Validation of complex radiotherapy techniques using polymer gel dosimetry

Modern radiotherapy delivers highly conformal dose distributions to irregularly shaped target volumes while sparing the surrounding normal tissue. Due to the complex planning and delivery techniques, dose verification and validation of the whole treatment workflow by end-to-end tests became much more important and polymer gel dosimeters are one of the few possibilities to capture the delivered dose distribution in 3D. The basic principles and formulations of gel dosimetry and its evaluation methods are described and the available studies validating device-specific geometrical parameters as well as the dose delivery by advanced radiotherapy techniques, such as 3D-CRT/IMRT and stereotactic radiosurgery treatments, the treatment of moving targets, online-adaptive magnetic resonance-guided radiotherapy as well as proton and ion beam treatments, are reviewed. The present status and limitations as well as future challenges of polymer gel dosimetry for the validation of complex radiotherapy techniques are discussed.

Evolution of dosimetric treatment planning for pediatric total lymphoid irradiation (TLI): a single-institution experience

Background

Total lymphoid irradiation (TLI) is a conditioning regimen in allogeneic hematopoietic stem cell transplantation (allo-HSCT) which may reduce long-term toxicities attributed to other techniques, such as total body irradiation (TBI). At our institution, TLI treatments were first planned with the three-dimensional conformal radiation therapy (3D-CRT) technique and later with volumetric modulated arc therapy (VMAT). With the recent availability of a basic helical tomotherapy (HT), the possible dosimetric gain of the latter for TLI is studied.

Materials and methods

22 pediatric patients were planned for VMAT and HT, prescribed to 8 Gy in 4 fractions. VMAT was planned with template based on a single cost function, using the Monaco treatment planning system (TPS). HT plans were planned using Accuray Precision TPS for a basic HT without the dynamic jaws feature or VOLO-Ultra algorithm. Plan quality was analyzed based on four quality indices, mean and maximum doses to planning target volume (PTV) and organs at risk (OARs), dose gradient and integral doses. Differences were analyzed with Wilcoxon signed-rank test.

Results

HT plans resulted in improved conformity (CI) and homogeneity indices (HI) (p < 0.05) but less steep dose gradient (p = 0.181). VMAT plans created larger areas with high doses within the PTV, while comparable doses to OARs, except mainly for the spinal marrow, for which a reduction of 37.7% in D2% was obtained (p < 0.05). Integral dose for non-tumor tissue was 11.3% lower with the VMAT template (p < 0.05).

Conclusion

HT achieves better conformity and homogeneity even without its more advanced features. Nevertheless, the VMAT template achieves dosimetric results close to those of HT, both with similar clinical outcome.

AAPM Task Group Report 306: Quality control and assurance for tomotherapy: An update to Task Group Report 148

Since the publication of AAPM Task Group (TG) 148 on quality assurance (QA) for helical tomotherapy, there have been many new developments on the tomotherapy platform involving treatment delivery, on-board imaging options, motion management, and treatment planning systems (TPSs). In response to a need for guidance on quality control (QC) and QA for these technologies, the AAPM Therapy Physics Committee commissioned TG 306 to review these changes and make recommendations related to these technology updates. The specific objectives of this TG were (1) to update, as needed, recommendations on tolerance limits, frequencies and QC/QA testing methodology in TG 148, (2) address the commissioning and necessary QA checks, as a supplement to Medical Physics Practice Guidelines (MPPG) with respect to tomotherapy TPS and (3) to provide risk-based recommendations on the new technology implemented clinically and treatment delivery workflow. Detailed recommendations on QA tests and their tolerance levels are provided for dynamic jaws, binary multileaf collimators, and Synchrony motion management. A subset of TPS commissioning and QA checks in MPPG 5.a. applicable to tomotherapy are recommended. In addition, failure mode and effects analysis has been conducted among TG members to obtain multi-institutional analysis on tomotherapy-related failure modes and their effect ranking.

Effective clinical applications of Monte Carlo-based independent secondary dose verification software for helical tomotherapy

PURPOSE

To investigate the scope of the effective clinical application of Monte Carlo (MC)-based independent dose verification software for helical tomotherapy.

METHODS

DoseCHECK was selected as the MC-based dose calculation software. First, the dose calculation accuracy of DoseCHECK was evaluated with film and chamber measurements in a water-equivalent phantom. Second, the dose calculation accuracy was examined in several heterogeneous materials. Finally, dosimetric comparisons between DoseCHECK and the treatment planning system (TPS) were performed for clinical patient plans. Prostate IMRT, head and neck IMRT (HN), total body irradiation (TBI), and brain stereotactic radiotherapy (SRT) were evaluated.

RESULT

The DoseCHECK calculations agreed with the chamber and film measurements in the homogenous phantom. For heterogeneous phantom cases, the dose differences between DoseCHECK and TPS were within 3 %, except in air, in which large dose differences of 20 % were observed. In clinical patient plans, the median dose differences between the lung D_mean in TBI cases and the normal brain D_mean in brain SRT cases were significantly >3 %. For HN and brain SRT cases, the median target dose differences were >3 %.

CONCLUSION

Our results show that independent dose verification with the MC algorithm can detect systematic errors caused by the lack of heterogeneity correction in the TPS. In particular, MC-based independent dose verification is required for HN, TBI, and brain SRT cases in helical tomotherapy.

Using 4D dose accumulation to calculate organ-at-risk dose deviations from motion-synchronized liver and lung tomotherapy treatments

Tracking systems such as Radixact Synchrony change the planned delivery of radiation during treatment to follow the target. This is typically achieved without considering the location changes of organs at risk (OARs). The goal of this work was to develop a novel 4D dose accumulation framework to quantify OAR dose deviations due to the motion and tracked treatment. The framework obtains deformation information and the target motion pattern from a four-dimensional computed tomography dataset. The helical tomotherapy treatment plan is split into 10 plans and motion correction is applied separately to the jaw pattern and multi-leaf collimator (MLC) sinogram for each phase based on the location of the target in each phase. Deformable image registration (DIR) is calculated from each phase to the references phase using a commercial algorithm, and doses are accumulated according to the DIR. The effect of motion synchronization on OAR dose was analyzed for five lung and five liver subjects by comparing planned versus synchrony-accumulated dose. The motion was compensated by an average of 1.6 cm of jaw sway and by an average of 5.7% of leaf openings modified, indicating that most of the motion compensation was from jaw sway and not MLC changes. OAR dose deviations as large as 19 Gy were observed, and for all 10 cases, dose deviations greater than 7 Gy were observed. Target dose remained relatively constant (D95% within 3 Gy), confirming that motion-synchronization achieved the goal of maintaining target dose. Dose deviations provided by the framework can be leveraged during the treatment planning process by identifying cases where OAR doses may change significantly from their planned values with respect to the critical constraints. The framework is specific to synchronized helical tomotherapy treatments, but the OAR dose deviations apply to any real-time tracking technique that does not consider location changes of OARs.

