Tomotherapy Treatment Planning System Research Articles

SU‐FF‐T‐139: Dosimetric Accuracy of Low Dose Region (V5‐V10) in TomoTherapy Planning of Lung IMRT

Purpose: To quantify the dosimetric accuracy of a TomoTherapy treatment planning system in areas of low dose (V5 to V10 regions) and in the presence of tissue heterogeneities in lung IMRT. Materials and Methods: CIRS anthropomorphic heterogeneous thorax phantom was used to generate clinical treatment plans using TomoTherapy's treatment planning system (TPS). Three plans were developed for treatment delivery using three different field widths (1.0, 2.5 and 5.0 cm) with standard planning parameters used in our clinic. A designated tumor volume was contoured in the phantom lung volume and was planned to receive 200 cGy fractions to a total prescription dose of 7000 cGy. Point measurements with a PTW Farmer chamber were conducted at two locations in the contralateral lung which fell with in the V5 to V10 region of low dose. Comparison of the experimental findings to the TPS results was performed to assess the dosimetric accuracy of TomoTherapy's convolution/superposition algorithm in low dose regions. Results: Comparison of the ion chamber measurements to the TPS dose calculation demonstrated an average difference of +4.5% and −3.0% at the medial and posterior points of measurement in the contralateral lung of the same axial slice. Variation as a function of field width was found to be 1–2% for chamber measurements. Over the total course of treatment of 35 fractions, differences between the TPS and measurement were 35 cGy regardless of field width or point of measurement. Conclusions: At low dose levels (V5 to V10 regions), dosimetric accuracy of conventional TPS systems is often underestimated, but our experimental results suggest the TomoTherapy TPS is reasonably accurate under the given conditions. Tomotherapy's TPS provides acceptable dosimetric accuracy in lung region areas of low dose.

Skin dose measurements using MOSFET and TLD for head and neck patients treated with tomotherapy

The purpose of this work was to estimate skin dose for the patients treated with tomotherapy using metal oxide semiconductor field-effect transistors (MOSFETs) and thermoluminescent dosimeters (TLDs). In vivo measurements were performed for two head and neck patients treated with tomotherapy and compared to TLD measurements. The measurements were subsequently carried out for five days to estimate the inter-fraction deviations in MOSFET measurements. The variation between skin dose measured with MOSFET and TLD for first patient was 2.2%. Similarly, the variation of 2.3% was observed between skin dose measured with MOSFET and TLD for second patient. The tomotherapy treatment planning system overestimated the skin dose as much as by 10–12% when compared to both MOSFET and TLD. However, the MOSFET measured patient skin doses also had good reproducibility, with inter-fraction deviations ranging from 1% to 1.4%. MOSFETs may be used as a viable dosimeter for measuring skin dose in areas where the treatment planning system may not be accurate.

Helical Tomotherapy Planning for Left-Sided Breast Cancer Patients With Positive Lymph Nodes: Comparison to Conventional Multiport Breast Technique

To evaluate the feasibility of using helical tomotherapy for locally advanced left-sided breast cancer. Treatment plans were generated for 10 left-sided breast cancer patients with positive lymph nodes comparing a multiport breast (three-dimensional) technique with the tomotherapy treatment planning system. The planning target volumes, including the chest wall/breast, supraclavicular, axillary, and internal mammary lymph nodes, were contoured. The treatment plans were generated on the tomotherapy treatment planning system to deliver 50.4 Gy to the planning target volume. To spare the contralateral tissues, directional blocking was applied to the right breast and right lung. The optimization goals were to protect the lungs, heart, and right breast. The tomotherapy plans increased the minimal dose to the planning target volume (minimal dose received by 99% of target volume = 46.2 +/- 1.3 Gy vs. 27.9 +/- 17.1 Gy) while improving the dose homogeneity (dose difference between the minimal dose received by 5% and 95% of the planning target volume = 7.5 +/- 1.8 Gy vs. 37.5 +/- 26.9 Gy). The mean percentage of the left lung volume receiving >or=20 Gy in the tomotherapy plans decreased from 32.6% +/- 4.1% to 17.6% +/- 3.5%, while restricting the right-lung mean dose to <5 Gy. However, the mean percentage of volume receiving >or=5 Gy for the total lung increased from 25.2% +/- 4.2% for the three-dimensional technique to 46.9% +/- 8.4% for the tomotherapy plan. The mean volume receiving >or=35 Gy for the heart decreased from 5.6% +/- 4.8% to 2.2% +/- 1.5% in the tomotherapy plans. However, the mean heart dose for tomotherapy delivery increased from 7.5 +/- 3.4 Gy to 12.2 +/- 1.8 Gy. The tomotherapy plans provided better dose conformity and homogeneity than did the three-dimensional plans for treatment of left-sided breast tumors with regional lymph node involvement, while allowing greater sparing of the heart and left lung from doses associated with increased complications.

