J Cygler

Radiotherapy dosimetry using a commercial OSL system

A commercial optically stimulated luminescence (OSL) system developed for radiation protection dosimetry by Landauer, Inc., the InLight microStar reader, was tested for dosimetry procedures in radiotherapy. The system uses carbon-doped aluminum oxide, Al2O3:C, as a radiation detector material. Using this OSL system, a percent depth dose curve for 60Co gamma radiation was measured in solid water. Field size and SSD dependences of the detector response were also evaluated. The dose response relationship was investigated between 25 and 400 cGy. The decay of the response with time following irradiation and the energy dependence of the Al2O3:C OSL detectors were also measured. The results obtained using OSL dosimeters show good agreement with ionization chamber and diode measurements carried out under the same conditions. Reproducibility studies show that the response of the OSL system to repeated exposures is 2.5% (1sd), indicating a real possibility of applying the Landauer OSL commercial system for radiotherapy dosimetric procedures.

Evaluation of a novel 4D in vivo dosimetry system

A prototype of a new 4D in vivo dosimetry system capable of simultaneous real-time position monitoring and dose measurement has been developed. The radiation positioning system (RADPOS) is controlled by a computer and combines two technologies: MOSFET radiation detector coupled with an electromagnetic positioning device. Special software has been developed that allows sampling position and dose either manually or automatically in user-defined time intervals. Preliminary tests of the new device include a dosimetric evaluation of the detector in 60Co, 6 MV, and 18 MV beams and measurements of spatial position stability and accuracy. In addition, the effect of metals and other materials on the performance of the positioning system has been investigated. Results show that the RADPOS system can measure in-air dose profiles that agree, on average, within 3%-5% of diode measurements for the energies tested. The response of the detector is isotropic within 1.6% (1 SD) with a maximum deviation of +/- 4.0% over 360 degrees. The maximum variation in the calibration coefficient over field sizes from 6 x 6 to 25 x 25 cm2 was 2.3% for RADPOS probe with the high sensitivity MOSFET and 4.6% for the probe with the standard sensitivity MOSFET. Of the materials tested, only aluminum, lead, and brass caused shifts in the RADPOS read position. The magnitude of the shift varied between materials and size of the material sample. Nonmagnetic stainless steel (Grade 304) caused a distortion of less than 2 mm when placed within 10 mm of the detector; therefore, it can provide a reasonable alternative to other metals if required. The results of the preliminary tests indicate that the device can be used for in vivo dosimetry in 60Co and high-energy beams from linear accelerators.

4D dose-position verification in radiation therapy using the RADPOS system in a deformable lung phantom

PURPOSEnA novel 4D in vivo dosimetry system (RADPOS), in conjunction with a deformable lung phantom, has been evaluated as a potential quality assurance tool for 4D radiotherapy.nnnMETHODSnRADPOS detectors, which consist of a MOSFET dosimeter combined with an electromagnetic positioning probe, were placed inside the deformable lung phantom. One detector was positioned directly inside a tumor embedded in the lung phantom and another was positioned inside the lung portion of the phantom, outside the tumor. CT scans were taken with the phantom at three breathing phases, and for each phase, the detector position inside the phantom was read with the RADPOS software and compared to the position as determined from the CT data. These values were also compared to RADPOS measurements taken with the phantom on the couch of a Varian Clinac 6EX linac. The deformable phantom and the RADPOS system were also used in two radiation delivery scenarios: (1) A simulation of a free-breathing delivery and (2) a simulation of an adaptive treatment.nnnRESULTSnCompared to CT imaging, the RADPOS positional accuracy was found to be better than 2.5 mm. The radial displacement measurements taken in the CT and linac rooms agreed to within an average of (0.7 +/- 0.3) mm. Hence, the system can provide relative displacement measurements in the treatment room, consistent with measurements made in the CT room. For the free-breathing delivery, the total dose reported by RADPOS agreed to within 4% and 5% of the treatment planning doses in the tumor and the lung portion of the phantom, respectively. The RADPOS-measured dose values for the adaptive delivery were within 1.5% of the treatment plan values, which was well within the estimated experimental uncertainties.nnnCONCLUSIONSnThis work has shown that the deformable lung phantom-RADPOS system can be an efficient quality assurance tool for 4D radiation therapy.