Incorporation of tumor motion directionality in margin recipe: The directional MidP strategy

PURPOSE

Planning target volume (PTV) definition based on Mid-Position (Mid-P) strategy typically integrates breathing motion from tumor positions variances along the conventional axes of the DICOM coordinate system. Tumor motion directionality is thus neglected even though it is one of its stable characteristics in time. We therefore propose the directional MidP approach (MidP dir), which allows motion directionality to be incorporated into PTV margins. A second objective consists in assessing the ability of the proposed method to better take care of respiratory motion uncertainty.

METHODS

11 lung tumors from 10 patients with supra-centimetric motion were included. PTV were generated according to the MidP and MidP dir strategies starting from planning 4D CT.

RESULTS

PTV_{MidP dir} volume didn't differ from the PTV_MidP volume: 31351 mm³ IC95% [17242-45459] vs. 31003 mm³ IC95% [ 17347-44659], p = 0.477 respectively. PTV_{MidP dir} morphology was different and appeared more oblong along the main motion axis. The relative difference between 3D and 4D doses was on average 1.09%, p = 0.011 and 0.74%, p = 0.032 improved with directional MidP for D_99% and D_95%. D_2% was not significantly different between both approaches. The improvement in dosimetric coverage fluctuated substantially from one lesion to another and was all the more important as motion showed a large amplitude, some obliquity with respect to conventional axes and small hysteresis.

CONCLUSIONS

Directional MidP method allows tumor motion to be taken into account more tightly as a geometrical uncertainty without increasing the irradiation volume.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

A preliminary study of a photon dose calculation algorithm using a convolutional neural network

The aim of dose calculation algorithm research is to improve the calculation accuracy while maximizing the calculation efficiency. In this study, the three-dimensional distribution of total energy release per unit mass (TERMA) and the electron density (ED) distribution are considered inputs in a method for calculating the three-dimensional dose distribution based on a convolutional neural network (CNN). Attempts are made to improve the efficiency of the collapsed cone convolution/superposition (CCCS) algorithm while providing an approach to improve the efficiency of other traditional dose calculation algorithms. Twelve sets of computed tomography (CT) images were employed for training. Data sets were generated by the CCCS algorithm with a random beam configuration. For each monoenergetic photon model, 7500 samples were generated for the training set, and 1500 samples were generated for the validation set. Training occurred for 0.5 MeV, 1 MeV, 2 MeV, 3 MeV, 4 MeV, 5 MeV, and 6 MeV monoenergetic photon models. To evaluate the usability under linac conditions, a comparison between CCCS and CNN-Dose was performed for the Mohan 6-MV spectrum for 12 additional new sets of CT images with different anatomies. A total of 1512 test samples were generated. For all anatomies, the mean value, 95% lower confidence limit (LCL) and 95% upper confidence limit (UCL) were 99.56%, 99.51% and 99.61%, respectively, at the 3%/2 mm criteria. The mean value, 95% LCL and 95% UCL were 98.57%, 98.46% and 98.67%, respectively, at the 2%/2 mm criteria. The results meet the relevant clinical requirements. In the proposed methods, the dose distribution of clinical energy can be obtained by TERMA, and the electronic density can be obtained with a CNN. This method can also be used for other traditional dose algorithms and displays potential in treatment planning, adaptive radiation therapy, and in vivo verification.

Commissioning of the Mobius3D independent dose verification system for TomoTherapy

In radiation therapy, a secondary independent dose verification is an important component of a quality control system. Mobius3D calculates three‐dimensional (3D) patient dose using reference beam data and a collapsed cone convolution algorithm and analyzes dose‐volume histogram automatically. There are currently no published data on commissioning and determining tolerance levels of Mobius3D for TomoTherapy. To verify the calculation accuracy and adjust the parameters of this system, we compared the measured dose using an ion chamber and film in a phantom with the dose calculated using Mobius3D for nine helical intensity‐modulated radiation therapy plans, each with three nominal field widths. We also compared 126 treatment plans used in our institution to treat prostate, head‐and‐neck, and esophagus tumors based on dose calculations by treatment planning system for given dose indices and 3D gamma passing rates with those produced by Mobius3D. On the basis of these results, we showed that the action and tolerance levels at the average dose for the planning target volume (PTV) at each treatment site are at μ ± 2σ and μ ± 3σ, respectively. After adjusting parameters, the dose difference ratio on average was −0.2 ± 0.6% using ion chamber and gamma passing rate with the criteria of 3% and 3 mm on average was 98.8 ± 1.4% using film. We also established action and tolerance levels for the PTV at the prostate, head‐and‐neck, esophagus, and for the organ at risk at all treatment sites. Mobius3D calculations thus provide an accurate secondary dose verification system that can be commissioned easily and immediately after installation. Before clinical use, the Mobius3D system needs to be commissioned using the treatment plans for patients treated in each institution to determine the calculational accuracy and establish tolerances for each treatment site and dose index.

Skin DVHs predict cutaneous toxicity in Head and Neck Cancer patients treated with Tomotherapy

PURPOSE

To explore the association between planning skin dose-volume data and acute cutaneous toxicity after Radio-chemotherapy for Head and Neck (HN) cancer patients.

METHODS

Seventy HN patients were treated with Helical Tomotherapy (HT) with radical intent (SIB technique: 54/66 Gy to PTV1/PTV2 in 30fr) ± chemotherapy superficial body layer 2 mm thick (SL2) was delineated on planning CT. CTCAE v4.0 acute skin toxicity data were available. Absolute average Dose-Volume Histograms (DVH) of SL2 were calculated for patients with severe (G3) and severe/moderate (G3/G2) skin acute toxicities. Differences against patients with none/mild toxicity (G0/G1) were analyzed to define the most discriminative regions of SL2 DVH; univariable and multivariable logistic analyses were performed on DVH values, CTV volume, age, sex, chemotherapy.