Intensity-Modulated Proton Therapy Versus Helical Tomotherapy in Nasopharynx Cancer: Planning Comparison and NTCP Evaluation

To compare intensity-modulated proton therapy (IMPT) and helical tomotherapy (HT) treatment plans for nasopharynx cancer using a simultaneous integrated boost approach. The data from 6 patients who had previously been treated with HT were used. A three-beam IMPT technique was optimized in the Hyperion treatment planning system, simulating a "beam scanning" technique. HT was planned using the tomotherapy treatment planning system. Both techniques were optimized to simultaneously deliver 66 Gy in 30 fractions to planning target volume (PTV1; GTV and enlarged nodes) and 54 Gy to PTV2 subclinical, electively treated nodes. Normal tissue complication probability calculation was performed for the parotids and larynx. Very similar PTVs coverage and homogeneity of the target dose distribution for IMPT and HT were found. The conformity index was significantly lower for protons than for photons (1.19 vs. 1.42, respectively). The mean dose to the ipsilateral and contralateral parotid glands decreased by 6.4 Gy and 5.6 Gy, respectively, with IMPT. The volume of mucosa and esophagus receiving > or =20 Gy and > or =30 Gy with IMPT was significantly lower than with HT. The average volume of larynx receiving > or =50 Gy was significantly lower with HT, while for thyroid, it was comparable. The volume receiving > or =30, > or =20, and > or =10 Gy in total body volume decreased with IMPT by 14.5%, 19.4%, and 23.1%, respectively. The normal tissue complication probability for the parotid glands was significantly lower with IMPT for all sets of parameters; however, we also estimated an almost full recovery of the contralateral parotid with HT. The normal tissue complication probability for the larynx was not significantly different between the two irradiation techniques. Excellent target coverage, homogeneity within the PTVs, and sparing of the organs at risk were reached with both modalities. IMPT allows for better sparing of most organs at risk at medium-to-low doses.

Monte Carlo calculation of helical tomotherapy dose delivery

Helical tomotherapy delivers intensity modulated radiation therapy using a binary multileaf collimator (MLC) to modulate a fan beam of radiation. This delivery occurs while the linac gantry and treatment couch are both in constant motion, so the beam describes, from a patient/phantom perspective, a spiral or helix of dose. The planning system models this continuous delivery as a large number (51) of discrete gantry positions per rotation, and given the small jaw/fan width setting typically used (1 or 2.5 cm) and the number of overlapping rotations used to cover the target (pitch often <0.5), the treatment planning system (TPS) potentially employs a very large number of static beam directions and leaf opening configurations to model the modulated fields. All dose calculations performed by the system employ a convolution/superposition model. In this work the authors perform a full Monte Carlo (MC) dose calculation of tomotherapy deliveries to phantom computed tomography (CT) data sets to verify the TPS calculations. All MC calculations are performed with the EGSnrc-based MC simulation codes, BEAMnrc and DOSXYZnrc. Simulations are performed by taking the sinogram (leaf opening versus time) of the treatment plan and decomposing it into 51 different projections per rotation, as does the TPS, each of which is segmented further into multiple MLC opening configurations, each with different weights that correspond to leaf opening times. Then the projection is simulated by the summing of all of the opening configurations, and the overall rotational treatment is simulated by the summing of all of the projection simulations. Commissioning of the source model was verified by comparing measured and simulated values for the percent depth dose and beam profiles shapes for various jaw settings. The accuracy of the MLC leaf width and tongue and groove spacing were verified by comparing measured and simulated values for the MLC leakage and a picket fence pattern. The validated source and MLC configuration were then used to simulate a complex modulated delivery from fixed gantry angle. Further, a preliminary rotational treatment plan to a delivery quality assurance phantom (the "cheese" phantom) CT data set was simulated. Simulations were compared with measured results taken with an A1SL ionization chamber or EDR2 film measurements in a water tank or in a solid water phantom, respectively. The source and MLC MC simulations agree with the film measurements, with an acceptable number of pixels passing the 2%/1 mm gamma criterion. 99.8% of voxels of the MC calculation in the planning target volume (PTV) of the preliminary plan passed the 2%/2 mm gamma value test. 87.0% and 66.2% of the voxels in two organs at risk (OARs) passed the 2%/2 mm tests. For a 3%/3 mm criterion, the PTV and OARs show 100%, 93.2%, and 86.6% agreement, respectively. All voxels passed the gamma value test with a criterion of 5%/3 mm. The Tomo-Therapy TPS showed comparable results.

