Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size

Small field output factor measurement and verification for CyberKnife robotic radiotherapy and radiosurgery system using 3D polymer gel, ionization chamber, diode, diamond and scintillator detectors, Gafchromic film and Monte Carlo simulation

The dosimetry of small fields has become tremendously important with the advent of intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery, where small field segments or very small fields are used to treat tumors. With high dose gradients in the stereotactic radiosurgery or radiotherapy treatment, small field dosimetry becomes challenging due to the lack of lateral electronic equilibrium in the field, x-ray source occlusion, and detector volume averaging. Small volume and tissue-equivalent detectors are recommended to overcome the challenges. With the lack of a perfect radiation detector, studies on available detectors are ongoing with reasonable disagreement and uncertainties. The joint IAEA and AAPM international code of practice (CoP) for small field dosimetry, TRS 483 (Alfonso et al., 2017) provides guidelines and recommendations for the dosimetry of small static fields in external beam radiotherapy. The CoP provides a methodology for field output factor (FOF) measurements and use of field output correction factors for a series of small field detectors and strongly recommends additional measurements, data collection and verification for CyberKnife (CK) robotic stereotactic radiotherapy/radiosurgery system using the listed detectors and more new detectors so that the FOFs can be implemented clinically. The present investigation is focused on using 3D gel along with some other commercially available detectors for the measurement and verification of field output factors (FOFs) for the small fields available in the CK system. The FOF verification was performed through a comparison with published data and Monte Carlo simulation. The results of this study have proved the suitability of an in-house developed 3D polymer gel dosimeter, several commercially available detectors, and Gafchromic films as a part of small field dosimetric measurements for the CK system.

An evaluation of solid state detectors for the relative dosimetry of Kilovoltage x-ray beams

INTRODUCTION

Kilovoltage (kV) x-ray beams are an essential modality in radiotherapy. Solid state detectors are widely available in radiotherapy departments but their use for kV dosimetry has been limited to date. This study aimed to evaluate the dosimetric performance of a range of solid state detectors for kV dosimetry.

METHOD

Percentage depth doses (PDDs) and Relative Output Factors (ROFs) were measured on an XStrahl 300 unit (XStrahl-Ltd., UK) using 60, 100, 150 and 300 kVp x-ray beams. The fields were defined by circular applicators with field sizes of 2, 5, 8 and 10 cm diameter and square applicators of field sizes 10×10 and 20×20 cm² . The following PTW dosimeters were used for measurements: Advanced Markus, PinPoint 3D and Semiflex ionization chambers; Photon, Electron and SRS diodes plus the microDiamond detector. All PDDs were normalized at 5 mm depth and ROFs were measured at 3 mm depth to avoid collisions with the end of the applicators. ROFs measured using chambers were corrected for polarity and ion-recombination effects.

RESULTS AND DISCUSSION

PDD measurements for 60,100 and 150 kVp beam exhibited good agreement between all diodes and the ionization chambers over the entire range of depths except in the first few millimeters near the surface. However, for the 300 kVp, all diode detectors exhibited an over-responding behaviour compared to reference depth dose data measured with the Advanced Markus chamber. Relative output factors with the diodes were higher than the Advanced Markus chamber at low energy, and the magnitude of these differences is inversely proportional to the field sizes. The PTW P diode showed the highest variation of up to 15% in the output factor compared to the Advanced Markus chamber.

CONCLUSION

This study evaluated the dosimetric performance of a range of solid state detectors in kV relative dosimetry. This study showed that diode detectors are a suitable replacement for ionization chambers for the PDD measurement of low energy kV beams (60-150 kVp) except for the PDD of 60 kVp with the smaller field sizes. However, an over-responding behaviour of diode detectors at 300 kVp beams shows that diode detectors are not suitable for the PDD measurement of high energy kV beams. Generally, all solid state detectors over responded to ROF measurements, indicating that it is not suitable for ROF measurements. In general, both shielded and unshielded diodes produced a similar dosimetric response, which demonstrates that the energy dependence of solid state detectors should be considered before they are used for any kV relative dosimetric measurements. This article is protected by copyright. All rights reserved.