The effect of different bleaching wavelengths on the sensitivity of Al2O3:C optically stimulated luminescence detectors (OSLDs) exposed to 6 MV photon beams

PURPOSEnTo determine the effect of different bleaching wavelengths on the response of Al(2)O(3):C optically stimulated luminescence detectors (OSLDs) exposed to accumulated doses of 6 MV photon beams.nnnMETHODSnIn this study the authors used nanoDot OSLDs readout with a MicroStar reader. The authors first characterized the dose-response, fading, and OSL signal loss of OSLDs exposed to doses from 0.5 to 10 Gy. To determine the effect of different bleaching wavelengths on the OSLDs response, the authors optically treated the OSLDs with 26 W fluorescent lamps in two modes: (i) directly under the lamps for 10, 120, and 600 min and (ii) with a long-pass filter for 55, 600, and 2000 min. Changes in the OSLDs sensitivity were determined for an irradiation-readout-bleaching-readout cycle after irradiations with 1 and 10 Gy dose fractions.nnnRESULTSnThe OSLDs presented supralinearity for doses of 2 Gy and above. The signal loss rates for sequential readouts were (0.287 ± 0.007)% per readout in the readers strong-stimulation mode, and (0.019 ± 0.002)% and (0.035 ± 0.007)% per readout for doses of 0.2 and 10 Gy, respectively, in the readers weak-stimulation mode. Fading half-life values ranged from (0.98 ± 0.14) min to (1.77 ± 0.24) min and fading showed dose dependence for the first 10-min interval. For 10 and 55 min bleaching using modes (i) and (ii), the OSL signal increased 14% for an accumulated dose of 7 Gy (1 Gy fractions). For OSLDs exposed to 10 Gy fractions, the OSL signal increased 30% and 25% for bleaching modes (i) and (ii) and accumulated dose of 70 Gy, respectively. For 120 and 600 min bleaching using modes (i) and (ii), the OSL signal increased 2.7% and 1.5% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 14% for bleaching mode (i) (120 min bleaching) and decreased 1.3% for bleaching mode (ii) (600 min bleaching) for an accumulated dose of 70 Gy. For 600 and 2000 min bleaching using modes (i) and (ii), the signal increased 2.3% and 1.8% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 10% for mode (i) (600 min bleaching) and decreased 2.5% for mode (ii) (2000 min bleaching) for an accumulated dose of 70 Gy.nnnCONCLUSIONSnThe dose-response of nanoDot OSLDs read using the MicroStar reader presented supralinearity for doses of 2 Gy and above. The signal loss as a function of sequential readouts depended on dose. Fading also depended on dose for the first 10-min interval. For dose fractions of 1 and 10 Gy, OSLDs may be reused within 3% and 5% accuracies up to the maximum accumulated dose of 7 and 70 Gy investigated in this study, respectively. These accuracies were obtained after the OSLDs were bleached with a light source with wavelengths above about 495 nm. The authors also concluded that changes in sensitivity of OSLDs depended on bleaching time, accumulated dose, and wavelength spectrum of the bleaching source.

