Diameter Field Size Research Articles

Discordance in acute gastrointestinal toxicity between synchrotron-based proton and linac-based electron ultra-high dose rate irradiation.

Proton FLASH has been investigated using cyclotron and synchrocyclotron beamlines but not synchrotron beamlines. We evaluated the impact of dose rate (ultra-high [UHDR] vs. conventional [CONV]) and beam configuration (shoot-through [ST] vs. spread-out-Bragg-peak [SOBP]) on acute radiation-induced gastrointestinal toxicity (RIGIT) in mice. We also compared RIGIT between synchrotron-based protons and linac-based electrons with matched mean dose rates. We administered abdominal irradiation (12-14 Gy single fraction) to female C57BL/6J mice with an 87 MeV synchrotron-based proton beamline (2 cm diameter field size as a lateral beam). Dose rates were 0.2 Gy/s (S-T pCONV), 0.3 Gy/s (SOBP pCONV), 150 Gy/s (S-T pFLASH), and 230 Gy/s (SOBP pFLASH). RIGIT was assessed by the jejunal regenerating crypt assay and survival. We also compared responses to proton [pFLASH and pCONV] with responses to electron CONV (eCONV, 0.4 Gy/s) and electron FLASH (eFLASH, 188-205 Gy/s). The number of regenerating jejunal crypts at each matched dose was lowest for pFLASH (similar between S-T and SOBP), greater and similar between pCONV (S-T and SOBP) and eCONV, and greatest for eFLASH. Correspondingly, mice that received pFLASH SOBP had the lowest survival rates (50% at 50 days), followed by pFLASH S-T (80%), and pCONV SOBP (90%), but 100% of mice receiving pCONV S-T survived (log-rank P = 0.047 for the four groups). Our findings are consistent with an increase in RIGIT after synchrotron-based pFLASH versus pCONV. This negative proton-specific FLASH effect versus linac-based electron irradiation underscores the importance of understanding the physical and biological factors that will allow safe and effective clinical translation.

23. Plastic scintillating fiber dosimetry for small animal irradiation

Introduction Small animal image-guided irradiation systems for preclinical research deliver millimetric beams of medium energy X-rays ( Materials and methods Our dosimeter prototype, called Dosirat, is composed of a 1mm diameter and 15mm long polystyrene based plastic scintillating fiber coupled to a clear optical fiber. The light output signal induced by irradiation is measured by a photodiode coupled to an electrometer. All irradiations have been performed with the image-guided X-Rad 225Cx irradiator (PXI Inc.) installed in Caen. Repeatability and linearity measurements have been performed. Dosirat response to energy has then been evaluated for different beam qualities and compared to measurements with an ionization chamber and to Monte Carlo simulations. Relative output factors (ROF) have been evaluated with a 4 mm long scintillating fiber for 10 × 10 cm 2 to 1 mm diameter field sizes and compared to the corresponding ROFs of EBT3 dosimetric films. Results Our dosimeter has shown excellent repeatability ( 2 field size, is consistent at 4% with the ROF curve obtained by the EBT3 dosimetric films, up to 5mm diameter field size. Conclusion These results show interesting performances of Dosirat at medium energy. Accurate dosimetry can be achieved by calibrating the dosimeter for any required beam quality. Hence, Dosirat is a promising dosimeter for small animal irradiation, particularly for in vivo dosimetry which concerns our next study. Scintillating fiber dosimeters could also be used in the clinical areas using medium X-ray energies such as diagnostic radiology, brachytherapy or intraoperative radiotherapy.

Characterization of a microDiamond detector in high-dose-per-pulse electron beams for intra operative radiation therapy

PurposeTo characterize a synthetic diamond dosimeter (PTW Freiburg microDiamond 60019) in high dose-per-pulse electron beams produced by an Intra Operative Radiation Therapy (IORT) dedicated accelerator. MethodsThe dosimetric properties of the microDiamond were assessed under 6, 8 and 9 MeV electron beams by a NOVAC11 mobile accelerator (Sordina IORT Technologies S.p.A.).The characterization was carried out with dose-per-pulse ranging from 26 to 105 mGy per pulse. The microDiamond performance was compared with an Advanced Markus ionization chamber and a PTW silicon diode E in terms of dose linearity, percentage depth dose (PDD) curves, beam profiles and output factors. ResultsA good linearity of the microDiamond response was verified in the dose range from 0.2 Gy to 28 Gy. A sensitivity of 1.29 nC/Gy was measured under IORT electron beams, resulting within 1% with respect to the one obtained in reference condition under 60Co gamma irradiation. PDD measurements were found in agreement with the ones by the reference dosimeters, with differences in R50 values below 0.3 mm. Profile measurements evidenced a high spatial resolution of the microDiamond, slightly worse than the one of the silicon diode. The penumbra widths measured by the microDiamond resulted approximately 0.5 mm larger than the ones by the Silicon diode. Output factors measured by the microDiamond were found within 2% with those obtained by the Advanced Markus down to 3 cm diameter field sizes. ConclusionsThe microDiamond dosimeter was demonstrated to be suitable for precise dosimetry in IORT applications under high dose-per-pulse conditions.