Dosimetry of a 6 MV flattening filter-free small photon beam using various detectors

The present study aimed to dosimetrically evaluate the small-fields of a 6 MV flattening filter-free (FFF) photon beam using different detectors.The 6 MV FFF photon beam was used for measurement of output factor, depth dose, and beam profile of small-fields of sizes 0.6 cm × 0.6 cm to 6.0 cm × 6.0 cm. The five detectors used were SNC125c, PinPoint, EDGE, EBT3, and TLD-100. All measurements were performed as per the International Atomic Energy Agency TRS 483 protocol. Output factors measured using different detectors as direct reading ratios showed significant variation for the smallest fields, whereas after correcting them according to TRS 483, all sets of output factors were nearly compatible with each other when measurement uncertainty was also considered. The beam profile measured using SNC125c showed the largest penumbra for all field sizes, whereas the smallest was recorded with EDGE. Compared with that of EBT3, the surface dose was found to be much higher for all the other detectors. PinPoint, EBT3, TLD-100, and EDGE were found to be the detector of choice for small-field output factor measurements; however, PinPoint needs special attention when used for the smallest field size (0.6 cm × 0.6 cm). EDGE and EBT3 are optimal for measuring beam profiles. EBT3, PinPoint, and EDGE can be selected for depth dose measurements, and EBT3 is suitable for surface dose estimation.

Reference dosimetry of modulated and dynamic photon beams

In the late 1980s, a new technique was proposed that would revolutionize radiotherapy. Now referred to as intensity-modulated radiotherapy, it is at the core of state-of-the-art photon beam delivery techniques, such as helical tomotherapy and volumetric modulated arc therapy. Despite over two decades of clinical application, there are still no established guidelines on the calibration of dynamic modulated photon beams. In 2008, the IAEA-AAPM work group on nonstandard photon beam dosimetry published a formalism to support the development of a new generation of protocols applicable to nonstandard beam reference dosimetry (Alfonso et al 2008 Med. Phys. 35 5179-86). The recent IAEA Code of Practice TRS-483 was published as a result of this initiative and addresses exclusively small static beams. But the plan-class specific reference calibration route proposed by Alfonso et al (2008 Med. Phys. 35 5179-86) is a change of paradigm that is yet to be implemented in radiotherapy clinics. The main goals of this paper are to provide a literature review on the dosimetry of nonstandard photon beams, including dynamic deliveries, and to discuss anticipated benefits and challenges in a future implementation of the IAEA-AAPM formalism on dynamic photon beams.

Physics and biology of ultrahigh dose-rate (FLASH) radiotherapy: a topical review

Ultrahigh dose-rate radiotherapy (RT), or 'FLASH' therapy, has gained significant momentum following various in vivo studies published since 2014 which have demonstrated a reduction in normal tissue toxicity and similar tumor control for FLASH-RT when compared with conventional dose-rate RT. Subsequent studies have sought to investigate the potential for FLASH normal tissue protection and the literature has been since been inundated with publications on FLASH therapies. Today, FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. The goal of this review article is to present the current state of this intriguing RT technique and to review existing publications on FLASH-RT in terms of its physical and biological aspects. In the physics section, the current landscape of ultrahigh dose-rate radiation delivery and dosimetry is presented. Specifically, electron, photon and proton radiation sources capable of delivering ultrahigh dose-rates along with their beam delivery parameters are thoroughly discussed. Additionally, the benefits and drawbacks of radiation detectors suitable for dosimetry in FLASH-RT are presented. The biology section comprises a summary of pioneering in vitro ultrahigh dose-rate studies performed in the 1960s and early 1970s and continues with a summary of the recent literature investigating normal and tumor tissue responses in electron, photon and proton beams. The section is concluded with possible mechanistic explanations of the FLASH normal-tissue protection effect (FLASH effect). Finally, challenges associated with clinical translation of FLASH-RT and its future prospects are critically discussed; specifically, proposed treatment machines and publications on treatment planning for FLASH-RT are reviewed.

