry technique. Three OptiSeed103 sources (model 1032P) with known air-kerma strength were used in this study. The photon energy spectrum emitted along the radial direction on the source's bisector was measured in air using a high-resolution intrinsic germanium spectrometer designed and established for low-energy brachytherapy source spectrometry. The dose rate constant of each source was determined from its emitted energy spectrum and the spatial distribution of radioactivity in the source. Unlike other sources made with traditional titanium encapsulation, the photons emitted by the OptiSeed103 sources exhibited only slight spectral hardening, yielding a relative energy spectrum closer to that emitted by bare 103Pd. The dose rate constant determined by the photon spectrometry technique for water was 0.664 +/- 0.025 cGy h(-1) U(-1). This value agreed, within experimental uncertainties, with the Monte Carlo-calculated value (MCLambda) of 0.665 +/- 0.014 cGy h(-1) U(-1) and the TLD-measured value (with a Monte Carlo-calculated solid-phantom-to-water conversion factor) of 0.675 +/- 0.051 cGy h(-1) U(-1) reported by Wang and Hertel [Appl. Radiat. Isot. 63, 311-321 (2005)]. However, it differed by -6.7% from the McLambda of 0.712 +/- 0.043 cGy h(-1) U(-1) reported by Bernard and Vynckier [Phys. Med. Biol. 50, 1493-1504 (2005)]. The results obtained in this work provide additional information needed for establishing a consensus value for the dose rate constant for the OptiSeed103 source. It suggests that an eventual consensus value of Lambda for the OptiSeed103 source is likely to be closer to a value of 0.668 cGy h(-1) U(-1) rather than 0.693 cGy h(-1) U(-1) as initially recommended by the source manufacturer based on the two previously published results.

Silver fluorescent x-ray yield and its influence on the dose rate constant for nine low-energy brachytherapy source models.

The physical characteristics of the photons emitted by a low-energy brachytherapy source are strongly dependent on the source's construction. Aside from absorption and scattering caused by the internal structures and the source encapsulation, the photoelectric interactions occurred in certain type of source-construction materials can generate additional energetic characteristic x rays with energies different from those emitted by the bare radionuclide. As a result, the same radionuclide encapsulated in different source models can result in dose rate constants and other dosimetric parameters that are strikingly different from each other. The aim of this work was to perform a systematic study on the yield of silver fluorescent x rays produced in nine 125I sources that are known to contain silver and its impact on the dose-rate constant. Using a high-resolution germanium spectrometer, the relative 125I spectra emitted by the nine sources on its bisector were measured and found to be similar to each other (the maximum variation in the 125I-Kbeta relative intensity was less than 4%). On the other hand, the measured silver fluorescent x-ray spectra exhibited much greater variations from model to model; the maximum change in the measured Ag-Kalpha relative intensity was over 95%. This larger variation in the measured silver fluorescent x-ray yield was caused by (1) the different amount of silver that was directly exposed to the 125I radionuclide in different source models and (2) the stronger influence of the source's internal geometry on the silver fluorescent x rays. Because the addition of silver fluorescent x rays can significantly alter the photon characteristics emitted by the radioactive sources, a precise knowledge on the silver fluorescent x-ray yield is needed in theoretical calculations of the sources' intrinsic dosimetric properties. This study concludes that the differences in silver fluorescent yield are the primary causes of the variable dose rate constant observed among these source models.

Photon spectrometry for the determination of the dose-rate constant of low-energy photon-emitting brachytherapy sources.

Accurate determination of dose-rate constant (lambda) for interstitial brachytherapy sources emitting low-energy photons (< 50 keV) has remained a challenge in radiation dosimetry because of the lack of a suitable absolute dosimeter for accurate measurement of the dose rates near these sources. Indeed, a consensus value of lambda taken as the arithmetic mean of the dose-rate constants determined by different research groups and dosimetry techniques has to be used at present for each source model in order to minimize the uncertainties associated with individual determinations of lambda. Because the dosimetric properties of a source are fundamentally determined by the characteristics of the photons emitted by the source, a new technique based on photon spectrometry was developed in this work for the determination of dose-rate constant. The photon spectrometry technique utilized a high-resolution gamma-ray spectrometer to measure source-specific photon characteristics emitted by the low-energy sources and determine their dose-rate constants based on the measured photon-energy spectra and known dose-deposition properties of mono-energetic photons in water. This technique eliminates many of the difficulties arising from detector size, the energy dependence of detector sensitivity, and the use of non-water-equivalent solid phantoms in absolute dose rate measurements. It also circumvents the uncertainties that might be associated with the source modeling in Monte Carlo simulation techniques. It was shown that the estimated overall uncertainty of the photon spectrometry technique was less than 4%, which is significantly smaller than the reported 8-10% uncertainty associated with the current thermo-luminescent dosimetry technique. In addition, the photon spectrometry technique was found to be stable and quick in lambda determination after initial setup and calibration. A dose-rate constant can be determined in less than two hours for each source. These features make it ideal to determine the dose-rate constant of each source model from a larger and more representative sample of actual sources and to use it as a quality assurance resource for periodic monitoring of the constancy of lambda for brachytherapy sources used in patient treatments.

