Total skin electron irradiation (TSEI) is a special radiotherapy technique which has generally been used for treating adult patients with mycosis fungoides. Recently, two infants presented with leukemia cutis isolated to the skin requiring TSEI. This work discusses the commissioning and quality assurance (QA) methods for implementing a modified Stanford technique using a rotating harness system to position sedated pediatric patients treated with electrons to the total skin.
Methods and Results
Commissioning of pediatric TSEI consisted of absolute calibration, measurement of dosimetric parameters, and subsequent verification in a pediatric patient sized cylindrical phantom using radiographic film and optically stimulated luminance (OSL) dosimeters. The depth of dose penetration under TSEI treatment condition was evaluated using radiographic film sandwiched in the phantom and demonstrated a 2 cm penetration depth with the maximum dose located at the phantom surface. Dosimetry measurements on the cylindrical phantom and in-vivo measurements from the patients suggested that, the factor relating the skin and calibration point doses (i.e., the B-factor) was larger for the pediatric TSEI treatments as compared to adult TSEI treatments. Custom made equipment, including a rotating plate and harness, was fabricated and added to a standard total body irradiation stand and tested to facilitate patient setup under sedated condition. A pediatric TSEI QA program, consisting of daily output, energy, flatness, and symmetry measurements as well as in-vivo dosimetry verification for the first cycle was developed. With a long interval between pediatric TSEI cases, absolute dosimetry was also repeated as part of the QA program. In-vivo dosimetry for the first two infants showed that a dose of ± 10% of the prescription dose can be achieved over the entire patient body.
Though pediatric leukemia cutis and the subsequent need for TSEI are rare, the ability to commission the technique on a modified TBI stand is appealing for clinical implementation and has been successfully used for the treatment of two pediatric patients at our institution.
Pediatric total skin electron irradiation; Commissioning; Quality assurance; Leukemia cutis
An anthropomorphic phantom was used to investigate a treatment technique and analyze the dose distributions for helical irradiation of the total skin (HITS) by helical tomotherapy (HT). Hypothetical bolus of thicknesses of 0, 10, and 15 mm was added around the phantom body to account for the dose homogeneity and setup uncertainty. A central core structure was assigned as a “complete block” to force the dose tangential delivery. HITS technique with prescribed dose (Dp) of 36 Gy in 36 fractions was generated. The radiochromic EBT2 films were used for the dose measurements. The target region with 95.0% of the Dp received by more than 95% of the PTV was obtained. The calculated mean doses for the organs at risk (OARs) were 4.69, 3.10, 3.20, and 2.94 Gy for the lung, heart, liver, and kidneys, respectively. The measurement doses on a phantom surface for a plan with 10 mm hypothetical bolus and bolus thicknesses of 0, 1, 2, and 3 mm are 89.5%, 111.4%, 116.9%, and 117.7% of Dp, respectively. HITS can provide an accurate and uniform treatment dose in the skin with limited doses to OARs and is safe to replace a total skin electron beam regimen.
Ionizing radiation in the form of x-ray therapy is the best modality of treatment available at the present time for single, isolated lesions of mycosis fungoides. However, for generalized mycosis fungoides, generalized x-ray therapy is technically difficult and dangerous. It is now possible to employ electron beam therapy for generalized mycosis fungoides, using energies which confine the dose to the superficial layers of the skin and thus avoid hematopoietic injury. A technique for wide field electron beam therapy has been developed for this purpose which has been effective and well tolerated in limited trials to date.
To analyse limits and capabilities in dose calculation of collapsed-cone-convolution (CCC) algorithm implemented in helical tomotherapy (HT) treatment planning system for thorax lesions.
The agreement between measured and calculated dose was verified both in homogeneous (Cheese Phantom) and in a custom-made inhomogeneous phantom. The inhomogeneous phantom was employed to mimic a patient's thorax region with lung density encountered in extreme cases and acrylic inserts of various dimensions and positions inside the lung cavity. For both phantoms, different lung treatment plans (single or multiple metastases and targets in the mediastinum) using HT technique were simulated and verified. Point and planar dose measurements, both with radiographic extended-dose-range (EDR2) and radiochromic external-beam-therapy (EBT2) films, were performed. Absolute point dose measurements, dose profile comparisons and quantitative analysis of gamma function distributions were analyzed.