RESULTS

Sixty-one % of patients experienced G2/G3 toxicity (rate of G3 = 19%). Differences in skin DVHs were significant in the range 53-68Gy (p-values: 0.005-0.01). V56/V64 were the most predictive parameters for G2/G3 (OR = 1.12, 95%CI = 1.03-1.21, p = 0.001) and G3 (OR = 1.13, 95%CI = 1.01-1.26, p = 0.027) with best cut-off of 7.7cc and 2.7cc respectively. The logistic model for V56 was well calibrated being both, slope and R2, close to 1. Average V64 were 2.2cc and 6cc for the two groups (G3 vs G0-G2 toxicity); the logistic model for V64 was quite well calibrated, with a slope close to 1 and R2 equal to 0.60.

CONCLUSION

SL2 DVH is associated with the risk of acute skin toxicity. Constraining V64 < 3cc (equivalent to a 4x4cm2 skin surface) should keep the risk of G3 toxicity below or around 10%.

Skin dose calculation during radiotherapy of head and neck cancer using deformable image registration of planning and mega-voltage computed tomography scans

Background and Purpose

Head-Neck (HN) patients may experience severe acute skin complications that can cause treatment interruption and increase the risk of late fibrosis. This study assessed a method for accurately monitoring skin dose changes during helical tomotherapy for HN cancer based on deformable image registration of planning computed tomography (CT) and mega-voltage CT (MVCT).

Materials and Methods

Planning CTs of nine patients were deformably registered to mid-treatment MVCT (MV15) images resulting in CTdef images. The original plans were recalculated on both CTdef and mid-treatment kilo-voltage CT (CT15) taken as ground truth. Superficial layers (SL) of the body with thicknesses of 2, 3 and 5 mm (SL2, SL3, SL5) were considered as derma surrogates. SL V95%, V97%, V98%, V100%, V102%, V105% and V107% of the prescribed PTV dose were extracted for CT15/CTdef and compared (considering patients with skin dose > 95%). For comparison, doses were calculated directly on the calibrated MVCT and analyzed in the same way.

Results

Differences between SL2/SL3/SL5 V95%-V107% in CT15/CTdef were very small: for eight of nine patients the difference between the considered SL2 Vd% computed on CTdef and CT15 was less than 1.4 cm³ for all d%. A larger value was found when using MVCT for skin dose calculation (4.8 cm³ for SL2), although CTdef body contour matched CT15 body with accuracy similar to that of MV15.

Conclusions

Deforming the planning CT-to-MVCT was shown to be accurate considering external body contours and skin DVHs. The method was able to accurately identify superficial overdosing.

Performance Characteristics of an Independent Dose Verification Program for Helical Tomotherapy

Helical tomotherapy with its advanced method of intensity-modulated radiation therapy delivery has been used clinically for over 20 years. The standard delivery quality assurance procedure to measure the accuracy of delivered radiation dose from each treatment plan to a phantom is time-consuming. RadCalc^®, a radiotherapy dose verification software, has released specifically for beta testing a module for tomotherapy plan dose calculations. RadCalc^®'s accuracy for tomotherapy dose calculations was evaluated through examination of point doses in ten lung and ten prostate clinical plans. Doses calculated by the TomoHDA™ tomotherapy treatment planning system were used as the baseline. For lung cases, RadCalc^® overestimated point doses in the lung by an average of 13%. Doses within the spinal cord and esophagus were overestimated by 10%. Prostate plans showed better agreement, with overestimations of 6% in the prostate, bladder, and rectum. The systematic overestimation likely resulted from limitations of the pencil beam dose calculation algorithm implemented by RadCalc^®. Limitations were more severe in areas of greater inhomogeneity and less prominent in regions of homogeneity with densities closer to 1 g/cm³. Recommendations for RadCalc^® dose calculation algorithms and anatomical representation were provided based on the results of the study.

Radiation dose escalation based on FDG-PET driven dose painting by numbers in oropharyngeal squamous cell carcinoma: a dosimetric comparison between TomoTherapy-HA and RapidArc

Purpose

Validation of dose escalation through FDG-PET dose painting (DP) for oropharyngeal squamous cell carcinoma (SCC) requires randomized clinical trials with large sample size, potentially involving different treatment planning and delivery systems. As a first step of a joint clinical study of DP, a planning comparison was performed between Tomotherapy HiArt® (HT) and Varian RapidArc® (RA).

Methods

The planning study was conducted on five patients with oropharyngeal SCC. Elective and therapeutic CTVs were delineated based on anatomic information, and the respective PTVs (CTVs + 4 mm) were prescribed a dose of 56 (PTV₅₆) and 70 Gy (PTV₇₀). A gradient-based method was used to delineate automatically the external contours of the FDG-PET volume (GTV_PET). Variation of the FDG uptake within the GTV_PET was linearly converted into a prescription between 70 and 86 Gy. A dilation of the voxel-by-voxel prescription of 2.5 mm was applied to account for geometric errors in dose delivery (PTV_PET). The study was divided in two planning phases aiming at maximizing target coverage (phase I) and lowering doses to OAR (phase II). A Quality-Volume Histogram (QVH) assessed conformity with the DP prescription inside the PTV_PET.

Results

In phase I, for both HT and RA, all plans achieved comparable target coverage for PTV₅₆ and PTV₇₀, respecting the planning objectives. A median value of 99.9 and 97.2% of all voxels in the PTV_PET received at least 95% of the prescribed dose for RA and HT, respectively. A median value of 0.0% and 3.7% of the voxels in the PTV_PET received 105% or more of prescribed dose for RA and HT, respectively. In phase II, no significant differences were found in OAR sparing. Median treatment times were 13.7 min for HT and 5 min for RA.

Conclusions

Both HT and RA can generate similar dose distributions for FDG-PET based dose escalation and dose painting in oropharyngeal SCC patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-017-0793-0) contains supplementary material, which is available to authorized users.

