Tomotherapy Treatment Planning System Research Articles

Adaptive radiotherapy dosimetry in a challenging geometry: A model gas-filled tissue expander in a helical TomoTherapy beam

Helical TomoTherapy treatment and delivery systems (Accuray Inc, Sunnyvale, USA) allow off-line adaptation of radiotherapy treatments, with dose calculations that use MV computed tomography (CT) data acquired at treatment. This study aimed to assess the potential dosimetric effects of a gas-filled temporary tissue expander (TTE) on the accuracy of breast radiotherapy dose calculations from both the TomoTherapy treatment planning system (TPS), which uses kV CT data, and the TomoTherapy adaptive radiotherapy (ART) system, which uses MV CT data. A TomoTherapy treatment plan was created and delivered to a 3D-printed rectilinear model of a breast with implanted gas-filled TTE, including a stainless steel CO2 container, and film measurements of the delivered dose were compared against dose calculations from the TPS and ART systems. The film measurements showed that the TomoTherapy TPS provided comparatively accurate dose calculations in the ~550 cm2 volume of air that modelled the gas filling of the TTE and within the surrounding tissue-equivalent materials, except in regions where the beam was transmitted through the stainless steel CO2 container, possibly due to the volume of stainless steel being over-estimated in the kV CT images that were used to generate the treatment plan. The ART system provided more accurate dose calculations than the TPS in regions affected by the stainless steel container, but also over-estimated the dose in the air within the TTE. These results suggest that the TomoTherapy TPS and ART systems could be used to produce reliable dose calculations of breast treatments in the presence of gas-filled TTEs, if kV CT imaging options are chosen to avoid artefacts and minimise the need for density over-rides and if treatment targets that include only clinically relevant tissues, and exclude all TTE components, are used to evaluate and compare the doses calculated by both systems.

Couch modeling optimization for tomotherapy planning and delivery.

We sought to validate new couch modeling optimization for tomotherapy planning and delivery. We constructed simplified virtual structures just above a default setting couch through a planning support system (MIM Maestro, version 8.2, MIM Software Inc, Cleveland, OH, USA). Based on ionization chamber measurements, we performed interactive optimization and determined the most appropriate physical density of these virtual structures in a treatment planning system (TPS). To validate this couch optimization, Gamma analysis and these statistical analyses between a three‐dimensional diode array QA system (ArcCHECK, Sun Nuclear, Melbourne, FL, USA) results and calculations from ionization chamber measurements were performed at 3%/2 mm criteria with a threshold of 10% in clinical QA plans. Using a virtual model consisting of a center slab density of 4.2 g/cm3 and both side slabs density of 1.9 g/cm3, we demonstrated close agreement between measured dose and the TPS calculated dose. Agreement was within 1% for all gantry angles at the isocenter and within 2% in off‐axis plans. In validation of the couch modeling in a clinical QA plan, the average gamma passing rate improved approximately 0.6%–5.1%. It was statistically significant (P < 0.05) for all treatment sites. We successfully generated an accurate couch model for a TomoTherapy TPS by interactively optimizing the physical density of the couch using a planning support system. This modeling proved to be an efficient way of correcting the dosimetric effects of the treatment couch in tomotherapy planning and delivery.

Utilising virtual bolus in superficial planning target volume dose optimisation (TomoTherapy): a phantom study

