Faculty Bibliography

Purpose: For the Cyberknife (Accuray Inc. Sunnyvale CA) stereotactic radiosurgery and stereotactic body radiotherapy treatment unit, a series of collimating cones are used with diameters ranging from 5 mm to 60 mm. The measurement of output factors (OFs) in these small radiation fields is challenging due to limitations in detector size. Currently multiple detectors are used for the measurement of output factors, but if not water equivalent, Monte Carlo corrections are required. The Exradin W2 (Standard Imaging Inc., Middleton, WI) plastic scintillation device (PSD) has an active volume of 0.0008 cm³ and is water equivalent. This work reports OFs and scanning data measured by the W2 PSD and a diode in Cyberknife.
Method(s): A Cyberknife M6 and a W2 PSD in a MP3 water phantom (PTW, Freiberg, Germany) were used to take TPR, profile, and OF measurements. The W2 comes with a specialized electrometer, MAX SD, used to measure the Cerenkov light ratio using the manufacturer's recommendation. Profiles, tissue phantom ratios (TPR), and OF values were taken using the clinically relevant (5, 7.5 and 10) mm cones, with OFs normalized to the 60 mm cone and scanning measurements normalized to maximum dose. All results were compared with PTW 60018 diode measurements taken during commissioning.
Result(s): The W2 scintillator required the slowest scan speed (0.1 mm/s) in order to eliminate noise, and TPR data was smoothed using a moving average filter. Comparison between diode and W2 profiles and TPR data showed good agreement. Monte Carlo adjusted diode OFs and W2 measured OFs were within 2.6%.
Conclusion(s): This work demonstrated the feasibility of using a water equivalent, small volume commercial W2 PSD in Cyberknife. Scanning data was near equivalent, but showed significant noise and required smoothing. W2 measured OFs agreed with MC-corrected diode measurements taken during commissioning

Purpose: Radiation induced interstitial pneumonitis and late renal dysfunction are major concerns for patients undergoing total body irradiation (TBI). The purpose of this work is to evaluate the dosimetry of VMAT-based TBI plans generated using Varian Eclipse scripting.
Method(s): Three full-body CT datasets (two patients, one anthropomorphic CIRS phantom) were used. An in-house Eclipse script was developed to generate optimized field arrangements using the body contour, user origin, and couch longitudinal travel. Plans consisted of a lower-body AP/PA portion and an upper-body VMAT portion (8 full arcs with 4-isocenters). Treatment plans to 1320 cGy (165 cGy x 8fx) were generated with dose directives: [PTV V100% >=90- 95%; Total lung Dmean <900 cGy; Kidneys Dmean <1100 cGy]. All plans used 6MV photons and were calculated using the AAA algorithm. Upperbody VMAT plan dosimetry was evaluated 'in-phantom' placing 12 OSLDs in different key locations (lung, kidneys, bone, and soft tissue). Additionally, dosimetric verification was performed for the three plans using Varian portal dosimetry, PerFraction(SNC) and ArcCheck(SNC) with a global gamma criterion of 2%/2 mm.
Result(s): Planning objectives were met for the three treatment plans with the following averages: PTV V100% = 94.02%, total lung Dmean = 872.9 cGy, and kidneys Dmean = 1075.8 cGy. The dose deviation between Eclipse and the OSLDs (relative to the prescribed dose) averaged 0.98%, with each individual dose deviation within +/-4%. Dose ranged between 52.5 cGy (lung) and 187.5 cGy (bone) for OSLD measurements. The average passing rate for all 24 fields (8 per plan) was 98.0%, 99.76% and 98.6% for portal dosimetry, PerFraction and ArcCheck respectively. The lowest passing rate of any individual field was 95.4%, 99.0% and 91.8% for portal dosimetry, PerFraction and ArcCheck respectively.
Conclusion(s): Eclipse scripting can assist in creating robust multi-isocentric VMATbased TBI treatment plans to block lungs and kidneys without compromising target coverage. Dosimetric accuracy and deliverability was confirmed using in-phantom OSLD dosimetry, Varian portal dosimetry, PerFraction and Arc-Check verification

