Measuring foetal dose from tomotherapy treatments.

INTRODUCTION: Treating pregnant women in the radiotherapy clinic is a rare occurrence. When it does occur, it is vital that the dose received by the developing embryo or foetus is understood as fully as possible. This study presents the first investigation of foetal doses delivered during helical tomotherapy treatments. Six treatment plans were delivered to an anthropomorphic phantom using a tomotherapy machine. These included treatments of the brain, unilateral and bilateral head-and-neck, chest wall, and upper lung. Measurements of foetal dose were made with an ionisation chamber positioned at various locations longitudinally within the phantom to simulate a variety of patient anatomies. All measurements were below the established limit of 100 mGy for a high risk of damage during the first trimester. The largest dose encountered was 75 mGy (0.125% of prescription dose). The majority of treatments with measurement positions less than 30 cm fell into the range of uncertain risk (50 - 100 mGy). All treatments with measurement positions beyond 30 cm fell into the low risk category (< 50 mGy). For the cases in this study, tomotherapy resulted in foetal doses that are at least on par with, if not significantly lower than, similar 3D conformal or intensity-modulated treatments delivered with other devices. Recommendations were also provided for estimating foetal doses from tomotherapy plans.

Impact of radiopacified bone cement on radiotherapy dose calculation.

BACKGROUND AND PURPOSE: Radiopacifiers are introduced to bone cements to provide the appearance of bone in kilovoltage (kV) radiographic images. For higher energy megavoltage (MV) radiotherapy treatment beams, however, these radiopacifiers do not cause a bone-like perturbation of dose. This study therefore aimed to determine the impact of the barium-contrasted plastic-based cement materials on radiotherapy dose calculations. MATERIALS AND METHODS: The radiological properties of a physical sample of bone cement were characterised by computed tomography (CT) imaging and transmission measurements. Monte Carlo simulations of percentage depth-dose profiles were performed to determine the possible dose error for MV treatment beams. Dose differences were then investigated for clinical volumetric modulated radiotherapy treatment plans, with and without density overrides applied. RESULTS: Differences of up to 7% were observed at the downstream interface of a 0.6 cm thick bone cement layer, compared to bone. Differences in planning target volume dose-volume metrics varied between -0.5% and 2.0%. CONCLUSION: Before planning radiotherapy treatments for patients who have undergone cranioplasty, every effort should be made to identify whether a radiopacified bone cement has been implanted. Density overrides should be applied to minimise dose calculation errors, whenever bone cement is used.

Tomotherapy treatment site specific planning using statistical process control.

BACKGROUND: This study investigated planned MLC distribution and treatment region specific plan parameters to recommend optimal delivery parameters based on statistical process techniques. METHODS: A cohort of 28 head and neck, 19 pelvic and 23 brain pre-treatment plans were delivered on a helical tomotherapy system using 2.5 cm field width. Parameters such as gantry period, leaf open time (LOT), actual modulation factor, LOT sonogram, treatment duration and couch travel were investigated to derive optimal range for plans that passed acceptable delivery quality assurance. The results were compared against vendor recommendations and previous publications. RESULTS: No correlation was observed between vendor recommended gantry period and percentage of minimum leaf open times. The range of gantry period (min-max) observed was 16-21 s for head and neck, 15-22 s for pelvis and 13-18 s for brain plans respectively. It was also noted that the highest percentage (average (X-) ±â€¯SD) of leaf open times for a minimum time of 100 ms was seen for brain plans (53.9 ±â€¯9.2%) compared to its corresponding head and neck (34.5 ±â€¯4.2%) and pelvic (32.0 ±â€¯9.4%) plans respectively. CONCLUSIONS: We have proposed that treatment site specific delivery parameters be used during planning that are based on the treatment centre and have detailed recommendations and limitations for the studied cohort. This may enable to improve efficiency of treatment deliveries by reducing inaccuracies in MLC distribution.

Characterisation of radiological properties of a brachytherapy moulding material.

The use of a non-water-equivalent personalised mould for gynaecological brachytherapy treatments can result in a substantial dose reduction at the treatment site, compared to calculated dose, in lieu of a dose calculation algorithm capable of modelling non-water-equivalent materials. This study describes the characterisation of the radiological properties of a brachytherapy applicator moulding material. Simple line source correction factors for an 192Ir source are obtained through Monte Carlo simulations and verified by film measurements. The dwell position corrections are used to estimate aggregate correction factors for dose deliveries that involve multiple dwell positions, in terms of treatment length, applicator radii and depth of reference dose. For the Fricotan moulding material used locally, the dose reductions varied from 1% for an applicator radius of 0.5 cm to > 4% for radii exceeding 2 cm. The method described in this paper could be used to develop correction factors for other non-water-equivalent moulding materials, in a TG-43UI dose calculation environment.