An excellent agreement between measured and calculated dose distributions was found in homogeneous media, both for point and planar dose measurements. Absolute dose deviations <3% were found for all considered measurement points, both inside the PTV and in critical structures. Very good results were also found for planar dose distribution comparisons, where at least 96% of all points satisfied the gamma acceptance criteria (3%-3 mm), both for EDR2 and for EBT2 films. Acceptable results were also reported for the inhomogeneous phantom. Similar point dose deviations were found with slightly worse agreement for the planar dose distribution comparison: 96% of all points passed the gamma analysis test with acceptable levels of 4%-4 mm and 5%-4 mm, for EDR2 and EBT2 films respectively. Lower accuracy was observed in high dose/low density regions, where CCC seems to overestimate the measured dose around 4-5%.
Very acceptable accuracy was found for complex lung treatment plans calculated with CCC algorithm implemented in the tomotherapy TPS even in the heterogeneous phantom with very low lung-density.
This paper describes the problems and solutions in using 18 MeV linear accelerator, with minimum 6 MeV electron capability, for total skin irradiation for mycosis fungoides. The 6 MeV electron energy can be degraded to acceptable electron energy of 3.2 MeV by interposing a plexiglass sheet of 9.6 mm in the beam. To minimize the bremsstrahlung, the degrading plexiglass should be kept away from the machine head. A wide area with uniform dose distribution over single plane can be achieved by using dual fields but homogenous dose distribution over irregular body surface cannot be achieved mainly because of self-shielding. The nails and the ocular lens can be easily shielded from the low energy electrons with 1.5 mm lead shield.
To characterize the dose distribution for a range of cone beam CT (CBCT) units, investigating different field of view sizes, central and off-axis geometries, full or partial rotations of the X-ray tube and different clinically applied beam qualities. The implications of the dose distributions on the definition and practicality of a CBCT dose index were assessed.
Dose measurements on CBCT devices were performed by scanning cylindrical head-size water and polymethyl methacrylate phantoms, using thermoluminescent dosemeters, a small-volume ion chamber and radiochromic films.
It was found that the dose distribution can be asymmetrical for dental CBCT exposures throughout a homogeneous phantom, owing to an asymmetrical positioning of the isocentre and/or partial rotation of the X-ray source. Furthermore, the scatter tail along the z-axis was found to have a distinct shape, generally resulting in a strong drop (90%) in absorbed dose outside the primary beam.
There is no optimal dose index available owing to the complicated exposure geometry of CBCT and the practical aspects of quality control measurements. Practical validation of different possible dose indices is needed, as well as the definition of conversion factors to patient dose.
cone beam computed tomography; radiation dosimetry; thermoluminescent dosimetry; quality control
Estimating organ residence times is an essential part of patient-specific dosimetry for radioimmunotherapy (RIT). Quantitative imaging methods for RIT are often evaluated using a single physical or simulated phantom but are intended to be applied clinically where there is variability in patient anatomy, biodistribution, and biokinetics. To provide a more relevant evaluation, the authors have thus developed a population of phantoms with realistic variations in these factors and applied it to the evaluation of quantitative imaging methods both to find the best method and to demonstrate the effects of these variations. Using whole body scans and SPECT/CT images, organ shapes and time-activity curves of 111In ibritumomab tiuxetan were measured in dosimetrically important organs in seven patients undergoing a high dose therapy regimen. Based on these measurements, we created a 3D NURBS-based cardiac-torso (NCAT)-based phantom population. SPECT and planar data at realistic count levels were then simulated using previously validated Monte Carlo simulation tools. The projections from the population were used to evaluate the accuracy and variation in accuracy of residence time estimation methods that used a time series of SPECT and planar scans. Quantitative SPECT (QSPECT) reconstruction methods were used that compensated for attenuation, scatter, and the collimator-detector response. Planar images were processed with