Energy response of diamond sensor to beta radiation

This paper demonstrates the ability of diamond sensors to respond to beta radiation. A Chemical Vapor Deposition (CVD) single crystal diamond was used in this work. The diamond crystal has a dimension of 4.5×4.5 by 0.5 mm thick. Metal contacts were fabricated on both sides of the diamond using titanium and palladium metals with thicknesses of 50 nm and 150 nm, respectively. The energy response of the diamond sensor was experimentally measured using three beta isotopes that cover the entire range of beta energy: ¹⁴⁷Pm, a weak beta radiation with a maximum energy of 0.225 MeV, ²°⁴Tl, a medium energy beta radiation with a maximum energy of 0.763 MeV, and ⁹°Sr/⁹°Y, with both a medium energy beta radiation with a maximum energy of 0.546 MeV, and a high energy beta radiation with a maximum energy of 2.274 MeV. The beta measurements indicate that diamond sensors are sensitive to beta radiation and are suitable for beta spectroscopy. This is important in estimating dose since diamond is tissue equivalent, and the absorbed dose is easily determined from the energy and the mass of the active volume. The high energy betas from ²°⁴Tl and ⁹⁰Sr/⁹⁰Y penetrates the sensor without depositing sufficient energy in the active area because their range is larger than the thickness of sensor. The sensitivity of the detector is limited because of its small volume and can be improved by combining smaller area sensors since growing large size diamond is currently a challenge.

Dose and slice thickness evaluation with nMAG gel dosimeters in computed tomography

Computed tomography (CT) has been widely used in clinical diagnosis. It is important to estimate radiation dose and perform image quality assurance procedures for CT scans. In this study, nMAG gel dosimeters were used to simultaneously measure the 300-mm weighted CT dose index (CTDI) and slice sensitivity profile (SSP) for multiple detector CT (MDCT). Magnetic resonance imaging (MRI) was performed on the irradiated gel to create R₂‒dose response curves for the tube voltages of 120 and 140 kVp. The gel dosimeters were loaded in three home-made cylindrical phantoms to obtain CTDI₁₀₀ and CTDI₃₀₀. The full width at half maximum (FWHM) for 2, 5, 10, 14.4, and 38.4-mm slice thicknesses was measured and compared with the result obtained by radiochromic films. The difference in weighted CTDI₁₀₀ obtained by the gel dosimeter and ionization chamber was less than 1%. The CTDI efficiency at 120 and 140 kVp was in the range of 80.1%-82.5%. The FWHM of SSP measured by the gel dosimeter matched very well with the nominal slice thickness. The use of nMAG gel dosimeters combined with the home-made cylindrical phantoms can provide 300-mm weighted CTDI and slice thickness information, showing potential for quality assurance and clinical applications in MDCT.

Measurement of Total Scatter Factor for Stereotactic Cones with Plastic Scintillation Detector

Advanced radiotherapy modalities such as stereotactic radiosurgery (SRS) and image-guided radiotherapy may employ very small beam apertures for accurate localized high dose to target. Accurate measurement of small radiation fields is a well-known challenge for many dosimeters. The purpose of this study was to measure total scatter factors for stereotactic cones with plastic scintillation detector and its comparison against diode detector and theoretical estimates. Measurements were performed on Novalis Tx^™ linear accelerator for 6MV SRS beam with stereotactic cones of diameter 6 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm. The advantage of plastic scintillator detector is in its energy dependence. The total scatter factor was measured in water at the depth of dose maximum. Total scatter factor with plastic scintillation detector was determined by normalizing the readings to field size of 10 cm × 10 cm. To overcome energy dependence of diode detector for the determination of scatter factor with diode detector, daisy chaining method was used. The plastic scintillator detector was calibrated against the ionization chamber, and the reproducibility in the measured doses was found to be within ± 1%. Total scatter factor measured with plastic scintillation detector was 0.728 ± 0.3, 0.783 ± 0.05, 0.866 ± 0.55, 0.885 ± 0.5, and 0.910 ± 0.06 for cone sizes of 6 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm, respectively. Total scatter factor measured with diode detector was 0.733 ± 0.03, 0.782 ± 0.02, 0.834 ± 0.07, 0.854 ± 0.02, and 0.872 ± 0.02 for cone sizes of 6 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm, respectively. The variation in the measurement of total scatter factor with published Monte Carlo data was found to be −1.3%, 1.9%, −0.4%, and 0.4% for cone sizes of 7.5 mm, 10 mm, 12.5 mm, and 15 mm, respectively. We conclude that total scatter factor measurements for stereotactic cones can be adequately carried out with a plastic scintillation detector. Our results show a high level of consistency within our data and compared well with published data.