SU‐GG‐T‐491: Dose Summation for Multi‐Phase Helical Tomotherapy Plans

Purpose: Develop a method to combine dose matrices from multi‐phase Tomotherapy plans to facilitate dose escalation clinical trials. Method and Materials: For many clinical sites it is customary to use treatment plans consisting of 2–3 phases. Currently the Tomotherapy treatment planning system doesn't provide a method to combine these phases, thus hampering the ability of clinicians to review dose volume information. We propose a simple method to create a composite plan which could be evaluated on a tertiary treatment planning system (TPS). Dose matrices from approved Tomotherapy plans for each phase are extracted and exported to a temporary location. A software algorithm was developed to read each distribution and perform a pixel by pixel summation. The final dose matrix was reviewed and placed in a format compatible with Pinnacle‐TPS. The dose matrix was then registered with the image set in Pinnacle facilitating composite plan evaluation. The accuracy of the method was tested using ion chamber and film measurements. Film dosimetry was performed using EBT‐Gafchromic film, Epson flatbed scanner and our in‐house film dosimetry protocol. The measured dose distributions were compared with values calculated by Tomotherapy at the location of the film and summed using this method. Dose distributions were compared using the γ‐index. Results: The x,y,z registration errors between the composite dose matrix and the image set of the TPS were found to be 0.6mm for x‐y axes and 1.5mm for z‐axis. Ion chamber measurements were within 1–2%. Good agreement (low gamma, high conformity index) was recorded using film dosimetry Conclusion: This summation method provides a fast, accurate, and simple way to evaluate multiphase composite treatment plans. The method can be used to sum dose distributions provided by different treatment planning system or to perform dosimetric comparisons between plans from these systems.

TH-D-352-01: Helical Tomotherapy Patient Specific Quality Assurance in An Anthropomorphic Phantom

Purpose: The purpose of this study was to evaluate the dosimetric accuracy of a helical tomotherapy system using an anthropomorphic RANDO phantom for patient‐specific QA. The results of this study will allow a benchmark to be established for dosimetric accuracy in heterogeneous materials. Method and Materials: Patient‐specific IMRT dosimetric verification was performed for 24 patients using a male RANDO Phantom. Treatment plans were created using the Hi‐Art TomoTherapy treatment planning and delivery system. The phantom was placed in the Vac‐Lok bag with radiographic film placed in a transverse plane between two of the phantom slices. The films were scanned and analyzed using both the Tomotherapy IMRT QA and the RIT dosimetry software. Horizontal profiles, vertical profiles, Gamma pass/fail analysis, and Gamma histograms were calculated for each test case. The reproducibility of the film was tested for both a prostate and lung case. Results: On average, the lung patients had 27.2% of the pixels exceeding gamma, while the prostate patients had 14.7%. For the prostate test cases, only two of the films had greater than 20% of the pixels exceeding the gamma threshold, while only four of the lung test cases were below the 20% pixel threshold. The reproducibility of film dosimetry found that the standard deviation for the number of pixels exceeding the gamma threshold was 3.9%. Conclusion: In this study, an anthropomorphic RANDO phantom was used to evaluate the doses calculated by a helical tomotherapy system. The dosimetric agreement between calculated and measured was worse than published results in homogenous phantoms, especially for target volumes located in the lung. Further investigation will be needed to determine if these errors are due to film response at heterogeneous tissue interfaces or from actual dose calculation errors.