Dose-Volume Predictors of Esophagitis After Thoracic Stereotactic Body Radiation Therapy.

OBJECTIVES: Esophageal toxicity has become a major concern as stereotactic hypofractionated radiation therapy is increasingly utilized for central pulmonary tumors. Our purpose was to define esophageal dosimetric parameters that predict potentially dose-limiting toxicities. MATERIALS AND METHODS: In total, 157 patients with a planning target volume ≤5 cm from the esophagus were selected from an institutional database. Toxicity was scored with the CTCAE v4.0. Esophageal Dmax and Dv (dose D in Gy covering volume v in mL) in 0.5 mL increments were collected. Corresponding biologically effective dose (BED) was calculated for α/ß=10,3 (BED10, BED3). Normal tissue complication probability was computed with conventionally fractionated radiotherapy parameters and equivalent dose in 2 Gy per fraction (EQD2). Dosimetric predictors were identified with multivariate logistic regression with a manual forward stepwise selection technique. RESULTS: The grade≥2 esophagitis rate was 5.7%. BED10 to 1.5 mL was the best predictor of esophagitis. BED10 to 0.5, 1.0, 2.0, 3.0, and 3.5 mL were also predictive but less strong. Results were similar when BED3 and physical dose were examined. Tumor-esophageal distance correlated with esophagitis (10.5% risk of≥grade 2 events with distance≤3.9 cm vs. 1.3% when>3.9 cm, P=0.016). BED10 to 1.5 mL correlated well with EQD2 normal tissue complication probability estimates. CONCLUSIONS: BED to 1.5 mL was the strongest predictor of grade≥2 esophagitis (independent of α/ß ratio) with a 10.6% toxicity risk when BED10>21.1 Gy (14.3 Gy in 3 fractions, 16.0 Gy in 5). The overall rate of severe toxicity is low, suggesting that higher doses may be tolerable.

State of dose prescription and compliance to international standard (ICRU-83) in intensity modulated radiation therapy among academic institutions.

PURPOSE: The purpose of this study was to evaluate dose prescription and recording compliance to international standard (International Commission on Radiation Units & Measurements [ICRU]-83) in patients treated with intensity modulated radiation therapy (IMRT) among academic institutions. METHODS AND MATERIALS: Ten institutions participated in this study to collect IMRT data to evaluate compliance to ICRU-83. Under institutional review board clearance, data from 5094 patients-including treatment site, technique, planner, physician, prescribed dose, target volume, monitor units, planning system, and dose calculation algorithm-were collected anonymously. The dose-volume histogram of each patient, as well as dose points, doses delivered to 100% (D100), 98% (D98), 95% (D95), 50% (D50), and 2% (D2), of sites was collected and sent to a central location for analysis. Homogeneity index (HI) as a measure of the steepness of target and is a measure of the shape of the dose-volume histogram was calculated for every patient and analyzed. RESULTS: In general, ICRU recommendations for naming the target, reporting dose prescription, and achieving desired levels of dose to target were relatively poor. The nomenclature for the target in the dose prescription had large variations, having every permutation of name and number contrary to ICRU recommendations. There was statistically significant variability in D95, D50, and HI among institutions, tumor site, and technique with P values < .01. Nearly 95% of patients had D50 higher than 100% (103.5 ± 6.9) of prescribed dose and varied among institutions. On the other hand, D95 was close to 100% (97.1 ± 9.4) of prescribed dose. Liver and lung sites had a higher D50 compared with other sites. Pelvic sites had a lower variability indicated by HI (0.13 ± 1.21). Variability in D50 is 101.2 ± 8.5, 103.4 ± 6.8, 103.4 ± 8.2, and 109.5 ± 11.5 for IMRT, tomotherapy, volume modulated arc therapy, and stereotactic body radiation therapy with IMRT, respectively. CONCLUSIONS: Nearly 95% of patient treatments deviated from the ICRU-83 recommended D50 prescription dose delivery. This variability is significant (P < .01) in terms of treatment site, technique, and institution. To reduce dosimetric and associated radiation outcome variability, dose prescription in every clinical trial should be unified with international guidelines.

