Glass beads are a novel TL dosimeter in radiotherapy. An important characteristic of TL dosimeters is
their energy response, especially when intended for use in radiotherapy applications over a wide range
of energies (typically from X-rays generated at 80 kVp up to 25 MV photon and MeV electron beams). In
this paper, the energy response of glass beads (Mill Hill, Japan) is investigated for their TL response to kV
X-rays from an orthovoltage radiotherapy unit and also for MV photon and MeV electron beams from a
medical linear accelerator. The experimental findings show that for photon and electron beams, the TL
response of this particular glass bead, normalised to unity for 6 MV X-rays (TPR20/10¼0.670), decreases
to 0.9670.02 for 15 MV X-rays (TPR20/10¼0.761) and to 0.9570.01 for 20 MeV electron beams
(R50,D¼8.35 cm). This compares favourably with other TLD materials such as LiF and also alanine
dosimeters that are readout with an EPR system. For kV X-rays, the response increases to 4.5270.05 for
80 kV X-rays (HVL¼2.4 mm Al) which approaches 3 times that of LiF TLDs and 5 times that of alanine.
In conclusion, the particular glass beads, when used as a dosimeter material, show a relatively small
energy dependence over the megavoltage range of clinically relevant radiation qualities, being clearly
advantageous for accurate dosimetry. Conversely, the energy response is significant for photon beam
energies covering the kV range. In both circumstances, in dosimetric evaluations the energy response
needs to be taken into account.
Bradley DA, Abdul Sani S, Alalawi A, Jafari S, Nisbet A, Noor N, Hairul Azhar A, Mahdiraji G, Tamchek N, Ghosh S, Paul M, Alzimami K, Maah M (2014) Development of tailor-made silica fibres for TL dosimetry,Radiation Physics and Chemistry104pp. 3-9 The Ge dopant in commercially available silica optical fibres gives rise to appreciable thermoluminscence (TL), weight-for-weight offering sensitivity to MV X-rays several times that of the LiF dosimeter TLD100. The response of these fibres to UV radiation, X-rays, electrons, protons, neutrons and alpha particles, with doses from a fraction of 1Gy up to 10kGy, have stimulated further investigation of the magnitude of the TL signal for intrinsic and doped SiO fibres. We represent a consortium effort between Malaysian partners and the University of Surrey, aimed at production of silica fibres with specific TL dosimetry applications, utilizing modified chemical vapour deposition (MCVD) doped silica-glass production and fibre-pulling facilities. The work is informed by defect and dopant concentration and various production dependences including pulling parameters such as temperature, speed and tension; the fibres also provide for spatial resolutions down to <10¼m, confronting many limitations faced in use of conventional (TL) dosimetry. Early results are shown for high spatial resolution (~0.1mm) single-core Ge-doped TL sensors, suited to radiotherapy applications. Preliminary results are also shown for undoped flat optical fibres of mm dimensions and Ge-B doped flat optical fibres of sub-mm dimensions, with potential for measurement of doses in medical diagnostic applications. © 2014 Elsevier Ltd.
Purpose:
Investigating the feasibility of using low-cost commercially available silica beads as novel thermo-luminescence dosimeters (TLD) for postal dosimetry audit.
Methods:
A mail-based dosimetry audit was designed to assess the positional and dosimetric accuracy of SABR-lung treatment delivery using alanine and EBT3-film, placed in a CIRS-anthropomorphic thorax phantom. In conjunction, the silica beads were dosimetrically characterised as TLDs and cross-calibrated against the alanine. A CT-scan of the phantom with pre-delineated volumes was sent to 20 RT centres and used to create a SABR plan using local current protocols and techniques. The silica beads were held in an insert, designed to match that of the alanine holder and ionisation chamber to give the same measurement length. The doses determined by the silica beads were compared to those measured by alanine, the local ionisation chamber, film and the TPS calculation.
Results:
The mean percentage difference between the doses measured by the silica beads and the calculated doses by the TPS was found to be 0.7% and differed by 0.6%, 0.7%, and 1.3% from the alanine, film and local ionisation chamber measurements respectively.