a conventional (CPlanar) method that used geometric mean attenuation and triple-energy window scatter compensation and a quantitative planar (QPlanar) processing method that used model-based compensation for image degrading effects. Residence times were estimated from activity estimates made at each of five time points. The authors also evaluated hybrid methods that used CPlanar or QPlanar time-activity curves rescaled to the activity estimated from a single QSPECT image. The methods were evaluated in terms of mean relative error and standard deviation of the relative error in the residence time estimates taken over the phantom population. The mean errors in the residence time estimates over all the organs were <9.9% (pure QSPECT), <13.2% (pure QPLanar), <7.2% (hybrid QPlanar/QSPECT), <19.2% (hybrid CPlanar/QSPECT), and 7%–159% (pure CPlanar). The standard deviations of the errors for all the organs over all the phantoms were <9.9%, <11.9%, <10.8%, <22.0%, and <107.9% for the same methods, respectively. The processing methods differed both in terms of their average accuracy and the variation of the accuracy over the population of phantoms, thus demonstrating the importance of using a phantom population in evaluating quantitative imaging methods. Hybrid CPlanar/QSPECT provided improved accuracy compared to pure CPlanar and required the addition of only a single SPECT acquisition. The QPlanar or hybrid QPlanar/QSPECT methods had mean errors and standard deviations of errors that approached those of pure QSPECT while providing simplified image acquisition protocols, and thus may be more clinically practical.
absolute quantitation; phantom population; quantitative SPECT; radioimmunotherapy
The aim of this study was to assess the characteristics of an optically stimulated luminescence dosemeter (OSLD) for use in diagnostic radiology and to apply the OSLD in measuring the organ doses by panoramic radiography.
The dose linearity, energy dependency and angular dependency of aluminium oxide-based OSLDs were examined using an X-ray generator to simulate various exposure settings in diagnostic radiology. The organ doses were then measured by inserting the dosemeters into an anthropomorphic phantom while using three panoramic machines.
The dosemeters demonstrated consistent dose linearity (coefficient of variation<1.5%) and no significant energy dependency (coefficient of variation<1.5%) under the applied exposure conditions. They also exhibited negligible angular dependency (≤10%). The organ doses of the X-ray as a result of panoramic imaging by three machines were calculated using the dosemeters.
OSLDs can be utilized to measure the organ doses in diagnostic radiology. The availability of these dosemeters in strip form proves to be reliably advantageous.
optically stimulated luminescence dosemeter; dosimetry; organ dose; diagnostic radiology
Because of the adverse effects of ionizing radiation on fetuses, prior to radiotherapy of pregnant patients, fetal dose should be estimated. Fetal dose has been studied by several authors in different depths in phantoms with various abdomen thicknesses (ATs). In this study, the effect of maternal AT and depth in fetal dosimetry was investigated, using peripheral dose (PD) distribution evaluations. A BEAMnrc model of Oncor linac using out of beam components was used for dose calculations in out of field border. A 6 MV photon beam was used to irradiate a chest phantom. Measurements were done using EBT2 radiochromic film in a RW3 phantom as abdomen. The followings were measured for different ATs: Depth PD profiles at two distances from the field's edge, and in-plane PD profiles at two depths. The results of this study show that PD is depth dependent near the field's edge. The increase in AT does not change PD depth of maximum and its distribution as a function of distance from the field's edge. It is concluded that estimating the maximum fetal dose, using a flat phantom, i.e., without taking into account the AT, is possible. Furthermore, an in-plane profile measured at any depth can represent the dose variation as a function of distance. However, in order to estimate the maximum PD the depth of Dmax in out of field should be used for in-plane profile measurement.
Fetal dosimetry; Monte Carlo; peripheral dose; radiotherapy
It is well accepted that collimation is a cost-effective dose-reducing tool for X-ray examinations. This phantom-based study investigated the impact of X-ray beam collimation on radiation dose to the lenses of the eyes and thyroid along with the effect on image quality in facial bone radiography.
