A Cherpak

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

PURPOSE A 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. METHODS RADPOS 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. RESULTS Compared 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. CONCLUSIONS This work has shown that the deformable lung phantom-RADPOS system can be an efficient quality assurance tool for 4D radiation therapy.

SU‐E‐T‐390: Real‐Time Measurement of Urethral Dose and Position Using a RADPOS Array during Permanent Seed Implantation for Prostate Brachytherapy

Purpose: The new in vivo dosimetry tool, RADPOS, combines MOSFETdosimetry with an electromagnetic positioning sensor. It has recently been modified to include a MOSFET array rather than a single MOSFET for dose monitoring at five points along the detector axis. The detector is in use as part of 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. Methods: The RADPOS detector was secured inside a Foley catheter inside the patients urethra. Spatial coordinates of the RADPOS detector were read every 0.5 s and the timing of events such as needle insertion was noted. MOSFET readings were taken over two ten minute periods once all seeds had been implanted both before and after the TRUS probe was removed. Results: Locations of the dosimeters could be inferred using a marker located at the end of the detector wire which is visible on fluoroscopyimages. Although detector position varied slightly between patients, on average one dose point was inside the bladder, two/three dose points were within the prostatic urethra, and one/two dose points were at or inferior to the apex of the prostate. Maximum integral dose in the prostatic urethral ranged from 110–195 Gy, and it was found that the dose can change up to 63 cGy depending on whether the rectal probe is in place. The average change in displacement from the beginning of the procedure to the end for all patients was Δr=(5.5 ± 2.2) mm, with changes in individual coordinates of Δx (right/left) = (0.1–2.6) mm, Δy (superior/inferior) = (0.3–8.7) mm, and Δz (anterior/posterior) = (1.2–5.0) mm. Conclusions: The modified RADPOS is a powerful tool that can provide accurate real time dose and position information for use during prostate brachytherapy. This project has been supported by grants from The Health Technology Exchange, Ontario Research Funds RE‐04‐026, The Ottawa Hospital Cancer Centre Foundation and by financial and technical support from Best Medical Canada and Ascension Technology Corporation.

WE‐G‐BRB‐07: Application of RADPOS in Vivo Dosimetry for QA of High Dose Rate Brachytherapy

PURPOSE The RADPOS in vivo dosimetry system combines an electromagnetic positioning sensor with MOSFET dosimetry, allowing for simultaneous online measurements of dose and spatial position. In this work, we assess the potential use of RADPOS for measurements of motion and dose during prostate HDR brachytherapy. METHODS The RADPOS was positioned in the centre of a thin plastic tube supported by two parallel acrylic plates submerged in water. The detector was calibrated by sending an Ir-192 source into an adjacent tube, to positions ranging from 8.0 to -8.0 cm along the vertical axis in increments of 0.5 cm. The dwell time at each position was 20 s and the change in threshold voltage of the RADPOS dosimeter was recorded for each position. The expected dose for each source position was calculated and compared to RADPOS measurements to obtain a detector calibration coefficient (cGy/mV). The procedure was then repeated sending the Ir-192 source into 5 other tubes, located 1 to 10 cm away from the RADPOS. Source positions, dwell times, and position of the RADPOS detector were the same as for the calibration. The dose for each source position was determined by RADPOS measurements and calibration coefficient and compared to the expected dose. RESULTS An average calibration coefficient of 0.74±0.11 cGy/mV was calculated for RADPOS measurements of Ir-192 irradiations. The absolute difference between RADPOS values and expected dose for subsequent measurements averaged over all source positions was 0.7±5.4 cGy. CONCLUSIONS In vivo dosimetry can potentially signal errors in catheter placement or numbering before entire dose is delivered. The demonstrated accuracy of RADPOS dose measurements and its ability to simultaneously measure displacement makes it a powerful tool for HDR brachytherapy treatments for prostate cancer, where high dose gradients and movement of the prostate gland can present unique in vivo dosimetry challenges. Financial and technical support has been received from Best Medical Canada and Ascension Technology Corporation.

SU‐E‐T‐565: Patient‐Specific Evaluation of the Need for Adaptive Therapy in Lung SBRT

Purpose: To develop a process to track respiratory motion during lungSBRT treatments and evaluate the dosimetric impact of this motion. Methods: Breathing patterns were recorded at time of 4DCT acquisition both with RADPOS and Bellows. Subsequently, breathing patterns were recorded solely with RADPOS during all fractions of lungSBRT. Differences in breathing pattern, relative to that acquired at time of 4DCT were quantified. Results: Comparative analyses of the breathing patterns acquired at 4DCT to those acquired during treatment revealed significant differences in both the amplitude and period Conclusions: This work describes a method to track breathing motion during lungSBRT, and demonstrates the potential need to adapt the treatment plan when the motion is different from that during 4DCT acquisition. Future work will involve applying the acquired motion parameters to the contoured tumours in order to evaluate the possible dosimetric effects of these differences.