Clinical use of a novel in vivo 4D monitoring system for simultaneous patient motion and dose measurements

PURPOSEnA new 4D in vivo dosimetry tool, RADPOS, has been used on lung cancer patients to evaluate the feasibility of using the detectors to characterize variations in patient breathing patterns as well as to monitor daily variations in dose.nnnMETHODS AND MATERIALSnThe RADPOS system combines a MOSFET dosimeter with an electromagnetic positioning sensor for simultaneous measurement of real-time dose and spatial coordinates. Three RADPOS sensors were placed on patients chest and abdomen during a 4DCT and daily treatments. A fourth detector was also placed on the couch as reference. Position data were collected in real-time and total dose was read at the end of each fraction.nnnRESULTSnSignificant deviations in surface motion have been found between the day of 4DCT and treatment fractions in 9 of 10 patients. Variations in daily dose ranged from 2.5 to 13.7 cGy (2.8-14.0%) and results agreed with treatment plan values for all but three points.nnnCONCLUSIONSnChanges in breathing motion have been found that emphasize a need for continued position monitoring. RADPOS measurements can be used to monitor such variations as well as to measure surface dose without any disruption to the treatment schedule or discomfort to patients.

Real-time measurement of urethral dose and position during permanent seed implantation for prostate brachytherapy

PURPOSEnThe inxa0vivo dosimetry tool, RADPOS, has been modified to include a metal oxide-silicon semiconductor field effect transistor (MOSFET) array with an electromagnetic positioning sensor. This allows dose monitoring at five points rather than just at single dose point as in the other versions of the device. The detector has been used in a clinical trial, which is the first to measure both urethral dose and internal motion concurrently during permanent seed implantation for prostate brachytherapy using a single probe.nnnMETHODS AND MATERIALSnThe RADPOS detector was secured inside a Foley catheter inside the patients urethra. Spatial coordinates of the RADPOS detector were read every 0.5s, and the timing of events such as needle insertion was noted. The MOSFET readings were taken over two 10-min periods; once all seeds had been implanted both before and after the transrectal ultrasound (TRUS), the probe was removed. Measurements were completed for 16 patients.nnnRESULTSnMaximum integral dose in the prostatic urethral ranged from 89 to 195Gy, and dose varied from -66% to 36% depending on the rectal probe position. The change in position of the RADPOS sensor owing to the removal of the TRUS probe ranged from 1.4 to 9.7mm.nnnCONCLUSIONSnThe modified RADPOS detector with MOSFET array is able to provide real-time dose information, which can be used to monitor dose rates while implantation is performed and to estimate the total integrated dose. Changes in position including those owing to the TRUS probe can be significant and should be quantified to evaluate the influence on dose distributions.

Experimental verification of 4D Monte Carlo simulations of dose delivery to a moving anatomy

Purpose: To evaluate a novel 4D Monte Carlo simulation tool by comparing calculations to physical measurements using a respiratory motion phantom. Methods: We used a dynamic Quasar phantom in both stationary and breathing states (sinusoidal motion of amplitude of 1.8 cm and period of 3.3 s) for dose measurements on an Elekta Agility linear accelerator. Gafchromic EBT3 film and the RADPOS 4D dosimetry system were placed inside the lung insert of the phantom to measure dose profiles and point‐dose values at the center of the spherical tumor inside the insert. Both a static 4 × 4 cm2 field and a VMAT plan were delivered. Static and 4D Monte Carlo simulations of the treatment deliveries were performed using DOSXYZnrc and a modified version of the defDOSXYZnrc user code that allows modeling of the continuous motion of both machine and patient. DICOM treatment plan files and linac delivery log files were used to generate corresponding input files. The phantom motion recorded by RADPOS during beam delivery was incorporated into the input files for the 4DdefDOSXYZnrc simulations. Results: For stationary phantom simulations, all point‐dose values from MC simulations at the tumor center agreed within 1% with film and within 2% with RADPOS. More than 98% of the voxels from simulated dose profiles passed a 1D gamma of 2%/2‐mm criteria against measured dose profiles. Similar results were observed when applying a 2D gamma analysis with a 2%/2‐mm criteria to compare 2D dose distributions of Monte Carlo simulations against measurements. For simulations on the moving phantom, MC‐calculated dose values at the center of the tumor were found to be within 1% of film and within 2σ of experimental uncertainties which are 2.8% of the RADPOS measurements. 1D gamma comparisons of the dose profiles were better than 91%, and 2D gamma comparisons of the 2D dose distributions were found to be better than 94%. Conclusion: Our 4D Monte Carlo method using defDOSXYZnrc can be used to accurately calculate the dose distribution in continuously moving anatomy for various treatment techniques. This work, if extended to deformable anatomies, can be used to reconstruct patient delivered dose for use in adaptive radiation therapy.

