Markus Parallel Plate Ionization Chamber Research Articles

Surface Dose and Skin Toxicity in Low-Magnetic Field MR-Linac

<h3>Purpose/Objective(s)</h3> Radiation induced skin toxicity is a common side effect that results in poor patient satisfaction and unplanned treatment breaks thus prolonging the overall treatment time with poor outcome. It is hypothesized that treatments on low-field MR-Linac will not produce higher surface dose which is investigated in this study. <h3>Materials/Methods</h3> Skin dose has always been an area of active clinical interest that is dependent on many parameters including beam energy, field size, material thickness and atomic number in the beam, material distance from surface, source to surface distance, beam angle (obliquity factor), concomitant chemotherapy etc. Along with these parameters, magnetic field is also a critical parameter that affect surface dose. With the development of the MRL system focus has shifted towards Lawrence force giving rise to electron return effect along with other treatment parameters affecting the skin dose. A Linac equipped with 6MV FFF photons with 90 cm source to axis distance is used. A Markus parallel plate ionization chamber is used in a solid water phantom with proper correction factor to investigate surface and buildup dose with six different gantry angles 15⁰ apart gantry (equivalent of standard Linac 0⁰, 15⁰, 30⁰, 45⁰, 60⁰, and 75⁰) for field sizes of 1.66 × 1.66 cm² (small), 9.96 × 9.9 cm² (reference) and 24.1 × 24.1 cm² (maximum). Measurements were made with increasing buildup thickness in steps of 1 mm to 20 mm of solid water in 0.35 T magnetic field. <h3>Results</h3> In traditional linear accelerators surface dose is 30-40% for a 6MV photon beam, without any modifier. This value increases significantly with beam angle. In MR-Linac, surface dose is relatively low, 12.6%, 14.9% and 16.7% for small, reference and maximum fields at zero-degree gantry angle, respectively. These values are 12% to 2% higher (at 0⁰ and 75⁰ respectively) for maximum field size compared to the reference field size. For lower field sizes minimum variation of 10% was observed at 75° for reference field size. The surface dose increases with increasing radiation field size but magnitude is still lower as the case with traditional lilacs. Table 1 shows the surface dose with field size and gantry angles. <h3>Conclusion</h3> Surface dose in low-field magnetic environment is relatively lower than traditional accelerators. It is concluded that low-field MR-Linac system may not add any additional skin toxicity compared to conventional Linac. However, the effect may be different for high-field MR-Linac system, and appropriate skin evaluation needs to be performed to differentiate between radiation induced toxicity specially for the oblique beams versus RF-thermal effect.

SU‐F‐T‐515: Increased Skin Dose in Supine Craniospinal Irradiation Due to Carbon Fiber Couch and Vacuum Bag Immobilization Device

Purpose:To measure the surface dose for supine craniospinal irradiation employing posterior beams, treating through an imaging couch and BlueBag immobilization device.Methods:The percentage depth dose (PDD) in the buildup region of a clinical 6 MV photon beam was measured using an Advanced Markus parallel plate ionization chamber in a solid water phantom. The PDD from a 10×10 cm2 anterior beam was measured at 100 cm SSD, simulating a traditional prone craniospinal technique. The measurements were compared to commissioning and treatment planning system data. The PDD was also measured in a posterior setup with the phantom surface laying directly on the Brainlab carbon fiber imaging couch, with the phantom surface 100 cm from the source, simulating a supine craniospinal setup. The posterior measurements were repeated with a BlueBag vacuum immobilization device between the couch and phantom, with thicknesses of 1.7 cm and 5 cm. The PDD from a 10×10 cm2 field and a typical 6×30 cm2 craniospinal field were also compared. The PDDs were normalized at 5 cm to reflect typical craniospinal prescription dose normalization.Results:The measured PDD curve from the anterior setup agreed well with commissioning and treatment planning data, with surface doses of 19.9%, 28.8% and 27.7%, respectively. The surface doses of the 10×10 cm2 and 6×30 cm2 fields delivered through the imaging couch were both 122.4%. The supine setup yielded surface doses of 122.4%, 121.6%, and 119.6% for the couch only, 1.7 cm bag, and 5 cm bag setups, respectively.Conclusion:Delivering craniospinal irradiation through a carbon fiber couch removes the majority of skin sparing. The addition of a vacuum bag immobilization device restores some skin sparing, but the magnitude of this effect is negligible.