Conclusions:
Results obtained with the silica beads agree well with those obtained from conventional detectors including alanine, film and ionisation chambers. This together with the waterproof and inert characteristics and minimal dose fading associated with silica bead TLDs confirm their potential as a postal dosimetry audit tool in both water and plastic phantoms which could withstand challenges of temperature and humidity variation, as well as postal service delays.
Purpose: kV radiotherapy treatment calculations are based on flat, homogenous, full-scatter reference conditions. However, clinical treatments often include surface irregularities and inhomogeneities, causing uncertainty. Therefore, confirmation of actual delivered doses in vivo is valuable. The current study evaluates, and implements, radiochromic film and micro silica bead TLD for in vivo kV dosimetry.
Methods: The kV energy and dose response of EBT3 film and silica bead TLD was established and uncertainty budgets determined. In vivo dosimetry measurements were made for a consecutive series of 30 patients using the two dosimetry systems.
Results: Energy dependent calibration factors were required for both dosimetry systems. The standard
uncertainty estimate for in vivo measurement with film was 1.7% and for beads was 1.5%. The mean measured dose was ý2.1% for film and ý2.6% for beads compared to prescription. Deviations up to ý9% were found in cases of large surface irregularity, or with underlying air cavities or bone. Dose shielding by beads could be clinically relevant at low kV energies and superficial depths.
Conclusions: Both film and beads may be used to provide in vivo verification of delivered doses in kV
radiotherapy, particularly for complex situations that are not well represented by standard reference condition
calculations
Round W, Jafari S, Kron T, Azhari H, Chhom S, Hu Y, Mauldon G, Cheung K, Kuppusamy T, Pawiro S, Lubis L, Soejoko D, Haryanto F, Endo M, Han Y, Suh T, Ng K, Luvsan-Ish A, Maung S, Chaurasia P, Jafri S, Farrukh S, Peralta A, Toh H, Sarasanandarajah S, Shiau A, Krisanachinda A, Suriyapee S, Vinijsorn S, Nguyen T (2015) Brief histories of medical physics in Asia-Oceania,Australasian Physical & Engineering Sciences in Medicine38(3)pp. 381-398 link.springer.com
The history of medical physics in Asia-Oceania goes back to the late nineteenth century when X-ray imaging was introduced, although medical physicists were not appointed until much later. Medical physics developed very quickly in some countries, but in others the socio-economic situation as such prevented it being established for many years. In others, the political situation and war has impeded its development. In many countries their medical physics history has not been well recorded and there is a danger that it will be lost to future generations. In this paper, brief histories of the development of medical physics in most countries in Asia-Oceania are presented by a large number of authors to serve as a record. The histories are necessarily brief; otherwise the paper would quickly turn into a book of hundreds of pages. The emphasis in each history as recorded here varies as the focus and culture of the countries as well as the length of their histories varies considerably.
Purpose: External dosimetry audit is valuable for the assurance of radiotherapy quality. However, motion
management has not been rigorously audited, despite its complexity and importance for accuracy. We describe the first end-to-end dosimetry audit for non-SABR (stereotactic ablative body radiotherapy) lung treatments, measuring dose accumulation in a moving target, and assessing adequacy of target dose coverage.
Materials and methods: A respiratory motion lung-phantom with custom-designed insert was used. Dose was measured with radiochromic film, employing triple-channel dosimetry and uncertainty reduction. The host?s 4DCT scan, outlining and planning techniques were used. Measurements with the phantom static and then moving at treatment delivery separated inherent treatment uncertainties from motion effects. Calculated and measured dose distributions were compared by isodose overlay, gamma analysis, and we introduce the concept of ?dose plane histograms? for clinically relevant interpretation of film dosimetry.