A three-view series (occipitomental, occipitomental 30 and lateral) was investigated, and radiation doses to the lenses and thyroid were measured using an Unfors dosemeter. Images were assessed by six experienced observers using a visual grading analysis and a total of 5400 observations were made.
Strict collimation significantly (p<0.0001) reduced the radiation dose to the lenses of the eyes and thyroid when using a fixed projection-specific exposure. With a variable exposure technique (fixed exit dose, to simulate the behaviour of an automatic exposure control), while strict collimation was again shown to reduce thyroid dose, higher lens doses were demonstrated when compared with larger fields of exposure. Image quality was found to significantly improve using strict collimation, with observer preference being demonstrated using visual grading characteristic curves.
The complexities of optimising radiographic techniques have been shown and the data presented emphasise the importance of examining dose-reducing strategies in a comprehensive way.
The purpose of this study is to develop a total body irradiation technique that does not require additional devices or sophisticated processes to overcome the space limitation of a small treatment room. The technique aims to deliver a uniform dose to the entire body while keeping the lung dose within the tolerance level. The technique treats the patient lying on the floor anteriorly and posteriorly. For each AP/PA treatment, two complementary fields with dynamic field edges are matched over an overlapped region defined by the marks on the body surface. A compensator, a spoiler, and lung shielding blocks were used during the treatment. Moreover, electron beams were used to further boost the chest wall around the lungs. The technique was validated in a RANDO phantom using GAFCHROMIC films. Dose ratios at different body sites along the midline ranged from 0.945 to 1.076. The dose variation in the AP direction ranged from 96.0% to 104.6%. The dose distribution in the overlapped region ranged from 98.5% to 102.8%. Lateral dose profiles at abdomen and head revealed 109.8% and 111.7% high doses, respectively, at the body edges. The results confirmed that the technique is capable of delivering a uniform dose distribution to the midline of the body in a small treatment room while keeping the lung dose within the tolerance level.
The central axis absorbed dose data for x-ray and electron beams generated by linear accelerators in the energy range 4 thru 25 MV are being compiled. The compilation includes specific x-ray beam parameters (surface doses, output factors, percent depth doses, tissue-phantom ratios, and wedge factors) as well as electron beam parameters (percent depth doses and output factors). The compilation includes published data sets of these parameters and those obtained directly from over 100 institutions participating in the study.
The data are grouped by accelerator model and input into computer files that provide a standard format suitable for intercomparisons. The software used to construct the computer files and to manipulate the data is discussed. Selected examples of the average values of parameters obtained to date with the standard deviations, the coefficients of variation, and the maximum and minimum values will be presented for several different linear accelerator models.
In treating mycosis fungoides (MF) and Sezary syndrome patients with electron beam, the entire thickness and the area of the skin from crown to sole should be irradiated uniformly. To achieve irradiation of the entire thickness of the skin, electron beams of 3 - 4 MeV energy with 80 percent depth dose at 6 mm is sufficient. This unique property of limited penetration of electron beam does not cause any systemic toxicity during or after total body electron therapy. However, this property of limited penetration of electrons poses the problem of self-shielding in the curvaceous human body. The optic lens, which is within the range of penetrability of electron beam energy used for total body electron therapy, is to be shielded artificially.
The purpose of this paper is to discuss the problems of self and artificial shielding in the superficial total body electron therapy for MF and Sezary syndrome.
Cutaneous T-cell lymphoma is typically a clonal neoplasm of epidermotropic CD4+ T-lymphocytes that includes the entity mycosis fungoides (MF). After identification of patients with recurrent MF treated with total skin electron beam therapy (TSEBT) at the Yale University School of Medicine, this study attempted to compare T-cell receptor (TCR) gamma gene rearrangements via polymerase chain reaction (PCR) in both original and recurrent skin biopsies from these patients. Between 1974 and 1996, a total of 95 T2 MF patients were treated with TSEB, and four of these were identified for the study. Slides and tissue samples of both primary and recurrent skin biopsies for each patient were confirmed as being consistent with ME DNA for PCR was isolated from paraffin-embedded tissue samples. Using consensus primers that hybridize with conserved regions of the TCR gene, these regions of the genome were amplified. The PCR products were then analyzed by acrylamide gel electrophoresis. Of the primary and recurrent samples from four patients with a median disease-free interval (DFI) of 1222 days, only two showed evidence of a dominant TCR clone. A number of factors, including lack of sequence homology between the primers and the gene segments, the existence of multiple neoplastic cell lines, DNA degradation in the archival samples, and the presence of reactive as well as malignant lymphocytes, may have prevented the detection of dominant TCR rearranged clones in the samples. Despite the results of this study, TCR analysis via PCR and gel electrophoresis continues to be of utility in the evaluation of patients with MF when used in conjunction with other diagnostic modalities and in cases with nonspecific clinical, histopathological, and immunophenotyping findings.
