Markus Parallel Plate Chamber Research Articles

Evaluation of cutout factors with small and narrow fields using various dosimetry detectors in electron beam keloid radiotherapy.

The inherent problems in the existence of electron equilibrium and steep dose fall-off pose difficulties for small- and narrow-field dosimetry. To investigate the cutout factors for keloid electron radiotherapy using various dosimetry detectors for small and narrow fields. The measurements were performed in a solid water phantom with nine different cutout shapes. Five dosimetry detectors were used in the study: pinpoint 3D ionization chamber, Farmer chamber, semiflex chamber, Classic Markus parallel plate chamber, and EBT3 film. The results demonstrated good agreement between the semiflex and pinpoint chambers. Furthermore, there was no difference between the Farmer and pinpoint chambers for large cutouts. For the EBT3 film, half of the cases had differences greater than 1%, and the maximum discrepancy compared with the reference chamber was greater than 2% for the narrow field. The parallel plate, semiflex chamber and EBT3 film are suitable dosimeters that are comparable with pinpoint 3D chambers in small and narrow electron fields. Notably, a semiflex chamber could be an alternative option to a pinpoint 3D chamber for cutout widths≥3 cm. It is very important to perform patient-specific cutout factor calibration with an appropriate dosimeter for keloid radiotherapy.

Double-strand breaks measured along a 160 MeV proton Bragg curve using a novel FIESTA-DNA probe in a cell-free environment

Proton radiotherapy treatment planning systems use a constant relative biological effectiveness (RBE) = 1.1 to convert proton absorbed dose into biologically equivalent high-energy photon dose. This method ignores linear energy transfer (LET) distributions, and RBE is known to change as a function of LET. Variable RBE approaches have been proposed for proton planning optimization. Experimental validation of models underlying these approaches is a pre-requisite for their clinical implementation. This validation has to probe every level in the evolution of radiation-induced biological damage leading to cell death, starting from DNA double-strand breaks (DSB). Using a novel FIESTA-DNA probe, we measured the probability of double-strand break (P DSB) along a 160 MeV proton Bragg curve at two dose levels (30 and 60 Gy (RBE)) and compared it to measurements in a 6 MV photon beam. A machined setup that held an Advanced Markus parallel plate chamber for proton dose verification alongside the probes was fabricated. Each sample set consisted of five 10 μl probes suspended inside plastic microcapillary tubes. These were irradiated with protons to 30 Gy (RBE) at depths of 5–17.5 cm and 60 Gy (RBE) at depths of 10–17.2 cm with 1 mm resolution around Bragg peak. Sample sets were also irradiated using 6MV photons to 20, 40, 60, and 80 Gy. For the 30 Gy (RBE) measurements, increases in P DSB/Gy were observed at 17.0 cm followed by decreases at larger depth. For the 60 Gy (RBE) measurements, no increase in P DSB/Gy was observed, but there was a decrease after 17.0 cm. Dose-response for P DSB between 30 and 60 Gy (RBE) showed less than doubling of P DSB when dose was doubled. Proton RBE effect from DSB, RBEP,DSB, was <1 except at the Bragg peak. The experiment showed that the novel probe can be used to perform DNA DSB measurements in a proton beam. To establish relevance to clinical environment, further investigation of the probe’s chemical scavenging needs to be performed.

Dosimetric impact and modeling of prone breast positioning device and couch structures

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 200, −100, and −900 for the insert, the board “base shell” and “base inside,” respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.

Understanding Parameters Affecting Surface Dose to Reduce Skin Toxicity in Patients Treated with SBRT