SU‐E‐T‐706: Small Field Dosimetry: Output Factors for Fixed Cones, Iris Collimator and MLC Fields for the CyberKnife M6™ System with Newly Released InCise™ Multileaf Collimator

Purpose:Accuray has recently released a new collimator, the InCise™ Multileaf Collimator (MLC), for clinical use with the CyberKnife M6™ System. This work reports the results of measurements of output factors (OF) for fields shaped by the InCise™ Multileaf collimator, Iris collimator and fixed cones.Methods:The MLC consists of 41 pairs of 2.5 mm wide leaves projecting a minimum and maximum field size of 7.6 mm × 7.5 mm and 110 mm × 97.5 mm at 800 mm SAD. OF measurements were made using 6 different detectors: PTW stereotactic and electron diodes, PTW microDiamond detector, Sun Nuclear Edge diode, IBA SFD diode and PFD 3G photon diode. Measurements were made for 14 MLC field sizes and 12 cone/Iris field sizes (5mm to 60 mm diameter field sizes). All measurements were made in a Wellhofer water tank at the depth of maximum dose. Correction factors from Francescon et al (PMB 59, 2014, N11‐N17; 17, 2012 3741–3748) were used to correct the diode responses for the fixed cone and Iris measurements.Results:For the MLC fields no correction factors are currently available. For the cones and Iris collimator, the average of corrected OF for all diodes ranged from 0.651 ± 0.014 (5mm cone) to 0.994 ± 0.001 (50 mm cone); for the Iris collimator the corresponding values are 0.510 ±0.004 and 0.997 ±0.000 respectively. The OF for the MLC field sizes ranged from 0.806 ±0.009 for the 7.6 mm × 7.5 mm field to 1.023± 0.003 for the 110 mm × 97.5 mm field size.Conclusion:The present results are the first set of output factor data reported for the InCise™ MLC, Iris collimator and the fixed cones for the CyberKnife M6™ system. Good agreement is obtained among all the corrected data for the cones and Iris data.

ITAR: A modified TAR method to determine depth dose distribution for an ophthalmic device that performs kilovoltage x‐ray pencil‐beam stereotaxy

New technology has been developed to treat age-related macular degeneration (AMD) using 100 kVp pencil-beams that enter the patient through the radio-resistant sclera with a depth of interest between 1.6 and 2.6 cm. Measurement of reference and relative dose in a kilovoltage x-ray beam with a 0.42 cm diameter field size and a 15 cm source to axis distance (SAD) is a challenge that is not fully addressed in current guidelines to medical physicists. AAPM's TG-61 gives dosimetry recommendations for low and medium energy x-rays, but not all of them are feasible to follow for this modality. An investigation was conducted to select appropriate equipment for the application. PTW's Type 34013 Soft X-Ray Chamber (Freiburg, Germany) and CIRS's Plastic Water LR (Norfolk, VA) were found to be the best available options. Attenuation curves were measured with minimal scatter contribution and thus called Low Scatter Tissue Air Ratio (LSTAR). A scatter conversion coefficient (C(scat)) was derived through Monte Carlo radiation transport simulation using MCNPX (LANL, Los Alamos, NM) to quantify the difference between a traditional TAR curve and the LSTAR curve. A material conversion coefficient (C(mat)) was determined through experimentation to evaluate the difference in attenuation properties between water and Plastic Water LR. Validity of performing direct dosimetry measurements with a source to detector distance other than the treatment distance, and therefore a different field size due to a fixed collimator, was explored. A method--Integrated Tissue Air Ratio (ITAR)--has been developed that isolates each of the three main radiological effects (distance from source, attenuation, and scatter) during measurement, and integrates them to determine the dose rate to the macula during treatment. LSTAR curves were determined to be field size independent within the range explored, indicating that direct dosimetry measurements may be performed with a source to detector distance of 20 cm even though the SAD is 15 cm during treatment. C(scat) varied from 1.102 to 1.106 within the range of depths of interest. The experimental variance among repeated measurements of C(mat) was larger than depth dependence, so C(mat) was estimated as1.019 for all depths of interest. Equipment selection, measurement techniques, and formalism for the determination of dose rate to the macula during stereotaxy for AMD have been determined and are strongly recommended by the authors of this paper to be used by clinical medical physicists.

