S Jang

Comparing Gamma Knife and CyberKnife in patients with brain metastases

The authors compared the relative dosimetric merits of Gamma Knife (GK) and CyberKnife (CK) in 15 patients with 26 brain metastases. All patients were initially treated with the Leksell GK 4C. The same patients were used to generate comparative CK treatment plans. The tissue volume receiving more than 12 Gy (V12), the difference between V12 and tumor volume (V12net), homogeneity index (HI), and gradient indices (GI25, GI50) were calculated. Peripheral dose falloff and three conformity indices were compared. The median tumor volume was 2.50 cm3 (range, 0. 044‐19.9). A median dose of 18 Gy (range, 15‐22) was prescribed. In GK and CK plans, doses were prescribed to the 40‐50% and 77‐92% isodose lines, respectively. Comparing GK to CK, the respective parametric values (median±standard deviation) were: minimum dose (18.2±3.4 vs. 17.6±2.4 Gy, p=0.395); mean dose (29.6±5.1 vs.20.6±2.8 Gy, p<0.00001); maximum dose (40.3±6.5 vs.22.7±3.3 Gy, p<0.00001); and HI (2.22±0.19 vs. 1.18±0.06, p<0.00001). The median dosimetric indices (GK vs. CK, with range) were: RTOG_CI, 1.76 (1.12‐4.14) vs. 1.53 (1.16‐2.12), p=0.0220; CI, 1.76 (1.15‐4.14) vs. 1.55 (1.18‐2.21), p=0.050; nCI, 1.76 (1.59‐4.14) vs. 1.57 (1.20‐2.30), p=0.082; GI50, 2.91 (2.48‐3.67) vs. 4.90 (3.42‐11.68), p<0.00001; GI25, 6.58 (4.18‐10.20) vs. 14.85 (8.80‐48.37), p<0.00001. Average volume ratio (AVR) differences favored GK at multiple normalized isodose levels (p<0.00001). We concluded that in patients with brain metastases, CK and GK resulted in dosimetrically comparable plans that were nearly equivalent in several metrics, including target coverage and minimum dose within the target. Compared to GK, CK produced more homogenous plans with significantly lower mean and maximum doses, and achieved more conformal plans by RTOG_CI criteria. By GI and AVR analyses, GK plans had sharper peripheral dose falloff in most cases. PACS number: 89.20.‐a

An introduction to molecular imaging in radiation oncology: A report by the AAPM Working Group on Molecular Imaging in Radiation Oncology (WGMIR)

Molecular imaging is the direct or indirect noninvasive monitoring and recording of the spatial and temporal distribution of in vivo molecular, genetic, and/or cellular processes for biochemical, biological, diagnostic, or therapeutic applications. Molecular images that indicate the presence of malignancy can be acquired using optical, ultrasonic, radiologic, radionuclide, and magnetic resonance techniques. For the radiation oncology physicist in particular, these methods and their roles in molecular imaging of oncologic processes are reviewed with respect to their physical bases and imaging characteristics, including signal intensity, spatial scale, and spatial resolution. Relevant molecular terminology is defined as an educational assist. Current and future clinical applications in oncologic diagnosis and treatment are discussed. National initiatives for the development of basic science and clinical molecular imaging techniques and expertise are reviewed, illustrating research opportunities in as well as the importance of this growing field.

The impact of CyberKnife's prescription isodose percentage on intracranial target planning