Skin toxicity has been reported with stereotactic body radiation therapy (SBRT) treatments. Skin dose is a complex function of many variables including beam energy, treatment distance (SSD), materials underlying beam path (t), and field size (FS). Various skin dose constraints have been reported and toxicity is noted despite meeting the published/proposed dose constraints predicted by the standard treatment planning systems. In vivo dosimetry assessments support that toxicity may be related to the bolus effect from the immobilization device. This study was undertaken to analyze the above parameters in the context of the BodyFix® (BF) system and their impact on skin dose. A custom BF device was created with three thicknesses (2.5, 5.0, 12.5 cm). Measurements were taken using solid water phantom with and without the BF using a PTW Markus parallel-plate chamber. The impact of BF on skin dose was investigated as a function of beam energy (6MV, 10MV, 10FFF), source to detector distance (SDD) (75, 85, 95cm), thickness (2.5, 7.5, 12.5cm), and depth (0, 1, 2, 3, 5mm & dmax) at clinically relevant field FS for SBRT (2x2 to 5x5cm2). Changes in percentage depth dose (PDD) with the same SDD were analyzed in the presence of BF, as well as relative skin dose (RSD), defined as dose with BF to dose without BF. The impact of BF on skin dose was significant for all beam energies as shown in Table. Comparing a 6MV beam without and with BF (6MV-BF), the PDD at 0mm depth (PDD0mm) ranged from 18.6% to 20.6% (61.0% to 83.5%) for all tested FS at 75cm SDD. The 10MV (10MV-BF) beam ranged from 10.8% to 12.6% (46.0% to 69.3%), and 10FFF (10FFF-BF) ranged from 16.6% to 17.7% (55.3% to 77.8%). RSD increased due to the presence of BF for all beams, increasing by 4-6x, 2-3x, and 1-2x at 0mm, 1mm, and 2mm depth, respectively. The effect of relative increase in RSD with BF decreased with depth. PDD decreased with larger BF thickness from 2.5-12.5 cm for all tested energies and FS. Compared to 2.5cm BF thickness, the PDD0mm decreased by ∼13% for 12.5cm thickness at 3x3cm2 FS for 10MV, indicating dose deposition is inside the BF for thicker BF. For 2.5cm of BF, more dose deposition is close to skin, resulting in increased skin dose. The RSD increased for all photon energies with increasing treatment distance. RSD at 0mm depth was 1.10 for 95cm SDD compared to 75cm SDD for t-2.5 cm & 2x2cm2. This increase is primarily due to the inverse square factor. BF parameters combined with dosimetric variables can create up to 6-fold increase in skin dose, which may result in acute skin toxicity. Using higher beam energy (up to 10x), larger BF thickness, and smaller SSD will help reduce skin dose in SBRT treatments. Accounting for immobilization devices and modifying planning parameters to reduce the bolus effect can potentially reduce skin dose and lead to lower toxicity.Abstract 3806; Table 13x3cm2PDD0mmPDD2mmPDD5mm6MV+BF72.9987.5897.256MV18.7260.0683.7110MV+BF55.3470.7184.4610MV10.6239.4364.0310FFF +BF63.5877.5188.9110FFF16.2550.4672.37 Open table in a new tab

A study on the suitability of the PTW microDiamond detector for kilovoltage x-ray beam dosimetry

Kilovoltage x-ray beams are widely used in treating skin cancers and in biological irradiators. In this work, we have evaluated four dosimeters (ionization chambers and solid state detectors) in their suitability for relative dosimetry of kilovoltage x-ray beams in the energy range of 50 – 280kVp. The solid state detectors, which have not been investigated with low energy x-rays, were the PTW 60019 microDiamond synthetic diamond detector and the PTW 60012 diode. The two ionization chambers used were the PTW Advanced Markus parallel plate chamber and the PTW PinPoint small volume chamber. For each of the dosimeters, percentage depth doses were measured in water over the full range of x-ray beams and for field sizes ranging from 2cm diameter to 12 × 12cm. In addition, depth doses were measured for a narrow aperture (7mm diameter) using the PTW microDiamond detector. For comparison, the measured data was compared with Monte Carlo calculated doses using the EGSnrc Monte Carlo package. The depth dose results indicate that the Advanced Markus parallel plate and PinPoint ionization chambers were suitable for depth dose measurements in the beam quality range with an uncertainty of less than 3%, including in the regions closer to the surface of the water as compared with Monte Carlo depth dose data for all six energy beams. The response of the PTW Diode E detector was accurate to within 4% for all field sizes in the energy range of 50–125kVp but showed larger variations for higher energies of up to 12% with the 12 × 12cm field size. In comparison, the microDiamond detector had good agreement over all energies for both smaller and larger field sizes generally within 1% as compared to the Advanced Markus chamber field and Monte Carlo calculations. The only exceptions were in measuring the dose at the surface of the water phantom where larger differences were found. For the 7mm diameter field, the agreement between the microDiamond detector and Monte Carlo calculations was good being better than 1% except at the surface. Based on these results, the PTW microDiamond detector has shown to be a suitable detector for relative dosimetry of low energy x-ray beams over a wide range of x-ray beam energies.