Surface and Buildup Region Dose Measurements with Markus Parallel-Plate Ionization Chamber, GafChromic EBT3 Film, and MOSFET Detector for High-Energy Photon Beams

The aim of the study was to investigate surface and buildup region doses for 6 MV and 15 MV photon beams using a Markus parallel-plate ionization chamber, GafChromic EBT3 film, and MOSFET detector for different field sizes and beam angles. The measurements were made in a water equivalent solid phantom at the surface and in the buildup region of the 6 MV and 15 MV photon beams at 100 cm source-detector distance for 5 × 5, 10 × 10, and 20 × 20 cm2field sizes and 0°, 30°, 60°, and 80° beam angles. The surface doses using 6 MV photon beams for 10 × 10 cm2field size were found to be 20.3%, 18.8%, and 25.5% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface doses using 15 MV photon beams for 10 × 10 cm2field size were found to be 14.9%, 13.4%, and 16.4% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface dose increased with field size for all dosimeters. As the angle of the incident radiation beam became more oblique, the surface dose increased. The effective measurement depths of dosimeters vary; thus, the results of the measurements could be different. This issue can lead to mistakes at surface and buildup dosimetry and must be taken into account.

Characterization of a MOSkin detector for in vivo skin dose measurements during interventional radiology procedures.

The MOSkin is a MOSFET detector designed especially for skin dose measurements. This detector has been characterized for various factors affecting its response for megavoltage photon beams and has been used for patient dose measurements during radiotherapy procedures. However, the characteristics of this detector in kilovoltage photon beams and low dose ranges have not been studied. The purpose of this study was to characterize the MOSkin detector to determine its suitability for in vivo entrance skin dose measurements during interventional radiology procedures. The calibration and reproducibility of the MOSkin detector and its dependency on different radiation beam qualities were carried out using RQR standard radiation qualities in free-in-air geometry. Studies of the other characterization parameters, such as the dose linearity and dependency on exposure angle, field size, frame rate, depth-dose, and source-to-surface distance (SSD), were carried out using a solid water phantom under a clinical x-ray unit. The MOSkin detector showed good reproducibility (94%) and dose linearity (99%) for the dose range of 2 to 213 cGy. The sensitivity did not significantly change with the variation of SSD (± 1%), field size (± 1%), frame rate (± 3%), or beam energy (± 5%). The detector angular dependence was within ± 5% over 360° and the dose recorded by the MOSkin detector in different depths of a solid water phantom was in good agreement with the Markus parallel plate ionization chamber to within ± 3%. The MOSkin detector proved to be reliable when exposed to different field sizes, SSDs, depths in solid water, dose rates, frame rates, and radiation incident angles within a clinical x-ray beam. The MOSkin detector with water equivalent depth equal to 0.07 mm is a suitable detector for in vivo skin dosimetry during interventional radiology procedures.

Evaluation of the dosimetric properties of a synthetic single crystal diamond detector in high energy clinical proton beams