AbstractAim:This is a phantom study to evaluate the dosimetry effects of using virtual bolus (VB) in TomoTherapy Treatment Planning System (TPS) optimisation for superficial planning target volume (PTV) that extends to the body surface. Without VB, the inverse-planning TPS will continuously boost the photon fluence at the surface of the superficial PTV due to lack of build-up region. VB is used during TPS optimisation only and will not be present in actual treatment delivery.Materials and methods:In this study, a dummy planning target was contoured on a cylindrical phantom which extends to the phantom surface, and VB of various combinations of thickness and density was used in treatment planning optimisation with TomoTherapy TPS. The plans were then delivered with the treatment modality TomoTherapy. Radiochromic films (Gafchromic EBT3) were calibrated and used for dose profiles measurements. TomoTherapy Planned-Adaptive software was used to analyse the delivered Dose-Volume Histograms (DVHs).Results:The use of 2 mm VB was not providing adequate build-up area and was unable to reduce the hot spots during treatment planning and actual delivery. The use of 4 mm VB was able to negate the photon fluence boosting effect by the TPS, and the actual delivery showed relatively small deviations from the treatment plan. The use of 6 mm VB caused significant dose overestimation by the TPS in the superficial regions resulting in insufficient dose coverage delivered.Findings:VB with the combination of 4 mm thickness and 1·0 g/cc density provides the most robust solution for the TomoTherapy TPS optimisation of superficial PTV.

Integration of Diffusion Magnetic Resonance Tractography into tomotherapy radiation treatment planning for high-grade gliomas

IntroductionFractionated radiotherapy in brain tumors is commonly associated with several detrimental effects, largely related to the higher radiosensitivity of the white matter (WM) with respect to gray matter. However, no dose constraints are applied to preserve WM structures at present. Magnetic Resonance (MR) Tractography is the only technique that allows to visualize in vivo the course of WM eloquent tracts in the brain. In this study, the feasibility of integrating MR Tractography in tomotherapy treatment planning has been investigated, with the aim to spare eloquent WM regions from the dose delivered during treatment. MethodsNineteen high grade glioma patients treated with fractionated radiotherapy were enrolled. All the patients underwent pre-treatment MR imaging protocol including Diffusion Tensor Imaging (DTI) acquisitions for MR Tractography analysis. Bilateral tracts involved in several motor, language, cognitive functions were reconstructed and these fiber bundles were integrated into the Tomotherapy Treatment planning system. The original plans without tracts were compared with the optimized plans incorporating the fibers, to evaluate doses to WM structures in the two differently optimized plans. ResultsNo significant differences were found between plans in terms of planning target volume (PTV) coverage between the original plans and the optimized plans incorporating fiber tracts. Comparing the mean as well as the maximal dose (Dmean and Dmax), a significant dose reduction was found for most of the tracts. The dose sparing was more relevant for contralateral tracts (P < 0.0001). ConclusionThe integration of MR Tractography into radiotherapy planning is feasible and beneficial to preserve important WM structures without reducing the clinical goal of radiation treatment.

Relevance in Standardizing Hearing Apparatus Contours

The potential for sensorineural hearing loss (SNHL) is an unfortunate reality with patients who receive radiation therapy (RT) to the head and neck (H&N) region. A review of the literature seems to indicate there is a lack in standards for contours associated with SNHL. The range of contoured structures reported by institutions include the tympanic cavity, Eustachian tube, mastoid process, cochlea, vestibule, inter auditory canal (IAC), and various combinations of the aforementioned items. At our institution, we have historically contoured the cochlea, vestibule, and a portion of the inner auditory canal as a single volume for each side (i.e. the bony labyrinth of the inner ear) and used that structure for optimization and dosimetric analysis. This retrospective study examines how much dose the cochlea, vestibule, and IAC individually received when the bony labyrinth of the inner ear was contoured and used as an avoidance structure during planning. Twenty consecutive patients who received IMRT for H&N cancer in 2015 at our institution were selected for this study. A Phillips Brilliance Big Bore 16 slice CT scanner was used for the RT simulation and images were acquired at a slice thickness of 3 mm. Contouring was done on a Phillips Pinnacle treatment planning system (TPS) and planned on an Accuray TomoTherapy TPS. The dose was transferred from the TomoTherapy TPS to the Pinnacle TPS where dose statistics were collected. A paired t-test analysis was performed with 40 data points of each structure (20 entries from each anatomical side) using SPSS software. Mean dose to our planning bony labyrinth organs at risk (OAR) was 12.95 Gy (standard deviation (SD) 9.96; standard error mean (SEM) 1.58). Cochlea mean dose = 13.66 Gy (10.42 SD; 1.65 SEM); Vestibule mean dose 12.05 Gy (9.57 SD, 1.51 SEM); and IAC mean dose = 14.24 Gy (11.35 SD, 1.80 SEM). Statistically significant differences were seen as we compared mean doses between our bony labyrinth to the cochlea (P=.0168) and the vestibule (P = .0001), but not to the IAC (P = .0877). Our results showed significant differences between the mean dose of the bony labyrinth versus the mean doses to the cochlea and vestibule but not with the IAC when the bony labyrinth of the inner ear is contoured as a single structure instead of having separate delineation of its components. Considering that there is ongoing research investigating the potential differences in tolerances for the various hearing structures, it is important to keep in mind that non-standard or ill-defined contouring of these structures may result in variations of dosimetric data between studies. This may lead to false correlations between dose given to OAR’s and clinical effect.