Intravascular brachytherapy (IVBT) is a useful treatment modality for the recurrence of in-stent restenosis following drug-eluting stents (DES) or IVBT failure. The objective of this study was to measure the dose rate of 90Sr/90Y IVBT sources for comparison with that given by the manufacturer and to control the dose uniformities of these sources along the source axis. The dose rates of 90Sr/90Y beta sources were measured with a radiochromic film in a custom-made phantom. The films for calibration were irradiated using 60Co photon beams. The results for the three sources were 4.5%, 2.3%, and 3.5% higher than the corresponding certificate values. Maximum and minimum of the dose rates varied within ±10% of those at source center; and maximum dose discrepancy for the first 90Sr/90Y source train was 8.2%; for the second source train, 7.1%; and for the third source train, 5.1%. Our study showed that the dose rates given by the manufacturer for the three 90Sr/90Y IVBT sources were reliable and dose uniformities were within ±10% along two thirds of the treatment length.
90Sr/90Y; intravascular brachytherapy; radiochromic film dosimetry
Irradiation of whole blood and blood components before transfusion is currently the only accepted method to prevent Transfusion-Associated Graft-Versus-Host-Disease (TA-GVHD). However, choosing the appropriate technique to determine the dosimetric parameters associated with blood irradiation remains an issue. We propose a dosimetric system based on the standard Fricke Xylenol Gel (FXG) dosimeter and an appropriate phantom. The modified dosimeter was previously calibrated using a 60Co teletherapy unit and its validation was accomplished with a 137Cs blood irradiator. An ionization chamber, standard FXG, radiochromic film and thermoluminescent dosimeters (TLDs) were used as reference dosimeters to determine the dose response and dose rate of the 60Co unit. The dose distributions in a blood irradiator were determined with the modified FXG, the radiochromic film, and measurements by TLD dosimeters. A linear response for absorbed doses up to 54 Gy was obtained with our system. Additionally, the dose rate uncertainties carried out with gel dosimetry were lower than 5% and differences lower than 4% were noted when the absorbed dose responses were compared with ionization chamber, film and TLDs.
Beam-on time in Total Marrow Irradiation (TMI) delivery with helical tomotherapy is more than 30 minutes. The purpose of this study was to investigate extended time output variation in tomotherapy machine without dose servo system and its impact on the dosimetry of TMI planning.
Materials and methods
The calibration procedures with 1800 seconds delivery were conducted. The slab and cylindrical phantoms were used for static and rotational output variation measurements, respectively. All measurements were performed in 0.1 second interval with an Exradin A1SL ionization chamber (Standard Imaging Inc., Madison, WI, USA) connected to the tomoelectrometer supplied by the manufacture. Simulated TMI treatment planning with a slab phantom was delivered and verified with ion chamber and EDR-2 films.
The static output variations during 30 min averaged −2.9% ± 0.2%, -3.4% ± 0.3%, and −3.4% ± 0.3% at 10 min, 20 min, and 30 min, respectively. The rotational output variations from start averaged −2.5% ± 0.7%, -3.1% ± 0.7%, and −3.5% ± 0.8% at 10 min, 20 min, and 30 min, respectively. The maximum output variation was up to 4.5%. In a TMI planning model, in which beam-on time was over 30 min, planned dose and dose measured with ion chambers in both cranial and caudal sides agreed within 3%. Film measurements in cranial and caudal sides also showed the pass rates of 97.7% and 92.2% with the criteria of 3 mm/3% in gamma analysis.