Purpose: Concurrent chemotherapy with radiotherapy is the standard of care for locally advanced oropharyngeal cancer patients. However, the main drawback of this approach is the high toxicities experienced by the patients. This has motivated new clinical trials that investigate the role of imaging biomarkers in dose de-escalation to mitigate the side effects of treatments.
Method(s): Ten patients from an institutional phase II clinical trial were CE-CT (Contrast-Enhanced-CT) simulated prior to starting radiotherapy treatment and at week-four as part of the protocol. A radiation oncologist manually contoured the GTVn (primary nodal disease) on both scans. Based on GTVn volume variation (>=40%) patients were eligible/ineligible for dose de-escalation. CE-CT scans and contours were transfer to IBEX for texture-feature calculation. The relative net change for 77 texture-features was calculated. The Pearson correlation coefficient (r) was used to correlate the volume change with the feature changes. Texture-features that presented an r >0.5 are possible candidates for treatment assessment. Significance level was evaluated using the t-test (P < 0.05) Results: Eight patients met criteria for mid-treatment nodal response and were de-escalated. For the two patients who proceeded with standard treatment, shape texture-features variation were low, ranging [-6% to 14%] when compared to de-escalated patients range [-0% to 60%]. Across all the patients two shape features (surface area and surface area density) showed high correlation with node-tumor volume changes, with r-values of 0.81 (P < 0.05) and -0.66 (P < 0.05). Histogram-based-likeskewness showed a medium correlation with r-value of 0.52 (P > 0.05), whereas dissimilarity feature from the Gray-Level-Occurrence-Matrix showed correlation of 0.63 (P < 0.05).
Conclusion(s): Features with high Pearson correlation values are potential candidates to be used as additional metrics for treatment assessment. The study includes other imaging modalities (e.g MRI and PET) which will be included as a future work. More analysis will be added to the study as more patients are continually enrolled in the protocol

PURPOSE/OBJECTIVE:Small field dosimetry has been an active area of research for over a decade. It is now known that large dosimetric errors can be introduced if proper detectors or correction factors are not used. The International Atomic Energy Agency (IAEA) through the technical report series No. 483 provides guidelines for small field dosimetry procedures as well as correction factors for most detectors available in the market. The plastic scintillator detector (PSD) Exradin W1 has been found to have a correction factor close to unity, however it is not designed for beam scanning. To overcome this limitation, the new PSD Exradin W2 has been developed to be used as a scanning as well as a relative dosimeter. Characterization of this detector in small field dosimetry is presented in this study. METHODS:A 6 MV beam from a Varian-Edge linac was used to collect data for the characterization of aW2 detector. Cerenkov light ratio (CLR) is corrected through a separate new electrometer system that comes with the W2 detector. The parameters investigated include the dose and dose rate linearity, beam profiles, percent depth doses (PDD), field output factors, and temperature response. The results were compared with Gafchromic film (EBT-3 film) for beam profiles. The field output factor and temperature response were compared to the Exradin W1 detector. RESULTS:field size when compared to the EBT-3 film profile. CONCLUSIONS:The W2 scintillator detector showed similar dosimetric and temperature properties to the W1 scintillator detector. The main advantage of the W2 detector among other plastic scintillators is the beam scanning capabilities that, combined with its correction factor of 1.0, make it an ideal detector for commissioning of SRS and SBRT techniques.

It is estimated that approximately 24% of the US population has at least one tattoo. However, tattoo ink ingredients include heavy metals (high atomic number Z) that are not regulated, which can cause skin reactions. This study investigates the dosimetric effects in surface dose due to high-Z elements in tattoo ink under electron beam irradiation. Four commercially available tattoo ink colors, black, red, yellow, and blue were chosen. The elemental composition of the tattoo ink samples was analyzed using X-ray Fluorescence (XRF). An ultrathin-window parallel plate ion chamber was used to measure the surface dose perturbation (ratio of ionizations with and without tattoo ink) for 6 - 20 MeV electron beams. The elemental concentration in the tattoo ink samples showed high-Z elements, with Z ranging from 11 to 92. The dose perturbation ranged from 1.4% up to 6% for the yellow ink for the 6 MeV electron beam, with similar values across the rest of the electron energies, whereas the black, red, and blue inks presented up to 3% dose perturbation for the same range of energies. Based on this initial study, we conclude that commercially available tattoo inks contain large amounts of high-Z metals that may contribute to dose perturbation. Therefore treatment of superficial lesions with electron beams in a tattooed area should be monitored for signs of early skin reaction during radiation therapy treatments.