Experimental Validation of Monte Carlo Simulations Based on a Virtual Source Model for TomoTherapy in a RANDO Phantom

A virtual source model for Monte Carlo simulations of helical TomoTherapy has been developed previously by the authors. The purpose of this work is to perform experiments in an anthropomorphic (RANDO) phantom with the same order of complexity as in clinical treatments to validate the virtual source model to be used for quality assurance secondary check on TomoTherapy patient planning dose. Helical TomoTherapy involves complex delivery pattern with irregular beam apertures and couch movement during irradiation. Monte Carlo simulation, as the most accurate dose algorithm, is desirable in radiation dosimetry. Current Monte Carlo simulations for helical TomoTherapy adopt the full Monte Carlo model, which includes detailed modeling of individual machine component, and thus, large phase space files are required at different scoring planes. As an alternative approach, we developed a virtual source model without using the large phase space files for the patient dose calculations previously. In this work, we apply the simulation system to recompute the patient doses, which were generated by the treatment planning system in an anthropomorphic phantom to mimic the real patient treatments. We performed thermoluminescence dosimeter point dose and film measurements to compare with Monte Carlo results. Thermoluminescence dosimeter measurements show that the relative difference in both Monte Carlo and treatment planning system is within 3%, with the largest difference less than 5% for both the test plans. The film measurements demonstrated 85.7% and 98.4% passing rate using the 3 mm/3% acceptance criterion for the head and neck and lung cases, respectively. Over 95% passing rate is achieved if 4 mm/4% criterion is applied. For the dose–volume histograms, very good agreement is obtained between the Monte Carlo and treatment planning system method for both cases. The experimental results demonstrate that the virtual source model Monte Carlo system can be a viable option for the accurate dose calculation of helical TomoTherapy.

In phantom assessment of superficial doses under TomoTherapy irradiation

PURPOSE

Aim of this work is the assessment of build-up and superficial doses of different clinical Head&Neck plans delivered with Helical TomoTherapy (HT) (Accuray, Sunnyvale, CA). Depth dose profiles and superficial dose points were measured in order to evaluate the Treatment Planning System (TPS) capability of an accurate dose modeling in regions of disequilibrium. Geometries and scattering conditions were investigated, similar to the ones generally encountered in clinical treatments.

METHODS

Measurements were performed with two dosimeters: Gafchromic® EBT3 films (Ashland Inc., Wayne, NJ) and a synthetic single crystal diamond detector (PTW-Frieburg microDiamond, MD). A modified version of the Alderson RANDO phantom was employed to house the detectors. A comparison with TPS data was carried out in terms of dose difference (DD) and distance-to-agreement (DTA).

RESULTS

DD between calculated data and MD measurements are within 4% even in points with high spatial dose variation. For depth profiles, EBT3 data show a DDmax of 3.3% and DTAmax of 2.2mm, in low and high gradient regions, respectively, and compare well with MD data. EBT3 superficial points always results in measured doses lower than TPS evaluated ones, with a maximum DTA value of 1.5mm.

CONCLUSIONS

Doses measured with the two devices are in good agreement and compare well with calculated data. The deviations found in the present work are within the reference tolerance level, suggesting that the HT TPS is capable of a precise dose estimation both in superficial regions and in correspondence with interfaces between air and PMMA.

Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: a tomotherapy investigation

PURPOSE

Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy(®) Treatment System plans under varying degrees of rotational beamlet symmetry.

METHODS

The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code geant4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes.

RESULTS

The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case.

CONCLUSIONS

This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

Phantomless patient-specific TomoTherapy QA via delivery performance monitoring and a secondary Monte Carlo dose calculation

PURPOSE

To describe the validation and implementation of a novel quality assurance (QA) system for TomoTherapy using a Monte Carlo (MC)-based secondary dose calculation and CT detector-based multileaf collimator (MLC) leaf opening time measurement QA verification. This system is capable of detecting plan transfer and delivery errors and evaluating the dosimetric impact of those errors.

METHODS

The authors' QA process, MCLogQA, utilizes an independent pretreatment MC secondary dose calculation and postdelivery TomoTherapy exit detector-based MLC sinogram comparison and log file examination to confirm accurate dose calculation, accurate dose delivery, and to verify machine performance. MC radiation transport simulations are performed to estimate patient dose utilizing prestored treatment machine-specific phase-space information, the patient's planning CT, and MLC sinogram data. Sinogram data are generated from both the treatment planning system (MC_TPS) and from beam delivery log files (MC_Log). TomoTherapy treatment planning dose (DTPS) is compared with DMC_TPS and DMC_Log via dose-volume metrics and mean region of interest dose statistics. For validation, in-phantom ionization chamber dose measurements (DIC) for ten sample patient plans are compared with the computed values.

RESULTS

Dose comparisons to in-phantom ion chamber measurements validate the capability of the MCLogQA method to detect delivery errors. DMC_Log agreed with DIC within 1%, while DTPS values varied by 2%-5% compared to DIC. The authors demonstrated that TomoTherapy treatments can be vulnerable to MLC deviations and interfraction output variations during treatment delivery. Interfractional Linac output variations for each patient were approximately 2% and average output was 1%-1.5% below the gold standard. While average MLC leaf opening time error from patient to patient varied from -0.6% to 1.6%, the MLC leaf errors varied little between fractions for the same patient plan, excluding one patient.

CONCLUSIONS

MCLogQA is a new TomoTherapy QA process that validates the planned dose before delivery and analyzes the delivered dose using the treatment exit detector and log file data. The MCLogQA procedure is an effective and efficient alternative to traditional phantom-based TomoTherapy plan-specific QA because it allows for comprehensive 3D dose verification, accounts for tissue heterogeneity, uses patient CT density tables, reduces total QA time, and provides for a comprehensive QA methodology for each treatment fraction.

Fast 3D dosimetric verifications based on an electronic portal imaging device using a GPU calculation engine

Purpose

To use a graphic processing unit (GPU) calculation engine to implement a fast 3D pre-treatment dosimetric verification procedure based on an electronic portal imaging device (EPID).

Methods

The GPU algorithm includes the deconvolution and convolution method for the fluence-map calculations, the collapsed-cone convolution/superposition (CCCS) algorithm for the 3D dose calculations and the 3D gamma evaluation calculations. The results of the GPU-based CCCS algorithm were compared to those of Monte Carlo simulations. The planned and EPID-based reconstructed dose distributions in overridden-to-water phantoms and the original patients were compared for 6 MV and 10 MV photon beams in intensity-modulated radiation therapy (IMRT) treatment plans based on dose differences and gamma analysis.