Pulmonary dose-volume predictors of radiation pneumonitis following stereotactic body radiation therapy.

PURPOSE: Radiation pneumonitis (RP) may be severe after stereotactic body radiation therapy. Our purpose was to identify pulmonary and cardiac dosimetric parameters that predicted for post-stereotactic body radiation therapy grade ≥2 RP. METHODS AND MATERIALS: A total of 335 patients with ≥3 months' follow-up were included. Normal pulmonary volume was total lungs minus gross tumor volume. Pulmonary maximum dose, mean lung dose (MLD), and the percent of lung receiving ≥x Gy for 5 to 50 Gy in 5-Gy increments were collected. Cardiac maximum dose, mean dose, volume of lung receiving ≥0.1 Gy (V0.1), V0.25 to V1, and V2.5 to V12.5 were recorded. Multivariable logistic regression with manual backward stepwise elimination was used to identify the best dosimetric predictors of toxicity. Optimal dose-volume cutoffs were isolated with recursive partitioning analysis (RPA). RESULTS: The grade ≥2 RP rate was 18.8%. Pulmonary V5 to V50, MLD, and cardiac V0.1 to V2.5 were significantly associated with toxicity on univariate analysis. On multivariable logistic regression, V10 was the strongest dosimetric predictor of grade ≥2 RP (odds ratio, 1.052; 95% confidence interval, 1.014-1.092; P = .007). RPA identified a 21.6% risk of grade ≥2 RP with V10 ≥6.14% (vs 3.8% with <6.14). MLD was the most significant predictor of grade ≥3 RP (odds ratio, 1.002; 95% confidence interval, 1.000-1.003; P = .031). RPA identified a 25.0% risk of grade ≥3 RP with MLD ≥7.84 Gy (vs 8.0% when <7.84 Gy). CONCLUSIONS: With a grade ≥2 RP rate of 18.8%, lung V10 was the best predictor of grade ≥2 toxicity. MLD was the best predictor of grade ≥3 RP.

Dosimetric comparison of two arc-based stereotactic body radiotherapy techniques for early-stage lung cancer.

To compare the dosimetric and delivery characteristics of two arc-based stereotactic body radiotherapy (SBRT) techniques for early-stage lung cancer treatment. SBRT treatment plans for lung tumors of different sizes and locations were designed using a single-isocenter multisegment dynamic conformal arc technique (SiMs-arc) and a volumetric modulated arc therapy technique (RapidArc) for 5 representative patients treated previously with lung SBRT. The SiMs-arc plans were generated with the isocenter located in the geometric center of patient׳s axial plane (which allows for collision-free gantry rotation around the patient) and 6 contiguous 60° arc segments spanning from 1° to 359°. 2 RapidArc plans, one using the same arc geometry as the SiMs-arc and the other using typical partial arcs (210°) with the isocenter inside planning target volume (PTV), were generated for each corresponding SiMs-arc plan. All plans were generated using the Varian Eclipse treatment planning system (V10.0) and were normalized with PTV V100 to 95%. PTV coverage, dose to organs at risk, and total monitor units (MUs) were then compared and analyzed. For PTV coverage, the RapidArc plans generally produced higher PTV D99 (by 1.0% to 3.3%) and higher minimum dose (by 2.7% to 12.7%), better PTV conformality index (by 1% to 8%), and less volume of 50% dose outside 2cm from PTV (by 0 to 20.8cm(3)) than the corresponding SiMs-arc plans. For normal tissues, no significant dose differences were observed for the lungs, trachea, chest wall, and heart; RapidArc using partial arcs produced lowest maximum dose to spinal cord. For dose delivery, the RapidArc plans typically required 50% to 90% more MUs than SiMs-arc plans to deliver the same prescribed dose. The additional intensity modulation afforded by variable gantry speed and dose rate and by overlapping arcs enabled RapidArc plans to produce dosimetrically improved plans for lung SBRT, but required more MUs (by a factor > 1.5) to deliver. The dosimetric improvements, most notably in PTV minimum dose and in dose conformality for irregularly shaped PTVs, may outweigh the increased MUs in using RapidArc. For small and peripherally located tumors, SiMs-arc produces comparable dosimetric quality and could be more efficient in both treatment planning and dose delivery.