Sci-Thurs PM: Planning-04: Evaluation of Dosimetric Differences between Dose-to-Water and Dose-to-Medium for Head and Neck Patients Treated with Electron Beams

Monte Carlo based treatment planning systems have made it possible to calculate and evaluate dose‐distributions in terms of dose‐to‐medium, D m , as opposed to the traditional dose‐to‐water, D w . For electron beams, differences between the two methods have been reported to exceed 10% in some cases. These differences are greatest in materials whose electron densities are furthest from water, such as bone and lung. This has raised the question of which approach is more appropriate. The purpose of this study is to investigate the dosimetric differences between plans calculated using the D m versus D w in clinical head and neck cases treated with electron beams. The analysis included plans for phantoms containing hard bone inhomogeneities as well as retrospective head and neck cases treated with electron beams. Differences between plans calculated using D m and D w were evaluated by means of the dose profiles and isodose distributions. For phantoms and head and neck patients, dose to the bone was larger by up to 10% when calculating dose‐to‐water rather than dose to medium. Differences between plans depend on beam energy, as well as the location of the tumor and organs at risk, and are consistent with the differences between water‐to‐bone stopping power ratios.

SU-E-T-627: Precision Modelling of the Leaf-Bank Rotation in Elekta’s Agility MLC: Is It Necessary?

Purpose: To demonstrate the method used to determine the leaf bank rotation angle (LBROT) as a parameter for modeling the Elekta Agility multi-leaf collimator (MLC) for Monte Carlo simulations and to evaluate the clinical impact of LBROT. Methods: A detailed model of an Elekta Infinity linac including an Agility MLC was built using the EGSnrc/BEAMnrc Monte Carlo code. The Agility 160-leaf MLC is modelled using the MLCE component module which allows for leaf bank rotation using the parameter LBROT. A precise value of LBROT is obtained by comparing measured and simulated profiles of a specific field, which has leaves arranged in a repeated pattern such that one leaf is opened and the adjacent one is closed. Profile measurements from an Agility linac are taken with gafchromic film, and an ion chamber is used to set the absolute dose. The measurements are compared to Monte Carlo (MC) simulations and the LBROT is adjusted until a match is found. The clinical impact of LBROT is evaluated by observing how an MC dose calculation changes with LBROT. A clinical Stereotactic Body Radiation Treatment (SBRT) plan is calculated using BEAMnrc/DOSXYZnrc simulations with different input values for LBROT. Results: Using the method outlined above, the LBROT is determined to be 9±1 mrad. Differences as high as 4% are observed in a clinical SBRT plan between the extreme case (LBROT not modeled) and the nominal case. Conclusion: In small-field radiation therapy treatment planning, it is important to properly account for LBROT as an input parameter for MC dose calculations with the Agility MLC. More work is ongoing to elucidate the observed differences by determining the contributions from transmission dose, change in field size, and source occlusion, which are all dependent on LBROT. This work was supported by OCAIRO (Ontario Consortium of Adaptive Interventions in Radiation Oncology), funded by the Ontario Research Fund.

EP-1462: Monte Carlo simulation of the Elekta Agility linear acelerator

doses were higher with VMAT plans for left breast treatments compared with tangent plans. Conclusions: VMAT improves homogeneity within the PTVWB and significantly improves conformality to PTVTB when compared with partial tangent fields to deliver the boost dose to the tumour bed in breast radiotherapy. In left breasts VMAT can result in higher heart doses depending on the location and size of the tumour bed however these doses are within established optimal constraints. VMAT is now used in this institution for breast boost radiotherapy. Breath hold techniques are being implemented for left breast patients to ensure the heart dose can be kept as low as practicable.