SU‐E‐T‐719: Surface Dose Evaluation of the Equivalent Quality Flattening Filter‐Free Photon Beams

Purpose:To provide data on the relative surface dose of photon beams of Versa HD linear accelerator designed to delivered the Flattening Filter Free (FFF) beam having beam quality equivalent to the corresponding Flattened beam.This study involves the comparative evaluation of the entrance dose of FFF and Flattened photon beams in the buildup region.Methods:Surface dose was measured using Markus parallel plate chamber with the thin flat entrance window thickness of 0.025mm of water equivalent material in solid water phantom. Data were collected for square fields of 2×2cm2, 3×3cm2,4×4cm2,5×5cm2,7×7cm2,10×10cm2 and 20×20cm2 created in Monaco Treatment Planning System.The surface dose was define for this study as the relative dose at depth of 0.5mm with respect to the dose at dmax.The same Markus chamber was used to investigate the dose in the buildup region for all the square fields.The buildup depths for 6MV flattened and FFF beams were 0 to 18mm and for 10MV Flattened and FFF beams were 0 to 24mm. All values were determined in relation to the depth at dose maximum dmax.Results:The relation of surface dose for both Flattened and FFF beams with field sizes were linear. The surface dose for smaller field size upto 10×10cm2 for both FFF beams were higher than Flattened beams but the variation was marginal. The difference was 0.3 percentage at 6MV and 1.9 percentage at 10MV for a 10×10cm2 field but for the larger field the relative surface dose was lower for FFF beam with difference of 3.6 percentage at 6MV and 4.3 percentage at 10MV for a 20×20cm2 field.Conclusion:The equivalent quality FFF Photons produce higher surface dose for smaller field sizes upto 10×10cm2 but lower surface dose for large field sizes than the conventional flattened beam. The percentage variation was marginal for smaller field sizes and may not be clinically significant.

Effects on the photon beam from an electromagnetic array used for patient localization and tumor tracking

One of the main components in a Calypso 4D localization and tracking system is an electromagnetic array placed above patients that is used for target monitoring during radiation treatment. The beam attenuation and beam spoiling properties of the Calypso electromagnetic array at various beam angles were investigated. Measurements were performed on a Varian Clinac iX linear accelerator with 6 MV and 15 MV photon beams. The narrow beam attenuation properties were measured under a field size of 1 cm×1 cm, with a photon diode placed in a cylindrical graphite buildup cap. The broad beam attenuation properties were measured under a field size of 10 cm×10 cm, with a 0.6 cc cylindrical Farmer chamber placed in a polystyrene buildup cap. Beam spoiling properties of the array were studied by measuring depth‐dose change from the array under a field size of 10 cm×10 cm cm in a water‐equivalent plastic phantom with an embedded Markus parallel plate chamber. Change in depth doses were measured with the array placed at distances of 2, 5, and 10 cm from the phantom surface. Narrow beam attenuation and broad beam attenuation from the array were found to be less than 2%–3% for both 6 MV and 15 MV beams at angles less than 40°, and were more pronounced at more oblique angles. Spoiling effects are appreciable at beam buildup region, but are insignificant at depths beyond dmax. Dose measurements in a QA phantom using patient IMRT and VMAT treatment plans were shown to have less than 2.5% dose difference with the Calypso array. The results indicate that the dose difference due to the placement of Calypso array is clinically insignificant.PACS number: 87.56.‐v

Surface dose measurements from air gaps under a bolus by using a MOSFET dosimeter in clinical oblique photon beams