To investigate the dosimetric properties of a synthetic single crystal diamond Schottky diode for accurate relative dose measurements in large and small field high-energy clinical proton beams. The dosimetric properties of a synthetic single crystal diamond detector were assessed by comparison with a reference Markus parallel plate ionization chamber, an Exradin A16 microionization chamber, and Exradin T1a ion chamber. The diamond detector was operated at zero bias voltage at all times. Comparative dose distribution measurements were performed by means of Fractional depth dose curves and lateral beam profiles in clinical proton beams of energies 155 and 250 MeV for a 14 cm square cerrobend aperture and 126 MeV for 3, 2, and 1 cm diameter circular brass collimators. ICRU Report No. 78 recommended beam parameters were used to compare fractional depth dose curves and beam profiles obtained using the diamond detector and the reference ionization chamber. Warm-up∕stability of the detector response and linearity with dose were evaluated in a 250 MeV proton beam and dose rate dependence was evaluated in a 126 MeV proton beam. Stem effect and the azimuthal angle dependence of the diode response were also evaluated. A maximum deviation in diamond detector signal from the average reading of less than 0.5% was found during the warm-up irradiation procedure. The detector response showed a good linear behavior as a function of dose with observed deviations below 0.5% over a dose range from 50 to 500 cGy. The detector response was dose rate independent, with deviations below 0.5% in the investigated dose rates ranging from 85 to 300 cGy∕min. Stem effect and azimuthal angle dependence of the diode signal were within 0.5%. Fractional depth dose curves and lateral beam profiles obtained with the diamond detector were in good agreement with those measured using reference dosimeters. The observed dosimetric properties of the synthetic single crystal diamond detector indicate that its behavior is proton energy independent and dose rate independent in the investigated energy and dose rate range and it is suitable for accurate relative dosimetric measurements in large as well as in small field high energy clinical proton beams.

Equivalent square formula for determining the surface dose of rectangular field from 6 MV therapeutic photon beam

The purpose of the study was to investigate the use of the equivalent square formula for determining the surface dose from a rectangular photon beam. A 6 MV therapeutic photon beam delivered from a Varian Clinac 23EX medical linear accelerator was modeled using the EGS4nrc Monte Carlo simulation package. It was then used to calculate the dose in the build‐up region from both square and rectangular fields. The field patterns were defined by various settings of the X‐ and Y‐collimator jaw ranging from 5 to 20 cm. Dose measurements were performed using a thermoluminescence dosimeter and a Markus parallel‐plate ionization chamber on the four square fields (5 × 5, 10 × 10, 15 × 15, and 20×20 cm2). The surface dose was acquired by extrapolating the build‐up doses to the surface. An equivalent square for a rectangular field was determined using the area‐to‐perimeter formula, and the surface dose of the equivalent square was estimated using the square‐field data. The surface dose of square field increased linearly from approximately 10% to 28% as the side of the square field increased from 5 to 20 cm. The influence of collimator exchange on the surface dose was found to be not significant. The difference in the percentage surface dose of the rectangular field compared to that of the relevant equivalent square was insignificant and can be clinically neglected. The use of the area‐to‐perimeter formula for an equivalent square field can provide a clinically acceptable surface dose estimation for a rectangular field from a 6 MV therapy photon beam.PACS number: 87.55.ne

SU-GG-T-611: Investigation of Different Carbon Fiber Tabletops Used in Radiation Therapy

Purpose: The dose changes in the buildup region and beam attenuation by several carbon fiber tabletops were investigated for 6 and 10 MV photon beams. Method and Materials: Measurements were performed for 3 × 3, 10 × 10, and 25 × 25 cm2 field sizes. The surface dose and percentage depth doses (PDD) were measured using a Markus parallel plate ionization chamber. A Farmer type ionization chamber with appropriate buildup caps have been used for attenuation measurement at several gantry angles (from 0° to 180°). Oncor's couch (Siemens schematic G5499), Primus' couch (MED-TEC model IL3005), and CT's couch (MED-TEC model IL3302) have been studied. Results: For a 6 MV beam Oncor couch increases the surface dose from 16.8% to 77.6% for small fields. Attenuation varies from 1% to 4.7% for 180° and 120° gantry angles. At 6 and 10 MV beams respectively, Siemen's couch increases the dose from 17.8% to 43.2% and from 10.0% to 28.5%, the beam attenuation varies from 0.1% to 8.6% and 0.2% to 3.1% for 180° and 120° gantry angles. The increase of the surface dose for the CT couch is from 17.5% to 77.8% and 9.6% to 58.9% for 6 and 10 MV beams respectively, and beam attenuation varies from 1% to 4.7% and 1% to 2.4% for 180° and 120° gantry angles. Conclusion: The carbon fiber tabletops significantly decrease the skin sparring effect. Our data show that the surface dose increase in the buildup region is about the same for the three couches. The dosimetric effect of the tabletop may be higher, especially for intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage. Keywords: Carbon fibre tabletops, Oncor, Primus, CT, PDD