Evaluation of TomoTherapy dose calculations with intrafractional motion and motion compensation.

Anatomical motion, both cyclical and aperiodic, can impact the dose delivered during external beam radiation. In this work, we evaluate the use of a research version of the clinical TomoTherapy® dose calculator to calculate dose with intrafraction rigid motion. We also evaluate the feasibility of a method of motion compensation for helical tomotherapy using the jaws and MLC. Treatment plans were created using the TomoTherapy treatment planning system. Dose was recalculated for several simple rigid motion traces including a 4 mm step motion applied either longitudinally or transversely, and a sinusoidal motion. The calculated dose volumes were compared to dose measurements that were performed by translating the phantom with the same motion traces used in the calculations. Measurements were made using film and ion chambers. Finally, the delivery plans were modified to compensate for the motion by sweeping the jaws for longitudinal motion and shifting the MLC leaves for transverse motion, and the calculations and measurements were repeated. A transverse step motion shifted the dose that was delivered after the step occurred, but otherwise did not impact the dose distribution. Film measurements agreed with dose calculations to within 2%/2 mm for 99% of dose points within the 50% isodose line. A shift in the MLC leaf delivery pattern successfully compensated for the step motion to within the 3 mm accuracy allowed by the finite leaf widths. A longitudinal step motion impacted the dose in the interior of the target volume to a degree that was dependent on the planning field width and step size. Film measurements agreed with dose calculations to within 2%/2 mm for 98% of dose points within the 50% isodose line. Shifts in the jaw position successfully compensated for the longitudinal step motion. Sinusoidal (breathing-like) motion was also studied, with similar results. A research version of the clinical TomoTherapy dose calculator has been shown to accurately calculate the dose from treatment plans delivered in the presence of arbitrary rigid motion. Modifications to the delivery plan using jaw and MLC leaf shifts that follow the motion can successfully compensate for the target motion.

Hippocampal-sparing and target volume coverage in treating 3 to 10 brain metastases: A comparison of Gamma Knife, single-isocenter VMAT, CyberKnife, and TomoTherapy stereotactic radiosurgery