These results suggest that long TMI delivery by helical tomotherapy, even without dose servo system, does not pose a risk for significant deviations from the original treatment plan regardless of the output variation. However, very long time output variation should be checked before the first treatment.
Radiation therapy; Tomotherapy; Total marrow irradiation; Total body irradiation; Very long time output variation
Independent external audits play an important role in quality assurance programme in radiation oncology. The audit supported by the IAEA in Serbia was designed to review the whole chain of activities in 3D conformal radiotherapy (3D-CRT) workflow, from patient data acquisition to treatment planning and dose delivery. The audit was based on the IAEA recommendations and focused on dosimetry part of the treatment planning and delivery processes.
The audit was conducted in three radiotherapy departments of Serbia. An anthropomorphic phantom was scanned with a computed tomography unit (CT) and treatment plans for eight different test cases involving various beam configurations suggested by the IAEA were prepared on local treatment planning systems (TPSs). The phantom was irradiated following the treatment plans for these test cases and doses in specific points were measured with an ionization chamber. The differences between the measured and calculated doses were reported.
The measurements were conducted for different photon beam energies and TPS calculation algorithms. The deviation between the measured and calculated values for all test cases made with advanced algorithms were within the agreement criteria, while the larger deviations were observed for simpler algorithms. The number of measurements with results outside the agreement criteria increased with the increase of the beam energy and decreased with TPS calculation algorithm sophistication. Also, a few errors in the basic dosimetry data in TPS were detected and corrected.
The audit helped the users to better understand the operational features and limitations of their TPSs and resulted in increased confidence in dose calculation accuracy using TPSs. The audit results indicated the shortcomings of simpler algorithms for the test cases performed and, therefore the transition to more advanced algorithms is highly desirable.
Treatment planning systems; Quality assurance; Dose calculation algorithms
This article summarizes the evolution of microwave array applicators for heating large area chestwall disease as an adjuvant to external beam radiation, systemic chemotherapy, and potentially simultaneous brachytherapy.
Current devices used for thermotherapy of chestwall recurrence are reviewed. The largest conformal array applicator to date is evaluated in four studies: i) ability to conform to the torso is demonstrated with a CT scan of a torso phantom and MR scan of the conformal waterbolus component on a mastectomy patient; ii) Specific Absorption Rate (SAR) and temperature distributions are calculated with electromagnetic and thermal simulation software for a mastectomy patient; iii). SAR patterns are measured with a scanning SAR probe in liquid muscle phantom for a buried coplanar waveguide CMA; and iv) heating patterns and patient tolerance of CMA applicators are characterized in a clinical pilot study with 13 patients.
CT and MR scans demonstrate excellent conformity of CMA applicators to contoured anatomy. Simulations demonstrate effective control of heating over contoured anatomy. Measurements confirm effective coverage of large treatment areas with no gaps. In 42 hyperthermia treatments, CMA applicators provided well-tolerated effective heating of up to 500cm2 regions, achieving target temperatures of Tmin=41.4±0.7°C, T90=42.1±0.6°C, Tave=42.8±0.6°C, and Tmax=44.3±0.8°C as measured in an average of 90 points per treatment.
The CMA applicator is an effective thermal therapy device for heating large-area superficial disease such as diffuse chestwall recurrence. It is able to cover over three times the treatment area of conventional hyperthermia devices while conforming to typical body contours.
