Shadow Tray Research Articles

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy.

Intensity-modulated radiotherapy is a widely used technique for accurately targeting cancerous tumours in difficult locations using dynamically shaped beams. This is ideally accompanied by real-time independent verification. Monolithic active pixel sensors are a viable candidate for providing upstream beam monitoring during treatment. We have already demonstrated that a Monolithic Active Pixel Sensor (MAPS)-based system can fulfill all clinical requirements except for the minimum required size. Here, we report the performance of a large-scale demonstrator system consisting of a matrix of 2 × 2 sensors, which is large enough to cover almost all radiotherapy treatment fields when affixed to the shadow tray of the LINAC head. When building a matrix structure, a small dead area is inevitable. Here, we report that with a newly developed position algorithm, leaf positions can be reconstructed over the entire range with a position resolution of below ∼200 μm in the centre of the sensor, which worsens to just below 300 μm in the middle of the gap between two sensors. A leaf position resolution below 300 μm results in a dose error below 2%, which is good enough for clinical deployment.

8 Implementation of MOD-ISO-CBCT+ QA module from Qualiformed on Elekta Agility

Introduction The MOD-ISO-CBCT+ module from QUALIFORNED is dedicated to the QA of isocenter (size and position) and angles position of gantry, collimator and table. For SBRT or non-coplanar treatments some mechanical tolerances could be more restrictive [1] . This module contains phantoms (OTP-ISO+ sphere, OTP-Cross-MV and OTP-Cross-kV plates). The OTP-CROSS plates materialize a fix point coming from the source (MV or kV) [2] . We have studied several possible devices in order to evaluate their user-friendly, safe and efficient aspects. Methods For the OTP-CROSS-kV, we recycled a filter neither used in service or clinical mode. We removed the aluminum part, fixed inside a machined plastic slab to reproducibly insert the OTP-Cross-kV in S position. This filter was painted in red and used in service mode only. For OTP-CROSS-MV, 3 setups were tested: Two with the shadow tray and one with a Mylar crosswire screen. Setup 1: a coded acrylic tray was machined to align OTP-Cross-MV. Setup 2: axes determination for OTP alignment by doing a cross irradiation of a gafchromic film taped to the top acrylic tray. Setup 3: OTP-Cross-MV was taped down on an old Mylar screen, avoiding any contact with the current crosswire, and then it is superimposed, aligned and fixed with nut. Finally these 3 setups were compared by performing QA with the same sphere position. Results No difference was observed, suggesting that our shadow tray and Mylar crosswire screen are well aligned and don’t involve any additional mechanical movement. Setup 1: metal balls of OTP-CROSS-MV appear on the sphere surface, complicating their image detection. Setup 2: The Shadow tray is heavy, but its positioning is reproducible. It is not used clinically, anymore. Setup 3: the old Mylar crosswire screen requires a good alignment with the linac crosswire, which is not removed, meaning no risk for the UV light. Conclusions The motivation of this work was to develop a dedicated QA kit, plug and play, routinely usable, with reproducible positioning, and clinically safe. For the second setup, the difficulty was the accurate axes determination on the top removable acrylic tray and that was solved by irradiating a taped gafchromic film. This setup n°2 is retained. We are considering the introduction of a security key for setup 3. Finally by recycling some linac parts, the cost development is limited. Thanks to Mr. N. Dodd from Elekta for his help and advice.

Surface dose variations in 6 and 10 MV flattened and flattening filter-free (FFF) photon beams.

As the use of linear accelerators operating in flattening filter‐free (FFF) modes becomes more widespread, it is important to have an understanding of the surface doses delivered to patients with these beams. Flattening filter removal alters the beam quality and relative contributions of low‐energy X‐rays and contamination electrons in the beam. Having dosimetric data to describe the surface dose and buildup regions under a range of conditions for FFF beams is important if clinical decisions are to be made. An Elekta Synergy linac with standard MLCi head has been commissioned to run at 6 MV and 10 MV running with the flattening filter in or out. In this linac the 6 MV FFF beam has been energy‐matched to the clinical beam on the central axis (D10). The 10 MV beam energy has not been adjusted. The flattening filter in both cases is replaced by a thin (2 mm) stainless steel plate. A thin window parallel plate chamber has been used to measure a comprehensive set of surface dose data in these beams for variations in field size and SSD, and for the presence of attenuators (wedge, shadow tray, and treatment couch). Surface doses are generally higher in FFF beams for small field sizes and lower for large field sizes with a crossover at 10×10 cm2 at 6 MV and 25×25 cm2 at 10 MV. This trend is also seen in the presence of the wedge, shadow tray, and treatment couch. Only small differences (<0.5%) are seen between the beams on varying SSD. At both 6 and 10 MV the filter‐free beams show far less variation with field size than conventional beams. By removing the flattening filter, a source of contamination electrons is exchanged for a source of low‐energy photons (as these are no longer attenuated). In practice these two components almost balance out. No significant effects on surface dose are expected by the introduction of FFF delivery.PACS number(s): 87.53.Bn, 87.55.ne, 87.56.bd