PurposeOur purpose was to evaluate hippocampal doses and target volume coverage with and without hippocampal sparing when treating multiple brain metastases using various stereotactic radiosurgery (SRS) platforms. Methods and materialsWe selected 10 consecutive patients with 14 separate treatments who had been treated in our department for 3 to 10 brain metastases and added hippocampal avoidance contours. All 14 treatments were planned with GammaPlan for Gamma Knife, Eclipse for single isocenter volumetric modulated arc therapy (VMAT), TomoTherapy Treatment Planning System (TPS) for TomoTherapy, and MultiPlan for CyberKnife. Initial planning was performed with the goal of planning target volume coverage of V100 ≥95% without hippocampal avoidance. If the maximum hippocampal point dose (Dmax) was <6.6 Gy in a single fraction and <40% of the hippocampi received ≤4.5 Gy, no second plan was performed. If either constraint was not met, replanning was performed with these constraints. ResultsThere was a median of 6 metastases per plan, with an average total tumor volume of 7.32 mL per plan. The median hippocampal Dmax (in Gy) without sparing averaged 1.65, 9.81, 4.38, and 5.46, respectively (P < .0001). Of 14 plans, 3 Gamma Knife and CyberKnife plans required replanning, whereas 13 VMAT and 8 TomoTherapy plans required replanning. The hippocampal constraints were not achievable in 1 plan on any platform when the tumor was bordering the hippocampus. The mean volume of brain receiving 12 Gy (in mL), which has been associated with symptomatic radionecrosis, was 23.57 with Gamma Knife, 76.77 with VMAT, 40.86 with CyberKnife, and 104.06 with TomoTherapy (P = .01). The overall average conformity indices for all plans ranged from 0.36 to 0.52. ConclusionsEven with SRS, the hippocampi can receive a considerable dose; however, if the hippocampi are outlined as organs of risk, sparing these structures is feasible in nearly all situations with all 4 platforms, without detriment to target coverage, and should be considered in all patients undergoing SRS for multiple brain metastases. SummaryHippocampi play an important role in memory, and sparing of these structures in whole brain radiation can improve neurocognitive outcomes. The hippocampi are not routinely spared when using stereotactic radiosurgery. We evaluated the incidental dose to the hippocampi when treating multiple brain metastases and sought to examine if hippocampal sparing is feasible without detriment to target coverage. We found that hippocampal sparing is possible without affecting coverage or conformality in most cases across treatment platforms.

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.

51 - Evaluation of the Tomotherapy treatment planning system for out-of-field doses calculation

51 - Evaluation of the Tomotherapy treatment planning system for out-of-field doses calculation

P13.15 * USING IMAGE-GUIDED AND INTENSITY-MODULATED RADIOTHERAPY TO REDUCE SCALP DOSE AND RADIOTHERAPY-RELATED ALOPECIA FOR PATIENTS WITH INTRACRANIAL TUMOURS TREATED WITH TOMOTHERAPY

INTRODUCTION: Radiotherapy to intracranial tumours commonly results in permanent alopecia on the scalp, an important and distressing side-effect that has a negative impact on patient quality-of-life. We demonstrated that the radiotherapy dose to the scalp can be examined during radiotherapy planning and can be reduced by the adoption of image-guided and intensity-modulated radiotherapy techniques. METHOD: Twelve patients were selected who had been treated using TomoTherapy at Cambridge University Hospitals and had suffered resulting alopecia. A method of producing a realistic ‘hair-bearing scalp’ structure in the radiotherapy planning software was developed to record scalp dosimetry. Image-guided radiotherapy (IGRT) systems such as TomoTherapy enable correction of patient setup errors and can facilitate the reduction of radiotherapy planning margins. We produced new treatment plans for the 12 patients using both 5mm and 3mm CTV-PTV margins to examine the changes in scalp dose that would result. A standardised inverse-planning optimisation protocol was used for each plan with fixed priorities, penalties and iterations. We also investigated the use of the hair-bearing scalp structure by the IMRT inverse-planning software as an avoid structure. In-vivo TLD measurements were carried out for a separate cohort of three patients to examine the correlation between planning system dose and delivered dose to the patient surface. RESULTS: We were able to produce detailed DVH data for the ‘hair-bearing scalp’ structure and show that the dose to the scalp was reduced significantly when 3mm rather than 5mm CTV-PTV margins were used. The mean scalp dose was reduced on average by 8.7% (p<0.0001). The V42.9Gy, the volume of scalp at 50% or higher risk of permanent alopecia (TD50), was reduced on average by 20.3% (p = 0.0008). Use of the scalp as an avoid structure reduced mean dose by 8.0% and V42.9Gy by 18.8%. There was excellent correlation between planned and in-vivo measured surface dose (correlation coefficient 0.96). CONCLUSIONS: Outlining the scalp hair field enables personalised prediction of the risk of permanent alopecia. Use of smaller CTV-PTV margins leads to significant reductions in scalp dose. Use of the scalp as a planning ‘avoid’ structure also may reduce dose to the scalp hair field. Alone or together these interventions may reduce the risk or extent of permanent alopecia. The TomoTherapy treatment planning system calculates skin doses accurately.