Photon optimizer (PO) vs progressive resolution optimizer (PRO): a conformality- and complexity-based comparison for intensity-modulated arc therapy plans.

This study aimed to provide guidance on the advantages and limitations of a new optimizer, "photon optimizer" (PO), when compared with its predecessor, "progressive resolution optimizer" (PRO), for intensity-modulated arc therapy (IMAT) plans. Eleven study plans that included a cohort of prostate, head and neck, and brain treatment sites were optimized using both PRO and PO algorithms. A plan template using the same objectives for the same number of iterations was used for each optimized plan to obtain hypothetical treatment plans that would be comparable with a clinical plan. Analysis was performed using plan conformity-based parameters such as target volume coverage factor, conformation number and homogeneity indices, and plan complexity assessment parameters such as small aperture score, modulation indices, and monitor unit variation with arc angle for prostate, brain and head, and neck IMAT treatment plans. Plan conformality analysis demonstrated that conformation numbers, target volume coverage factors, and homogeneity indices produced by the 2 optimizers were comparable for most anatomic sites. IMAT treatment plans produced using the PRO optimizer were found to be less complex than plans produced using the PO optimizer, in terms of multileaf collimator (MLC) leaf position variability and modulation complexity scores. Similarly, the PRO optimizer was shown to produce treatment plans that used fewer monitor units (and generally fewer monitor unit per degree of arc rotation) than PO optimizer. This study demonstrated that the PO optimizer can produce IMAT treatment plans with a similar degree of dose conformity to the target volume and generally improved organ at risk sparing, compared with the PRO optimizer. Better coverage to organs at risk produced by plans optimized using PO was observed to have higher MLC variability and monitor units. Therefore, careful evaluation of treatment plan conformity and complexity before assessing its deliverability is recommended when implementing the routine use of PO optimizer.

Statistical process control and verifying positional accuracy of a cobra motion couch using step-wedge quality assurance tool.

This study utilizes process control techniques to identify action limits for TomoTherapy couch positioning quality assurance tests. A test was introduced to monitor accuracy of the applied couch offset detection in the TomoTherapy Hi-Art treatment system using the TQA "Step-Wedge Helical" module and MVCT detector. Individual X-charts, process capability (cp), probability (P), and acceptability (cpk) indices were used to monitor a 4-year couch IEC offset data to detect systematic and random errors in the couch positional accuracy for different action levels. Process capability tests were also performed on the retrospective data to define tolerances based on user-specified levels. A second study was carried out whereby physical couch offsets were applied using the TQA module and the MVCT detector was used to detect the observed variations. Random and systematic variations were observed for the SPC-based upper and lower control limits, and investigations were carried out to maintain the ongoing stability of the process for a 4-year and a three-monthly period. Local trend analysis showed mean variations up to ±0.5 mm in the three-monthly analysis period for all IEC offset measurements. Variations were also observed in the detected versus applied offsets using the MVCT detector in the second study largely in the vertical direction, and actions were taken to remediate this error. Based on the results, it was recommended that imaging shifts in each coordinate direction be only applied after assessing the machine for applied versus detected test results using the step helical module. User-specified tolerance levels of at least ±2 mm were recommended for a test frequency of once every 3 months to improve couch positional accuracy. SPC enables detection of systematic variations prior to reaching machine tolerance levels. Couch encoding system recalibrations reduced variations to user-specified levels and a monitoring period of 3 months using SPC facilitated in detecting systematic and random variations. SPC analysis for couch positional accuracy enabled greater control in the identification of errors, thereby increasing confidence levels in daily treatment setups.

Investigating output and energy variations and their relationship to delivery QA results using Statistical Process Control for helical tomotherapy.

The aims of this study were to investigate machine beam parameters using the TomoTherapy quality assurance (TQA) tool, establish a correlation to patient delivery quality assurance results and to evaluate the relationship between energy variations detected using different TQA modules. TQA daily measurement results from two treatment machines for periods of up to 4years were acquired. Analyses of beam quality, helical and static output variations were made. Variations from planned dose were also analysed using Statistical Process Control (SPC) technique and their relationship to output trends were studied. Energy variations appeared to be one of the contributing factors to delivery output dose seen in the analysis. Ion chamber measurements were reliable indicators of energy and output variations and were linear with patient dose verifications.

Monitoring Daily QA 3 constancy for routine quality assurance on linear accelerators.