This study is intended to investigate the effects of surface dose from air gaps under the bolus in clinically used oblique photon beams by using a Markus parallel-plate chamber and a metal-oxide semiconductor field-effect transistor (MOSFET) dosimeter. To evaluate the performances of the two detectors, the percentage surface doses of the MOSFET dosimeters in without an air gap under the bolus material were measured and compared with those of the Markus parallel-plate chamber. MOSFET dosimeters at the surface provided results mostly in good agreement with the parallelplate chamber. The MOSFET dosimeters seemed suitable for surface dose measurements having excellent accuracy for clinical used photon beams. The relative surface doses were measured with air gaps (2, 5, 10 mm) and without an air gap under 3 different bolus setups: (1) unbolused (no bolus), (2) 5-mm bolus, and (3) 10-mm bolus. The reductions in the surface dose substantially increased with small field size, thick bolus, and large air gap. The absolute difference in the reductions of the surface dose between the MOSFET dosimeter and the Markus parallel-plate chamber was less than 1.1%. Results at oblique angles of incidence showed larger reductions in surface dose with increasing angle of incidence. The largest reduction in surface dose was recorded for a 6 × 6 cm2 field at a 60° angle of incidence with an 10-mm air gap under a 10-mm bolus. When a 10-mm bolus was used, a reduction in the surface dose with an air gap of up to 10.5% could be achieved by varying the field size and the incident angle. Therefore, air gaps under the bolus should be avoided in radiotherapy treatment, especially for photon beam with highly oblique angles of incidence.

SU-E-T-269: The Evaluation of Copper as an Alternative for Cerrobend Electron Shielding.

To evaluate the replacement of Cerrobend by copper for electron beam cutouts. The dosimetric comparisons for circular copper-and Cerrobend-cutouts with diameters (1.0, 2.0, 3.0, 5.0, 7.5, 10.0, and 12.5 cm) were made using electron beams with energies (6, 9, 12, 16, and 20 MeV) from 3 Varian accelerators. A PTW Farmer chamber (0.125cc-volume) was used for larger cutouts (diameters > 2cm), and an electron-diode for the 2 smallest cutouts. Also a Markus parallel plate chamber was used. (1) The tests showed little difference for the electron dosimetric characteristics, Eo, Eop, R50, Rp, and dmax. For larger cutout, the parameters were virtually the same for copper and Cerrobend. for smaller cutout (diameter = 3cm), small discrepancies were observed i.e. differences < 1mm for R50, Rp and dmax, =0.1MeV for Eop, and =0.3MeV for Eo. (2) The larger-cutout outputs at dmax were also virtually the same (difference = 0.6%). For smaller cutouts (diameters = 3cm), the copper outputs were 2.0%∼5.0% higher than Cerrobend. (3) For lower energy electrons (<12MeV), more larger-angle scattered electrons from higher-Z Cerrobend raise the Cerrobend percentage-depth-dose (PDD) curve at shallow-depths, and more forward scatter dose after dmax from lower-Z copper shifts the copper PDD slightly away from the one of Cerrobend. for higher energy electrons (= 12MeV), the shallow-dose difference becomes smaller for both cutouts, but even more forward-scattered dose from copper shifts copper's PDD further away from Cerrobend's. (4) The higher X-ray transmission through copper is also observable; i.e. 12%, 10%, and 7% for 20MeV, 16MeV, and 12MeV, respectively, but such small transmitted amount is clinically insignificant. Except for a higher x-ray transmission, other dosimetric differences brought in by the replacement of Cerrobend by copper cutout are negligible.

TU‐A‐BRB‐07: Characteristics of Diode Detectors in Proton Beam Dosimetry

Purpose: Small and narrow field dosimetry in proton beam therapy is difficult due to fluence disequilibrium. Diode detectors could be a suitable choice for high resolution proton dosimetry due to their relatively small size and high sensitivity. Unfortunately, semiconductor detectors are prone to radiation damage due to charged particle interactions. Characteristics of some of the commercial diodes in proton beam dosimetry are investigated in this study specially a prototype diode from PTW. Methods: The characteristics of several diodes from different vendors were evaluated in proton beams. All diodes were irradiated in a water phantom under reference condition (10cm diameter field, 16cm range, 10 cm SOBP) at various dosages to investigate the signal loss. Based on radiation response, the prototype diode was used for dose, dose rate, depth dose and profile measurements. The results were compared with Markus parallel plate chamber for proton energies in the range of 6cm to 27cm in water. Results: The initial signal loss in various diode detectors range from 0.4%/Gy to 0.05%/Gy. The PTW prototype exhibited the smallest amount of signal degradation that was investigated up to a dose of 1.4 kGy. Relative dose measurements (depth dose and profiles) are in good agreement with the measured data from Markus chamber. The magnitude of Bragg peaks differed by −2% from the results obtained with Markus chamber for all energies. Conclusions: Diode reading is a function of pre‐irradiation dose, beam energy, dose rate and total dose. Ignoring signal degradation and pre‐treatment dose, diodes can be used for relative dose measurements such as depth dose and profile. For absolute dose measurements, especially in small fields, special attention should be given to account for the loss of signal. The LET dependence is minimal for the prototype and can be used in a wide range of beam energy.