Effect of various physical parameters on surface and build-up dose for 15-MV X-rays

The purpose of this study was to find out the effect of various physical parameters on the skin and build-up doses of 15-MV photon beams. The effects of field dimensions, acrylic shadow tray, focus to-skin distance (FSD) on surface and buildup dose were determined for open, motorized 60° wedge (MW) and blocked fields. A ‘Markus’ plane parallel plate chamber was used for these measurements in an Elekta (6–15MV) linear accelerator. The surface dose for MW fields was lower than the dose for an open field, but the trend reversed for large fields and higher degree wedges. With the use of an acrylic shadow tray, the surface dose increased for all field sizes, but the increase was dominant for large fields. The surface dose for blocked fields was lower than the dose for open fields. The percentage depth dose of 10 × 10 cm2 field at surface (PDD0) for open beam were 13.89%, 11.71%, and 10.74% at 80 cm, 100 cm, and 120 cm FSD, respectively. The blocking tray increased PDD0 of 10 × 10 cm2 field to 26.29%, 14.01%, and 11.53%, while the motorized 60° wedge decreased PDD0 to 11.32%, 9.7%, and 8.9 % at these FSDs. The maximum PDD difference seen at surface (i.e., skin) for 5 × 5 cm2, 15 × 15 cm2, and 30 × 30 cm2 are 0.5%, 4.6%, and 5.6% for open field and 0.9%, 4.7%, and 7.2% for motorized 60° wedge field, when FSDs varied from 80 cm to 120 cm. The maximum PDD difference seen at surface for 5 × 5 cm2, 15 × 15 cm2, and 30 × 30 cm2 fields are 5.6%, 22.8%, and 29.6%, respectively, for a 1.0-cm perspex-blocking tray as the FSD is changed. The maximum PDD difference was seen at the surface (i.e., skin) and this decreased with increasing depth.

Ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

A new method to measure the effect of the backscatter into the beam monitor chambers in linear accelerators is introduced from first principles. The technique, applicable to high-energy photon beams, is similar to the well-known telescopic method although here the heavy blocks are replaced by a very small, centred block on the shadow tray, thus the name ‘ecliptic method’. This effect, caused mainly by backscattering from the secondary collimators, is known to be an output factor constituent and must be accounted for when detailed calculations involving the machine's head are required. Since its magnitude is generally small, experimental errors might obscure the behaviour of the phenomenon. Consequently, the procedure introduced goes along with an uncertainty assessment. Our theory was confirmed via measurements in cobalt-60 beams, where the studied effect does not contribute to the output factor. Measurements were also performed on our Saturne 41 linear accelerator and the results were qualitatively similar to those described elsewhere. The collimation systems were studied separately by varying one jaw setting while keeping the other at its maximum value. In the light of these results, we deduced an algorithm that can correlate the former data with the effect of backscattering to the beam monitor chambers for any rectangular field within 0.5%, which is of the order of the experimental uncertainty (0.6%). As we show, the experimental procedure is safe, simple, not invasive for the linac and requires only basic dosimetry equipment.

A self-correcting method for improving the precision of beam blocks

A technique for manufacturing precise custom blocks is described. Using the tracing stylus of the mold making machine, reference markers are cut into the lateral borders of the polystyrene mold after the cavities for the blocks have been made. These markers are aligned with the central ray cross hair of the shadow tray when the blocks are mounted on the tray. The ability of the technique to enhance precision has been verified in laboratory tests by intentionally introducing small imperfections into a mold making machine and checking the positional accuracy of the mounted blocks. The clinical performance has been tested by evaluating 47 check films of blocks for 16 randomly selected patients. The average positional error of individual blocks, projected to the isocenter, was less than one mm. The average time needed to cut the reference markers was 25 seconds. Implementing the technique required only minor modifications of a commercial mold making machine. PACS number(s): 87.53.–j, 87.56.–v

Design, construction, and performance of an asymmetrical collimator for a cobalt unit

