H Song

A method to implement full six-degree target shift corrections for rigid body in image-guided radiotherapy

Treatment position setup errors often introduce temporal variations in the position of target relative to the planned external radiation beams. The errors can be introduced by the movement of a target relative to external setup marks or to other relevant landmarks that are used to position a patient for radiotherapy. Those variations can cause dose deviations from the planned doses and result in suboptimal treatments where part of the target is not fully irradiated or a critical structure receives more than desired radiation doses. Clinically available technology for image-guided radiotherapy can detect variations of target position. In this study, a method has been developed to correct for target position variations and restore the original beam geometries relative to the target. The technique involves three matrix transformations: (1) transformation of beams from the machine coordinate system to the patient coordinate system as in the patient geometry in the approved dosimetric plan; (2) transformation of beams from the patient coordinate system in the approved plan to the patient coordinate system that is identified at the time of treatment; (3) transformation of beams from the patient coordinate system at the time of treatment in the treatment patient geometry back to the machine coordinate system. The transformation matrix used for the second transformation is determined through the use of image-guided radiotherapy technology and image registration. By using these matrix transformations, the isocenter shift, the gantry, couch and collimator angles of the beams for the treatment, adjusted for the target shift, can be derived. With the new beam parameters, the beams will possess the same positions and orientations relative to the target as in the plan for a rigid body. This method was applied to a head phantom study, and it was found that the target shift was fully corrected in treatment and excellent agreement was found in target dose coverage between the plan and the treatment.

Comparison of the IAEA TRS-398 and AAPM TG-51 absorbed dose to water protocols in the dosimetry of high-energy photon and electron beams

The International Atomic Energy Agency (IAEA TRS-398) and the American Association of Physicists in Medicine (AAPM TG-51) have published new protocols for the calibration of radiotherapy beams. These protocols are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standards laboratorys reference quality beam. This paper compares the recommendations of the two protocols in two ways: (i) by analysing in detail the differences in the basic data included in the two protocols for photon and electron beam dosimetry and (ii) by performing measurements in clinical photon and electron beams and determining the absorbed dose to water following the recommendations of the two protocols. Measurements were made with two Farmer-type ionization chambers and three plane-parallel ionization chamber types in 6, 18 and 25 MV photon beams and 6, 8, 10, 12, 15 and 18 MeV electron beams. The Farmer-type chambers used were NE 2571 and PTW 30001, and the plane-parallel chambers were a Scanditronix-Wellhöfer NACP and Roos, and a PTW Markus chamber. For photon beams, the measured ratios TG-51/TRS-398 of absorbed dose to water Dw ranged between 0.997 and 1.001, with a mean value of 0.999. The ratios for the beam quality correction factors kQ were found to agree to within about +/-0.2% despite significant differences in the method of beam quality specification for photon beams and in the basic data entering into kQ. For electron beams, dose measurements were made using direct N(D,w) calibrations of cylindrical and plane-parallel chambers in a 60Co gamma-ray beam, as well as cross-calibrations of plane-parallel chambers in a high-energy electron beam. For the direct N(D,w) calibrations the ratios TG-51/TRS-398 of absorbed dose to water Dw were found to lie between 0.994 and 1.018 depending upon the chamber and electron beam energy used, with mean values of 0.996, 1.006, and 1.017, respectively, for the cylindrical, well-guarded and not well-guarded plane-parallel chambers. The Dw ratios measured for the cross-calibration procedures varied between 0.993 and 0.997. The largest discrepancies for electron beams between the two protocols arise from the use of different data for the perturbation correction factors p(wall) and p(dis) of cylindrical and plane-parallel chambers, all in 60Co. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and the quantities in the two protocols.

Protocols for the dosimetry of high-energy photon and electron beams: a comparison of the IAEA TRS-398 and previous international Codes of Practice