To the Editor: Recently, a detailed comparative study regarding intracranial Gamma Knife (GK) vs. CyberKnife (CK) intracranial dosimetry has been published by your Journal.(1) In a group of 15 patients with 26 brain metastases, we showed that CK produced more homogeneous and conformal plans, while the GK plans had sharper peripheral dose falloff in most cases. In the CK plans, by convention, the applied range of prescription isodose percentage (PIP) was 77%–92%, with a median value of 86%. Intrigued by the results, we hypothesized that lowering the PIP in CK planning would improve peripheral dose falloff, without compromising the excellent dosimetric conformality which had previously been achieved. Secondarily, it was expected that, as PIP decreased, the stereotactic radiosurgical (SRS) plans would become less homogeneous as maximum dose within the target increased. We thank you for the opportunity to share with you the additional results that were generated from this investigation. Parts of the methods and materials have been previously described.(1) We compared the relative dosimetric merit of various prescription isodose levels in CK’s MultiPlan (Accuray Inc, Sunnyvale, CA). The same 15-patient series was used for dosimetric planning. For each tumor, the PIP was varied at three levels averaging approximately 50, 65, and 85% (CK50, CK65 and CK85; Table 1). The homogeneity (HI) and gradient (GI) indices, modified conformity index (mCI, the ratio of the prescription isodose volume to the tumor volume receiving at least the prescription dose), and an MPS-defined quantity called “new CI” (nCI, the ratio of mCI to target coverage, also the inverse of van’t Riet’s Conformation Number) were computed. For peripheral dose falloff, GI50 was calculated as the ratio of the volume enclosed by the isodose at 50% of the prescription dose level to the volume enclosed by the original prescription isodose. GI25, GI40, GI60, and GI80 were calculated in a similar manner. Statistical analyses were performed using analysis of variance (ANOVA) and nonparametric Kruskal-Wallis tests. We found that the mean tumor volume was 4.4 cm3; a median dose of 18 Gy was prescribed. For CK50, CK65, and CK85 series, the coverage was maintained at 96%–100% in all cases. Optimized plans in each scenario across various PIPs were computed, and dosimetric constraints of critical organ structures were all met. Minimum, average, maximum doses, HI, nCI, GI25, GI50, and MU were reported (Table 2). Comparing across the CK50, CK65, and CK85 series, the median mCIs were: 1.48, 1.36, and 1.52, p =.086, respectively. The remaining gradient indices were: GI40 (5.8, 6.9, and 7.6, p = 0.0008); for GI60 (2.8, 3.4, and 3.8), GI80 (1.6, 1.9, and 2.2), and GI90 (1.3, 1.4, and 1.6), p < 0.00001 in all cases. In our study, as expected, the selection of a PIP had a statistically significant impact on HI, mean, and maximum doses. By both mCI and nCI, CK65 produced the most conformal plans which nearly reached statistical significance. Importantly, CK50 had significantly sharpest dose falloff at all gradient index levels, with the exception of GI25. However, dosimetric plans prescribing to CK50 required significantly longer treatment times by MU estimation. In current clinical practices, deciding a prescription isodose level in CK varies by individual plan and preference, and clearly no consensus exists. The CK is a relatively new modality for SRS, which is stereotactically capable for extracranial indications, as well. For GK and linacbased intracranial SRS, it is common to prescribe to 40%–60% and 80%–95% of PIP, respectively. It is then customarily believed that the PIPs of CK should fall between those of the GK and linac-based SRS plans, as CK shares features of both. For CK dosimetric planning, some JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 15, NUMBER 5, 2014

SU‐F‐J‐187: The Statistical NTCP and TCP Models in the Proton Therapy

PURPOSE The statistical models (SM) are typically used as a subjective description of a population for which there is only limited sample data, and especially in cases where the relationship between variables is known. The normal tissue complications and tumor control are frequently stochastic effects in the Radiotherapy (RT). Based on probabilistic treatments, it recently has been formulated new NTCP and TCP models for the RT. Investigating the particular requirements for their clinical use in the proton therapy (PT) is the goal of this work. METHODS The SM can be used as phenomenological or mechanistic models. The former way allows fitting real data and getting theirparameters. In the latter one, we should do efforts for determining the parameters through the acceptable estimations, measurements, and/or simulation experiments. Experimental methodologies for determination of the parameters have been developed from the fraction cells surviving the proton irradiation curves in tumor and OAR, and precise RBE models are used for calculating the variable of effective dose. As the executions of these methodologies have a high costs, so we have developed computer tools enable to perform simulation experiments as complement to limitations of the real ones. RESULTS The requirements for the use of the SM in the PT, such as validation and improvement of the elaborated and existent methodologies for determining the SM parameters and effective dose respectively, were determined. CONCLUSION The SM realistically simulates the main processes in the PT, and for this reason these can be implemented in this therapy, which are simples, computable and they have other advantages over some current models. It has been determined some negative aspects for some currently used probabilistic models in the RT, like the LKB NTCP and others derived from logistic functions; which can be improved with the proposed methods in this study.