Purpose: Small field dosimetry is challenging due to the range of secondary electrons and detector size producing variable perturbations. TRS-483 (IAEA Vienna, 2017) provided valuable information, however there are subtle differences that are not addressed properly for small clinical fields (fclin) compared to machine specific reference field (fmsr). The discrepancies in field output factor, OMEGA(fclin, fmsr) is especially significant among various publications. This could be possibly due to differences in treatment setup, depth and detectors which are studied in this investigation. Methods: Measurements were performed in a water phantom for OMEGA(fclin, fmsr) with 100 cm source to surface distance (SSD) and source to axis distance (SAD) setups at various depths (dmax-20 cm) for plastic scintillator W1 and microDiamond detectors presuming k(fclin, fmsr) closed to 1.0 for all fields. A 6 MV beam was used from a Varian Edge machine that has high reliability in small fields. Results: The measured values of OMEGA(fclin, fmsr) are nearly identical for both detectors confirming that k(fclin, fmsr) = 1.0 except at 0.5x0.5 cm² field at dmax. The differences between SSD and SAD at same depth are <2% which is within limit of experimental accuracy, thus either of the setup techniques can be used. However, OMEGA(fclin, fmsr) varies greatly with depth. Compared to reference depth (10 cm), OMEGA differs by 28%, 10.1% and -13.5% for dmax, 5 and 20 cm depths, respectively, for 0.5 x0.5 cm² field and decreases linearly to 0% for large fields. These differences are inherent and can be resolved using correction factor based on the ratio of percent depth dose (PDD) and output at dmax. Conclusion: The OMEGA(fclin, fmsr) is independent of the SSD and SAD setup for the same depth. A large variation in OF is observed with depth that can be successfully corrected. Care should be taken in selecting data for proper depth in clinical use

Purpose: The publication of TRS-483 provided code of practice and the data for most detectors in the market. It is found that plastic scintillator detector (PSD) W1 is an ideal choice; however this cannot be used for beam scanning. Therefore, the goal of this study is to characterize the Exradin plastic scintillator (W2) for small field scanning dosimetry. Methods: A 6 MV beam from a Varian-Edge linac was used to collect data for characterization of a W2 detector. Cerenkov light radiation (CLR) is corrected through a separate new electrometer system. The parameters investigated include the dose and dose rate linearity, dose profile, output factors, and temperature response. These are compared with the EBT-3 film for depth dose and profiles for fields in the range of 0.5-5.0 cm² square fields. Additionally, characteristics are compared with W1 for a wide range of field sizes. Results: The dose linearity measured with 600 MU/min dose rate showed very low variations (<0.5%) even for 1 MU. Similar results were seen for dose rate linearity. The output factors comparison between the W2 and W1 showed up to 1% variation, except for the 0.5x0.5 cm² field size with -1.4% differences. Indicating that W2 is suitable for small field dosimetry and provides similar characteristics to W1. The temperature response showed up to 3% variation when the temperature was varied from 15degreeC to 36degreeC, normalized to 20degreeC. The profile using the W2 and EBT-3 film showed good correlation in the penumbra and umbra region. Conclusion: The new scanning W2 scintillator showed superb constancy, and has similar radiation response characteristics for small field dosimetry as W1 scintillator. The W2 detector satisfactorily provides opportunity for photon beam scanning

Purpose: Esophageal cancer patients undergoing radiotherapy usually receive daily or weekly cone-beam CT (CBCT) imaging to verify positioning before treatment. The purpose of this study is to evaluate the reproducibility of texture features extracted from CBCT and its correlation with CT features for their potential use as early predictors of esophageal cancer response during the course of RT. Methods: Ten patients treated for esophageal cancer that received daily CBCT were retrospectively evaluated (Varian TrueBeam with same Thorax imaging technique: half-fan, full-trajectory, 125 kVp, 270 mAs). The planning CT (CTP) and the two initial CBCTs (day 1 and 2 of treatment) were exported from Eclipse TPS to an in-house processing pipeline. This included edge detection for couch removal, CBCT resampling and automatic 3D rigid-registration of CBCT to CTP using Mattes mutual information metric. GTVs for each patient were exported and texture features were extracted from CTP and the registered CBCTs using the Imaging Biomarker Explorer (IBEX) software. Thirty-three texture features using cooccurrence and run-length matrices were extracted. Texture reproducibility between consecutive CBCTs was evaluated using intraclass correlation (ICC). CTP and CBCTs were evaluated using Pearson's correlation. Significance level was corrected for multiple testing using Bonferroni adjustment. Results: Registration results were deemed satisfactory using mutual information as well as visual inspection within the volume that encompassed the GTV. Out of the initial 33 texture features considered, 13 presented excellent (ICC > 0.9) reproducibility between the two initial CBCTs. Out of these 13 features, 5 presented statistically significant Pearson's correlation adjusted for multiple statistical tests (P < 0.004) ranging from 0.86 to 0.89 (Table). Conclusion: Several texture features from CBCT showed reproducibility between the first two days of treatment and strongly correlated between CTP and CBCT. Therefore, derived texture features could be investigated as predictors of treatment response during the course of treatment