Results: 12 radiotherapy centres and 19 plans were audited: conformal, IMRT (intensity modulated radiotherapy) and VMAT (volumetric modulated radiotherapy). Excellent agreement between planned and
static-phantom results were seen (mean gamma pass 98.7% at 3% 2 mm). Dose blurring was evident in the moving-phantom measurements (mean gamma pass 88.2% at 3% 2 mm). Planning techniques for motion management were adequate to deliver the intended moving-target dose coverage.
Conclusions: A novel, clinically-relevant, end-to-end dosimetry audit of motion management strategies in
radiotherapy is reported.
Glass beads have recently been proposed for use as radiation therapy dosimeters. Glass beads have a number of characteristics that make them suitable for in vivo skin dose measurements, including an ability to be worn on a string, and therefore avoid possible patient discomfort that may result from the use of adhesives. In this study, their use for in vivo dose measurements in total skin electron irradiation treatments has been tested. First, the dosimetric properties of cylindrical beads with a 3 mm diameter were characterised using electron fields produced by a linear accelerator. The mean individual bead reproducibility was demonstrated to be within 3%; and a batch variation of 7% was observed. The beads were shown to have a linear dose response, and both dose rate and beam energy independence, within the measurement uncertainty. Phantom measurements were then
performed for a total skin electron irradiation beam arrangement, and results compared against optically
stimulated luminescent dosimeters at five anatomical sites. For a majority of measurement locations, agreement
within 3% was observed between the two dosimetry techniques, demonstrating the feasibility of glass beads as in vivo dosimeters for total skin electron irradiation; though further investigation may be needed to minimise uncertainty in results.
Examinations have been made of a low cost commercially available material which is potentially useful as a dosimeter in radiotherapy. An investigation of the thermoluminescent (TL) yield and electron paramagnetic resonance (EPR) signal was performed by irradiating acid-washed glass jewellery beads to MV photons using a medical linear-accelerator and 60Co gamma rays. For
comparison, irradiation exposures were also carried
out on 5 mm length of Ge-doped optical fibres that
have been widely investigated for their TL properties
[1].
The dose response was linear for the investigated
dose range of 1 to 2500 cGy, with an R2 correlation
coefficient of > 0.999 and reproducibility of 1.7%.
The results suggest the potential for use of glass
beads as TL dosimeters in radiotherapy.
While commercial jewellery glass beads offer the basis of novel radiotherapy TL dosimetry (Jafari et al.
2014a,b,c, 2015a,b), detailed study of TL variation is required for the products from various manufacturers.
Investigation is made for glass beads from four manufacturers from four countries: China (Rocaille), Japan (Mill Hill), Indonesia (TOHO") and Czech Republic (Czech). Sample composition was determined using an energy-dispersive X-ray unit coupled to a scanning electron microscope. Values of mass attenuation coefficient, ¼/Á, as a function of photon energy were then calculated for photons of energy 1 keV to 10 MeV, using the National Institute of Standards and Technology XCOM program. Radiation and energy response were determined using X-rays generated at accelerating potentials from 80 kVp to 6 MV (TPR20/10¼0.670).
All bead types showed TL to be linear with dose (R240.999). Glow curve dosimetric peaks reached a
maximum value at 300 °C for the Toho and 290 °C for the Czech and Mill Hill products but was between
200?250 °C for the Rocaille product. Radiation sensitivity following mass normalisation varied within an
order of magnitude; Toho samples showed the greatest and Rocaille the least sensitivity. For the Toho,
Czech, Rocaille and Mill Hill samples the energy responses at 80 kVp were 5.0, 4.0, 3.6 and 3.3 times that
obtained at 6 MV. All four glass bead types offer potential use as TL dosimeters over doses commonly
applied in radiotherapy. Energy response variation was o1% at 6 MV but significant variation was found
for photon beam energies covering the kV range; careful characterisation is required if use at this range is
intended.