LET dependence of GafChromic films and an ion chamber in low-energy proton dosimetry

Dosimetry using a PMMA phantom was performed in 15 and 29 MeV proton beams from the Birmingham cyclotron, with a Markus parallel-plate ionization chamber and GafChromic EBT and MD-V2-55 film. Simulations of the depth–dose curves were performed with FLUKA 2008.3 and MCNPX 2.5.0, which agreed almost perfectly with each other in range and only differed by 2% in the Bragg peak (BP) region. FLUKA was also used to calculate kQ factors for Markus chamber measurements as an improvement to the IAEA TRS-398 values in low-energy beams. FLUKA depth–dose simulations overestimate the BP height measured by ion chamber by about 10%, where the initial proton energy spread was estimated by fitting to the slope of the measured BP distal edge. Both GafChromic films showed an under-response in the BP compared to ion chamber; however, EBT exhibits this effect at lower energies than MD-V2-55. A possible reason for this is attributed to the shape and arrangement of the monomer particles being different in the active components of EBT and MD-V2-55. Relative effectiveness (RE) of both films is presented as functions of residual range Rres in water and peak proton energy determined by FLUKA, with considerations for the spatial separation of the two active layers in each film. The proton energies at which RE reduces to 90% of maximum film response are 6.7 and 3.2 MeV for MD-V2-55 and EBT, respectively. Additionally, a beam quality correction factor () is suggested for both GafChromic films, involving water-to-film stopping power ratios evaluated using ICRU recommendations, and a polymer yield factor . RE in this work is equated to the reciprocal of the polymer yield factor. The calculated values of are constant within 2.1% and 1.2% across the proton energy range of 1–300 MeV for EBT and MD-V2-55, respectively, so it is concluded that the polymer yield factor is the dominant factor causing the LET quenching effect.

Sci‐Sat AM (2) Therapy‐03: Characterization of Intermediate Energy X‐Ray Photons (0.2–1.0 MeV) for stereotactic radiosurgery: experimental demonstration of reduced radiological penumbra

Introduction: Stereotactic radiosurgery is used to treat intracranial lesions with a high degree of accuracy. At the present time, x‐ray energies at or above Co‐60 gamma rays are used. We hypothesize that intermediate energy x‐ray photons (IEP's), combined with small field sizes, produce a reduced radiological penumbra leading to a sharper dose gradient. The purpose of this work is three‐fold: 1.) to produce IEP's using a medical linear accelerator 2.) to characterize the x‐ray energy 3.) to demonstrate reduced radiological penumbra for IEP's compared to 6MV x‐rays.Materials &amp; Methods: A Siemens linear accelerator was modified to produce IEP's. PDD versus depth measurements were done in solid water using a Markus parallel plate ionization chamber (PTW Freiburg) at SSD=100cm for a 2×2cm2 field size. Monte Carlo computer simulations were done using MCNP‐4C. A penumbra measurement device was constructed to examine radiological penumbra for various photon energies, field sizes and depths. Film (Gafchromic EBT) was used to record field edge profiles. These films were scanned using a digital microscope (spatial resolution of 1.87 microns/pixel). Film irradiations were done using SSD=100cm, depth= 2cm, FS=1.1cm2.Results: For the IEP's, PDD values were 55.4%(surface), 62.6%(5cm), and 34.7%(10cm). Monte Carlo simulations suggest a nominal x‐ray energy of 800kV. The 80%–20% penumbra widths were 2.10mm (6MV) and 0.345mm (IEP's).Conclusions: A novel intermediate energy x‐ray beam (800kV) has been produced using a linear accelerator. There is more than a 5‐fold reduction in radiological penumbra for the IEP's versus 6MV x‐rays for the small field size examined.