Results

The total single-field dose computation time was less than 8 s, and the gamma evaluation for a 0.1-cm grid resolution was completed in approximately 1 s. The results of the GPU-based CCCS algorithm exhibited good agreement with those of the Monte Carlo simulations. The gamma analysis indicated good agreement between the planned and reconstructed dose distributions for the treatment plans. For the target volume, the differences in the mean dose were less than 1.8%, and the differences in the maximum dose were less than 2.5%. For the critical organs, minor differences were observed between the reconstructed and planned doses.

Conclusions

The GPU calculation engine was used to boost the speed of 3D dose and gamma evaluation calculations, thus offering the possibility of true real-time 3D dosimetric verification.

Monte Carlo evaluation of the dose calculation algorithm of TomoTherapy for clinical cases in dynamic jaws mode

PURPOSE

For the TomoTherapy(®) system, longitudinal conformation can be improved by selecting a smaller field width but at the expense of longer treatment time. Recently, the TomoEdge(®) feature has been released with the possibility to move dynamically the jaws at the edges of the target volume, improving longitudinal penumbra and enabling faster treatments. Such delivery scheme requires additional modeling of treatment delivery. Using a previously validated Monte Carlo model (TomoPen), we evaluated the accuracy of the implementation of TomoEdge in the new dose engine of TomoTherapy for 15 clinical cases.

METHODS

TomoPen is based on PENELOPE. Particle tracking in the treatment head is performed almost instantaneously by 1) reading a particle from a phase-space file corresponding to the largest field and 2) correcting the weight of the particle depending on the actual jaw and MLC configurations using Monte Carlo pre-generated data. 15 clinical plans (5 head-and-neck, 5 lung and 5 prostate tumors) planned with TomoEdge and with the last release of the treatment planning system (VoLO(®)) were re-computed with TomoPen. The resulting dose-volume histograms were compared.

RESULTS

Good agreement was achieved overall, with deviations for the target volumes typically within 2% (D95), excepted for small lung tumors (17 cm(3)) where a maximum deviation of 4.4% was observed for D95. The results were consistent with previously reported values for static field widths.

CONCLUSIONS

For the clinical cases considered in the present study, the introduction of TomoEdge did not impact significantly the accuracy of the computed dose distributions.

Generation of prescriptions robust against geometric uncertainties in dose painting by numbers

BACKGROUND

In the context of dose painting by numbers delivered with intensity-modulated radiotherapy, the robustness of dose distributions against geometric uncertainties can be ensured by robust optimization. As robust optimization is seldom available in treatment planning systems (TPS), we propose an alternative method that reaches the same goal by modifying the heterogeneous dose prescription (based on (18)FDG-PET) and guarantees coverage in spite of systematic and random errors with known standard deviations Σ and σ, respectively.

MATERIAL AND METHODS

The objective was that 95% of all voxels in the GTVPET received at least 95% of the prescribed dose despite geometric errors. The prescription was modified by a geometric dilation of αΣ for systematic errors and a deconvolution by a Gaussian function of width σ for random errors. For a 90% confidence interval, α = 2.5. Planning was performed on a TomoTherapy system, such that 95% of the voxels received at least 95% of the modified prescription and less than 5% of the voxels received more than 105% of the modified prescription. The applicability of the method was illustrated for two head-and-neck tumors.

RESULTS

Systematic and random displacements larger than αΣ and σ degraded coverage. Down to 62.8% of the points received at least 95% of prescribed dose for the largest considered displacements (5 mm systematic translation and 3 mm standard deviation for random errors). When systematic and random displacements were smaller than αΣ and σ, no degradation of target coverage was observed.

CONCLUSIONS

The method led to treatment plans with target coverage robust against geometric uncertainties without the need to incorporate these in the optimizer of the TPS. The methodology was illustrated for head-and-neck cancer but can be potentially extended to all treatment sites.

A virtual source model for Monte Carlo simulation of helical tomotherapy

The purpose of this study was to present a Monte Carlo (MC) simulation method based on a virtual source, jaw, and MLC model to calculate dose in patient for helical tomotherapy without the need of calculating phase‐space files (PSFs). Current studies on the tomotherapy MC simulation adopt a full MC model, which includes extensive modeling of radiation source, primary and secondary jaws, and multileaf collimator (MLC). In the full MC model, PSFs need to be created at different scoring planes to facilitate the patient dose calculations. In the present work, the virtual source model (VSM) we established was based on the gold standard beam data of a tomotherapy unit, which can be exported from the treatment planning station (TPS). The TPS‐generated sinograms were extracted from the archived patient XML (eXtensible Markup Language) files. The fluence map for the MC sampling was created by incorporating the percentage leaf open time (LOT) with leaf filter, jaw penumbra, and leaf latency contained from sinogram files. The VSM was validated for various geometry setups and clinical situations involving heterogeneous media and delivery quality assurance (DQA) cases. An agreement of <1% was obtained between the measured and simulated results for percent depth doses (PDDs) and open beam profiles for all three jaw settings in the VSM commissioning. The accuracy of the VSM leaf filter model was verified in comparing the measured and simulated results for a Picket Fence pattern. An agreement of <2% was achieved between the presented VSM and a published full MC model for heterogeneous phantoms. For complex clinical head and neck (HN) cases, the VSM‐based MC simulation of DQA plans agreed with the film measurement with 98% of planar dose pixels passing on the 2%/2 mm gamma criteria. For patient treatment plans, results showed comparable dose‐volume histograms (DVHs) for planning target volumes (PTVs) and organs at risk (OARs). Deviations observed in this study were consistent with literature. The VSM‐based MC simulation approach can be feasibly built from the gold standard beam model of a tomotherapy unit. The accuracy of the VSM was validated against measurements in homogeneous media, as well as published full MC model in heterogeneous media.

PACS numbers: 87.53.‐j, 87.55.K‐

Independent calculation of dose distributions for helical tomotherapy using a conventional treatment planning system

PURPOSE

The dosimetric verification of treatment plans in helical tomotherapy usually is carried out via verification measurements. In this study, a method for independent dose calculation of tomotherapy treatment plans is presented, that uses a conventional treatment planning system with a pencil kernel dose calculation algorithm for generation of verification dose distributions based on patient CT data.

