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.

Early Ophthalmology Findings in Nonsyndromic Craniosynostosis.

Craniosynostosis (CS) occurs 1 in 2500 births and surgical intervention is indicated partly due to risk for elevated intracranial pressure (EICP). Ophthalmological examinations help identify EICP and additional vision concerns. This study describes preoperative and postoperative ophthalmic findings in CS patients (N=314) from chart review. Patients included nonsyndromic CS: multisuture (6.1%), bicoronal (7.3%), sagittal (41.4%), unicoronal (22.6%), metopic (20.4%), and lambdoidal (2.2%). Preoperative ophthalmology visits were at M =8.9±14.1 months for 36% of patients and surgery was at M =8.3±4.2 months. Postoperative ophthalmology visits were at age M =18.7±12.6 months for 42% with follow-up at M =27.1±15.1 months for 29% of patients. A marker for EICP was found for a patient with isolated sagittal CS. Only a third of patients with unicoronal CS had normal eye exams (30.4%) with hyperopia (38.2%) and anisometropia (16.7%) at higher rates than the general population. Most children with sagittal CS had normal exams (74.2%) with higher than expected hyperopia (10.8%) and exotropia (9.7%). The majority of patients with metopic CS had normal eye exams (84.8%). About half of patients with bicoronal CS had normal eye exams (48.5%) and findings included: exotropia (33.3%), hyperopia (27.3%), astigmatism (6%), and anisometropia (3%). Over half of children with nonsyndromic multisuture CS had normal exams (60.7%) with findings of: hyperopia (7.1%), corneal scarring (7.1%), exotropia (3.6%), anisometropia (3.6%), hypertropia (3.6%), esotropia (3.6%), and keratopathy (3.6%). Given the range of findings, early referral to ophthalmology and ongoing monitoring is recommended as part of CS care.

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.

Extended duration of dilator use beyond 1 year may reduce vaginal stenosis after intravaginal high-dose-rate brachytherapy.

BACKGROUND: Vaginal dilators (VD) are recommended following vaginal or pelvic radiotherapy for patients with endometrial carcinoma (EC) to prevent vaginal stenosis (VS). The time course of VS is not fully understood and the optimal duration of VD use is unknown. METHODS: We reviewed 243 stage IA-II EC patients who received adjuvant brachytherapy (BT) at an academic tertiary referral center. Patients were instructed to use their VD three times per week for at least 1-year duration. The primary outcome was development of grade ≥ 1 VS using CTCAEv4 criteria during the follow-up period. The log-rank test and multivariable Cox proportional hazards modeling were used to evaluate the effect of VD use (noncompliance vs. standard compliance [up to 1 year] vs. extended compliance [over 1 year]) on VS. RESULTS: The median follow-up was 15.2 months over the 5-year study period. At 15 months, the incidence of VS was 38.8% for noncompliant patients, 33.5% for those with standard compliance, and 21.4% for those with extended compliance (median time to grade ≥ 1 VS was 17.5 months, 26.7 months, and not yet reached for these groups, respectively). On multivariable Cox regression analysis, extended compliance remained a significant predictor of reduced VS risk when compared to both noncompliance (HR 0.38, 95% CI 0.18-0.80, p = 0.012) and standard compliance (HR 0.43, 95% CI 0.20-0.89, p = 0.023). CONCLUSIONS: The risk of VS persists beyond 1 year after BT. Extended VD compliance beyond 1 year may mitigate this risk.

The AAPM/ASTRO 2023 Core Physics Curriculum for Radiation Oncology Residents.

PURPOSE: The American Association of Physicists in Medicine Radiation Oncology Medical Physics Education Subcommittee (ROMPES) has updated the radiation oncology physics core curriculum for medical residents in the radiation oncology specialty. METHODS AND MATERIALS: Thirteen physicists from the United States and Canada involved in radiation oncology resident education were recruited to ROMPES. The group included doctorates and master's of physicists with a range of clinical or academic roles. Radiation oncology physician and resident representatives were also consulted in the development of this curriculum. In addition to modernizing the material to include new technology, the updated curriculum is consistent with the format of the American Board of Radiology Physics Study Guide Working Group to promote concordance between current resident educational guidelines and examination preparation guidelines. RESULTS: The revised core curriculum recommends 56 hours of didactic education like the 2015 curriculum but was restructured to provide resident education that facilitates best clinical practice and scientific advancement in radiation oncology. The reference list, glossary, and practical modules were reviewed and updated to include recent literature and clinical practice examples. CONCLUSIONS: ROMPES has updated the core physics curriculum for radiation oncology residents. In addition to providing a comprehensive curriculum to promote best practice for radiation oncology practitioners, the updated curriculum aligns with recommendations from the American Board of Radiology Physics Study Guide Working Group. New technology has been integrated into the curriculum. The updated curriculum provides a framework to appropriately cover the educational topics for radiation oncology residents in preparation for their subsequent career development.