Bradley D, Siti Shafiqah A, Siti Rozaila Z, Sabtu S, Abdul Sani S, Alanazi A, Jafari S, Amouzad Mahdiraji G, Mahamd Adikan F, Maah M, Nisbet A, Tamchek N, Abdul Rashid H, Alkhorayef M, Alzimami K (2016) Developments in production of silica-based thermoluminescence dosimeters,Radiation Physics and Chemistry137pp. 37-44 http://www.elsevier.com/
This work addresses purpose-made thermoluminescence dosimeters (TLD) based on doped silica fibres and sol?gel nanoparticles, produced via Modified Chemical Vapour Deposition (MCVD) and wet chemistry techniques respectively. These seek to improve upon the versatility offered by conventional phosphor-based TLD forms such as that of doped LiF. Fabrication and irradiation-dependent factors are seen to produce defects of differing origin, influencing the luminescence of the media. In coming to a close, we illustrate the utility of Ge-doped silica media for ionizing radiation dosimetry, first showing results from gamma-irradiated Ag-decorated nanoparticles, in the particular instance pointing to an extended dynamic range of dose. For the fibres, at radiotherapy dose levels, we show high spatial resolution (0.1 mm) depth-dose results for proton irradiations. For novel microstructured fibres (photonic crystal fibres, PCFs) we show first results from a study of undisturbed and technologically modified naturally occurring radioactivity environments, measuring doses of some 10 s of ¼Gy over a period of several months.
Abdul Sani S, Hammond R, Jafari S, Wahab N, Amouzad Mahdiraji G, Siti Shafiqah A, Abdul Rashid H, Maah M, Aldousari H, Alkhorayef M, Alzimami M, Bradley D (2016) Measurement of a wide-range of X-ray doses using specialty doped silica fibres,Radiation Physics and Chemistry137pp. 49-55 http://www.elsevier.com/
Using six types of tailor-made doped optical fibres, we carry out thermoluminescent (TL) studies of X-rays, investigating the TL yield for doses from 20 mGy through to 50 Gy. Dosimetric parameters were investigated for nominal 8 wt% Ge doped fibres that in two cases were co-doped, using B in one case and Br in the other. A comparative measurement of surface analysis has also been made for non-annealed and annealed capillary fibres, use being made of X-ray Photoelectron Spectroscopy (XPS) analysis. Comparison was made with the conventional TL phosphor LiF in the form of the proprietary product TLD-100, including dose response and glow curves investigated for X-rays generated at 60 kVp over a dose range
from 2 cGy to 50 Gy. The energy response of the fibres was also performed for X-rays generated at peak
accelerating potentials of 80 kVp, 140 kVp, 250 kVp and 6 MV photons for an absorbed dose of 2 Gy. Present results show the samples to be suitable for use as TL dosimeters, with good linearity of response and a simple glow curve (simple trap) distribution. It has been established that the TL performance of an irradiated fibre is not only influenced by radiation parameters such as energy, dose-rate and total dose but also the type of fibre.
Alalawi A, Jafari S, Najem M, Clark K, Nisbet A, Abolaban F, Hugtenburg R, Hussein M, Alzimami K, Spyrou N, Bradley D (2013) Comparison of Ge-doped TL optical fibres and glass beads with ion chamber and Gafchromic film for small field photon dosimetry,1st International Conference on Dosimetry and its Applications - Book of abstractspp. pp1-206 ?eské vysoké u
ení technické v Praze Fakulta jaderná a fyzikáln? in~enýrská
Small field (d 4 × 4 cm) photon radiotherapy treatments include intensity-modulated radiation therapy (IMRT)and stereotactic body radiation therapy (SBRT). These require small, high spatial resolution dosimeters of adequate dynamic range. In this study, field sizes of 1 cm ×
1 cm, 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm, and 10 cm × 10 cm have been investigated using commercially available silica-based fibres and glass beads (GB) as TL dosimeters and a Varian linear accelerator operating at 6, 10 and 15 MV. Ge-doped SiO2 fibres have previously been shown by this group to offer a viable system for use as dosimeters. The fibres and GB, offer good spatial resolution (<120 ¼m and
2 mm respectively), large dynamic dose range (with linearity from tens of mGy up to well in excess of many tens of Gy), a non-hygroscopic nature and low cost. The
main aim of this present work is to investigate the use of Ge-doped optical fibres and GBs as thermoluminescence dosimeters in small photon fields for different photon beam energies, comparing the measurements against Gafchromic films, hospital commissioning data obtained from small
ionisation chambers and photon diodes and Monte Carlo simulations with FLUKA and BEAMnrc.
