The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

PURPOSE: We have experimentally and computationally characterized the PTW microSilicon 60023-type diode's performance in 6 and 15 MV photon fields ≥5 × 5 mm2 projected to isocenter. We tested the detector on- and off-axis at 5 and 15 cm depths in water, and investigated whether its response could be improved by including within it a thin airgap. METHODS: Experimentally, detector readings were taken in fields generated by a Varian TrueBeam linac and compared with doses-to-water measured using Gafchromic film and ionization chambers. An unmodified 60023-type diode was tested along with detectors modified to include 0.6, 0.8, and 1.0 mm thick airgaps. Computationally, doses absorbed by water and detectors’ sensitive volumes were calculated using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Detector response was characterized using K^{fdin, 4cm}_{Qclin, 4cm}, a factor that corrects for differences in the ratio of dose-to-water to detector reading between small fields and the reference condition, in this study 5 cm deep on-axis in a 4 × 4 cm2 field. RESULTS: The greatest errors in measurements of small field doses made using uncorrected readings from the unmodified 60023-type detector were over-responses of 2.6% ± 0.5% and 5.3% ± 2.0% determined computationally and experimentally, relative to the reading-per-dose in the reference field. Corresponding largest errors for the earlier 60017-type detector were 11.9% ± 0.6% and 11.7% ± 1.4% over-responses. Adding even the thinnest, 0.6 mm, airgap to the 60023-type detector over-corrected it, leading to under-responses of up to 4.8% ± 0.6% and 5.0% ± 1.8% determined computationally and experimentally. Further, Monte Carlo calculations indicate that a detector with a 0.3 mm airgap would read correctly to within 1.3% on-axis. The ratio of doses at 15 and 5 cm depths in water in a 6 MV 4 × 4 cm2 field was measured more accurately using the unmodified 60023-type detector than using the 60017-type detector, and was within 0.3% of the ratio measured using an ion chamber. The 60023-type diode's sensitivity also varied negligibly as dose-rate was reduced from 13 to 4 Gy min–1 by decreasing the linac pulse repetition frequency, whereas the sensitivity of the 60017-type detector fell by 1.5%. CONCLUSIONS: The 60023-type detector performed well in small fields across a wide range of beam energies, field sizes, depths, and off-axis positions. Its response can potentially be further improved by adding a thin, 0.3 mm, airgap

Density compensated diodes for small field dosimetry: Comprehensive testing and implications for design

Purpose. In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6-1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally. Methods. Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥ 0.5 0.5 cm2. Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the formalism, choosing a 4 4 cm2 reference field. Results. For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5 0.5 cm2 field, and 11.7% experimentally at -0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter's effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer. Conclusions. The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities

A systematic review of clinical studies on variable proton Relative Biological Effectiveness (RBE)

Recently, a number of clinical studies have explored links between possible Relative Biological Effectiveness (RBE) elevations and patient toxicities and/or image changes following proton therapy. Our objective was to perform a systematic review of such studies. We applied a “Problem [RBE], Intervention [Protons], Population [Patients], Outcome [Side effect]” search strategy to the PubMed database. From our search, we retrieved studies which: (a) performed novel voxel-wise analyses of patient effects versus physical dose and LET (n = 13), and (b) compared image changes between proton and photon cohorts with regard to proton RBE (n = 9). For each retrieved study, we extracted data regarding: primary tumour type; size of patient cohort; type of image change studied; image-registration method (deformable or rigid); LET calculation method, and statistical methodology. We compared and contrasted their methods in order to discuss the weight of clinical evidence for variable proton RBE. We concluded that clinical evidence for variable proton RBE remains statistically weak at present. Our principal recommendation is that proton centres and clinical trial teams collaborate to standardize follow-up protocols and statistical analysis methods, so that larger patient cohorts can ultimately be considered for RBE analyses

Density compensated diodes for small field dosimetry: comprehensive testing and implications for design

Purpose: In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6-1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally. Methods: Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥0.5x0.5 cm2. Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the kQ clin ,Qmsr fclin,fmsr formalism, choosing a 4x4 cm2 reference field. Results: For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by kQ clin ,Qmsr fclin,fmsr factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5x0.5 cm2 field, and 11.7% experimentally at -0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter's effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer. Conclusions: The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities.</br

Abstract C091: Racial disparities in pancreatic cancer pancreatic cancer patients in Florida and an investigation into a possible role of cancer cachexia

Abstract Background: Five-year survival for pancreatic cancer remains low at 8%. While pancreatic cancer health disparities exist among different racial groups, these disparities have not been well investigated in the State of Florida. We aimed to investigate these disparities and hypothesized that cancer cachexia may play a role. Methods: A retrospective review of data from the Florida Cancer Data System and Florida Agency for Healthcare administration was performed to assess for PC disparities between racial groups in the State of Florida. A cohort of patients at a single center was analyzed for differences in cachexia indicators such as psoas muscle index (PSI) and albumin at presentation. Results: African Americans (AA) had significantly higher mean age-adjusted PC incidence (12.5/100,000) and mortality rates (10.97/100,000) than NHW (incidence=11.2/100,000; mortality=10.3/100,000) and Hispanics (incidence=9.6/100,000; mortality=8.7/100,000). Of the 67 counties in the State of Florida, 43 (64.2%) observed higher PC incidence rates in AA than NHW and Hispanics. AA are often diagnosed with PC at a younger age than NHW. AA and Hispanics are more likely to be insured by Medicaid compared to NHW (16% and 14% vs 7%, respectively) and less likely to undergo surgical treatment for their condition (31% vs. 37%). AA present with significantly lower serum albumin levels (3.2 vs. 3.7 g/dL). Serum albumin levels < 3.5 correlated with significantly lower survival. When compared to healthy controls presenting for cholecystectomy, AA patients present with a more significant reduction in psoas muscle index compared to Caucasians. Conclusion: African-Americans with PC have higher incidence rates and mortality than their NHW and Hispanics counterparts. AA are also younger at age of diagnosis, more likely to be insured by Medicaid, and less likely to undergo potential curative surgical treatment for PC. We demonstrated that AA had significantly lower albumin levels and that this correlated with worse survival. AA also present with a significantly greater reduction in psoas muscle index when compared to healthy controls. Further investigation into potential reasons for this disparity in cancer cachexia is warranted. Citation Format: Patrick Underwood, Miles Cameron, Ashley Clark Daly, Tracey Barnett, Clement Gwede, Anthony Magliocco, Barbara Centeno, Dung-Tsa Chen, Jung Choi, Daniel Jeong, Robert Gillies, Mokenge Malafa, Andrew Judge, Nipun Merchant, Jennifer Permuth, Jose Trevino. Racial disparities in pancreatic cancer pancreatic cancer patients in Florida and an investigation into a possible role of cancer cachexia [abstract]. In: Proceedings of the Eleventh AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2018 Nov 2-5; New Orleans, LA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl):Abstract nr C091