Measurement of superficial dose from a static tomotherapy beam

Superficial doses were measured for static TomoTherapy Hi-Art beams for normal and oblique incidence. Dose was measured at depths < or = 2 cm along the central axis of 40 x 5 cm2 and 40 x 2.5 cm2 beams at normal incidence for source to detector distances (SDDs) of 55, 70, and 85 cm. Measurements were also made at depths normal to the phantom surface for the same beams at oblique angles of 30 degrees, 45 degrees, 60 degrees, 75 degrees, and 83 degrees from the normal. Data were collected with a Gammex/RMI model 449 parallel-plate chamber embedded in a solid water phantom and with LiF thermoluminescent dosimeters (TLDs) in the form of powder. For comparison, measurements were made on a conventional 6 MV beam (Varian Clinac 2100C) at normal incidence and at an oblique angle of 60 degrees from the normal. TomoTherapy surface dose varied with the distance from the source and the angle of incidence. For normal incidence, surface dose increased from 0.16 to 0.43 cGy/MU as the distance from the source decreased from 85 to 55 cm for the 40 x 5 cm2 field and increased from 0.12 to 0.32 cGy/MU for the 40 x 2.5 cm2 field. As the angle of incidence increased from 0 degrees to 83 degrees, surface dose increased from 0.24 to 0.63 cGy/MU for the 40 x 5 cm2 field and from 0.18 to 0.58 cGy/MU for the 40 x 2.5 cm2 field. For normal incidence at 55 cm SDD, the surface dose relative to the dose at d(max) for the 40 x 5 cm2 TomoTherapy Hi-Art beam was 31% less than that from a conventional, flattening filter based linear accelerator. These data should prove useful in accessing the accuracy of the TomoTherapy treatment planning system to predict the dose at superficial depths for a static beam delivery.

Helical tomotherapy superficial dose measurements

Helical tomotherapy is a treatment technique that is delivered from a 6 MV fan beam that traces a helical path while the couch moves linearly into the bore. In order to increase the treatment delivery dose rate, helical tomotherapy systems do not have a flattening filter. As such, the dose distributions near the surface of the patient may be considerably different from other forms of intensity-modulated delivery. The purpose of this study was to measure the dose distributions near the surface for helical tomotherapy plans with a varying separation between the target volume and the surface of an anthropomorphic phantom. A hypothetical planning target volume (PTV) was defined on an anthropomorphic head phantom to simulate a 2.0 Gy per fraction IMRT parotid-sparing head and neck treatment of the upper neck nodes. A total of six target volumes were created with 0, 1, 2, 3, 4, and 5 mm of separation between the surface of the phantom and the outer edge of the PTV. Superficial doses were measured for each of the treatment deliveries using film placed in the head phantom and thermoluminescent dosimeters (TLDs) placed on the phantom's surface underneath an immobilization mask. In the 0 mm test case where the PTV extends to the phantom surface, the mean TLD dose was 1.73 +/- 0.10 Gy (or 86.6 +/- 5.1% of the prescribed dose). The measured superficial dose decreases to 1.23 +/- 0.10 Gy (61.5 +/- 5.1% of the prescribed dose) for a PTV-surface separation of 5 mm. The doses measured by the TLDs indicated that the tomotherapy treatment planning system overestimates superficial doses by 8.9 +/- 3.2%. The radiographic film dose for the 0 mm test case was 1.73 +/- 0.07 Gy, as compared to the calculated dose of 1.78 +/- 0.05 Gy. Given the results of the TLD and film measurements, the superficial calculated doses are overestimated between 3% and 13%. Without the use of bolus, tumor volumes that extend to the surface may be underdosed. As such, it is recommended that bolus be added for these clinical cases. For cases where the target volume is located 1 to 5 mm below the surface, the tumor volume coverage can be achieved with surface doses ranging from 56% to 93% of the prescribed dose.