Purpose/Objective: To investigate the feasibility of using glass beads as a novel thermoluminescence dosimeter (TLD) in clinical proton radiotherapy. The glass beads have several physical characteristics which suggest their suitability for use as TLDs in this area: a spherical physical shape with a hole in the middle that facilitate their use in 2D and 3D arrangements; chemically inert nature; small size of 1.5 mm diameter and 1 mm thickness; inexpensive and readily available; reusable; and importantly their TL light transparency with negligible self-attenuation which is very important for high LET beams. Proton beams have high LET and therefore can deposit dose nonuniformly across a detector. Readout of TL detectors can be influenced if any opacity is present causing self-attenuation of TL light [1], [2]. The transparency of glass beads to TL light means the beads have the potential to avoid such issues.
Purpose/Objective: To examine whether the impact of respiratory motion on the delivered dose distributions for a range of tumour sizes in SABR, differs between 3DCRT,sliding window IMRT and RapidArc.
Materials and Methods: Three patient data sets with different lung tumour sizes were selected: T1=6cc, T2=31cc and T3=60cc. Each was planned in Eclipse for SABR using 3DCRT, sliding window IMRT and VMAT,
creating 9 treatment plans which were then delivered to a dynamic thorax phantom. The phantom was programmed to move at a typical patient breathing amplitude of 15mm with a period of 5 seconds and
Varian linacs were used for the delivery. EBT3 Gafchromic film was used in coronal and sagittal planes for measuring dose distributions. Static phantom measurements were compared with the TPS calculated plans to
establish agreement between expected and measured dose distributions without motion, using the software OmniPro I'mRT. Comparisons of static and dynamic phantom measurements followed. Global gamma analysis
was used to carry out a relative comparison between the three delivery techniques. Five regions of the gamma index map (Middle, Proximal Left, Proximal Right, Distal Left, Distal Right) were analysed to quantify the
differences along the axis of motion. The criteria used for the gamma analysis were 3%/3mm, with a threshold of 20%.
Results: The setup and delivery accuracy was confirmed by the agreement between planned and static delivered dose distributions. The average percentage of pixels passing was 100% (T1),100% (T2) and 98.46%
(T3). The comparison of films with and without motion gave lower percentages of pixels passing, ranging between 33.68 - 59.94% (T1), 47.86 - 61.77% (T2) and 43.44 - 64.32% (T3). Comparison of the delivery
technique, showed passing rates of 33.63 - 52.25% (3DCRT), 43.44 - 64.52% (IMRT) and 46.58- 56.08% (VMAT). Analysis of the five regions for all delivery techniques gave averages of 93.76% (Middle), 58.7%
(Proximal) and 12.8% (Distal). For 3DCRT results were 87.08% (Middle), 46.52% (Proximal) and 12.54% (Distal), for IMRT were 96.45%, 69.20%, 14.14% and for VMAT 97.75%, 60.39% and 11.71%, respectively.
Conclusions: The results are indicative of the intra-fractional respiratory motion-induced dosimetric inaccuracies caused in three SABR delivery techniques.On average, the impact is greatest in the distal regions,
significant in the proximal regions, whereas the middle region is less susceptible to these effects. No noticeable difference was observedbetween coronal and sagittal planes. The results also suggest that the effect of motion is greater in the proximal regions for 3DCRT in relation to the other techniques, particularly with smaller tumour sizes.
Purpose/Objective:
The UK SABR Lung Consortium dose audit
was designed to assess the positional and dosimetric accuracy of SABR lung treatment delivery. The audit has been carried out in 21 radiotherapy centres between October 2013 and July 2014 in order to provide an independent check of safe implementation and to identify problems in the modelling and delivery of SABR lung treatment.