A new international Code of Practice for radiotherapy dosimetry co-sponsored by several international organizations has been published by the IAEA, TRS-398. It is based on standards of absorbed dose to water, whereas previous protocols (TRS-381 and TRS-277) were based on air kerma standards. To estimate the changes in beam calibration caused by the introduction of TRS-398, a detailed experimental comparison of the dose determination in reference conditions in high-energy photon and electron beams has been made using the different IAEA protocols. A summary of the formulation and reference conditions in the various Codes of Practice, as well as of their basic data, is presented first. Accurate measurements have been made in 25 photon and electron beams from 10 clinical accelerators using 12 different cylindrical and plane-parallel chambers, and dose ratios under different conditions of TRS-398 to the other protocols determined. A strict step-by-step checklist was followed by the two participating clinical institutions to ascertain that the resulting calculations agreed within tenths of a per cent. The maximum differences found between TRS-398 and the previous Codes of Practice TRS-277 (2nd edn) and TRS-381 are of the order of 1.5-2.0%. TRS-398 yields absorbed doses larger than the previous protocols, around 1.0% for photons (TRS-277) and for electrons (TRS-381 and TRS-277) when plane-parallel chambers are cross-calibrated. For the Markus chamber, results show a very large variation, although a fortuitous cancellation of the old stopping powers with the ND,w/NK ratios makes the overall discrepancy between TRS-398 and TRS-277 in this case smaller than for well-guarded plane-parallel chambers. Chambers of the Roos-type with a 60Co ND,w calibration yield the maximum discrepancy in absorbed dose, which varies between 1.0% and 1.5% for TRS-381 and between 1.5% and 2.0% for TRS-277. Photon beam calibrations using directly measured or calculated TPR20,10 from a percentage dose data at SSD = 100 cm were found to be indistinguishable. Considering that approximately 0.8% of the differences between TRS-398 and the NK-based protocols are caused by the change to the new type of standards, the remaining difference in absolute dose is due either to a close similarity in basic data or to a fortuitous cancellation of the discrepancies in data and type of chamber calibration. It is emphasized that the NK-ND,air and ND,w formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.

Computed tomography dose index and dose length product for cone‐beam CT: Monte Carlo simulations of a commercial system

Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone (1) showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose index (CTDIw) and DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On‐Board Imager (OBI) system was measured with radiochromic films (model: XR‐QA). The MC model of the OBI X‐ray tube, validated in a previous study, was used to acquire the phase space files of the full‐fan and half‐fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone‐beam CTDI (CTDIw,l) was calculated from DPI values and mean CTDIw,l(CTDIw,l)¯ and DLP were derived. We also evaluated the differences of CTDIw values between MC simulations and point dose measurements using standard CT phantoms. In results, it was found that CTDIw,600¯ was 8.74±0.01 cGy for head and CTDIw,900¯ was 4.26±0.01 cGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans. PACS number: 87.57.uq

Reference dosimetry in clinical high‐energy electron beams: Comparison of the AAPM TG‐51 and AAPM TG‐21 dosimetry protocols

A comparison of the determination of absorbed dose to water in reference conditions with high-energy electron beams (Enominal of 6, 8, 10, 12, 15, and 18 MeV) following the recommendations given in the AAPM TG-51 and in the original TG-21 dosimetry protocols has been made. Six different ionization chamber types have been used, two Farmer-type cylindrical (PTW 30001, PMMA wall; NE 2571, graphite wall) and four plane parallel (PTW Markus, and Scanditronix-Wellhöfer NACP, PPC-05 and Roos PPC-40). Depending upon the cylindrical chamber type used and the beam energy, the doses at dmax determined with TG-51 were higher than with TG-21 by about 1%-3%. Approximately 1% of this difference is due to the differences in the data given in the two protocols; another 1.1%-1.2% difference is due to the change of standards, from air-kerma to absorbed dose to water. For plane-parallel chambers, absorbed doses were determined by using two chamber calibration methods: (i) direct use of the ADCL calibration factors N(60Co)D,w and Nx for each chamber type in the appropriate equations for dose determination recommended by each protocol, and (ii) cross-calibration techniques in a high-energy electron beam, as recommended by TG-21, TG-39, and TG-51. Depending upon the plane-parallel chamber type used and the beam energy, the doses at dmax determined with TG-51 were higher than with TG-21 by about 0.7%-2.9% for the direct calibration procedures and by 0.8%-3.2% for the cross-calibration techniques. Measured values of photon-electron conversion kecal, for the NACP and Markus chambers were found to be 0.3% higher and 1.7% lower than the corresponding values given in TG-51. For the PPC-05 and PPC-40 (Roos) chamber types, the values of kecal were measured to be 0.889 and 0.893, respectively. The uncertainty for the entire calibration chain, starting from the calibration of the ionization chamber in the standards laboratory to the determination of absorbed dose to water in the user beam, has been analyzed for the two formalisms. For cylindrical chambers, the observed differences between the two protocols are within the estimated combined uncertainty of the ratios of absorbed doses for 6 and 8 MeV; however, at higher energies (10< or =E< or =18 MeV), the differences are larger than the estimated combined uncertainties by about 1%. For plane-parallel chambers, the observed differences are within the estimated combined uncertainties for the direct calibration technique; for the cross-calibration technique the differences are within the uncertainty estimates at low energies whereas they are comparable to the uncertainty estimates at higher energies. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors, and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration.