SU-FF-T-118: Comparison of Calculated and Measured Superficial Depth Doses for Static TomoTherapy Beams

Purpose: To compare computed superficial doses from the TomoTherapy treatment planning system (TPS) to measured doses from a static TomoTherapy beam. Method and Materials: Simulated CT datasets of flat phantoms were created to calculate static TomoTherapy beams at normal and oblique incidence. Calculated doses for depths ⩽ 2cm were compared with superficial dose measurements made with a Gammex/RMI model 449 parallel‐plate chamber and LiF TLDpowder for a static TomoTherapy beam at normal incidence for source to detector distances (SDDs) of 55, 70, and 85cm and oblique angles of 30°, 45°, 60°, 75° and 83° for typical TomoTherapy jaw sizes (40×2.5cm2 and 40×5cm2). Results: For all measurement conditions, when accounting for the effective depth of each calculation point, the TPS under predicted the dose at the surface. For all normally incident conditions, and 30° and 45°, the TPS under predicted the dose at a depth of 0.1cm by more than 5% of the maximum dose. For 55cm SDD, the TPS under predicted the dose at a depth of 0.2cm by more than 5% of the maximum dose. For highly oblique angles (75° and 83°), the TPS over predicted the dose at superficial depths (i.e., depths between 0.1 and 0.3cm) by as much as 7% of the maximum dose. For all measurement conditions, the measured and calculated data agreed to within 5% of the maximum dose at a depth of 0.3cm and beyond. Conclusion: The disagreement between calculated and measuredsurface doses was expected because the TomoTherapy treatment planning system does not currently model electron contamination. The disagreement found at depths between 0.1 and 0.3cm for the highly oblique angles is likely due to the failure of the convolution/superposition algorithm to account for reduced scatter dose near the tissue/air interface. Supported in part by a research agreement with TomoTherapy, Inc.

SU-FF-T-217: Evaluation of the Dosimetric Accuracy of a Commercial Adaptive Radiotherapy Process

Purpose: To evaluate the dosimetric accuracy and reproducibility of the TomoTherapy Planned Adaptive system using MVCT datasets. Method and Materials: Comparisons were made between reference dose distributions computed with the TomoTherapy treatment planningsystem (TPS) and verification dose distributions computed on the TomoTherapy Planned Adaptive (PA) system. Both calculations were performed on TomoTherapy megavoltage CT (MVCT) datasets to eliminate differences associated with variations in CT‐density tables. Simulated PTVs drawn on an MVCT of an anthropomorphic radiosurgery head phantom (CIRS, Inc.) were used as input to calculate doses on the TomoTherapy TPS. After plan optimization, a post‐plan MVCT image was taken and used to calculate dose on the TomoTherapy PA system. Initially, calculations were made using the same MVCT dataset as input to both programs to investigate the agreement between the algorithms. Next, calculations were made using registered pre‐ and post‐plan MVCT images to investigate the variation with MVCT datasets. Comparisons were made using the tools available on the PA system and in‐house software to compare doses outside of contoured structures. Results: For calculations on identical datasets, isodose and DVH comparisons between the TomoTherapy TPS and PA systems showed insignificant dose differences (< 0.2%) to all contoured structures. Dose discrepancies within the phantom but outside of contoured structures showed slightly larger variations, although well within 0.5%. Similar results were found for dose calculations using different MVCT datasets. In both cases, the doses computed in air outside the phantom showed the largest differences (up to 21%). Conclusion: Comparisons between TomoTherapy's TPS and PA system MVCT dose calculations showed clinically acceptable agreement everywhere within the phantom. Differences between doses calculated in air are due to differences in the way the algorithms mask low density voxels. Research sponsored by TomoTherapy, Inc.