Materials and Methods:
A mail based audit using EBT3
GafChromic film and alanine dosimeters was designed. A CIRS
Model 002LFC anthropomorphic thorax phantom which
contained 9 adjacent alanine pellets in the tip of a Farmer
chamber shaped insert was scanned, structure sets for the
ITV and alanine pellets were pre-delineated, and was sent to
radiotherapy centres to be loaded into their treatment
planning system. Each centre used this CT scan set to create
a SABR plan using their current planning protocol (including
dose, fractionation and coverage) and technique. The centres
used their own margin to create the PTV. A range of delivery
techniques were used including conformal, VMAT and
Cyberknife and calculated using local algorithms (AAA,
Collapsed Cone, Monte Carlo and Pencil beam). The doses
determined by the alanine dosimeters were compared to
expected doses determined by treatment plan system (TPS)
calculation, film and local ionisation chamber measurements.
Results:
The mean % difference between the alanine
measured doses, the TPS calculated doses, and the local
chamber measurements found to be within 2% (1 SD) as given
in table 1. As shown, alanine findings were supported by the
film results.
Purpose/Objective:
The UK SABR Consortium QA group
conducted a postal dosimetry audit of SABR lung plans at 21
UK centres. The purpose of this was to verify the accuracy of
calculated dose distributions, improve confidence of centres
in the early stages of implementing lung SABR and to
establish a benchmark QA method. Here the results of the
GafChromic film relative dosimetry arm of the audit are
given.
Materials and Methods:
Individual centres were asked to plan
a treatment to a pre-defined PTV in the CIRS Thorax phantom, using their clinical method and prescription dose.
EBT3 GafChromic film was used to measure an axial plane of
dose. Pins in the phantom facilitated alignment of the film
and calculated dose planes. Gantry linac and Cyberknife
centres were audited, using a variety of TPS with pencil
beam, AAA, CCC, Acuros and Monte Carlo algorithms.
Scanned films were compared to dose distributions calculated
by the individual centres, using single red-channel dosimetry
and a purpose-built Matlab application. Centres were also
asked to irradiate additional calibration films to provide
output-normalised optical density to dose calibration.
Measured and calculated isodoses corresponding to 120, 100,
70 and 50% of prescription dose were compared (figure 1),
and conformity and maximum distance to agreement were
measured. For the areas bound by the 100, 50 and 30%
calculated isodoses, local gamma analysis, mean gamma and
gamma pass rate (at 3%, 2mm) and a mean dose comparison
was performed. The latter was compared to the alanine
dosimetry results.
Results:
The dosimetry of the calibration films was
reproducible to ±0.9% (1.S.D), for doses ranging from 4.3 to
26.9 Gy.
The audit relative dosimetry results are reported in table 1.
Mean dose differences within the 100% calculated isodose
line agreed well with alanine dosimetry; -0.1 ± 2.0 % (1.S.D).
Gamma pass rates (%) and mean gamma results varied with
some outlying measurements, mostly caused by small dose
deviations within the PTV or at low doses. Isodose line
agreement (figure 1) was generally much closer at the 70 and
100% dose levels, indicated by the lower S.D. (table 1,
column 5). The exception was the centre using a pencil beam
algorithm, where the measured prescription dose covered a
significantly smaller area than that calculated, consistent
with the algorithm?s known limitations calculating dose in
low density lung surrounding tumour.
Conclusions:
Of the 21 UK centres audited, 74% of
measurements were within ±3% agreement compared to
calculated doses. Where appropriate, outlying centres have
been offered support from the QA Group to bring their results
into line. The EBT3 GafChromic film was found to be highly suited to a
postal audit, reliably giving detailed information about the
geometric and dosimetric accuracy of treatment.