The purpose of this study was to evaluate the suitability of the Daily QA 3 (Sun Nuclear Corporation, Melbourne, USA) device as a safe quality assurance device for control of machine specific parameters, such as linear accelerator output, beam quality and beam flatness and symmetry. Measurements were performed using three Varian 2300iX linear accelerators. The suitability of Daily QA 3 as a device for quality control of linear accelerator parameters was investigated for both 6 and 10MV photons and 6, 9, 12, 15 and 18MeV electrons. Measurements of machine specific using the Daily QA 3 device were compared to corresponding measurements using a simpler constancy meter, Farmer chamber and plane parallel ionisation chamber in a water tank. The Daily QA 3 device showed a linear dose response making it a suitable device for detection of output variations during routine measurements. It was noted that over estimations of variations compared with Farmer chamber readings were seen if the Daily QA 3 wasn't calibrated for output and sensitivity on a regular eight to ten monthly basis. Temperature-pressure correction factors calculated by Daily QA 3 also contributed towards larger short term variations seen in output measurements. Energy, symmetry and flatness variations detected by Daily QA 3 were consistent with measurements performed in water tank using a parallel plate chamber. It was concluded that the Daily QA 3 device is suitable for routine daily and fortnightly quality assurance of linear accelerator beam parameters however a regular eight-ten monthly dose and detector array calibration will improve error detection capabilities of the device.

Investigating the use of image thresholding in brachytherapy catheter reconstruction.

This study evaluated the accuracy of image thresholding in the reconstruction of catheters in brachytherapy treatment planning systems. Six test cases including four planar catheter configurations, an interstitial prostate and an intracavitary treatment plan were made use of in this study. The four planar CT scanned catheter arrangements included; catheters placed approximately 1, 0.5 cm apart, catheters closely arranged in a plan (<0.5 cm apart) and a loop arrangement. The intracavitary plan consisted of catheters arranged inside a mould configuration. All reconstruction methods were based on tracking wire markers placed inside the plastic catheters. Each of these catheter arrangements was reconstructed using an existing window adjustment technique (manual reconstruction) in the treatment planning system followed by a CT-based automated thresholding technique available in the same planning system. A corresponding reconstructed catheter was created using a segmented catheter structure using image thresholding from another planning system within the same department. Co-ordinates from all the reconstructed catheters were compared against each other to assess the geometric shift between manual and threshold based reconstruction on each transaxial image using in-house software and the maximum variations were recorded for assessment. It was observed in general that automated thresholding technique could assist in catheter reconstruction for catheters which are greater than 0.5 cm apart. The segmented thresholding method reported smaller variations when compared to the manual reconstruction using window adjustment technique. Automated reconstruction saves time in the brachytherapy planning, however it was noted that it is not feasible for closely spaced catheters. Segmented catheter reconstruction although time consuming, did provide a better alternative in most cases.

Effects of changing modulation and pitch parameters on tomotherapy delivery quality assurance plans.

This study was aimed at investigating delivery quality assurance (DQA) discrepancies observed for helical tomotherapy plans. A selection of tomotherapy plans that initially failed the DQA process was chosen for this investigation. These plans failed the fluence analysis as assessed using gamma criteria (3%, 3 mm) with radiographic film. Each of these plans was modified (keeping the planning constraints the same), beamlets rebatched and reoptimized. By increasing and decreasing the modulation factor, the fluence in a circumferential plane as measured with a diode array was assessed. A subset of these plans was investigated using varied pitch values. Metrics for each plan that were examined were point doses, fluences, leaf opening times, planned leaf sinograms, and uniformity indices. In order to ensure that the treatment constraints remained the same, the dose-volume histograms (DVHs) of all the modulated plans were compared to the original plan. It was observed that a large increase in the modulation factor did not significantly improve DVH uniformity, but reduced the gamma analysis pass rate. This also increased the treatment delivery time by slowing down the gantry rotation speed which then increases the maximum to mean non-zero leaf open time ratio. Increasing and decreasing the pitch value did not substantially change treatment time, but the delivery accuracy was adversely affected. This may be due to many other factors, such as the complexity of the treatment plan and site. Patient sites included in this study were head and neck, right breast, prostate, abdomen, adrenal, and brain. The impact of leaf timing inaccuracies on plans was greater with higher modulation factors. Point-dose measurements were seen to be less susceptible to changes in pitch and modulation factors. The initial modulation factor used by the optimizer, such that the TPS generated 'actual' modulation factor within the range of 1.4 to 2.5, resulted in an improved deliverable plan.

First experiences in using a dose control system on a TomoTherapy Hi·Art II.