SU-E-T-56: Characterization of OSLDs for Use in Small Field Photon Beam Dosimetry

Purpose: Use optically stimulated luminescent dosimeters (OSLD) whose visible active luminescent area were masked as a remote audit tool to measure small field photon doses down to a 7.5mm diameter field size. Methods: Different aperture mask sizes (1, 2, and 3mm) were made for the OSLD nanodots and tested to determine which had the best reproducibility. These masks defined the active area of the OSLDs. The OSLDs were characterized by determining correction factors for — linearity, fading, depletion, element, and energy, in order to calculate dose. OSLD readings were performed with an OSLD reader with a known LED light distribution. Dose rate versus field size for OSLDs were generated and compared with other dosimeters (film, diodes, ion chamber, and Monte Carlo). All measurements were performed using a 6MV photon beam. Results: The study determined the 2mm diameter aperture to be the smallest aperture with the best reproducible readings between each individual OSLD with coefficient of variation ranging between 1–1.5%. Linearity tests for the masked OSLDs showed correspondence with unmasked commissioning data to within 3% for both the 2mm and 3mm diameter aperture, for doses ranging from 50cGy to 200cGy. The 1mm masked OSLD will not be further investigated as it produced non-reproducible results and its linearity correction was not in agreement with the commissioning unmasked correction. Conclusion: The 2mm masked OSLDs can potentially be used as a remote audit dosimetry tool to measure small photon field sizes doses. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

SU-E-T-172: Output Factors of a Cyberknife System : Comparison Between Measurements and Monte Carlo Calculations

Purpose: Small photon beams used in radiotherapy are characterised by high dose gradients and a lack of lateral electron equilibrium. Thus, the measurement of output factors (OF) of these beams is problematic and OF measurements with different detectors for similar irradiation conditions can lead to a deviation up to 30% (Derreumaux et al., 2010). The aim of this study is to accurately determine the OF of a Cyberknife system thanks to measurements and Monte Carlo data. Methods: OF were measured on a Cyberknife for the 5, 7.5, 10, 20 and 60 mm diameter field sizes using active detectors (high resolution diodes (IBA SFD, PTW 60016, PTW 60017, Sun Nuclear EDGE), PTW 31014 Pinpoint chamber and PTW 60003 natural diamond) and passive dosemeters (EBT2 radiochromic films and TLD micro‐cubes). Correction factors for diodes (EDGE, PTW 60016, PTW 60017) and for the PTW 31014 chamber were calculated thanks to a recent methodology based on Monte Carlo data (Francescon et al., 2008 and 2009). The Cyberknife used in this study was modelled with Penelope and MCNPXMonte Carlo codes in order to determine the “true” OF. Results: For the smallest field size, a deviation of nearly 20% was observed between the values of the OF measured with the different detectors. The agreement between the passive dosemeters was better than 2%. After correction for diodes and for the chamber, the OF obtained with these active detectors agreed within 2% with the passive OF measurements. Conclusions: This study is encouraging concerning the determination of the output factors of the Cyberknife system. For the active detectors, the use of correction factors improves the accuracy in the OF measurements. The results obtained with the passive dosemeters in this study as well as in another study performed on a Novalis system (Bassinet et al., 2011) are promising.

The water equivalence of solid phantoms for low energy photon beams

To compare and evaluate the dosimetric water equivalence of several commonly used solid phantoms for low energy photon beams. A total of ten different solid phantom materials was used in the study. The PENELOPE Monte Carlo code was used to calculate depth doses and beam profiles in all the phantom materials as well as the dose to a small water voxel at the surface of the solid phantom. These doses were compared to the corresponding doses calculated in a water phantom. The primary photon beams used ranged in energy from 50 to 280 kVp. A number of phantom materials had excellent agreement in dose compared to water for all the x-ray beam energies studied. RMI457 Solid Water, Virtual Water, PAGAT, A150, and Plastic Water DT all had depth doses that agreed with those in water to within 2%. For these same phantom materials, the dose changes in the water voxel at the surface of the solid phantom were within 2%, except for A150, which agreed to within 2.7%. By comparison, the largest differences in depth doses occurred for Plastic Water (-21.7%) and polystyrene (17.6%) for the 50 kVp energy photon beam and 8 cm diameter field size. Plastic Water gave the largest difference in the normalized beam profiles with differences of up to 3.5% as compared to water. Surface dose changes, due to the presence of the solid phantom acting as the backscatter material, were found to be up to 9.1% for polystyrene with significant differences also found for Plastic Water, PMMA, and RW3 phantoms. The following solid phantoms can be considered water equivalent and are recommended for relative dosimetry of low energy photon beams: A150, PAGAT, Plastic Water DT, RMI457 Solid Water, and Virtual Water. However, the following solid phantoms give significant differences, compared to water, in depth doses, profiles, and/or in surface doses due to backscatter changes: Plastic Water, PMMA, polystyrene, PRESAGE, and RW3.