Purpose or Objective
kV radiotherapy continues to be an important modality in
modern radiotherapy, but has received less research
attention in recent years. There remains a challenge to
accurately calculate and verify treatment dose
distributions for clinical sites with significant surface
irregularity or where the treated region contains
inhomogeneities, e.g. nose and ear. The accuracy of
current treatment calculations has a significant level of
uncertainty [1, 2]. The objective of this work was to
characterise two novel detectors, micro-silica bead TLDs
and Gafchromic EBT3 film, for in-vivo measurements for
kV treatments, and to compare measured doses with
conventional treatment calculations.
[1. Currie (2007) Australas Phys Eng Sci Med, 2. Chow
(2012) Rep Pract Oncol Radiother.]
Material and Methods
Micro-silica bead TLDs (1 mm diam.) and Gafchromic EBT3
film were calibrated against an NPL traceably calibrated
ionisation chamber using an Xstrahl D3300 kV radiotherapy
treatment unit. Energy response was evaluated over 70 to
250 kV and compared to 6 MV, useable dose range was
evaluated from 0 to 25 Gy, and uncertainty budgets
determined. Silica beads were cleaned, annealed, and TL
response individually calibrated. EBT3 film was used with
triple-channel dosimetry via FilmQAPro® with procedures
to reduce uncertainties. Commissioning tests were
undertaken in standard conditions using Solid Water blocks
and in simulated clinical treatment condition using a
custom made ?wax face with nose? phantom. Pilot in vivo
measurements were made for a consecutive series of eight
clinical patient treatments, including cheek, ear, nose and
rib sites, over 70 to 250 kV, and 4 to 18 Gy. Results for the
two dosimetry systems were compared to conventional
treatment planning calculations.
Results
Energy response varied by 460% for beads and 9% for film,
from 70 kV to 6 MV, necessitating energy-specific
calibration. Both dosimeters were useable up to 25 Gy.
Standard uncertainty was 3.1% for beads, 2.1% for film.
The figure shows typical film and bead positions within the
lead cut-out of a kV treatment to the cheek. The table
provides calculated and measured doses. Average
deviation over 6 patients was -1.3% for beads, -0.9% for
film. 3 patients had larger deviations; See table note 1:
tumour sitting over the maxillary sinus may reduce dose.
Note 2: beads placed along surface of tumour into ear,
most distal bead received dose -17.5% from prescription,
doctor made compensation. Note 3: Increased uncertainty
due to curved surface, film required offset to corner as
patient sensitive to contact. Note 4: Uncertainty
increased due to large respiratory motion at treatment
site.
Conclusion
Both micro-silica bead TLDs and EBT3 film were
characterised as suitable for in vivo dosimetry in kV
radiotherapy, providing assurance of delivered doses. Film
is simpler to prepare, use and read. A line of beads allows
conformation to irregular anatomy across the field. A
clinical service is now available to verify dose delivery in
complex clinical sites.
Purpose or Objective
To develop a novel high resolution experimental method
for validating Monte Carlo-derived TG-43 brachytherapy
source data, and model-based treatment planning
systems. Experimental verification is recommended in
the ?Report of the High Energy Brachytherapy Source
Dosimetry (HEBD) Working Group, 2012?, however, the
steep dose gradients with a wide dynamic dose range and
rapid change in dose rate has been a limitation for the
ability of common detectors to obtain accurate
measurements. In this study, we tested a micro silica
bead TLDs recently developed at University of Surrey for
suitability to perform accurate dose measurements
around 60Co and 192Ir clinical HDR brachytherapy sources.
The micro silica beads proven dosimetric characteristics
(independency from dose rate and angle of radiation
incidence) accompanied by small ?donut shape? physical
dimensions (1.2 mm diameter and 0.9 mm thickness (Fig.
1 (a)) along with chemically inert nature, ease of use and
reusability were considered as a very promising detectors
for this application.
Material and Methods
Novel dosimeter positioning templates were designed and
produced using AutoCAD software. The micro silica bead
TLDs were threaded using cotton yarns and stitched onto
the template to accurately position the dosimeters within
± 0.1 mm, in a full-scatter water tank (Fig 1(b) and 1(c)).