Radiobiological evaluation of the stepping-source effect in single-fraction monotherapy high-dose-rate prostate brachytherapy.

INTRODUCTION: The continued reliance upon the traditional biologically effective dose (BEDT) formalism of BEDT=nd(1+d/(α/ß)) may be one possible contributor to the poor clinical outcomes observed with single-fraction 19-20 Gy prescriptions in prostate high-dose-rate (HDR) brachytherapy because BEDT does not consider intrafraction sublethal damage repair (iSLDR). This, along with low α/ß and repair half-times comparable to delivery time, could reduce the biological effect predicted using BEDT. METHODS AND MATERIALS: BED was recalculated with a model accounting for iSLDR, using time-averaged uniform dose rate (BEDg1) patterns and time-variable dose rate (BEDgss) patterns inherent to stepping-source delivery. An assortment of two-pulse delivery sequences assuming 19 Gy in 972 s was analyzed. Calculations were repeated for 17470 and 61050 U to investigate source strength dependence. RESULTS: BEDg1 and BEDgss was/were lower than BEDT by 16.9% and 11.1%-21.1%, respectively, for 40700 U. For 17470 U, BEDg1 and BEDgss was/were lower than BEDT by 32.5% and 21.5%-37.1%, respectively. For 61050 U, BEDg1 and BEDgss was/were lower than BEDT by 11.9% and 7.8%-15.3%, respectively. BEDgss was most dependent on pulse spacing with milder dependence on pulse onset time. BEDg1 served as a lower bound approximation of BEDgss for fast effective delivery time. CONCLUSIONS: Even for points with the same calculated dose, the biological dose was significantly reduced by iSLDR (as much as 37.1%). While BEDgss explicitly addressed the temporally-variable dose rate inherent to a stepping-source delivery, calculations were cumbersome. Under certain conditions, BEDg1 may serve as an approachable method to quickly assess "worst-case scenario" BED.

Implementing a simulation-based curriculum for hybrid intracavitary/interstitial brachytherapy using a new, commercially available, US/MR/CT-compatible gynecologic phantom.

PURPOSE: To establish a simulation-based workshop for teaching hybrid intracavitary/interstitial (IC/IS) brachytherapy skills using a new, commercially available US/CT/MRI-compatible gynecologic phantom prototype. METHODS AND MATERIALS: The VIOMERSE gynecological trainer phantom consisted of tissue-like colloid material and was US, CT, and MRI-compatible. We designed a hands-on workshop incorporating the phantom prototype to teach skills for hybrid IC/IS brachytherapy including pre-implant planning, procedural steps of implant placement, and post-implant evaluation. The workshop impact was evaluated with pre- and post-workshop surveys and responses were analyzed with Wilcoxon matched-pairs signed-rank test. RESULTS: Thirteen residents, divided into small groups, attended one of three separate 1-h workshop sessions held during the gynecologic education block. Workshop steps included IC applicator placement, tumor mapping and pre-planning desired location and depth of needles for hybrid IC/IS application, IS needle labelling and insertion, image review of needle placements, comparison of IC-alone versus IC/IS brachytherapy plans. Responses to questions ascertaining knowledge, confidence, and ability in planning and executing hybrid IC/IS procedures all demonstrated significant improvement from pre- to post-workshop. In response to whether the session was an effective learning experience, all residents rated the workshop with a score of 9 (n = 1) or 10 (n = 12) out of 10, with a score of 10 indicating "strongly agree." CONCLUSIONS: Implementation of a hybrid IC/IS brachytherapy workshop utilizing a new, commercially-available phantom prototype was successful, with clear, subjective benefit for the residents in our program. This experience may inform continued efforts towards standardizing brachytherapy curricula across programs and increasing trainee exposure to interstitial procedures.

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.

Single application hybrid interstitial brachytherapy for cervical cancer: an institutional approach during the COVID-19 pandemic.