Technical Note: Response measurement for select radiation detectors in magnetic fields

PURPOSE

Dose response to applied magnetic fields for ion chambers and solid state detectors has been investigated previously for the anticipated use in linear accelerator-magnetic resonance devices. In this investigation, the authors present the measured response of selected radiation detectors when the magnetic field is applied in the same direction as the radiation beam, i.e., a longitudinal magnetic field, to verify previous simulation only data.

METHODS

The dose response of a PR06C ion chamber, PTW60003 diamond detector, and IBA PFD diode detector is measured in a longitudinal magnetic field. The detectors are irradiated with buildup caps and their long axes either parallel or perpendicular to the incident photon beam. In each case, the magnetic field dose response is reported as the ratio of detector signals with to that without an applied longitudinal magnetic field. The magnetic field dose response for each unique orientation as a function of magnetic field strength was then compared to the previous simulation only studies.

RESULTS

The measured dose response of each detector in longitudinal magnetic fields shows no discernable response up to near 0.21 T. This result was expected and matches the previously published simulation only results, showing no appreciable dose response with magnetic field.

CONCLUSIONS

Low field longitudinal magnetic fields have been shown to have little or no effect on the dose response of the detectors investigated and further lend credibility to previous simulation only studies.

Analysis of small field percent depth dose and profiles: Comparison of measurements with various detectors and effects of detector orientation with different jaw settings

The advent of modern technologies in radiotherapy poses an increased challenge in the determination of dosimetric parameters of small fields that exhibit a high degree of uncertainty. Percent depth dose and beam profiles were acquired using different detectors in two different orientations. The parameters such as relative surface dose (D_S), depth of dose maximum (D_max), percentage dose at 10 cm (D₁₀), penumbral width, flatness, and symmetry were evaluated with different detectors. The dosimetric data were acquired for fields defined by jaws alone, multileaf collimator (MLC) alone, and by MLC while the jaws were positioned at 0, 0.25, 0.5, and 1.0 cm away from MLC leaf-end using a Varian linear accelerator with 6 MV photon beam. The accuracy in the measurement of dosimetric parameters with various detectors for three different field definitions was evaluated. The relative D_S(38.1%) with photon field diode in parallel orientation was higher than electron field diode (EFD) (27.9%) values for 1 cm ×1 cm field. An overestimation of 5.7% and 8.6% in D₁₀ depth were observed for 1 cm ×1 cm field with RK ion chamber in parallel and perpendicular orientation, respectively, for the fields defined by MLC while jaw positioned at the edge of the field when compared to EFD values in parallel orientation. For this field definition, the in-plane penumbral widths obtained with ion chamber in parallel and perpendicular orientation were 3.9 mm, 5.6 mm for 1 cm ×1 cm field, respectively. Among all detectors used in the study, the unshielded diodes were found to be an appropriate choice of detector for the measurement of beam parameters in small fields.

Dose response of selected solid state detectors in applied homogeneous transverse and longitudinal magnetic fields

PURPOSE

MR-Linac devices under development worldwide will require standard calibration, commissioning, and quality assurance. Solid state radiation detectors are often used for dose profiles and percent depth dose measurements. The dose response of selected solid state detectors is therefore evaluated in varying transverse and longitudinal magnetic fields for this purpose.

METHODS

The Monte Carlo code PENELOPE was used to model irradiation of a PTW 60003 diamond detector and IBA PFD diode detector in the presence of a magnetic field. The field itself was varied in strength, and oriented both transversely and longitudinally with respect to the incident photon beam. The long axis of the detectors was oriented either parallel or perpendicular to the photon beam. The dose to the active volume of each detector in air was scored, and its ratio to dose with zero magnetic field strength was determined as the "dose response" in magnetic field. Measurements at low fields for both detectors in transverse magnetic fields were taken to evaluate the accuracy of the simulations. Additional simulations were performed in a water phantom to obtain few representative points for beam profile and percent depth dose measurements.