Measurement setup for radial dose distribution and dose
linearity is shown on (Fig 1(c)) and (Fig 1(b)). The used
dose rates were form 10 to 4000 cGy/min and dose
ranged from 0.5 to 40 Gy. The results of dose distribution
measurements around the sources were compared to TG-
43 tabulated data and simultaneously irradiated EBT3
Gafchromic film. A TOLADO TL system was employed for
read out of the TLDs. Triple-channel dosimetry using
FilmQAPro with uncertainty reduction technique was used
for film dosimetry
Results
The novel experimental method suitably addressed the
dosimetry challenges. A linear dose response was
observed in the investigated range, 0.5?40 Gy, with a
correlation coefficient of R2 > 0.999. The ability of
detector to assess the high gradient dose distribution
with variable dose rate within the range of 10?4000
cGy/min around the sources was compared to the TG-43
data to that of EBT3 film and found to be within
experimental uncertainty (Fig 2 (a) and (b)).
Conclusion
A novel, high spatial resolution experimental method was
developed for validating brachytherapy dosimetry using
micro silica bead TLDs on high precision templates. The
measured radial dose distributions around both of the
60Co and 192Ir sources were comparable within the
experimental uncertainty to the relevant TG-43 data and
superior to that of EBT3 Gafchromic film measurement in
terms of the dynamic dose range evaluated. The
experimental method presented is suitable to address the
challenge of HDR brachytherapy dosimetry.
Jafari Shakardokht, Distefano G., Lee J., Gouldstone C., Mayles H., Bradley D. A., Spyrou N. M., Nisbet A., Clark C. H. (2015) Feasibility of using glass bead TLDs for a postal dosimetry audit of MV radiotherapy photon beams,Proceedings of the 8th UK Radiation Oncology Conference (UKRO) Varian Medical Systems, Inc.
Stereotactic radiosurgery (SRS), a non-invasive therapeutic technique, seeks delivery of elevated doses of ionizing radiation to precisely defined targets while at the same time preserving surrounding tissue viability. SRS was developed for treatment of various functional abnormalities, extending also to benign and malignant lesions (the latter sometimes referred to as stereotactic body radiation therapy, SBRT). Local tumour control for single and multiple brain metastases at low complication rates is one such outcome. Notable commercial SRS platforms include Gamma Knife and the linac-based systems, Novalis and Cyberknife. Such systems use imaging techniques that include computed tomography (CT) and magnetic resonance imaging (MRI) in localizing SRS targets, down to a small fraction of one mm. With a wide range of platforms for delivery of SRS, greater investigation and standardization is called for. Present work concerns a multi-centre dosimetric audit (20 centres in all), investigating the range of SRS machines and techniques for a single brain metastasis using a series of small dimension detectors (1.55/mm and less) and an anthropomorphic head phantom. With the lens as one of the more radiosensitive tissues, the aim has been to determine the scattered radiation lens dose received during an SRS treatment, as well as the imaging dose received during planning-stage CT-scanning. Custom-designed holders were fabricated to carry three types of thermoluminescence dosimeters: Ge-doped silica fibres, silica glass beads and TLD-100, the latter as a reference dosimeter (being also of larger dimension than the silica-based dosimeters). For reproducible placement of the TLD holders, a bespoke 3D-printed goggle insert was produced for the head phantom. International guidance is to seek reduction in lens dose down to 0.5/Gy. Present results show lens dose values below 0.5/Gy, albeit sometimes to modest degree, there being need to continue to exercise associated due care in SRS planning and delivery.
It is widely known that SiPM photodetectors have a strong temperature dependence. TrueInvivo is currently developing an automated TLD reader with the intention of replacing the PMT with a SiPM, taking advantage of the low power consumption, small form-factor, and low cost the SiPM has to offer. Here we discuss our initial investigations into the suitability of this switch in applied technology, present spectroscopic response as a function of SiPM temperature, and the effectiveness of some options to mitigate this concern. It was found that while the SiPM is indeed affected by variations in temperature a recovery of 50-60% in gain and resolution could be achieved with a rudimentary cooling system.