PURPOSE: To ease anesthesia and inpatient strain during the COVID-19 pandemic, our institution's policy for hybrid intracavitary-interstitial brachytherapy (IC/ISBT) for cervical cancer (CC) was modified from multiple applications (MA) treated over 2 separate weeks (7 Gy × 4) to a single-application (SA), treated within 1 week (8 Gy × 3). Here, we assessed dosimetric quality of the SA hybrid IC/ISBT approach and report our early outcomes. MATERIAL AND METHODS: This was an IRB-approved retrospective review of CC patients treated with magnetic resonance imaging (MRI)-guided hybrid IC/ISBT between April 1, 2020 and December 31, 2020 (COVID cohort). Treatment parameters and quality indicators were compared to hybrid IC/ISBT cases treated in 2 years prior (pre-COVID cohort). Differences between cohorts were evaluated with the Mann-Whitney U-test. RESULTS: In the COVID compared to pre-COVID cohort, median high-risk clinical target volume (HR-CTV) was similar: 33.3 vs. 33.9 cc, as was cumulative HR-CTV D90%: 81.2 vs. 80.9 Gy. Organ-at-risk D2cc values and recto-vaginal point doses were similar. Median number of channels loaded was increased to 6 vs. 4 (p = 0.002), but percentage of total dwell time allocated to needles remained similar: 13% vs. 15%. Median implant HR-CTV D90% was higher: 107.8% vs. 98.4% (p = 0.001), and there was a trend toward reduced overall treatment time (OTT): 44 days vs. 53 days (p = 0.1). Local control was achieved in all patients, but mucosal toxicity was higher in the COVID group, with grade 2 or higher vaginal, genitourinary, or gastrointestinal events recorded in 56% of the patients. CONCLUSIONS: The SA hybrid IC/ISBT approach utilized during the COVID-19 pandemic maintained similar plan characteristics as pre-pandemic MA hybrid cases, while simultaneously reducing anesthesia, inpatient resources, and OTT. Local control outcomes demonstrate the regimen was effective; however, given the increased risk of mucosal toxicity, we conclude that the SA regimen should be considered only when a MA schedule is not feasible.

A methodology for direct quantification of over-ranging length in helical computed tomography with real-time dosimetry.

In helical computed tomography (CT), reconstruction information from volumes adjacent to the clinical volume of interest (VOI) is required for proper reconstruction. Previous studies have relied upon either operator console readings or indirect extrapolation of measurements in order to determine the over-ranging length of a scan. This paper presents a methodology for the direct quantification of over-ranging dose contributions using real-time dosimetry. A Siemens SOMATOM Sensation 16 multislice helical CT scanner is used with a novel real-time "point" fiber-optic dosimeter system with 10 ms temporal resolution to measure over-ranging length, which is also expressed in dose-length-product (DLP). Film was used to benchmark the exact length of over-ranging. Over-ranging length varied from 4.38 cm at pitch of 0.5 to 6.72 cm at a pitch of 1.5, which corresponds to DLP of 131 to 202 mGy-cm. The dose-extrapolation method of Van der Molen et al. yielded results within 3%, while the console reading method of Tzedakis et al. yielded consistently larger over-ranging lengths. From film measurements, it was determined that Tzedakis et al. overestimated over-ranging lengths by one-half of beam collimation width. Over-ranging length measured as a function of reconstruction slice thicknesses produced two linear regions similar to previous publications. Over-ranging is quantified with both absolute length and DLP, which contributes about 60 mGy-cm or about 10% of DLP for a routine abdominal scan. This paper presents a direct physical measurement of over-ranging length within 10% of previous methodologies. Current uncertainties are less than 1%, in comparison with 5% in other methodologies. Clinical implantation can be increased by using only one dosimeter if codependence with console readings is acceptable, with an uncertainty of 1.1% This methodology will be applied to different vendors, models, and postprocessing methods--which have been shown to produce over-ranging lengths differing by 125%.

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.

Impact of tissue heterogeneity correction on Gamma Knife stereotactic radiosurgery of acoustic neuromas.

PURPOSE/OBJECTIVES: Treatment planning systems (TPS) for Gamma Knife stereotactic radiosurgery (GK-SRS) include TMR10 algorithms, which assumes tissue homogeneity equivalent to water, and collapsed-cone convolutional (CCC) algorithms, which accounts for tissue inhomogeneity. This study investigated dosimetric differences between TMR10 and CCC TPS for acoustic neuromas (ANs) treated with GK-SRS. MATERIALS/METHODS: A retrospective review of 56 AN treated with GK-SRS was performed. All patients underwent MRI and CT imaging during their initial treatment and were planned using TMR10. Each plan was recalculated with CCC using electron density extracted from CT. Parameters of interest included Dmax, Dmin, D50%, cochlea Dmax, mean cochlea dose, target size, and laterality (>20 mm from central axis). RESULTS: Median target volume of patients was 1.5 cc (0.3 cc-2.8 cc) with median dose of 12 Gy prescribed to the 50% isodose line. Compared to CCC algorithms, the TMR10 calculated dose was higher: Dmax was higher by an average 6.2% (p < 0.001), Dmin was higher by an average 3.1% (p < 0.032), D50% was higher by an average of 11.3%. For lateralized targets, calculated Dmax and D50% were higher by 7.1% (p < 0.001) and 10.6% (p < 0.001), respectively. For targets <1 cc, Dmax and D50% were higher by 8.9% (p ≤ 0.009) and 12.1% (p ≤ 0.001), respectively. Cochlea Dmax was higher, by an average of 20.1% (p < 0.001). CONCLUSION: There was a statistically significant dosimetric differences observed between TMR10 and CCC algorithms for AN GK-SRS, particularly in small and lateralized ANs. It may be important to note these differences when relating GK-SRS with standard heterogeneity-corrected SRS regimens.