Accuracy of out-of-field dose calculation of tomotherapy and cyberknife treatment planning systems: A dosimetric study

PurposeLate toxicities such as second cancer induction become more important as treatment outcome improves. Often the dose distribution calculated with a commercial treatment planning system (TPS) is used to estimate radiation carcinogenesis for the radiotherapy patient. However, for locations beyond the treatment field borders, the accuracy is not well known. The aim of this study was to perform detailed out-of-field-measurements for a typical radiotherapy treatment plan administered with a Cyberknife and a Tomotherapy machine and to compare the measurements to the predictions of the TPS. Materials and methodsIndividually calibrated thermoluminescent dosimeters were used to measure absorbed dose in an anthropomorphic phantom at 184 locations. The measured dose distributions from 6 MV intensity-modulated treatment beams for CyberKnife and TomoTherapy machines were compared to the dose calculations from the TPS. ResultsThe TPS are underestimating the dose far away from the target volume. Quantitatively the Cyberknife underestimates the dose at 40cm from the PTV border by a factor of 60, the Tomotherapy TPS by a factor of two. If a 50% dose uncertainty is accepted, the Cyberknife TPS can predict doses down to approximately 10 mGy/treatment Gy, the Tomotherapy-TPS down to 0.75 mGy/treatment Gy. The Cyberknife TPS can then be used up to 10cm from the PTV border the Tomotherapy up to 35cm. ConclusionsWe determined that the Cyberknife and Tomotherapy TPS underestimate substantially the doses far away from the treated volume. It is recommended not to use out-of-field doses from the Cyberknife TPS for applications like modeling of second cancer induction. The Tomotherapy TPS can be used up to 35cm from the PTV border (for a 390 cm3 large PTV).

Effective and Efficient Technique for Total Marrow Irradiation

The aim of the study was to increase total marrow irradiation (TMI) treatment effectiveness and efficiency while minimizing the dose for non-skeletal structures. Twenty patients with multiple myeloma were treated by the following TMI method. The upper body TMI plan (UTMI) covered the body from the head to 11 cm above knee joint, for which the helical tomotherapy technique was applied. The lower body TMI plan (LTMI) covered the body from the feet up to 6 cm below the knee joint, for which the direct tomotherapy technique was applied (two opposite fields of 90°-270°). All plans were implemented using the tomotherapy treatment planning system. The optimization goal was to achieve that a minimum of 85% of the target volume receives at least 99% of the prescription dose (12 Gy). Fractionation scheme was 3 x 4 Gy (three consecutive days, one irradiation a day). In 20 patients, the median target doses determined from treatment plans were following: target-UTMI 12.18 Gy and target-LTMI 12.37 Gy. Median organ doses are presented in Table. Mean irradiation time [mean ± SD, minutes] lasted for target-UTMI was 34.94 ± 3.35, and for target-LTMI was 12.61 ± 0.94. Applying the described hypofractionated TMI (4 Gy dose per fraction) a dose reduction to the organs at risk ranged from 39.50%-82.50% of the prescribed dose and the total time for one radiation session was 1 hour and 15 minutes, including patient positioning and imaging. The tomotherapy direct field angles arrangement instead of using the helical intensity-modulated radiation has allowed minimizing the delivery error associated with the setup of the leg positioning, especially in horizontal direction. Additionally, this way we avoided the beamlets propagated through the couch materials. The final benefit of the dose delivery to the leg bones without the intensity-modulated method was the shorter irradiation time. A disadvantage of the direct technique was poor conformality, as a result of which the non-skeletal structures in legs (mostly muscles and vessels) received a higher dose in order to ensure hitting the target. In conclusion, the applied 4 Gy fractionation is feasible using TomoHD system and time-efficient in a busy radiation therapy department, requiring only a single patient setup a day on three consecutive days. Through hypofractionation, the biological equivalent dose is also effectively increased. Further observation of patients treated with this scheme is necessary to evaluate the treatment response.Poster Viewing Abstract 3422; TableMedian organ doses determined from treatment plans for 20 patients with multiple myelomaOrganMedian dose [Gy]Lungs7.26Brain7.25Liver6.98Bowel6.58Heart6.37Bladder5.56Kidneys5.50Oral cavity4.38Lenses2.10 Open table in a new tab