METHODS

A pencil beam algorithm that directly uses measured beam data was configured for dose calculation for a tomotherapy machine. Tomotherapy treatment plans were converted into a format readable by an in-house treatment planning system by assigning each projection to one static treatment field and shifting the calculation isocenter for each field in order to account for the couch movement. The modulation of the fluence for each projection is read out of the delivery sinogram, and with the kernel-based dose calculation, this information can directly be used for dose calculation without the need for decomposition of the sinogram. The sinogram values are only corrected for leaf output and leaf latency. Using the converted treatment plans, dose was recalculated with the independent treatment planning system. Multiple treatment plans ranging from simple static fields to real patient treatment plans were calculated using the new approach and either compared to actual measurements or the 3D dose distribution calculated by the tomotherapy treatment planning system. In addition, dose-volume histograms were calculated for the patient plans.

RESULTS

Except for minor deviations at the maximum field size, the pencil beam dose calculation for static beams agreed with measurements in a water tank within 2%/2 mm. A mean deviation to point dose measurements in the cheese phantom of 0.89% ± 0.81% was found for unmodulated helical plans. A mean voxel-based deviation of -0.67% ± 1.11% for all voxels in the respective high dose region (dose values >80%), and a mean local voxel-based deviation of -2.41% ± 0.75% for all voxels with dose values >20% were found for 11 modulated plans in the cheese phantom. Averaged over nine patient plans, the deviations amounted to -0.14% ± 1.97% (voxels >80%) and -0.95% ± 2.27% (>20%, local deviations). For a lung case, mean voxel-based deviations of more than 4% were found, while for all other patient plans, all mean voxel-based deviations were within ± 2.4%.

CONCLUSIONS

The presented method is suitable for independent dose calculation for helical tomotherapy within the known limitations of the pencil beam algorithm. It can serve as verification of the primary dose calculation and thereby reduce the need for time-consuming measurements. By using the patient anatomy and generating full 3D dose data, and combined with measurements of additional machine parameters, it can substantially contribute to overall patient safety.

Risk of secondary cancers from scattered radiation during intensity-modulated radiotherapies for hepatocellular carcinoma

PURPOSE

To evaluate and compare the risks of secondary cancers from therapeutic doses received by patients with hepatocellular carcinoma (HCC) during intensity-modulated radiotherapy (IMRT), volumetric arc therapy (VMAT), and tomotherapy (TOMO).

METHODS

Treatments for five patients with hepatocellular carcinoma (HCC) were planned using IMRT, VMAT, and TOMO. Based on the Biological Effects of Ionizing Radiation VII method, the excess relative risk (ERR), excess absolute risk (EAR), and lifetime attributable risk (LAR) were evaluated from therapeutic doses, which were measured using radiophotoluminescence glass dosimeters (RPLGDs) for each organ inside a humanoid phantom.

RESULTS

The average organ equivalent doses (OEDs) of 5 patients were measured as 0.23, 1.18, 0.91, 0.95, 0.97, 0.24, and 0.20 Gy for the thyroid, lung, stomach, liver, small intestine, prostate (or ovary), and rectum, respectively. From the OED measurements, LAR incidence were calculated as 83, 46, 22, 30, 2 and 6 per 10(4) person for the lung, stomach, normal liver, small intestine, prostate (or ovary), and rectum.

CONCLUSIONS

We estimated the secondary cancer risks at various organs for patients with HCC who received different treatment modalities. We found that HCC treatment is associated with a high secondary cancer risk in the lung and stomach.

Impact of the number of discrete angles used during dose computation for TomoTherapy treatments

PURPOSE

To quantify systematically the effect on accuracy of discretizing gantry rotation during the dose calculation process of TomoTherapy treatments.

METHODS

Up to version 4.0.x included, TomoTherapy treatment planning system (TPS) approximates gantry rotation by computing dose from 51 discrete angles corresponding to the center of the projections used to control the binary multileaf collimator. Potential effects on dose computation accuracy for off-axis targets and low modulation factors have been shown previously for a few treatment configurations. In versions 4.1.x and later, TomoTherapy oversamples the projections to better account for gantry rotation, but only during full scatter optimization and final calculation (i.e., not during optimization in "beamlet" mode). The effect on accuracy of changing the number of angles was quantified with the following framework: (1) predict the impact of the discretization of gantry rotation for various modulation factors, target sizes, and off-axis positions using a simplified analytical algorithm; (2) perform regular quality assurance using measurements with EDR2 radiographic films; (3) isolating the effect of changing the number of discretized angles only (51, 153, and 459) using a previously validated Monte Carlo model (TomoPen). The diameters of the targets were 2, 3, and 5 cm; off-axis central positions of target volumes were 5, 10 and 15, and 17 cm (when accepted by the treatment unit); planned modulation factors were 1.3 and 2.0.

RESULTS

For extreme configurations (3 cm tumor, 1.3 modulation factor, 15 cm off-axis position), effects on dose distributions were significant with 89.3% and 95.4% of the points passing gamma tests with 2%∕2 mm and 3%∕3 mm criteria, respectively, for TPS software version 4.0.x (51 gantry angles). The passing rate was 100% for both gamma criteria for the 4.1.x version (153 gantry angles). Those differences could be attributed almost completely to gantry motion discretization using TomoPen. Using 51 gantry angles for dose computation, TomoPen reproduced within statistical uncertainties (<1% standard deviation) dose distributions computed with version 4.0.x. Using 153 and 459 gantry angles, TomoPen reproduced within statistical uncertainties measurements and dose distributions computed with version 4.1.x.

CONCLUSIONS

When low modulation factors and significant off-axis positions are used, accounting for gantry rotation during dose computation using at least 153 gantry angles is required to ensure optimal accuracy.

Helical tomotherapy for SIB and hypo-fractionated treatments in lung carcinomas: a 4D Monte Carlo treatment planning study

PURPOSE

To evaluate the impact of intra-fraction motion induced by regular breathing on treatment quality for helical tomotherapy treatments.

MATERIAL AND METHODS

Four patients treated by simultaneous-integrated boost (SIB) and three by hypo-fractionated stereotactic treatments (hypo-fractionated, 18 Gy/fraction) were included. All patients were coached to ensure regular breathing. For the SIB group, the tumor volume was delineated using CT information only (CTV(CT)) and the boost region was based on PET information (GTV(PET), no CTV extension). In the hypo-fractionated group, a GTV based on CT information was contoured. In both groups, ITVs were defined according to 4D data. The PTV included the ITV plus a setup error margin. The treatment was planned using the tomotherapy TPS on 3D CT images. In order to verify the impact of intra-fraction motion and interplay effects, dose calculations were performed using a previously validated Monte Carlo model of tomotherapy (TomoPen): first on the planning 3D CT ("planned dose") and second, on the 10 phases of the 4D scan. For the latter, two dose distributions, termed "interplay simulated" or "no interplay" were computed with and without beamlet-phase correlation over the 10 phases and combined using deformable dose registration.