SU‐FF‐T‐459: The Effect of Proton Therapy Beam Scanning Pattern Size On Secondary Neutron Dose Equivalent

Purpose: One way to deliver a proton therapy beam is to actively scan a uniform dose across a treatment field using a fast scanning magnet. In this study, we examine how proton therapy beam scanning patterns affect the scattered neutron dose equivalent in an active scanning proton therapy beam delivery nozzle. Method and Materials: The nominal scanning pattern size is a 324 cm2 rectangular pattern which projects a flat proton field of 12 × 12 cm at isocenter for a 12 cm snout size. However, it is possible to reduce the size of this scanning pattern for field sizes smaller than 12 cm and maintain beam flatness while minimizing the scattered neutron dose equivalent. Neutron measurements were taken using a SWENDI‐II (Thermo Electron Corporation™) neutron detector for a treatment field defined by apertures of 10 cm and 5 cm in diameter using protons of approximately 160 MeV. In each case, the field modulation in depth was 10 cm. A Scanditronix Matrixx™ panel was used to determine field flatness. Results: The maximum neutron dose equivalent was measured to be 0.65 mSv/Gy for the 10 cm field and 0.97 mSv/Gy for the 5 cm field 40 cm laterally from isocenter. By minimizing the rectangular scanning pattern, it was possible to reduce the secondary neutron dose equivalent by approximately 20% and 60% for the 10 cm and 5 cm diameter field sizes respectively and maintain acceptable treatment parameters. Conclusion: The efficiency of a proton treatment beam through the final patient collimator has a large influence on the creation of unwanted secondary neutrons. Increasing this efficiency using different scanning patterns minimizes neutron dose equivalent and maintains acceptable treatment parameters for the therapeutic proton beam.

Measurement of dose reductions for superficial x-rays backscattered from bone interfaces

Accurate measurement and knowledge of dose delivered during superficial x-ray radiotherapy is required for patient dose assessment. Some tumours treated near the surface (within the first few centimetres) can have large posterior bone structures. This can cause perturbations to dose delivered due to changed backscatter contributions from the bony structure as compared to full water or tissue scattering conditions. Measured results have shown that up to 7.5% of Dmax reductions in dose can occur near the water/bone interface for 100 kVp, using 10 cm diameter field sizes when a 1 cm thick slab of bone is located at 2 cm depth. At smaller field sizes such as 2 cm diameter these values reduce to 2% for the same energy. Larger variations (up to 12.5% of maximum) have been seen at the phantom surface when the bone layer is directly behind the point of interest (within 0.5 mm) and smaller effects (up to 5% of maximum) at depths down to 5 cm. Interesting to note is the fact that for larger field sizes, an increase in percentage dose is found at the water/bone interface due to the production of low energy backscattered electrons similar to the effect found in lead. However, they are much smaller in magnitude and thus would not cause any significant dosimetric effects. In the case where large bony structures lie relatively close to the surface and the tissue above this region is being treated, a dosimeter such as radiochromic film can be used to estimate the dose reduction that may occur due to the changed backscatter conditions.

Variations of blood flow at optic nerve head induced by sinusoidal flicker stimulation in cats.

1. The present investigation explored, in thirty-four anaesthetized cats, the blood flow changes at the optic nerve head elicited by sinusoidally modulated photic stimuli. 2. The stimuli were achromatic, diffuse and had 30 deg diameter field size; the stimulus frequency was varied from 0 to 100 Hz, modulation depth from 0 to 100% and mean retinal illuminance up to 50,000 trolands (td); the blood flow was measured with a near-infrared (810 nm) laser Doppler flowmeter. 3. At various frequencies, modulation depths and mean retinal illuminance, sinusoidal flicker stimulation always caused an increase in blood flow at the optic nerve head relative to steady stimulation. 4. The frequency response and temporal contrast sensitivity function of the blood flow changes had a bandpass shape; the high-frequency slope of the frequency response was 3 decades (dec) per decade and that of the temporal contrast sensitivity function was 1.7 dec per dec, close to the slope for cat 'on' ganglion cells (2.6 dec per dec). 5. In most cats, the magnitude of the increase in blood flow was a sigmoidal function of modulation depth; in the remainder, the relationship was close to linear. 6. The threshold of blood flow changes varied with respect to mean retinal illuminance similar to Ferry-Porter's law and the photopic linear slope was 50 Hz dec-1. 7. In comparison with reported psychophysical and electrophysiological responses elicited by similar stimulations, the results of the present study resemble more those obtained from ganglion cells than those from electroretinograms, visual-evoked potentials and psychophysics. It is suggested that the blood flow changes at the optic nerve head are induced by the activity of ganglion cells.