SU-E-T-19: A New End-To-End Test Method for ExacTrac for Radiation and Plan Isocenter Congruence

PURPOSE To combine and integrate quality assurance (QA) of target localization and radiation isocenter End to End (E2E) test of BrainLAB ExacTrac system, a new QA approach was devised using anthropomorphic head and neck phantom. This test insures the target localization as well as radiation isocenter congruence which is one step ahead the current ExacTrac QA procedures. METHODS The head and neck phantom typically used for CyberKnife E2E test was irradiated to the sphere target that was visible in CT-sim images. The CT-sim was performed using 1 mm thickness slice with helical scanning technique. The size of the sphere was 3-cm diameter and contoured as a target volume using iPlan V.4.5.2. A conformal arc plan was generated using MLC-based with 7 fields, and five of them were include couch rotations. The prescription dose was 5 Gy and 95% coverage to the target volume. For the irradiation, two Gafchromic films were perpendicularly inserted into the cube that hold sphere inside. The linac used for the irradiation was TrueBeam STx equipped with HD120 MLC. In order to use ExacTrac, infra-red head-array was used to correlate orthogonal X-ray images. RESULTS Using orthogonal X-rays of ExacTrac the phantom was positioned. For each field, phantom was check again with X-rays and re-positioned if necessary. After each setup using ExacTrac, the target was irradiated. The films were analyzed to determine the deviation of the radiation isocenter in all three dimensions: superior-inferior, left-right and anterior-posterior. The total combining error was found to be 0.76 mm ± 0.05 mm which was within sub-millimeter accuracy. CONCLUSION Until now, E2E test for ExacTrac was separately implemented to test image localization and radiation isocenter. This new method can be used for periodic QA procedures.

SU‐E‐T‐115: Collimator Scatter Factor (Sc) Measurements for IRIS in CyberKnife Using Build‐Up Caps

PURPOSE To investigate the impact of custom-made build-up caps for two different diode detectors in robotic radiosurgery radiation fields with variable collimator (IRIS) for collimator scatter factor (Sc) measurement. METHODS Two acrylic caps were custom-made to fit our SFD or PFD (IBA Dosimetry, Germany) diode detectors. The two caps have thicknesses of 1.5 and 5 cm, corresponding to the dmax of the 6 MV CyberKnife and a depth beyond electron contamination, respectively. A watertank (Blue Phantom, IBA Dosimetry) was used to position the detector at 80 cm source-to-detector distance. Measurements were performed with the SFD and PFD, with and without the build-up caps, for all 12 clinical IRIS settings ranging from 5 to 60 mm. RESULT As expected, the biggest discrepancy was found in the smallest (5mm) IRIS field size. The Sc factor with 5 cm build-up cap was 6.6% lower than that without build-up using the SFD detector while the PFD differed by 13%. However, when the 1.5 cm build-up cap was used, the largest difference (4%) was found using the 10 mm IRIS field size using the SFD while the maximum discrepancy using the PFD was still using the 5 mm IRIS field size (7.6%). For IRIS field sizes larger than 10 mm, maximum discrepancies were less than 1.3% and 3.7% for SFD and PFD, respectively. CONCLUSION Sc measurement data are critical components in advanced algorithms for treatment planning, such as Monte Carlo, in order to calculate the dose accurately. After incorporating build-up caps, we discovered differences of up to 6.6% and 13% in Sc factors in the SFD and PFD detectors, respectively, when compared against in-air measurements without build-up caps. These are significant differences and were obtained with IRIS settings routinely used for clinical treatment.