RESULTS

Simulations show significant dose response as a function of magnetic field in transverse field geometries. This response can be near 20% at 1.5 T, and it is highly dependent on the detectors' relative orientation to the magnetic field, the energy of the photon beam, and detector composition. Measurements at low transverse magnetic fields verify the simulations for both detectors in their relative orientations to radiation beam. Longitudinal magnetic fields, in contrast, show little dose response, rising slowly with magnetic field, and reaching 0.5%-1% at 1.5 T regardless of detector orientation. Water tank and in air simulation results were the same within simulation uncertainty where lateral electronic equilibrium is present and expectedly differed at the beam edge in transverse field orientations only. Due to the difference in design, the two detectors behaved differently.

CONCLUSIONS

When transverse magnetic fields are present, great care must be taken when using diamond or diode detectors. Dose response varies with relative detector orientation, magnetic field strength, and between detectors. This response can be considerable (∼20% for both detectors). Both detectors in longitudinal fields exhibit little to no dose response as a function of magnetic field. Water tank simulations seem to suggest that the diode detector is better suited to general beam commissioning, and each detector must be investigated separately.

Ionizing radiation detectors based on Ge-doped optical fibers inserted in resonant cavities

The measurement of ionizing radiation (IR) is a crucial issue in different areas of interest, from environmental safety and industrial monitoring to aerospace and medicine. Optical fiber sensors have recently proven good candidates as radiation dosimeters. Here we investigate the effect of IR on germanosilicate optical fibers. A piece of Ge-doped fiber enclosed between two fiber Bragg gratings (FBGs) is irradiated with gamma radiation generated by a 6 MV medical linear accelerator. With respect to other FBG-based IR dosimeters, here the sensor is only the bare fiber without any special internal structure. A near infrared laser is frequency locked to the cavity modes for high resolution measurement of radiation induced effects on the fiber optical parameters. In particular, we observe a variation of the fiber thermo-optic response with the radiation dose delivered, as expected from the interaction with Ge defect centers, and demonstrate a detection limit of 360 mGy. This method can have an impact in those contexts where low radiation doses have to be measured both in small volumes or over large areas, such as radiation therapy and radiation protection, while bare optical fibers are cheap and disposable.

The clinical impact of detector choice for beam scanning

Recently, the developers of Eclipse have recommended the use of ionization chambers for all profile scanning, including for the modeling of VMAT and stereotactic applications. The purpose of this study is to show the clinical impact caused by the choice of detector with respect to its ability to accurately measure dose in the penumbra and tail regions of a scanned profile. Using scan data acquired with several detectors, including an IBA CC13, a PTW 60012, and a Sun Nuclear EDGE Detector, three complete beam models are created, one for each respective detector. Next, using each beam model, dose volumes are retrospectively recalculated from actual anonymous patient plans. These plans include three full‐arc VMAT prostate plans, three left chest wall plans delivered using irregular compensators, two half‐arc VMAT lung plans, three MLC‐collimated static‐field pairs, and two SBRT liver plans. Finally, plans are reweighted to deliver the same number of monitor units, and mean dose‐to‐target volumes and organs at risk are calculated and compared. Penumbra width did not play a role. Dose in the tail region of the profile made the largest difference. By overresponding in the tail region of the profile, the 60012 diode detector scan data affected the beam model in such a way that target doses were reduced by as much as 0.4% (in comparison to CC13 and EDGE data). This overresponse also resulted in an overestimation of dose to peripheral critical structure, whose dose consisted mainly of scatter. This study shows that, for modeling the 6 MV beam of Acuros XB in Eclipse Version 11, the choice to use a CC13 scanning ion chamber or an EDGE Detector was an unimportant choice, providing nearly identical models in the treatment planning system.

PACS number: 87.55.kh

Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife® and linear accelerators equipped with microMLC and circular cones

PURPOSE

The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities: a CyberKnife(®) system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Qmsr (fclin,fmsr) correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors.