Evaluation of an electron Monte Carlo dose calculation algorithm for electron beam

The electron Monte Carlo (eMC) dose calculation algorithm of the Eclipse treatment planning system is based heavily upon Monte Carlo simulation of the linac head and modeling of the linac beam characteristics with minimal measurement of beam data. Commissioning of the eMC algorithm on multiple identical linacs provided a unique opportunity to systematically evaluate the algorithm with actual measurements of clinically relevant beam and dose parameters. In this study, measured and eMC calculated dose distributions were compared both along and perpendicular to electron beam direction for electron energy/applicator/depth combination using measurement data from four Varian CLINAC 21EX linear accelerators (Varian Medical Systems, Palo Alto, CA). Cutout factors for sizes down to 3×3 cm were also compared. Comparisons between the measurement and the eMC calculated values show that the R90, R80, R50, and R10 values mostly agree within 3 mm. Measure and calculated bremsstrahlung dose Dx correlates well statistically although eMC calculated Dx values are consistently smaller than the measured, with maximum discrepancy of 1% for the 20 MeV electron beams. Surface dose agrees mostly within 2%. Field width and penumbra agree mostly within 3 mm. Calculation grid size is found to have a significant effect on the dose calculation. A grid size of 5 mm can produce erroneous dose distributions. Using a grid size of 2.5 mm and a 3% accuracy specified for the eMC to stop calculation iteration, the absolute output agrees with measurements within 3% for field sizes of 5×5 cm or larger. For cutout of 3×3 cm, however, the output disagreement can reach 8%. Our results indicate that the eMC algorithm in Eclipse provides acceptable agreement with measurement data for most clinical situations. Calculation grid size of 2.5 mm or smaller is recommended.

Racial Differences in Adipose Tissue Distribution and Risk of Aggressive Prostate Cancer among Men Undergoing Radiotherapy

Background: Although elevated body mass index (BMI) has been associated with increased risk of aggressive prostate cancer, the importance of adipose tissue distribution is not well understood. We examined associations between overall and visceral obesity and aggressive prostate cancer risk. Moreover, given racial differences in adipose tissue distribution, we examined whether race modified these associations. Methods: We conducted a cross-sectional analysis of 308 radiotherapy-treated patients with prostate cancer within the Durham VA from 2005 to 2011. Multivariable logistic regression examined the association between BMI categories and tertiles of waist circumference (WC), visceral fat area (VFA), and periprostatic adipose tissue area (PPAT) with high-grade prostate cancer risk (Gleason score ≥7 vs. ≤6). Models stratified by race examined whether these associations differed between black and nonblack men. Results: Both elevated BMI (Ptrend = 0.054) and WC (Ptrend = 0.040) were associated with increased high-grade prostate cancer risk, with similar results between races, although the association with BMI was not statistically significant. In contrast, elevated VFA was associated with increased aggressive prostate cancer risk in black men (Ptrend = 0.002) but not nonblack men (Ptrend = 0.831), with a significant interaction between race and VFA (Pinteraction = 0.035). Though similar patterns were observed for PPAT, none was statistically significant. Conclusions: Among men undergoing radiotherapy for prostate cancer, visceral obesity is associated with increased aggressive prostate cancer risk, particularly among black men. If confirmed in future studies, these results suggest that adipose tissue distribution differences may contribute to prostate cancer racial disparity. Impact: These findings highlight the need to elucidate mechanisms contributing to racial differences in the association between visceral obesity and aggressive prostate cancer. Cancer Epidemiol Biomarkers Prev; 23(11); 2404–12. ©2014 AACR.