SU-E-T-57: Assessment of Systematic Uncertainties On Beam Data Collection Using Blue Phantom HelixTM Tomotherapy Scanning System

Purpose: Tomotherapy users are not allowed to modify the beam‐data or parameters for beam modeling in Tomotherapy treatment planning system(TPS).The gold beam‐data in TPS for modeling and commissioning was measured with TomoScannerTM system(TS).This study investigates the use of the Blue Phantom HelixTM(BPH) as an alternative scanner to establish a benchmark dataset for commissioning and quantifies systematic differences between TS and BPH. Methods: Reproducibility of scanning with BPH was tested by 3 experienced physicists taking 5 measurements over 3 month period.Several enhancements of BPH over TS were included a 3D scanning arm which acquires beam‐data with one tank setup,a universal chamber mount,and the OmniPro software,which allows provides online data collections and processing.Discrepancy was estimated by acquiring datasets with each tank under the most recent target conditions.The possible uncertainty due to TPS modeling was quantified by comparison to the golden beam‐data.The total systematic uncertainty defined as the combination of scanning system and beam modeling uncertainties, was determined through numerical analysis,and tabulated according to scan type.OmniPro was used for all analysis to eliminate variances of data processing. Results: The setup reproducibility of BPH remained within 0.5mm/0.5%.For PDDs,the systematic uncertainties were within 1.4mm/2.1% except for the buildup region up to 7.0±2.5%,which was attributed to output instabilities at surfaces and differences of chamber alignments and dimensions.The differences in field width and penumbra of in‐line profiles between BPH,TS and golden were within 0.7±0.9mm.The displacements of field width were increased in cross‐line profiles at depth >=10cm up to 3.5%/3.5mm among three datasets.Use of BPH reduced measurement time by 1–2 hrs per session. Conclusion: The uncertainty magnitudes of BPH have been quantified as an efficient,reproducible and accurate scanning system capable of providing a reliable benchmark beam data. Without the flexibility of replacing the TPS beam‐data with BPH measurements,further investigation of dose uncertainties in beam modeling from different measurement datasets is suggested

SU‐E‐T‐383: 3D Dose Reconstruction From Tomotherapy Detector Response of the Pre‐Treatment QA

Purpose: To evaluate the reconstructed 3D dose from the Tomotherapy detector response as a pre‐treatment patient‐specific dose verification tool by comparing the reconstructed dose with the 3D dose from the Tomotherapy treatment planning system (TPS). Methods: A simple dry‐run of a patient plan was performed to capture the fluence sinogram with the detectors. Then, DosimetryCheck™ (DC) was used to calculate the 3D dose using the detector sinogram, CT images, and treatment delivery parameters. The couch attenuation was corrected mathematically. The reconstructed dose was compared with the Tomotherapy TPS 3D dose in the patient anatomy. Results: TPS dose and DC reconstructed dose were compared for 5 prostate plans, 1 head and neck plan, and 1 SRS plan. For the prostate plans, the average dose differences of the bladder, rectum, femur, and prostate were 3.0+−1.8%, 4.2+−1.1%, 3.0+−2.0%, and 0.1+−1.1% (+−S.D.). For the head and neck plan, the average differences of the spinal cord, parotid, mandible, and PTV were 6.9%, 8.6%, 1.2%, and 0.2%. For the SRS plan, the average differences of the brain stem, optical chiasm, left eye, and the PTV were −0.5%, 0.6%, 1.5%, and −3.4%. The overall gamma factor (3%, 3mm) passing rate was &gt;90% for all the cases. Conclusion: It is feasible to perform patient specific QA without Tomotherapy phantom DQA. The QA is more meaningful beca use the dose difference is visualized over patient anatomy instead of over a phantom. The statistics with prostate plans show a consistent match between the measured and planning dose. The head and neck plan suggests that DC Pencil Beam algorithm may need improvement in accuracy for areas with dramatic electron density changes. The SRS plan indicates DC accuracy is independent of jaw width and pitch. All the differences between the Tomotherapy TPS and the reconstructed doses were within clinical tolerance.