METHODS

Small field sizes were defined either by microMLC down to 6 × 6 mm(2) or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber, and PTW 60003 natural diamond). Two types of passive dosimeters were used: LiF microcubes and EBT2 radiochromic films.

RESULTS

Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters.

CONCLUSIONS

Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers.

Small field segments surrounded by large areas only shielded by a multileaf collimator: comparison of experiments and dose calculation

PURPOSE

Complex radiotherapy fields delivered using a tertiary multileaf collimator (MLC) often feature small open segments surrounded by large areas of the beam only shielded by the MLC. The aim of this study was to test the ability of two modern dose calculation algorithms to accurately calculate the dose in these fields which would be common, for example, in volumetric modulated arc treatment (VMAT) and study the impact of variations in dosimetric leaf gap (DLG), focal spot size, and MLC transmission in the beam models.

METHODS

Nine test fields with small fields (0.6-3 cm side length) surrounded by large MLC shielded areas (secondary collimator 12 × 12 cm(2)) were created using a 6 MV beam from a Varian Clinac iX linear accelerator with 120 leaf MLC. Measurements of output factors and profiles were performed using a diamond detector (PTW) and compared to two dose calculations algorithms anisotropic analytical algorithm [(AAA) and Acuros XB] implemented on a commercial radiotherapy treatment planning system (Varian Eclipse 10).

RESULTS

Both calculation algorithms predicted output factors within 1% for field sizes larger than 1 × 1 cm(2). For smaller fields AAA tended to underestimate the dose. Profiles were predicted well for all fields except for problems of Acuros XB to model the secondary penumbra between MLC shielded fields and the secondary collimator. A focal spot size of 1 mm or less, DLG 1.4 mm and MLC transmission of 1.4% provided a generally good model for our experimental setup.

CONCLUSIONS

AAA and Acuros XB were found to predict the dose under small MLC defined field segments well. While DLG and focal spot affect mostly the penumbra, the choice of correct MLC transmission will be essential to model treatments such as VMAT accurately.

Development and optimization of a beam shaper device for a mobile dedicated IOERT accelerator

PURPOSE

The aim of this study was to design and build a prototype beam shaper to be used on a dedicated mobile accelerator that protects organs at risk within the radiation field and conforms the beam to the target geometry during intraoperative electron radiotherapy (IOERT). A dosimetric characterization of the beam shaper device was performed based on Monte Carlo (MC) simulations, as well as experimental data, at different energies, field sizes, and source to skin distances.

METHODS

A mobile light intraoperative accelerator (LIAC(®), Sordina, Italy) was used. The design of the beam shaper prototype was based on MC simulations (BEAMnrc∕OMEGA and DOSXYZnrc code) for a selection of materials and thicknesses, as well as for dosimetric characterization. Percentage depth dose (PDD) and profile measurements were performed using a p-type silicon diode and a commercial water phantom, while output factors were measured using a PinPoint ion chamber in a PMMA phantom. Planar doses in planes of interest were carried out using radiochromic films (Gafchromic(TM) EBT and EBT2) in PMMA and in a Solid Water(®) phantom. Several experimental set-ups were investigated with the beam shaper device fixed on the top of the phantom, varying both the short side of the rectangular field and the air gap between the device and the phantom surface, simulating the clinical situation. The output factors (OFs) were determined using different geometrical set-ups and energies.

RESULTS

The beam shaper prototype consists of four blades sliding alongside each other and mounted on a special support at the end of the 10 cm diameter PMMA circular applicator. Each blade is made of an upper layer of 2.6 cm of Teflon(®) and a lower layer of 8 mm of stainless steel. All rectangles inscribed in a 5 cm diameter can be achieved in addition to any "squircle-shaped" field. When one side of the rectangular field is held constant and the second side is reduced, both R(50) and R(max) move towards the phantom surface. Comparing the PDDs obtained with the 5 cm circular applicator and with a 4.4 × 4.4 cm(2) square field (that is the equivalent square of the 5 cm circular field) obtained with the beam shaper, a different behavior was observed in the region extending from the surface to a depth of 50% of the maximum dose. Isodoses measured for rectangular fields used for clinical cases (i.e., 4 × 9 cm(2) 8 MeV) are shown, with different air gaps. For each energy investigated, the normalized OFs slowly increase, when the length of the side decreases down to about 4 cm, and then rapidly decreases for smaller field widths. MC simulation showed an excellent agreement with experimental data (<2%).