Purpose: Besides four linear accelerators, our institute has a 100-cm SAD cobalt unit which has some specific and useful features for radiotherapy. The unit is equipped with a electronic portal imaging device (EPID) and an asymmetrical collimator to perform similar treatment techniques as on the linear accelerators, using the same shielding and half-beam techniques. The design, construction, and performance of the asymmetrical collimator are described. Methods and Materials: The new design includes the doubling of the drive and read-outs for both X and Y jaws. Spare parts of the standard collimator provided by the manufacturer were used as much as possible. Special trimmers were designed to allow asymmetry to the center of the field. The trimmers are removable, but the design allows the use of trimmers, wedges, and shadow tray simultaneously. Results: Transmission through the trimmers is less than 5% of the dose at d max. The use of the trimmers extends the source to diaphragm distance to 53 cm, resulting in a smaller penumbra as compared to the standard collimator. Without the trimmers, the maximum field size remains unchanged, i.e., 40 × 40 cm 2; with the trimmers, it is 38 × 36 cm 2. Percentage depth doses and output factors were similar to the standard collimator. Conclusion: The modified collimator is compact and allows use of the same shielding blocks as the linear accelerators, providing the same treatment techniques and capabilities. This simplifies an exchange of patients between our treatment machines.

Thyroid dose in children undergoing prophylactic cranial irradiation

Purpose: To determine the radiation dose received by the thyroid gland as a result of prophylactic cranial irradiation (PCI) in childhood leukemia and the factors influencing that dose.Methods and Materials: The dose to the thyroid resulting from simulated cranial irradiation with parallel opposed lateral fields of an adult anthropomorphic (ART) phantom with both 6 MV X-rays and Cobalt-60 γ-rays was measured using thermoluminescent dosimeters (TLDs). The dependence of thyroid dose on the distance of the field from the thyroid and the proportions of thyroid dose from stray radiation (leakage, scatter from jaws, etc.) and tissue scattered radiation were measured. The effects of a shadow tray and shielding blocks were also determined. Calculation of thyroid dose using the Clarkson scatter integration method was performed for 6 MV X-rays to compare with the measured doses. In vivo thyroid dose estimates were made using TLD measurements for three children receiving PCI with 6 MV X-rays.Results: Using open, unshielded fields, the thyroid region of the phantom received 1.2–1.4% of the prescribed cranial dose for 6 MV X-rays and 1.5–1.7% for Cobalt-60. For both treatment units, stray radiation accounted for approximately two thirds of the thyroid dose and tissue scatter accounted for the remaining one third. The thyroid dose increased as the field moved closer to the thyroid, with an increasing proportion of the dose due to tissue scatter. Placement of a thyroid shielding block on a shadow tray reduced the thyroid dose by only 20% compared with the open, unshielded setup. Thyroid dose from 6 MV using open fields was affected by the orientation of the collimator. When the inferior field edge was defined by the lower jaw, the dose was reduced by 27% compared with the upper jaw. Good correlation of dose to the thyroid region was obtained between phantom measured doses, in vivo measured doses and calculation of dose using the Clarkson method.Conclusion: For PCI doses of 1800 or 2400 cGy in the adult phantom, the dose to the thyroid was 20–40 cGy (1–2%). For small children this could rise to approximately 5% of the prescribed dose, of which half was due to stray radiation. As the thyroid in children is very sensitive to radiation and the dose-response curve for thyroid tumor induction is linear, attempts to shield the thyroid during cranial irradiation are mandatory. Cobalt-60 units should not be used, as the thyroid dose was higher than using 6 MV X-rays. Collimator orientation and the use of shadow trays and shielding were important factors in determining thyroid dose.

192 シャドウトレイ使用時のTPRの乱れの増感紙による改善

192 シャドウトレイ使用時のTPRの乱れの増感紙による改善

173. 10MV X 線の Shadow Tray による線量分布影響について(治療-照射法・付属器具)

173. 10MV X 線の Shadow Tray による線量分布影響について(治療-照射法・付属器具)

343. 頭頸部用シャドウトレイの開発とその性能評価(治療-9 附属器具)

343. 頭頸部用シャドウトレイの開発とその性能評価(治療-9 附属器具)

343. 頭頸部用シャドウトレイの開発とその性能評価(第 48 回総会学術大会一般研究発表予稿)

343. 頭頸部用シャドウトレイの開発とその性能評価(第 48 回総会学術大会一般研究発表予稿)