RESULTS

In all cases, DVHs of "interplay simulated" dose distributions complied within 1% of the original clinical objectives used for planning, defined according to ICRU (report 83) and RTOG (trials 0236 and 0618) recommendations, for SIB and hypo-fractionated groups, respectively. For one patient in the hypo-fractionated group, D(mean) to the CTV(CT) was 2.6% and 2.5% higher than "planned" for "interplay simulated" and "no interplay", respectively.

CONCLUSION

For the patients included in this study, assuming regular breathing, the results showed that interplay of breathing and tomotherapy delivery motions did not affect significantly plan delivery accuracy. Hence, accounting for intra-fraction motion through the definition of an ITV volume was sufficient to ensure tumor coverage.

Validation of GPU based TomoTherapy dose calculation engine

PURPOSE

The graphic processing unit (GPU) based TomoTherapy convolution/superposition(C/S) dose engine (GPU dose engine) achieves a dramatic performance improvement over the traditional CPU-cluster based TomoTherapy dose engine (CPU dose engine). Besides the architecture difference between the GPU and CPU, there are several algorithm changes from the CPU dose engine to the GPU dose engine. These changes made the GPU dose slightly different from the CPU-cluster dose. In order for the commercial release of the GPU dose engine, its accuracy has to be validated.

METHODS

Thirty eight TomoTherapy phantom plans and 19 patient plans were calculated with both dose engines to evaluate the equivalency between the two dose engines. Gamma indices (Γ) were used for the equivalency evaluation. The GPU dose was further verified with the absolute point dose measurement with ion chamber and film measurements for phantom plans. Monte Carlo calculation was used as a reference for both dose engines in the accuracy evaluation in heterogeneous phantom and actual patients.

RESULTS

The GPU dose engine showed excellent agreement with the current CPU dose engine. The majority of cases had over 99.99% of voxels with Γ(1%, 1 mm) < 1. The worst case observed in the phantom had 0.22% voxels violating the criterion. In patient cases, the worst percentage of voxels violating the criterion was 0.57%. For absolute point dose verification, all cases agreed with measurement to within ±3% with average error magnitude within 1%. All cases passed the acceptance criterion that more than 95% of the pixels have Γ(3%, 3 mm) < 1 in film measurement, and the average passing pixel percentage is 98.5%-99%. The GPU dose engine also showed similar degree of accuracy in heterogeneous media as the current TomoTherapy dose engine.

CONCLUSIONS

It is verified and validated that the ultrafast TomoTherapy GPU dose engine can safely replace the existing TomoTherapy cluster based dose engine without degradation in dose accuracy.

Impact of the calculation resolution of AAA for small fields and RapidArc treatment plans

PURPOSE

To investigate the impact of the calculation resolution of the anisotropic analytical algorithms (AAA) for a variety of small fields in homogeneous and heterogeneous media and for RapidArc plans.

METHODS

Dose distributions calculated using AAA version 8.6.15 (AAA8) and 10.0.25 (AAA10) were compared to measurements performed with GafChromic EBT film, using phantoms made of polystyrene or a combination of polystyrene and cork. The accuracy of the algorithms calculated using grid resolutions of 2.5 and 1.0 mm was investigated for different field sizes, and for a limited selection of RapidArc plans (head and neck, small meningioma, and lung). Additional plans were optimized to create excessive multileaf collimator modulation and measured on a homogenous phantom. Gamma evaluation criterion of 3% dose difference and 2- or 1-mm distance to agreement (DTA) were applied to evaluate the accuracy of the algorithms.

RESULTS

For fields < or = 3 x 3 cm2, both versions of AAA predicted lower peak doses and broader penumbra widths than the measurements. However, AAA10 and a finer calculation grid improved the agreement. For RapidArc plans with many small multileaf collimator (MLC) segments and relatively high number of monitor units (MU), AAA8 failed to identify small dose peaks within the target. Both versions performed better in polystyrene than in cork. In homogeneous cork layers, AAA8 underestimated the average target dose for a clinical lung plan. This was improved with AAA10 calculated using a 1 mm grid.

CONCLUSIONS

AAA10 improves the accuracy of dose calculations, and calculation grid of 1.0 mm is superior to using 2.5 mm, although calculation times increased by factor of 5. A suitable upper MU constraint should be assigned during optimization to avoid plans with high modulation. For plans with a relative high number of monitor units, calculations using 1 mm grid resolution are recommended. For planning target volume (PTV) which contains relatively large area of low density tissue, users should be aware of possible dose underestimation in the low density region and recalculation with AAA10 grid 1.0 mm is recommended.

Dosimetric and clinical review of helical tomotherapy

As a modality for delivering rotational therapy, helical tomotherapy offers dosimetric advantages by combining a continuously rotating gantry with a binary multileaf collimator. Helical tomotherapy, embodied in the TomoTherapy(®) Hi-Art II(®) system, delivers intensity-modulated fan beams in a helical pattern using binary multileaf collimator leaves while the couch is translated through the gantry. Helical tomotherapy offers the possibility of treating a variety of cases--from simple to complex--with improved target conformality and sensitive structure sparing compared with 3D or conventional static field IMRT plans, thereby allowing biologically effective dose escalation. For precise irradiation and possible treatment adaptation, the fully integrated on-board image-guidance system provides online volumetric images of patient anatomy using 3.5-MV x-ray beams and the xenon computed tomography detector. Several review articles were published before the year 2007 but emphasized the technical aspects of helical tomotherapy. In this article, we review very recent papers and focus on the dosimetric and clinical aspects of helical tomotherapy.

A slit method to determine the focal spot size and shape of TomoTherapy system

PURPOSE

To obtain accurate x-ray source profile measurements using a slit-collimator, the slit-collimator should have a narrow width, large height, and be positioned near the source. However, these conditions may not always be met. In this paper, the authors provide a detailed analysis of the slit measurement geometry and the relationship between the slit parameters and the measured x-ray source profile. The slit model allows the use of a shorter and more easily available slit-collimator, while accurate source profile measurements can still be obtained.