CONCLUSIONS

The beam shaper device is able to provide square∕rectangular∕squircle fields with adequate dose homogeneity for mobile dedicated accelerators, thus allowing conformal treatment with IOERT. Monte Carlo simulation can be a very useful tool to simulate any clinical set up and can be used to create a data set to calculate MUs, thereby increasing the accuracy of the delivered dose during IOERT procedures.

A feasibility study of using couch-based real time dosimetric device in external beam radiotherapy

PURPOSE

Measurement of actual dose delivered during radiotherapy treatment aids in checking the accuracy of dose delivered to the patient. In this study, a couch-based real time dosimetric device has been proposed to measure the exit or entrance dose to a patient during external beam radiotherapy. The utility and feasibility of such a device using a 2D array of diodes has been demonstrated.

METHODS

Two MAPCHECK devices: MAPCHECK (1175) and MAPCHECK 2 (both SunNuclear) were embedded in a foam block in the treatment couch of a Varian 21iX linear accelerator. The angular dependence of the detector response for both devices was studied before implementing the MAPCHECKs for experimental purposes. An Alderson Rando head phantom was scanned with the MAPCHECK and MAPCHECK 2 devices separately and four different treatment plans were generated with target volumes at three different positions simulating typical clinical situations. The analytical anisotropic algorithm (AAA) was used to compute the doses in an Eclipse treatment planning system (Varian Medical Systems). The Rando phantom with the MAPCHECK device was exposed in Clinac 21iX linear accelerator. The measured dose distribution was compared with the calculated dose distribution to check for the accuracy in dose delivery.

RESULTS

Measured and computed dose distribution were found to agree with more than 93% of pixels passing at 3% and 3 mm gamma criteria for all the treatment plans. The couch-based real time dosimetry system may also be applied for noncoplanar beams where electronic portal imaging device (EPID) is not practical to measure the dose. Other advantages include checking the beam stability during the patient treatment, performing routine morning quality assurance (QA) tests in the linear accelerator, and to perform pretreatment verification of intensity modulated radiation therapy (IMRT). One of the drawbacks of this system is that it cannot be used for measuring the dose at 90° or 270° gantry angles.

CONCLUSIONS

This preliminary study shows that a 2D array of detectors may be used as part of the treatment couch for real time patient dosimetry in studying the dose delivered to the patient in real time and also for performing routine quality assurance.

Caracterização dosimétrica de feixes de fótons com detector de diamante

OBJETIVO: Este trabalho foi realizado com o objetivo de caracterizar o detector de diamante, bem como observar seu comportamento. MATERIAIS E MÉTODOS: As características dosimétricas de feixes de fótons de 6 MV de energia foram medidas utilizando o sistema automático de varredura MP3 da PTW com um detector de diamante e foram comparadas com medidas usando uma câmara de ionização 31010 da PTW. RESULTADOS: As características dosimétricas do detector de diamante foram observadas por meio de medidas de linearidade com a dose, dependência com a taxa de dose e distribuições de dose em profundidade, bem como perfis. CONCLUSÃO: Algumas medidas com diamante e câmara de ionização foram validadas com resultados publicados na literatura, o que demonstrou um bom comportamento do detector de diamante na comparação com a câmara de ionização, muito utilizada para dosimetria em radioterapia, evidenciando que o diamante é uma boa escolha de detector para dosimetria de campos pequenos.

Assessment of the setup dependence of detector response functions for mega-voltage linear accelerators

PURPOSE

Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning.

METHODS

In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and 0.65 cm3 and a diode of surface area 0.64 mm2. The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a 10 x 10 cm2 field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed.

RESULTS

Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a +/- 0.7 mm variation in the 80%-20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements.