SU‐E‐T‐301: Evaluation of Simultaneously Integrated Boost (SIB) and Sequential IMRT Boost (SqIB) Treatments of Head and Neck Cancer Using Empirical Radiobiological Modeling

PURPOSE To evaluate and compare normal tissue complication probabilities (NTCP) in SIB (simultaneous integrated boost) and SqIB (sequential IMRT boost) IMRT methods in head and neck cancer using the radiobiological modeling of HART program (Histogram Analysis in Radiation Therapy; J Appl Clin Med Phys 11(1): 3013, 2010). METHODS Of the 40 SIB IMRT cases identified in a 2-year follow up study, 14 SIB (Rx range: 66-70Gy) cases that developed dysphagia(N=11) or xerostomia(N=10) or both types of complications(N=9) were studied. Similarly 10 SqIB cases (Rx=73.5Gy) was studied previously. The TCP and NTCPs were calculated from the dose-volume histogram (DVH) statistics using the Poisson Statistics (PS) and JT Lyman models respectively. Values for the volume parameter (n), slope parameter (m), tumor control dose (TCD=63.8Gy) and tolerance dose (TD50,5 = 46 and 47 Gy for bilateral parotids and esophagus, respectively) were selected from Luxton et al. (Phys. Med. Biol. 53, 23-36, 2008). RESULTS In the SIB method (N=14; Students t-test), TCP of tumor was estimated to be 0.78±0.02; while NTCP for parotids and esophagus were 0.16±0.10, and 0.20±0.06 respectively. The corresponding numbers in the SqIB method (N=10) were 0.83±0.02; 0.45±0.14 and 0.17±0.09 respectively. CONCLUSION In a 2-year follow up study with SIB treatments, the estimated values of NTCP of esophagus correlated with the severity of dysphagia. In addition, the hot spots were also reduced and better parotid sparing was found in SIB method than in SqIB method which may partially be related to smaller prescription doses. However JT Lyman model provided better correlation between severity of xerostomia and NTCP of parotids; and PS models for tumor progression free survivability in SqIB treatments. These findings are not in direct comparison due to the differences in tumors and stages. This novel methodology of radiobiological outcome-related analysis can be utilized to evaluate different treatment plan techniques.

SU-E-T-595: A Study of Sequential and Simultaneously Integrated Boost IMRT Methods in Head and Neck Cancer

PURPOSE The purpose of this study was to obtain the characteristics of the sequential (SqB) and simultaneous integrated boost (SIB) IMRT methods in head and neck cancer using HART (Histogram Analysis in Radiation Therapy) program. METHODS Ten SqB and seventeen SIB IMRT cases were studied retrospectively. A cumulative mean dose of 71.3 Gy was prescribed sequentially, and a mean dose of 66.2 Gy for SIB method. Homogeneity (HI), radiation conformality indices (RCI) and quality factor (QF) were calculated from dose-volume histograms (DVHs). In order to estimate the radiobiological outcomes of NTCP, DVH statistics for the critical and hot spots were utilized with Poisson statistics and JT Lyman models in HART. RESULTS HI, RCI, and QF were 1.11±0.02, 0.97±0.01, 1.00±0.02 in SIB method; and 1.10±0.01, 0.98±0.01, and 0.93±0.03 in SqB method, respectively. Critical spots for parotids, larynx, and esophagus were 0.75±0.03, 0.06±0.02, and 0.34±0.02 in SqB method, and 0.29±0.07, <0.01, and 0.22±0.06 in SIB method, respectively. Hot spots for parotids, larynx, and esophagus were 0.34±0.03, 0.54±0.02, and <0.01, respectively in SqB method whereas 0.10±0.05, <0.01, and 0.05±0.03 in SIB method, respectively. NTCP estimates for parotids, larynx, and esophagus were 0.45±0.14, 0.03±0.01, and 0.17±0.09 in SqB method, and 0.09±0.04, <0.01, and 0.18±0.04 in SIB method, respectively. CONCLUSION For both boost methods mean HIs were comparable while mean RCI was better with SqB than SIB method. QF was significantly better in SIB than in SqB. Critical spots and hot spots were reduced in SIB method. Both SqB and SIB methods yielded similar NTCP for larynx and esophagus. Although better parotid sparing with SIB method than SqB was observed; due to the differences in tumors, stages and doses more patient data and detailed analyses should be followed for comparison. The radiobiological outcome-related analysis using DVHs can be utilized to evaluate different treatment planning techniques.