METHODS

Measurements were performed with a variety of slit widths and/or slit to source distances. The relationship derived between the slit parameters and the measured profile was used to determine the true focal spot profile through a least square fit of the profile data. The model was verified by comparing the predicted profiles at a variety of slit-collimator parameters with the measured results on the TomoTherapy Hi-Art system.

RESULTS

Both the treatment beam and the imaging beam were measured. For treatment mode, it was found that a source consisting of one Gaussian with a 0.75 mm full-width-half-maximum (FWHM) and 72% peak amplitude and a second Gaussian with a 2.27 mm FWHM and 18% peak amplitude matched measurement profiles. The overall source profile has a FWHM of 0.93 mm, but with a higher amplitude in the tail region than a single Gaussian. For imaging mode, the source consists of one Gaussian with a 0.68 mm FWHM and 82% peak amplitude and a second Gaussian with a 1.83 mm FWHM and 18% peak amplitude. The overall source profile has a FWHM of 0.77 mm.

CONCLUSIONS

Our study of the focal spot measurement using slit-collimators showed that accurate source profile measurements can be achieved through fitting of measurement results at different slit widths and source-to-slit distances (SSD). Quantitative measurements of the TomoTherapy linac focal spot showed that the source distribution could be better described with a model consisting of two Gaussian components rather than a single Gaussian model as assumed in previous studies.

Ultrafast convolution/superposition using tabulated and exponential kernels on GPU

PURPOSE

Collapsed-cone convolution/superposition (CCCS) dose calculation is the workhorse for IMRT dose calculation. The authors present a novel algorithm for computing CCCS dose on the modern graphic processing unit (GPU).

METHODS

The GPU algorithm includes a novel TERMA calculation that has no write-conflicts and has linear computation complexity. The CCCS algorithm uses either tabulated or exponential cumulative-cumulative kernels (CCKs) as reported in literature. The authors have demonstrated that the use of exponential kernels can reduce the computation complexity by order of a dimension and achieve excellent accuracy. Special attentions are paid to the unique architecture of GPU, especially the memory accessing pattern, which increases performance by more than tenfold.

RESULTS

As a result, the tabulated kernel implementation in GPU is two to three times faster than other GPU implementations reported in literature. The implementation of CCCS showed significant speedup on GPU over single core CPU. On tabulated CCK, speedups as high as 70 are observed; on exponential CCK, speedups as high as 90 are observed.

CONCLUSIONS

Overall, the GPU algorithm using exponential CCK is 1000-3000 times faster over a highly optimized single-threaded CPU implementation using tabulated CCK, while the dose differences are within 0.5% and 0.5 mm. This ultrafast CCCS algorithm will allow many time-sensitive applications to use accurate dose calculation.

Dosimetric accuracy of tomotherapy dose calculation in thorax lesions

BACKGROUND

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.

METHODS

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.

RESULTS

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%.

CONCLUSIONS

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.

Helical tomotherapy versus single-arc intensity-modulated arc therapy: a collaborative dosimetric comparison between two institutions

PURPOSE

Both helical tomotherapy (HT) and single-arc intensity-modulated arc therapy (IMAT) deliver radiation using rotational beams with multileaf collimators. We report a dual-institution study comparing dosimetric aspects of these two modalities.

METHODS AND MATERIALS

Eight patients each were selected from the University of Maryland (UMM) and the University of Wisconsin Cancer Center Riverview (UWR), for a total of 16 cases. Four cancer sites including brain, head and neck (HN), lung, and prostate were selected. Single-arc IMAT plans were generated at UMM using Varian RapidArc (RA), and HT plans were generated at UWR using Hi-Art II TomoTherapy. All 16 cases were planned based on the identical anatomic contours, prescriptions, and planning objectives. All plans were swapped for analysis at the same time after final approval. Dose indices for targets and critical organs were compared based on dose-volume histograms, the beam-on time, monitor units, and estimated leakage dose. After the disclosure of comparison results, replanning was done for both techniques to minimize diversity in optimization focus from different operators.

RESULTS

For the 16 cases compared, the average beam-on time was 1.4 minutes for RA and 4.8 minutes for HT plans. HT provided better target dose homogeneity (7.6% for RA and 4.2% for HT) with a lower maximum dose (110% for RA and 105% for HT). Dose conformation numbers were comparable, with RA being superior to HT (0.67 vs. 0.60). The doses to normal tissues using these two techniques were comparable, with HT showing lower doses for more critical structures. After planning comparison results were exchanged, both techniques demonstrated improvements in dose distributions or treatment delivery times.

CONCLUSIONS

Both techniques created highly conformal plans that met or exceeded the planning goals. The delivery time and total monitor units were lower in RA than in HT plans, whereas HT provided higher target dose uniformity.

Helical TomoTherapy versus sterotactic Gamma Knife radiosurgery in the treatment of single and multiple brain tumors: a dosimetric comparison

The objective was to compare the dosimetry of Helical TomoTherapy (TOMO) and Gamma Knife (GK) treatment plans for tumor and normal brain in the treatment of single and multiple brain tumors. An anthropomorphic Rando Head phantom was used to compare the dosimetry of TOMO and GK. Eight brain tumors of various shapes, sizes and locations were used to generate 10 plans. The radiation dose was 20 Gy prescribed to the 100% isodose line for TOMO plans and to the 50% for the GK plans. Dose Volume Histograms for tumor and brain were compared. Equivalent Uniform Dose (gEUD), Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP) were performed and used for plan comparisons. Average minimum, mean, median and maximum tumor doses were 19.93, 27.83, 27.38, 39.60 Gy for GK and 20.17, 20.60, 20.59, 20.90 Gy for TOMO. Average gEUD values for tumor and normal brain were 25.0 and 7.2 Gy for GK and 20.7 and 8.1 Gy for TOMO. Conformity indices (CI) were similar for both modalities. Gradient indices (GI) were greater for TOMO. A combination plan was also generated using all eight tumors. TOMO was able to target all eight tumors simultaneously resulting in mean tumor and brain doses of 20.5 and 9.35 Gy, respectively. Due to the maximum limit of 50 beams per plan, GK was unable to provide a treatment plan for all eight tumors. GK provides an advantage for all tumor sizes with respect to tumor and normal brain dose. Clinical studies are needed to correlate these dosimetric findings with patient outcomes.

PACS number: 87.55‐x