Image Guided Hypofractionated Postprostatectomy Intensity Modulated Radiation Therapy for Prostate Cancer

PURPOSE Hypofractionated radiation therapy (RT) has promising long-term biochemical relapse-free survival (bRFS) with comparable toxicity for definitive treatment of prostate cancer. However, data reporting outcomes after adjuvant and salvage postprostatectomy hypofractionated RT are sparse. Therefore, we report the toxicity and clinical outcomes after postprostatectomy hypofractionated RT. METHODS AND MATERIALS From a prospectively maintained database, men receiving image guided hypofractionated intensity modulated RT (HIMRT) with 2.5-Gy fractions constituted our study population. Androgen deprivation therapy was used at the discretion of the radiation oncologist. Acute toxicities were graded according to the Common Terminology Criteria for Adverse Events version 4.0. Late toxicities were scored using the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer scale. Biochemical recurrence was defined as an increase of 0.1 in prostate-specific antigen (PSA) from posttreatment nadir or an increase in PSA despite treatment. The Kaplan-Meier method was used for the time-to-event outcomes. RESULTS Between April 2008 and April 2012, 56 men received postoperative HIMRT. The median follow-up time was 48 months (range, 21-67 months). Thirty percent had pre-RT PSA <0.1; the median pre-RT detectable PSA was 0.32 ng/mL. The median RT dose was 65 Gy (range, 57.5-65 Gy). Ten patients received neoadjuvant and concurrent hormone therapy. Posttreatment acute urinary toxicity was limited. There was no acute grade 3 toxicity. Late genitourinary (GU) toxicity of any grade was noted in 52% of patients, 40% of whom had pre-RT urinary incontinence. The 4-year actuarial rate of late grade 3 GU toxicity (exclusively gross hematuria) was 28% (95% confidence interval [CI], 16%-41%). Most grade 3 GU toxicity resolved; only 7% had persistent grade ≥3 toxicity at the last follow-up visit. Fourteen patients experienced biochemical recurrence at a median of 20 months after radiation. The 4-year bPFS rate was 75% (95% CI, 63%-87%). CONCLUSIONS The biochemical control in this series appears promising, although relatively short follow-up may lead to overestimation. Late grade 3 GU toxicity was higher than anticipated with hypofractionated radiation of 65 Gy to the prostate bed, although most resolved.

Characteristics of x-ray beams in two commercial multidetector computed tomography simulators: Monte Carlo simulations.

PURPOSE As multidetector computed tomography (MDCT) scanning is routinely performed for treatment planning in radiation oncology, understanding the characteristics of the MDCT x-ray beam is essential to accurately estimate patient dose. The purpose of this study is to characterize the x-ray beams of two commercial MDCT simulators widely used in radiation oncology by Monte Carlo (MC) simulations. METHODS X-ray tube systems of two wide bore MDCT scanners (GE LightSpeed RT 4 and Philips Brilliance Big Bore) were modeled in the BEAMNRC/EGSNRC MC system. All the tube components were modeled from targets to bowtie filters. To validate our MC models, the authors measured half-value layers (HVL) using aluminum sheets and multifunctional radiation detectors and compared them to those obtained from MC simulations for 120 kVp beams. The authors also compared x-ray spectra obtained from MC simulation to the data provided by manufacturers. Additionally, lateral/axial beam profiles were measured in-air using radiochromic films and compared to the MC results. To understand the scatter effect, the authors also derived the scatter-to-primary energy fluence ratio (SPR) profiles and calculated the total SPR for each CT system with the CT dose index (CTDI) head and body phantoms using the BEAMNRC system. RESULTS The authors found that the HVL, x-ray spectrum and beam profiles of the MC simulations agreed well with the manufacturer-specified data within 1%-10% on average for both scanners. The total SPR were ranged from 7.8 to 13.7% for the head phantom and from 10.7 to 18.9% for the body phantom. CONCLUSIONS The authors demonstrate the full MC simulations of two commercial MDCT simulators to characterize their x-ray beams. This study may be useful to establish a patient-specific dosimetry for the MDCT systems.

Tracking brachytherapy sources using emission imaging with one flat panel detector

This work proposes to use the radiation from brachytherapy sources to track their dwell positions in three-dimensional (3D) space. The prototype device uses a single flat panel detector and a BB tray. The BBs are arranged in a defined pattern. The shadow of the BBs on the flat panel is analyzed to derive the 3D coordinates of the illumination source, i.e., the dwell position of the brachytherapy source. A kilovoltage x-ray source located 3.3 m away was used to align the center BB with the center pixel on the flat panel detector. For a test plan of 11 dwell positions, with an Ir-192 high dose rate unit, one projection was taken for each dwell point, and locations of the BB shadows were manually identified on the projection images. The 3D coordinates for the 11 dwell positions were reconstructed based on two BBs. The distances between dwell points were compared with the expected values. The average difference was 0.07 cm with a standard deviation of 0.15 cm. With automated BB shadow recognition in the future, this technique possesses the potential of tracking the 3D trajectory and the dwell times of a brachytherapy source in real time, enabling real time source position verification.