TU‐E‐224A‐05: Intermediate Energy X‐Ray Photons (0.2 – 1.0 MeV) for Radiosurgery: Producing a Beam and Measurement of Radiological Penumbra

Purpose: The main advantage of stereotactic radiosurgery is the steep dose gradient associated with the intracranial dose distribution. We are examining the effects of x‐ray energy on penumbra and dose gradient. Specifically, we are exploring the reduction in radiological penumbra for intermediate energy x‐ray photons (0.2–1.0 MeV) or so called IEP's. The purpose of this work is two‐fold: 1.) to produce an IEP beam using a medical linear accelerator and 2.) to examine the radiological penumbra associated with this beam. Method and Materials: A Siemens medical linear accelerator was adapted to produce IEP's. PDD measurements versus depth (SSD=100cm,FS=2×2cm2) were done in solid water using a Markus parallel plate ionization chamber (PTW Freiburg). These were compared with Monte Carlo computer simulations (MCNP‐4C). Monte Carlo involved generating x‐ray spectra that impinged upon a SW phantom. A penumbra measurement device (consisting of a half‐beam block) was constructed to examine radiation beam edge profiles using film (Gafchromic EBT) at SSD=100cm,FS=1.1cm2 and depth=2cm. In a separate experiment, film irradiations were done collimating a 3×3mm2 beam using a 10cm thick brass block flush with the SW surface. In all cases, the geometric penumbra (due to the finite source size) was made negligible by having the collimation very close to the phantom. A high‐resolution digital microscope (Axiomat) was used to acquire film profiles. Results: Measured PDD values were 55.4%(surface), 62.6%(5cm), and 34.7%(10cm). Monte Carlo PDD's compared with measurement suggest a nominal 800kV x‐ray beam. For the half‐beam block, the 80%–20% film edge profiles were 0.345mm (IEP) and 2.10mm (6MV). For the 3×3mm2 field, there was a 5‐fold reduction in radiological penumbra (IEP vs. 6MV). Conclusions: A novel intermediate energy photon beam (of nominal energy 800kV) has been produced using a conventional linear accelerator. There is a substantial reduction in radiological penumbra when using IEP's with small fields.

Alanine dosimetry of proton therapy beams.

An analysis of some of the properties of the ESR-alanine dosimetry in therapeutic proton beams is reported. Thin alanine-based detectors (1 and 2 mm thick pellets and 0.25 mm thick films) were tested in a clinical 62 MeV proton beam. The response of the alanine detectors in unmodulated and modulated proton beams was studied in tissue equivalent phantoms. The dose assessed by alanine was compared to the dose provided by a Markus parallel plate ionization chamber, used for reference dosimetry. Experiments in the 5-250 Gy dose range showed linearity of alanine dose response and no significant differences in the alanine response per unit dose to gammas and protons. Agreement within the experimental uncertainties was found between the alanine and the Markus chamber depth dose curves, including the Bragg peak region.

Proton response of alanine based pellets and films

The performance of two different alanine-ESR systems (1 mm ISS pellets and NRI films) in therapy proton beams was checked against reference dosimetery provided by a Markus parallel plate ionization chamber. Irradiations were performed at the OPTIS facility of the PSI with a 62 MeV proton beam. The energy dependence of the response was derived from the analysis of film stacks irradiated with pristine beams. In fact, 250 μm films allowed for a high resolution sampling of the proton slowing down mechanisms. Moreover pellets were irradiated in unmodulated and modulated beams. Alanine dose distributions were always in good agreement with the PSI Markus chamber results. Simple density scaling allowed for the conversion of the results to the dose distributions in water, consistently with the good tissue equivalence properties of these detectors.