EP-1172: Dose calibration of the Tomotherapy treatment planning system based on a static tomotherapy plan

EP-1172: Dose calibration of the Tomotherapy treatment planning system based on a static tomotherapy plan

Dose Painting a Prostate Peripheral Zone Boost With Helical Tomotherapy IMRT

Despite applying an IMRT dose of 81 Gy to prostate cancer, a substantial fraction of men with intermediate - high risk prostate cancer may go on to develop local failure. There is a known dose - response relationship for prostate cancer radiation. We sought to determine if it was feasible to increase the dosage within the general peripheral zone region of the prostate gland, where prostate tumors preferentially reside, without increasing the dose to surrounding normal structures. Five consecutive prostate cancer patients were chosen, who had already undergone treatment with tomotherapy. A peripheral zone simultaneous integrated boost (SIB) volume was added to each case. This was done by making the SIB equal the prostate (CTV) contour, and then coming 5 mm off of the bladder, rectum, penile bulb, and the “anterior stripe”. The anterior stripe was drawn on each prostate slice as a 0.8cm spot in the mid gland that was then extended 3cm anteriorly. The SIB did not extend outside the prostate gland and was adjusted if necessary. PTV was based on standard expansion of the prostate (CTV) contour by 5 - 6mm, 3mm posteriorly. Contouring was solely CT based, and all patients had fiducial markers. Plan was rerun with the tomotherapy Treatment Planning System (TPS), administering 1.8 Gy x 45# = 81 Gy to the PTV, and the SIB was prescribed 2.0 Gy x 45# = 90 Gy. These plans were not used for treatment, but were compared with the patients' actual treatment plans. The average treatment volumes were 28.0 cc for the SIB, 60.3 cc for the CTV, and 114.8 cc for the PTV. The mean dose to the prostate (CTV) was 82.97 Gy in the standard plan, and 89.80 Gy in the dose painting plan (p < 0.00001), for an 8.2% increase. This resulted in a BED that was 11.6% increased, based on an alpha-beta value of 3.0. However, the rectal median dose was only increased by 2.4%, and the bladder median dose was only increased by 3.6%. Rectal V70 dropped from 9.4% to 8.1% with the dose painting plan, and the bladder V70 dropped from 13.9% to 13.4%. Penile bulb median dose was identical at 44 Gy. All these changes in normal tissue dose were statistically non-significant. Treatment delivery times were identical at 217 seconds. Dose painting a generic peripheral zone was very straightforward, allowing a simultaneous integrated boost to be prescribed. This resulted in a significantly higher BED to be administered to the prostate gland overall, yet the rectal and bladder dosages were not significantly increased. This may improve the therapeutic ratio. We plan to test this strategy for intermediate - high risk cases and also incorporate 3T MRI fusion to help improve demarcation of prostate contour, peripheral zone, urethra, and tumor nodule(s).