Poster — Thur Eve — 13: Monitoring the Breathing Patterns of Lung Patients throughout the Course of Treatment — Preliminary Experience with the RADPOS System

The RADPOS system is a new in vivodosimetry tool that combines a MOSFET dosimeter with an electromagnetic positioning sensor to allow for simultaneous measurement in real‐time of dose and spatial coordinates at a specific location. A study is currently underway using the RADPOS system during the 4DCT and external beam treatments of lungcancer patients. Each day, RADPOS detectors are positioned at marked points on the patients chest and abdomen while a fourth detector is placed on the CT or treatment couch for reference. Position coordinates of the sensors are read in real‐time at a rate of 20–25 Hz and total dose is read at the end of each treatment fraction. Measurements have been completed on 11 patients during the 4DCT and 7–16 treatment fractions. The standard deviation of the average dose measured at each point ranged from 3.0 to 13.7 cGy (7.7 to 14.0%) at CT zero and from 2.5 to 11.1 cGy (2.8 to 9.2%) at the site of the tumour. Variations in amplitude of breathing motion have been found to be patient‐specific. Some patients had very consistent breathing patterns, with interfraction variations in average amplitude and period as low as 11.4% and 4.2% respectively, while others had variations as high as 38.9% and 50.0% . Daily set‐up of the RADPOS system was completed quickly, requiring minimal additional time for each scheduled treatment fraction. Acknowledgements: This project is supported by grants from HTX and ORCC Foundation. Financial and technical support from Best Medical Canada is also acknowledged.

SU-GG-T-341: Initial Results of Real-Time External Surface Motion and Dose Monitoring Study for Lung Patients

Purpose: To describe results from an ongoing clinical trial that aims to evaluate the potential of the RADPOS system, which combines a MOSFET dosimeter and electromagnetic positioning sensor, for applications in external beam treatments for lungcancer patients. Method and Materials: Measurements were done at the time of each patients 4DCT and throughout the course of treatment. Each day, three RADPOS sensors were positioned at marked points on the patients chest and abdomen while a fourth detector was placed on the CT or treatment couch for reference. Position coordinates of the sensors and dose information can be read in real‐time, but for these trials the total dose was read after each treatment fraction. Results: Measurements have been completed on ten patients during 7–14 fractions each. The standard deviation of the average dose measured at each point ranged from 3.0–13.7 cGy at CT zero and 2.5–11.1 cGy at the site of the tumour. Large differences were sometimes seen between data collected during the 4DCT and treatment fractions. Most patients settled into a more consistent breathing pattern as treatment progressed, with maximum interfraction variations in average amplitude and period between 0.9–3.5 mm and 0.2–1.8 s during treatment. A cross‐correlation analysis comparing the displacements measured simultaneously at the three locations found that correlation varied between patients, as no two detectors were consistently the most correlated. The magnitude of the correlation coefficients also varied greatly, ranging from ρ=0.13–0.24 for Patient G and from ρ=0.74–0.81 for Patient E. Conclusion: RADPOS system can provide real‐time feedback regarding motion due to breathing, coughing or other patient movement. Variations in breathing patterns are patient‐specific and should be monitored to ensure accurate positioning and treatment delivery. Acknowledgements: This project is supported by grants from HTX and ORCC Foundation. Financial and technical support from Best Medical Canada is also acknowledged.

Quality assurance and application of a 4D in vivo dosimetry system using a deformable lung phantom

A novel 4D in-vivo dosimetry detector (RADPOS) in conjunction with a deformable lung phantom has been used as a quality assurance tool for 4D radiotherapy. A treatment plan was created and delivered with the phantom in two breathing states and the RADOS system was used to verify dose and displacement of the tumour and lung volumes. The RADPOS measured the movement within a maximum deviation of 1.5 mm, and the dose within 4.2% of the total dose. The latest quality assurance and characterization tests of the RADPOS 4-D in-vivo dosimetry system are also presented in this work.

SU‐FF‐T‐89: Comparison of RADPOS 4D in Vivo Dosimetry System with Phillips Bellows and Varian RPM Systems

Purpose: To present the latest data on the newly‐developed RADPOS 4D in vivodosimetry system including comparisons with the Phillips Bellows and Varian RPM systems. Methods and Materials: A 4D Quasar phantom was set‐up on the couch of a Phillips Brilliance CT scanner and set to move in a sinusoidal pattern with amplitude of 1 cm. The Phillips Bellows belt was secured around the moving translation stage and was used to record the motion of the stage. The same motion was also measured by a RADPOS detector. This procedure was repeated on the couch of a PET/CT scanner using the Varian RPM system. A RADPOS detector and the RPM optical block were placed on top of the phantoms translation stage and the motion was recorded by both systems. The feasibility of using the RAPDOS system for in vivo dosimetry during daily external beam radiation therapy is also being studied. Results: When used to measure simulated breathing motion on a 4D Quasar phantom, the RADPOS‐measured displacements were on average within 0.13 and 0.05 mm of those recorded by the Phillips Bellows and Varian RPM systems, respectively. The correlation between the RADPOS displacements and those measured by the Bellows and RPM systems were 0.96 and 0.99, respectively. Initial results for clinical use in radiation therapytreatments show that the RADPOS system can be set‐up quickly, requiring minimal additional time for each scheduled treatment fraction. Conclusions: In conclusion, the RADPOS system agrees well with currently accepted position monitoring systems and provides sufficient information to identify changes in the patients breathing pattern and other patient motion. Acknowledgements: This project is supported by grants from HTX and ORCC Foundation. Financial and technical support from Best medical Canada is also acknowledged.