硬質ナイロンネットを利用した Shadow Tray の考案

It is a well-known fact that skin injuries after high-energy photon beam radiotherapy are attributed to the secondary electron emitted from acrylic shadow trays. We applied, as a shadow tray material, a high-impact nylon net used generally for tennis rackets, Comparative studies on surface doses in radiotherapy with Linac 10 MV (Toshiba) in conjuction with acrylic trays or the novel nylon net tray were carried out, with the facts being reparted below. In a clinical setting (i.e., source-to-chamber distance 90 cm, tray-to-surface distance 17 cm, field size 15x 15 cm^2), surface doses at a peak depth 25 mm were normalized to the value in the use of conventional trays (5 mm acrylic+10 mm acrylic) respectively in the following tray occasions. a) 5 mm acrylic-high-impact nylon. b) 7 mm lead-acrylic-high-impact nylon. c) hight-impct nylon + high-impact nylon. Normalized data clearly showed improved reduction in the secondary electron of a) 23.6% b) 37.3% c) 42.6% The dramatical reduction (42.6%) in the surface doses thus far obtained by use of the high-impact nylon net tray would greatly benefit clinical radiotherapy.

Electron contamination in 60 Co gamma-ray beams.

All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron "contamination." The present study of a 60 Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5×5, 20×20, and 35×35-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a "clear" 60 Co γ-ray beam of 1-cm diameter.

The experimental measurements of the effects of field size and shadow tray on the relative surface doses for C0-60 and 10 MV X-ray beams

The experimental measurements of the effects of field size and shadow tray on the relative surface doses for C0-60 and 10 MV X-ray beams

Thin lead sheets as tissue compensators for larger field irradiation

This paper describes the use of a thin lead sheet as a tissue compensating filter when a large field that includes the supraclavicular and mediastinal regions is irradiated. The typical midplane depths between supraclavicular and mediastinal regions may vary between 6 to 12 cm. Flattening of the beam entry surface is necessary for dose uniformity; this is achieved with a thin lead sheet compensating filter on the shadow tray of a 4 MV Unit. The shadow tray also contains lead shielding blocks for lung, cervical spinal cord, and larynx. The advantages of using thin lead sheets include easy maneuverability of shaping and sizing for irregular fields, and the small dimensions that are needed. Dose uniformity is verified by measuring optical densities from the film that is taken with the actual tray containing this compensating filter. This compensating filter may be extended to many situations where there are marked dose variations between different locations within the same large radiation field. The electron contamination produced by the scattering medium being placed in the beam is less for lead than for aluminum and wax. This contamination is also insignificant when the scatterer is more than 20 cm. away from the patient's skin surface when Cobalt-60 and 4 NIV units are used.

Linear accelerator calibration monitor with a memory.

A calibration monitor has been designed for measuring the constancy of linear accelerator or cobalt unit output between full calibrations. This monitor is battery operated, light-weight and slides into the shadow tray attachment on a linear accelerator or cobalt unit for easy setup. It provides a digital readout of the dose delivered, and a consistency check can be made in less than two minutes. The precision of the monitor, determined by cobalt-60 irradiations over a 2 1/2 period, is +/- 0.6% (standard deviation). The monitor also retains the dose reading in a CMOS digital counter indefinitely, hence it can be used in the same fashion as mailed thermoluminescent dosimeters (TLD) for calibration checks at remote facilities without the complicated readout procedures associated with TLD. The monitor can be mailed to a remote facility, positioned without ambiguity, and irradiated; and the reading can be verified on return to the originating center simply by pressing a switch. The monitor can easily be set up to carry out a "blind" check in which the reading obtained is not known to the remote facility.

A device for insuring the congruency of opposed irregularly blocked fields

A device for insuring the congruency of opposed irregularly blocked fields has been developed which permits the treatment of both fields of a parallel opposed pair without the realignment of any field-shaping standard non-divergent blocks. The device is an adaptor which attaches directly to the shadow tray or wedge holder and provides 2 channels for 1 Lucite shadow tray. A mirrow-image field is obtained without disturbing the position of the blocking array. Set-up times are decreased.

A device for insuring the congruency of opposed irregularly blocked fields.

A device for insuring the congruency of opposed irregularly blocked fields has been developed which permits the treatment of both fields of a parallel opposed pair without the realignment of any field-shaping standard non-divergent blocks. The device is an adaptor which attaches directly to the shadow tray or wedge holder and provides 2 channels for 1 Lucite shadow tray. A mirrow-image field is obtained without disturbing the position of the blocking array. Set-up times are decreased.

Accessory beam flattening filter for the varian clinac-4 linear accelerator.

An accessory beam flattening filter (brass disk with critical angles) attaches to the shadow tray of the clinac-4 linear accelerator, intercepts the x-ray beam, and eliminates higher dose areas at the edges of large fields near the skin surface. This improves the x-ray beam dose distribution within the patient and results in a reduction of dose to normal tissues near the surface.