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.

Role of Gamma Knife Radiosurgery in Small Cell Lung Cancer: A Multi-Institutional Retrospective Study of the International Radiosurgery Research Foundation (IRRF).

BACKGROUND: Despite a high incidence of brain metastases in patients with small-cell lung cancer (SCLC), limited data exist on the use of stereotactic radiosurgery (SRS), specifically Gamma Knife™ radiosurgery (Elekta AB), for SCLC brain metastases. OBJECTIVE: To provide a detailed analysis of SCLC patients treated with SRS, focusing on local failure, distant brain failure, and overall survival (OS). METHODS: A multi-institutional retrospective review was performed on 293 patients undergoing SRS for SCLC brain metastases at 10 medical centers from 1991 to 2017. Data collection was performed according to individual institutional review boards, and analyses were performed using binary logistic regression, Cox-proportional hazard models, Kaplan-Meier survival analysis, and competing risks analysis. RESULTS: Two hundred thirty-two (79%) patients received SRS as salvage following prior whole-brain irradiation (WBRT) or prophylactic cranial irradiation, with a median marginal dose of 18 Gy. At median follow-up after SRS of 6.4 and 18.0 mo for surviving patients, the 1-yr local failure, distant brain failure, and OS were 31%, 49%, and 28%. The interval between WBRT and SRS was predictive of improved OS for patients receiving SRS more than 1 yr after initial treatment (21%, <1 yr vs 36%, >1 yr, P = .01). On multivariate analysis, older age was the only significant predictor for OS (hazard ratio 1.63, 95% CI 1.16-2.29, P = .005). CONCLUSION: SRS plays an important role in the management of brain metastases from SCLC, especially in salvage therapy following WBRT. Ongoing prospective trials will better assess the value of radiosurgery in the primary management of SCLC brain metastases and potentially challenge the standard application of WBRT in SCLC patients.

Construction of anthropomorphic phantoms for use in dosimetry studies.

This paper reports on the methodology and materials used to construct anthropomorphic phantoms for use in dosimetry studies, improving on methods and materials previously described by Jones et al. [Med Phys. 2006;33(9):3274-82]. To date, the methodology described has been successfully used to create a series of three different adult phantoms at the University of Florida (UF). All phantoms were constructed in 5 mm transverse slices using materials designed to mimic human tissue at diagnostic photon energies: soft tissue-equivalent substitute (STES), lung tissue-equivalent substitute (LTES), and bone tissue-equivalent substitute (BTES). While the formulation for BTES remains unchanged from the previous epoxy resin compound developed by Jones et al. [Med Phys. 2003;30(8):2072-81], both the STES and LTES were redesigned utilizing a urethane based compound which forms a pliable tissue-equivalent material. These urethane based materials were chosen in part for improved phantom durability and easier accommodation of real-time dosimeters. The production process has also been streamlined with the use of an automated machining system to create molds for the phantom slices from bitmap images based on the original segmented computed tomography (CT) datasets. Information regarding the new tissue-equivalent materials as well as images of the construction process and completed phantom are included.

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.

Incorporating patient-specific CT-based ophthalmic anatomy in modeling iodine-125 eye plaque brachytherapy dose distributions.

PURPOSE: To quantify the dosimetric impact of incorporating patient-specific CT-based models rather than the conventional stylized-standard model for eye plaque brachytherapy planning. METHODS AND MATERIALS: Plaque Simulator was used to plan 16 patients using both CT-based patient-specific eye model and stylized-standard (SS) eye models. Plaque position was initially based on the SS model and compared against their patient-specific model without changing the plaque loading pattern and seed strength. Dosimetric parameters were compared for tumor and healthy ocular structures. RESULTS: Patient-specific ocular parameters ranged from 0.40 to 1.38 of SS model values. If plaques were placed based on SS model eyelet positions, target volume receiving prescription dose (V100%) is overpredicted by 5.9% on average (max: 27%), and D95% is overpredicted by 17.2 Gy on average (max: 58.1 Gy). If the plaques were recentered, 13 of 16 patients had changes in V100% of less than 2%, whereas half of the patients still had optic disc dose difference greater than 5 Gy (max: 36.2 Gy). The largest differences were observed with a target-to-optic disk distance less than 6 mm. No substantial dose differences were observed for the tumor apex, fovea, lens, and opposing retina. CONCLUSIONS: Patient-specific modeling is recommended for clinical planning, especially with target-to-optic disk distances less than 6 mm, due to significant differences compared with SS model.