The purpose of this study was to investigate the impact of a dose control system (DCS) servo installed on two fully commissioned TomoTherapy Hi·Art II treatment units. This servo is designed to actively adjust machine parameters to control the output variation of a tomotherapy unit to within ± 0.5% of the nominal dose rate. Machine output, dose rate, and patient-specific quality assurance data were retrospectively analyzed for periods prior to and following the installation of the servo system. Quality assurance tests indicate a reduction in the rotational variation of the output during a procedure, where the peak-to-peak amplitude of the variation was ± 1.30 prior to DCS and equal to ± 0.4 with DCS. Comparing two tomotherapy unit static outputs over four years the percentage error was 1.05% ± 0.7% and -0.4% ± 0.66% and, once DCS was installed, was reduced to -0.22% ± 0.29% and -0.08% ± 0.16%. The results of the quality assurance tests indicate that the dose control system reduced the output variation of each machine for both static and rotational delivery, leading to an improvement in the overall performance of the machine and providing greater certainty in treatment delivery.

Constituent components of out-of-field scatter dose for 18-MV intensity modulated radiation therapy versus 3-dimensional conformal radiation therapy: a comparison with 6-MV and implications for carcinogenesis.

PURPOSE: To characterize and compare the components of out-of-field dose for 18-MV intensity modulated radiation therapy (IMRT) versus 3-dimensional conformal radiation therapy (3D-CRT) and their 6-MV counterparts and consider implications for second cancer induction. METHODS AND MATERIALS: Comparable plans for each technique/energy were delivered to a water phantom with a sloping wall; under full scatter conditions; with field edge abutting but outside the bath to prevent internal/phantom scatter; and with shielding below the linear accelerator head to attenuate head leakage. Neutron measurements were obtained from published studies. RESULTS: Eighteen-megavolt IMRT produces 1.7 times more out-of-field scatter than 18-MV 3D-CRT. In absolute terms, however, differences are just approximately 0.1% of central axis dose. Eighteen-megavolt IMRT reduces internal/patient scatter by 13%, but collimator scatter (C) is 2.6 times greater than 18-MV 3D-CRT. Head leakage (L) is minimal. Increased out-of-field photon scatter from 18-MV IMRT carries out-of-field second cancer risks of approximately 0.2% over and above the 0.4% from 18-MV 3D-CRT. Greater photoneutron dose from 18-MV IMRT may result in further maximal, absolute increased risk to peripheral tissue of approximately 1.2% over 18-MV 3D-CRT. Out-of-field photon scatter remains comparable for the same modality irrespective of beam energy. Machine scatter (C+L) from 18 versus 6 MV is 1.2 times higher for IMRT and 1.8 times for 3D-CRT. It is 4 times higher for 6-MV IMRT versus 3D-CRT. Reduction in internal scatter with 18 MV versus 6 MV is 27% for 3D-CRT and 29% for IMRT. Compared with 6-MV 3D-CRT, 18-MV IMRT increases out-of-field second cancer risk by 0.2% from photons and adds 0.28-2.2% from neutrons. CONCLUSIONS: Out-of-field photon dose seems to be independent of beam energy for both techniques. Eighteen-megavolt IMRT increases out-of-field scatter 1.7-fold over 3D-CRT because of greater collimator scatter despite reducing internal/patient scatter. Out-of-field carcinogenic risk is thus increased (but improved in-field dose conformity may offset this). Potentially increased carcinogenic risk should be weighed against any benefit 18-MV IMRT may provide.

A comparison of out-of-field dose and its constituent components for intensity-modulated radiation therapy versus conformal radiation therapy: implications for carcinogenesis.

PURPOSE: To investigate differences in scatter and leakage between 6-MV intensity-modulated radiation therapy (IMRT) and three-dimensional conformal radiation therapy (3DCRT); to describe the relative contributions of internal patient scatter, collimator scatter, and head leakage; and to discuss implications for second cancer induction. METHODS AND MATERIALS: Dose was measured at increasing distances from the field edge in a water bath with a sloping wall (1) under full scatter conditions, (2) with the field edge abutting but outside the bath to prevent internal (water) scatter, and (3) with the beam aperture plugged to reflect leakage only. RESULTS: Internal patient scatter from IMRT is 11% lower than 3DCRT, but collimator scatter and head leakage are five and three times higher, respectively. Ultimately, total scattered dose is 80% higher with IMRT; however this difference is small in absolute terms, being 0.14% of prescribed dose. Secondary dose from 3DCRT is mostly due to internal patient scatter, which contributes 70% of the total and predominates until 25 cm from the field edge. For IMRT, however, machine scatter/leakage is the dominant source, contributing 65% of the secondary dose. Internal scatter predominates for just the first 10 cm from field edge, collimator scatter for the next 10 cm, and head leakage thereafter. CONCLUSIONS: Out-of-field dose is 80% higher with IMRT, but differences are tiny in absolute terms. Reductions in internal patient scatter with IMRT are outweighed by increased machine scatter and leakage, at least for small fields. Reductions from IMRT in dose to tissues within the portals and in internal scatter, which predominates close to the field edge, means that calculations based solely on dose to distant tissues may overestimate carcinogenic risks.