CONCLUSIONS

The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs. cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged response function could be utilized to provide a good approximation of the true profile for large ionization chambers and for larger fields for which diode detectors are not recommended.

A silicon strip detector dose magnifying glass for IMRT dosimetry

PURPOSE

Intensity modulated radiation therapy (IMRT) allows the delivery of escalated radiation dose to tumor while sparing adjacent critical organs. In doing so, IMRT plans tend to incorporate steep dose gradients at interfaces between the target and the organs at risk. Current quality assurance (QA) verification tools such as 2D diode arrays, are limited by their spatial resolution and conventional films are nonreal time. In this article, the authors describe a novel silicon strip detector (CMRP DMG) of high spatial resolution (200 microm) suitable for measuring the high dose gradients in an IMRT delivery.

METHODS

A full characterization of the detector was performed, including dose per pulse effect, percent depth dose comparison with Farmer ion chamber measurements, stem effect, dose linearity, uniformity, energy response, angular response, and penumbra measurements. They also present the application of the CMRP DMG in the dosimetric verification of a clinical IMRT plan.

RESULTS

The detector response changed by 23% for a 390-fold change in the dose per pulse. A correction function is derived to correct for this effect. The strip detector depth dose curve agrees with the Farmer ion chamber within 0.8%. The stem effect was negligible (0.2%). The dose linearity was excellent for the dose range of 3-300 cGy. A uniformity correction method is described to correct for variations in the individual detector pixel responses. The detector showed an over-response relative to tissue dose at lower photon energies with the maximum dose response at 75 kVp nominal photon energy. Penumbra studies using a Varian Clinac 21EX at 1.5 and 10.0 cm depths were measured to be 2.77 and 3.94 mm for the secondary collimators, 3.52 and 5.60 mm for the multileaf collimator rounded leaf ends, respectively. Point doses measured with the strip detector were compared to doses measured with EBT film and doses predicted by the Philips Pinnacle treatment planning system. The differences were 1.1% +/- 1.8% and 1.0% +/- 1.6%, respectively. They demonstrated the high temporal resolution capability of the detector readout system, which will allow one to investigate the temporal dose pattern of IMRT and volumetric modulated are therapy (VMAT) deliveries.

CONCLUSIONS

The CMRP silicon strip detector dose magnifying glass interfaced to a TERA ASIC DAQ system has high spatial and temporal resolution. It is a novel and valuable tool for QA in IMRT dose delivery and for VMAT dose delivery.

Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison

Small photon fields are increasingly used in modern radiotherapy and especially in IMRT and SRS/SRT treatments. The uncertainties related to small field profile measurements can introduce significant systematic errors to the overall treatment process. These measurements are challenging mainly due to the absence of charged particle equilibrium conditions, detector size and composition effects, and positioning problems. In this work four different dosimetric methods have been used to measure the profiles of three small 6 MV circular fields having diameters of 7.5, 15.0, and 30.0 mm: a small sensitive volume air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and vinyl-pyrrolidone based polymer gel dosimeter. The results of this work support the validity of previous findings, suggesting that (a) air ion chambers are not suitable for small field dosimetry since they result in penumbra broadening and require significant corrections due to severe charged particle transport alterations; (b) diamond detectors provide high resolution and rather accurate small field profile measurements, as long as positioning problems can be addressed and the necessary dose rate corrections are correctly applied; and (c) the novel silicon-diode array (DOSI) used in this study seems to be adequate for small field profile measurements overcoming positioning problems. Polymer gel data were assumed as reference data to which the other measurement data were compared both qualitatively and quantitatively using the gamma-index concept. Polymer gels are both phantom and dosimeter, hence there are no beam perturbation effects. In addition, polymer gels are tissue equivalent and can provide high-spatial density and high-spatial resolution measurements without positioning problems, which makes them useful for small field dosimetry measurements. This work emphasizes the need to perform beam profile measurements of small fields (for acceptance, commissioning, treatment planning systems data feed, and periodic quality assurance purposes) using more than one dosimetric method. The authors believe this to be a safe way towards the reduction of the overall uncertainty related to SRS/SRT treatments.