Microwave array; conformal applicator; superficial hyperthermia; chestwall recurrence
The objective of this study was to evaluate the organ dose and effective dose to patients undergoing routine adult and paediatric CT examinations with 64-slice CT scanners and to compare the doses with those from 4-, 8- and 16-multislice CT scanners. Patient doses were measured with small (<7 mm wide) silicon photodiode dosemeters (34 in total), which were implanted at various tissue and organ positions within adult and 6-year-old child anthropomorphic phantoms. Output signals from photodiode dosemeters were read on a personal computer, from which organ and effective doses were computed. For the adult phantom, organ doses (for organs within the scan range) and effective doses were 8–35 mGy and 7–18 mSv, respectively, for chest CT, and 12–33 mGy and 10–21 mSv, respectively, for abdominopelvic CT. For the paediatric phantom, organ and effective doses were 4–17 mGy and 3–7 mSv, respectively, for chest CT, and 5–14 mGy and 3–9 mSv, respectively, for abdominopelvic CT. Doses to organs at the boundaries of the scan length were higher for 64-slice CT scanners using large beam widths and/or a large pitch because of the larger extent of over-ranging. The CT dose index (CTDIvol), dose–length product (DLP) and the effective dose values using 64-slice CT for the adult and paediatric phantoms were the same as those obtained using 4-, 8- and 16-slice CT. Conversion factors of DLP to the effective dose by International Commission on Radiological Protection 103 were 0.024 mSv⋅mGy−1⋅cm−1 and 0.019 mSv⋅mGy−1⋅cm−1 for adult chest and abdominopelvic CT scans, respectively.
To investigate the clinical usage of dose verification of Helical Tomotherapy plans by using 2D-array ion chambers, and to develop an efficient way to validate the dose delivered for the patients during treatments.
Materials and Methods:
A pixel-segmented ionisation chamber device, IMRT MatriXX™ and Multicube™ phantom from IBA were used on ten selected Tomotherapy IMRT/IGRT head and neck plans in this study. The combined phantom was set up to measure the dose distribution from coronal and sagittal planes. The setup of phantom was guided for verifying the correction position by pre-treatment Tomotherapy MVCT images. After the irradiation, the measured dose distributions of coronal and sagittal planes were compared with those from calculation by the planning system for cross verification. The results were evaluated by the absolute and relative doses as well as Gamma (γ) function. The feasibility of the different measuring methods was studied for this rotational treatment technique.
The dose distributions measured by the MatriXX 2D array were in good agreements with plans calculated by Tomotherapy planning system. The discrepancy between the measured dose and predicted dose in the selected points was within ±3%. In the comparison of the pixel-segmented ionisation chamber versus treatment planning system using the 3 mm/3% γ-function criteria, the mean passing rates of 2 mm dose grid with γ-parameter ≤1 were 97.37% and 96.91%, in two orthogonal planes (coronal and sagittal directions), respectively.
MatriXX with Multicube is a new system created for rotational delivery quality assurance (QA) and found to be reliable to measure both absolute dose and relative dose distributions, simultaneously. It achieves the goal of an efficient and accurate dosimetry validation method of the helical delivery pattern for the Helical Tomotherapy IMRT planning.
Tomotherapy; dose verification; IMRT; radiation therapy; QA
The aim of this study was to evaluate the influence of thyroid collars on radiation dose during cone beam CT (CBCT) scanning.
Average tissue-absorbed dose for a NewTom 9000 CBCT scanner (Quantitative Radiology, Verona, Italy) was measured using thermoluminescent dosemeter chips in a phantom. The scans were carried out with and without thyroid collars. Effective organ dose and total effective dose were derived using International Commission on Radiological Protection 2007 recommendations.
The effective organ doses for the thyroid gland and oesophagus were 31.0 µSv and 2.4 µSv, respectively, during CBCT scanning without a collar around the neck. When the thyroid collars were used loosely around the neck, no effective organ dose reduction was observed. When one thyroid collar was used tightly on the front of the neck, the effective organ dose for the thyroid gland and oesophagus were reduced to 15.9 µSv (48.7% reduction) and 1.4 µSv (41.7% reduction), respectively. Similar organ dose reduction (46.5% and 41.7%) was achieved when CBCT scanning was performed with two collars tightly on the front and back of the neck. However, the differences to the total effective dose were not significant among the scans with and without collars around the neck (p = 0.775).