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

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

The use of exit detector sinograms to detect anatomical variations for patients extending beyond the TomoTherapy field of view: A feasibility study

This work describes an independent method to use the TomoTherapy Hi-ART megavoltage CT imaging system for daily monitoring of anatomical changes of cancer patients whose anatomy extends beyond the imaging field of view. The imaging detector response to changes in attenuating media was measured using water-equivalent plastic. Weight loss was simulated using an anthropomorphic phantom and determining the system's ability to detect the weight loss. Layers of tissue-equivalent bolus were added to an anthropomorphic pelvis phantom and CT simulations of the phantom were conducted, one in which the phantom and bolus were both within the TomoTherapy imaging field of view, and another in which the couch was raised so that the bolus was outside the field of view. Gynecological treatment plans were developed using the TomoTherapy treatment planning system, and successive fractions of the plan were then delivered to the phantom. Weight loss was simulated by removing a 0.5 cm layer of bolus following each fraction. The exit detector sinograms were obtained from each fraction, and ratios of sinograms were calculated relative to a reference sinogram for which all bolus was in place. Histograms of ratio sinograms were determined and used to correlate with simulated weight loss. Exit detector sinograms and ratio histograms were also retrospectively analyzed for five patients all of whose anatomies extended beyond the imaging field of view and all of whom experienced weight variations exceeding 10% during treatment. Exit detector signal is well correlated to changes in attenuator thickness as demonstrated in both slab and anthropomorphic phantom geometries. Measured and expected signal increases agreed to within less than 2% for simulated weight loss on the anthropomorphic phantom. Exit detector signals for pelvic patients with significant weight loss variations were consistent with phantom measurements. The analysis of the ratio sinograms for the phantom measurements and real patients indicated that exit detector sinograms can be used to detect relative changes in patient anatomy for each fraction as a means of in vivo quality assurance.

Independent verification of transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines: a protocol for patient-specific quality assurance

The purpose of this study was to independently verify the transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines with the goal of eliminating redundant quality assurance (QA) measurements on the second machine. The equivalence of the two machines was evaluated based on both geometric and dosimetric beam characteristics, including measuring open field percent depth doses (PDD), longitudinal and transverse profiles and helical delivery of clinical patient treatment plans measured in phantoms. QA of 56 patient plans was studied. The delivery sinogram on the secondary machine was computed by accounting for the differences in the MLC characteristics of the two machines. Computed sinograms were compared against the transferred sinograms by tomotherapy's data management system for the same 56 patient plans. The PDD, transverse and longitudinal dose profiles agreed within ±1% between the two machines. Ionization chamber and planar dose measurements with the Iba MatriXX device on both machines for the 56 patients were found to be within ±3% of the doses computed by the tomotherapy treatment planning system. For all 56 patients, the differences between computed sinograms and DMS-converted sinograms were within ±2%. The matched tomotherapy machines had similar beam characteristics. The sinogram-based QA was validated using point and planar dose measurements and found to be acceptable for clinical use.

Improvements in dose calculation accuracy for small off‐axis targets in high dose per fraction tomotherapy

A recent field safety notice from TomoTherapy detailed the underdosing of small, off-axis targets when receiving high doses per fraction. This is due to angular undersampling in the dose calculation gantry angles. This study evaluates a correction method to reduce the underdosing, to be implemented in the current version (v4.1) of the TomoTherapy treatment planning software. The correction method, termed "Super Sampling" involved the tripling of the number of gantry angles from which the dose is calculated during optimization and dose calculation. Radiochromic film was used to measure the dose to small targets at various off-axis distances receiving a minimum of 21 Gy in one fraction. Measurements were also performed for single small targets at the center of the Lucy phantom, using radiochromic film and the dose magnifying glass (DMG). Without super sampling, the peak dose deficit increased from 0% to 18% for a 10 mm target and 0% to 30% for a 5 mm target as off-axis target distances increased from 0 to 16.5 cm. When super sampling was turned on, the dose deficit trend was removed and all peak doses were within 5% of the planned dose. For measurements in the Lucy phantom at 9.7 cm off-axis, the positional and dose magnitude accuracy using super sampling was verified using radiochromic film and the DMG. A correction method implemented in the TomoTherapy treatment planning system which triples the angular sampling of the gantry angles used during optimization and dose calculation removes the underdosing for targets as small as 5 mm diameter, up to 16.5 cm off-axis receiving up to 21 Gy.