SU‐E‐T‐570: Improvement to the Histogram Analysis in Radiation Therapy (HART): An Open Source Software System for the Multi‐Dimensional Dose‐ Volume Histogram Analysis in Digital Image Communication in Medicine ‐ Radiation Therapy (DICOM‐RT) Treatment Plans

PURPOSE Histogram Analysis in Radiation Therapy (HART) is an efficient and accurate dose-volume histogram (DVH) computational tool in radiotherapy research. Several applications of the program have been presented previously (J Appl Clin Med Phys 11(1): 3013, 2010; Med Phys 38(6), p.3678, 2011) for the Radiation Therapy Oncology Group (RTOG) users. The program has been further developed to incorporate various types of DVH analysis features to support the research using DICOM-RT plans. The main objective of this work was to present the improvement and compatibility of the program for the DICOM-RT plans. METHODS AND MATERIALS MATLAB based codes were primarily designed to read and write a simpler HART format from the standard DICOM-RT data objects exported from the Xio treatment planning system (CMS Inc., St. Louis, MO). This format employed an optimal polynomial fitting technique to interpolate the co-ordinates of the contours in the regions-of-interest. The format was efficient for the (a) precise extraction of the cumulative DVH (cDVH) and spatial DVH (sDVH; x-,y-, and z-DVHs respectively) data- statistics, (b) universal-plan indices evaluation, (c) biological modeling based outcome analyses (BMOA), (d) radiobiological dose-response modeling, and (e) physical parameterization modules. The fundamental DVH statistics were validated using the DVH statistics extracted from the Computational Environment for Radiotherapy Research program. RESULTS HART offers various types of DVH computational functionalities, several plan evaluation and radiobiological outcome analysis modules in a user- friendly software package for the RTOG and DICOM-RT planners. The cDVH and BMOA modules were found to be the most applicable features for the global researchers. CONCLUSIONS HART is a novel and universal multi-dimensional DVH analysis tool for the radiation therapy research. We further expect to develop HART for the space-time DVH analysis and proton therapy applications. The software is available online (http://www2.uic.edu/∼apyaku1) for the radiotherapy research. This work was partially supported by NIH-NIDCD grant.

SU‐E‐T‐487: TomoTherapy Plan Quality Metrics for Multiple Anatomical Sites

Purpose: TomoTherapy IMRT plans for various anatomical sites were analyzed in order to obtain dosimetric characteristics, and to determine plan quality and deliverability.Methods: Treatment plans for thirty patients treated with TomoTherapy were reviewed and analyzed. The distribution of treatment sites was: abdomen (n=4), brain (n=4), head and neck (HN n=6), lung (n=6), pelvis (n=5) and prostate (n=5). Prescription dose (PD), D90%, PTV, max, mean and min doses of PTV were extracted from the plans and DVH data. Metrics calculated include: homogeneity index (H.I.=max dose in PTV/ PD), mean dose of PTV over PD, max dose of PTV over min dose in PTV, and D90%/PD were calculated for the different treatment sites. In order to verify plan delivery, absolute and relative dose measurements were performed and analyzed using an ion chamber and GafChromic films. Results: Mean prescription isodose lines were 95.7% (n=30). The PTV ranged from 11.7 to 982.2 cc. For H.I. the prostate site yielded the smallest H.I. of 0.99 while the pelvis site yielded the greatest H.I. of 1.14. The prostate site produced the smallest mean dose over PD (0.97) and H&N produced the greatest (1.03). The overall H.I. was 1.06 and the overall ratio of mean dose over PD was 1.02. The overall max dose over min dose was 1.18. The mean difference for absolute point dose measurements was −0.9%. Measured film profiles demonstrated excellent agreement with calculated profiles in plan delivery QA Conclusions: For the treatment sites reviewed prostate had the most homogeneous plans, while lung had the most inhomogeneous plans in this study. The overall mean dose over PD and QA results were excellent in TomoTherapy. TomoTherapy is capable of planning and delivering high quality IMRTtreatments for a wide variety of treatment sites