Thyroid collars can effectively reduce the radiation dose to the thyroid and oesophagus if used appropriately.
cone beam computed tomography; radiation; radiation dosimetry; thyroid gland
Background and Purpose
The use of endo-rectal balloons as immobilisation devices in external beam radiotherapy for prostate cancer has lead to improved target position reproducibility and a decrease in rectal toxicity. The air cavity created by an endo-rectal balloon in photon radiotherapy perturbs the dose distribution. In this study, the effect of the balloon cavity on the dose distribution and the accuracy to which two treatment planning systems calculate the dose distribution was investigated.
Materials and Methods
Single beams as well as 3D conformal, conventional IMRT and helical tomotherapy treatment plans were investigated using a specifically constructed phantom. Radiochromic film was used to measure the cavity wall doses and cavity wall DVHs.
For a 70Gy prescription dose both the Pinnacle and TomoTherapy TPSs over-predicted the anterior cavity wall dose by 1.43Gy, 3.92Gy and 2.67Gy for 3D conformal, conventional IMRT and helical tomotherapy respectively. The posterior cavity wall dose was under-predicted by 2.62Gy, 2.01Gy and 4.79Gy for 3D conformal, conventional IMRT and helical tomotherapy respectively. An over-prediction by the Pinnacle RTPS of the V50, V60, V65 and V70 values for the cavity wall DVH was measured for the 3D conformal and conventional IMRT cases. These reductions may lead to a less than expected rectal toxicity. The TomoTherapy RTPS under-predicted the V50, V60, V65 and V70 values which may lead to higher rectal toxicity than predicted.
Calculation of dose around an air cavity created by an endo-rectal balloon provides a challenge for radiotherapy planning systems. Various electronic disequilibrium situations exist due to the cavity, which can lead to a lower anterior rectal wall and higher posterior rectal wall dose than calculated by planning systems. This has consequences for comparisons of dose volume constraints between different modalities.
endo-rectal balloon; cavity; radiochromic film; tomotherapy; IMRT
External photon beam modulation using compensators in order to achieve a desired dose distribution when brachytherapy treatment is followed by external beam radiation is a well-established technique. A compensator modulates the central part of the beam, and the dose beneath the thickest part of the compensator is delivered mostly by scattered, low energy photons. A two-dimensional detector with a good spatial resolution is needed for the verification of those beams. In this work, the influence of different types of detectors on the measured modulated dose distributions was examined.
Materials and methods
Dosimetric verification was performed using X-Omat V, Eastman Kodak radiographic films at different depths in a solid water phantom. The film measurements were compared with those made by ionization chambers. Photon beams were also modelled using EGSnrc Monte Carlo algorithm to explain the measured results.
Monte Carlo calculated over-response of the film under the thickest part of the compensator was over 15%, which was confirmed by measurements. The magnitude of over-response could be associated with changes in the spectra of photon energy in the beam.
The radiographic film can be used for the dosimetry of compensated high energy photon beams, with limitations in volumes where photon spectra are hardly degraded.
radiation therapy; dosimetry; compensators
To develop and characterize the accuracy and reproducibility of a quad-phantom dosimeter which will serve as an independent verification tool during commissioning of a PRESAGE/optical-CT 3D dosimetry system.
A 16cm × 12cm cylindrical quad-phantom was constructed from four pieces of solid polyurethane mimicking the PRESAGE material. Films were placed and anchored in orthogonal planes and the quad-phantom was fastened tightly together and placed in a water-filled Styrofoam container for irradiation. A simple, two-field plan consisting of 6×6cm anterior-posterior and right-lateral 6MV photon beams (400cGy) was delivered three times (fresh films inserted for each) with a Varian Clinac 600C. Image registration was performed in the Computational Environment for Radiological Research (CERR) and dose profiles and gamma analysis was performed in CERR and MATLAB.
Results & Discussion
Excellent reproducibility was observed during the irradiations, with ~2.3% standard deviation between all pixels. Using a 3%, 3mm gamma criteria, excellent dosimetric accuracy was observed, with 98.8% and 96.3% passing rates in the sagittal and axial planes, respectively.
The preliminary results indicate that the quad-phantom can serve as a reproducible and accurate system for high resolution dosimetry in orthogonal planes and should serve as an effective verification tool for PRESAGE/optical-CT in more challenging clinical scenarios.