SU‐FF‐T‐348: Beam Attenuation and Beam Spoiling Properties of An Electromagnetic Array Used for Patient Localization and Tumor Tracking

Purpose: To investigate the beam attenuation and beam spoiling properties of the Calypso electromagnetic array used for patient localization and tumor tracking during the radiation delivery Method and Materials: One of the main components of the Calypso 4D system is an electromagnetic array placed above patients who are implanted with transponders. We measured both the narrow and board beam attenuation properties of the array including the effect of beam angle (0 to 90 degree). A photon diode placed in cylindrical graphite buildup cap was used for narrow geometry, field size of 1 cm ×1 cm, and a 0.6 cc cylindrical farmer chamber placed in a polystyrene buildup cap for broad geometry, field size of 10 cm × 10 cm. For broad beam geometry, distances of 2, 5, and 10 cm between the chamber and the array were used. Measurements were performed using a Varian Clinac IX linear accelerator with 6 MV and 15 MV photon beams. Beam spoiling properties of the array was studied by placing the array above a water equivalent phantom with an imbedded Markus parallel plate chamber and measuring depth doses. Depth doses were measured for both 6 MV and 15 MV photon beams with 10 cm × 10 cm field size for distances of 2, 5, and 10 cm between array and phantom surface. Results: Narrow beam geometry attenuation is 1.5 % and 1.3% for 6 MV and 15 MV beam respectively. Beam angle dependence is more pronounced at angles greater than 70 degrees with attenuation greater than 5%. Broad beam attenuation, in clinical cases, is nonexistent due to scatter contribution. Spoiling properties could be appreciable. Conclusion: Attenuation properties of the array may be ignored for treatment delivery for most clinical cases. Spoiling effect could be appreciable depending on array distance to patient surface.

SU‐FF‐T‐325: Dose Perturbation Study in a Multichannel Breast Brachytherapy Device

Purpose: To determine the dosimetric effects due to air pockets and high Z contrast within a new multichannel breast brachytherapy device Method and Materials: The 5‐6cm diameter Contura brachytherapy device was inflated using 37cm3 of saline to a 40 mm diameter. Baseline dose falloff from an HDR Iridium‐192 source was measured in water using a Markus parallel plate chamber supported on a motorized vertical drive. Ionization measurements with the Iridium source centered in the central Contura channel and an anterior off‐center channel were performed. Data were collected at distances from 1 to 50 mm. Comparison studies were conducted with identical inflated volume containing varying air pocket volumes (1–4 cm3) and varying concentrations of Isovue contrast solution (3, 6, and 9% by volume). Dose perturbation factors (DPF) were computed and evaluated. Results: Dose perturbations due to air pockets and contrast solutions were observed. As the volume of air increased the DPF caused increased dose at a rate of approximately 2.25%/cm3. The effect was relatively consistent for both channels throughout the distance range. The effects due to contrast were more complex. Increased dose was observed from the 3% solution, by 2% from the source in the central channel (♯5) and by 4% with the source in the off‐center channel (♯1). A dose reduction of 1.0% from channel ♯5 and 1.5% dose increase from channel ♯1 occurred from the 6% contrast solution. The 9% contrast solution caused dose reductions by 3.5% (channel ♯5) and 2.5% (channel ♯1). The DPF from all contrast solutions moderated with increasing distance. Conclusion: Dose perturbations due to air pockets and high Z contrast solution can be significant. Treatment planning systems do not account for these. It is important to minimize internal air pockets and limit contrast solution concentration to avoid dose errors to the target and normal tissue.

Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

The dose changes in the buildup region and beam attenuation by a carbon fiber tabletop were investigated for 6-and 18-MV photon beams. Measurements were performed for 2 x 2 cm to 40 x 40 cm field sizes. The surface dose and percentage depth doses (PDD) were measured by a Markus parallel plate chamber. Attenuation measurements were made at the cylindrical phantom for 180 degrees rotation of the beam. A carbon fiber tabletop increases the surface dose from 7.5% to 63.0% and from 4% to 43% for small fields at 6 and 18 MV, respectively. The increase was nearly fivefold for the 10 x 10 cm field and nearly twofold for the 40 x 40 cm field. Beam attenuation of the tabletop varies from 3.0% to 5.6% for 180 degrees and 120 degrees gantry angles for 6 MV. The carbon fiber tabletop significantly decreases the skin-sparing effect. The dosimetric effect of the tabletop may be higher, especially for the 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.

An experimental evaluation of recent electron dosimetry codes of practice

As from the 1 January 1997, the recent IPEMB code of practice for electron dosimetry is the recommended protocol for electron beam dosimetry in the UK, replacing the previous HPA code of practice and its IPSM addendum. New recommendations for electron beam dosimetry have also been formulated recently by the AAPM and the IAEA on the use of parallel-plate ionization chambers in high-energy electron beams. Against this background, the procedures recommended in each of these codes of practice have been followed from intercomparison of the field instrument ionization chamber with a secondary standard through to the determination of absorbed dose at the reference position in the electron beam. Absorbed doses have been determined for a number of electron beam energies ranging from nominal 5 MeV through to 17 MeV, and for four different types of field instrument ionization chamber: an NE2571 graphite walled cylindrical chamber; an NACP parallel-plate chamber; a Markus parallel-plate chamber; and a Roos parallel-plate chamber. The differences in the determination of absorbed dose between the IPEMB protocol and the HPA/IPSM protocol vary from to at the depth of maximum dose. In addition the IPEMB measured doses are 0.2% larger than those measured following the IAEA code of practice. It may also be stated that the IPEMB measured doses at the depth of maximum dose are up to 1.5%, but generally less than 1.0%, lower than those measured by the AAPM protocol.

Polarity and ion recombination correction factors for ionization chambers employed in electron beam dosimetry

Polarity and ion recombination correction factors for the NACP (type-02) design parallel-plate ionization chamber employed in a recent UK national electron beam dosimetry intercomparison are derived over the full range of energies and measurement conditions encountered. In addition, these effects have been studied for a further four NACP chambers, a Markus parallel-plate chamber, a Roos parallel-plate chamber and a NE2571 graphite walled cylindrical ionization chamber.

Evaluation of the dosimetric characteristics of a diamond detector for photon beam measurements

The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a silicon p-type photon diode and a 0.14 cm3 ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0 degree (parallel to the detector stem) to a direction transverse to the detector stem (90 degrees), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%-22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3 ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter < or = 4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.

Determination of build‐up region over‐response corrections for a Markus‐type chamber

Over-response corrections for a widely used parallel plate ionization chamber were determined using contemporaneous measurement of build up for 4, 6, 10 and 18-MV photon beams utilizing a commercially available extrapolation chamber (PTW model 23392). The resultant over-response corrections were essentially independent of field size (5 X 5 to 30 X 30 cm) and were less for increased depth (from the surface) and higher x-ray energy. The over-response of the parallel plate chamber (Markus-type, PTW model 329) was 13.8, 10.7, 6.2 and 4.7 percent (absolute) at the surface for 4, 6, 10 and 18-MV x-ray energies. At only 2 mm depth, the over-response of the Markus chamber under investigation decreased to 50% of surface value over-response. These corrections, so derived, are applied to Markus parallel plate chamber build-up measurements by simple subtraction of the derived corrections. For example, a 6-MV percentage depth dose of 37.5% measured at the surface with the Markus chamber is reduced by 10.7% so to agree with the surface dose indicated by the PTW extrapolation chamber for this x-ray field, that is, 26.8%.