SU‐FF‐T‐254: IMRT — Tomotherapy Vs Conventional Linear Accelerator

Purpose: To compare IMRT plans delivered with a conventional linear accelerator (Varian 2100EX) and a Helical Hi‐Art Tomotherapy accelerator and the dose differences to critical organs and structures surrounding the tumor volume. Method and Materials: Five patient plans were generated using both the Varian Eclipse treatment planning system and the Tomotherapy treatment planning system. A sampling of different treatment sites were chosen for the study, which include an esophagus, a hilum/lung, a para‐aortic, a pelvis, and an orbital case. IMRT plans were created with the goal of reducing dose to surrounding critical organs and structures while achieving similar satisfactory target coverage. Radiation doses were taken at three volumes of 25%, 50% and 75% for comparison. Also a mean dose and a max dose for spinal cord are provided. The plans generated represent the best dosimetric effort within clinically permitted time frame. Results: The critical organs for each case are highlighted in yellow in the supporting documentation. Tomotherapy plans show an average improvement of mean doses to the critical organs by 31.83% over the conventional linear accelerator plans. For spinal cord doses, the average improvement of maximum doses shows an 18.75% advantage to the Tomotherapy plan over the conventional linac. Conclusions: Examination of the dose volume histograms determined that the Tomotherapy unit was superior for thoracic cases, significantly reducing total lung doses. In most cases, the data show that the Tomotherapy plans provide better overall dosimetric characteristics. Also note that only five cases were compared in this study but similar results can be expected in most other cases.

WE‐D‐AUD‐01: Clincal Evaluation of An Independent Dose Check Algorithm for Helical TomoTherapy

Purpose: To determine the clinical acceptability of an algorithm developed to independently verify doses calculated by the TomoTherapy treatment planning system. Method and Materials: Point doses from treatment plans for 97 patients treated on a TomoTherapy system were compared to an independent dose calculation algorithm developed at our facility. All patient plans for which treatment sinograms were available were included in the comparison, which represented a variety of treatment sites and plan parameters. For each treatment plan, the calculation point was selected to be in the geometric center of the primary PTV, where beam modulation and high dose‐gradients were expected to be minimal. If this process put the point into a high dose‐gradient region or very near a tissue interface region, the point was manually repositioned. Results: Comparisons of our calculation to the TomoTherapy‐predicted point doses for the patient treatment plans showed good agreement. For sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Our independent calculations within the lung or superficial PTVs overestimated the dose by an average of 3.1% (σ =2.4%) and 3.2% (σ=2.2%), respectively. Conclusion: Systematic errors at points within lung are probably the result of the known weakness of the radiological path length method for correcting for missing tissue and/or tissue density heterogeneities. Errors encountered at points within superficial PTVs are probably the result of our algorithm overestimating the scatter dose at points near the surface of the patient. For the majority of cases, the algorithm demonstrates sufficient accuracy for clinical use and may be used to independently verify patient treatment plans.Supported in part by a research agreement with TomoTherapy, Inc.

SU‐FF‐T‐420: Tomotherapy as a Solution to Minimize Dose to Critical Structures in Previously Irradiatied Areas

Purpose: Patients who have had prior radiation therapy often present with recurrent/residual disease in the previously irradiated area. However, additional radiation is often not feasible because the maximum tolerated dose for normal critical organs have already been reached. We present a case of a patient with metastatic endometrial cancer who was retreated to the ninth rib/vertebral body with Tomotherapy although the spine had already received 4500 cGy. Method and Materials: A patient with metastatic disease to the ninth right rib and vertebral body was treated with 4400 cGy in 2003. The patient was recently found to have recurrent/progressive disease in the area of prior treatment. A PET scan was taken prior to treatment and an PET/CT fusion was performed in order to aid in the contouring of the tumor volume. The tumor volume(rib) wrapped around the liver and ended at the ninth vertebral body right next to the spinal cord. A Tomotherapy plan was generated to maximally cover the tumor volume while minimizing the dose to the spinal cord and liver. The positioning of the patient was verified daily with an MVCT. Results: Treatment plans generated using the Tomotherapy treatment planning system and the ability of the Tomotherapy machine to verify patient set‐up daily show that the Tomotherapy unit was able to deliver 6000 cGy to the tumor volume while only delivering a maximum dose of 575 cGy to the spine with an average of only 179 cGy. Dose the liver was kept to a minimum with an average of 1804 cGy. Conclusions: Examination of the dose volume histograms determined that the Tomotherapy unit shows was able to keep at a minimum the doses to the spinal cord and liver and surrounding structures thus making Tomotherapy a useful tool in treating patients with prior radiation.

SU-FF-J-108: Dose Variation of Critical Structures in Daily Treatment of Nasopharyngeal Cancer Patients Using Helical Tomotherapy Megavoltage CT System

Purpose: To study daily spinal cord and parotid gland dose variation for nasopharyngeal cancer patients undergoing helical tomotherapy treatments, as a result of patient weight loss during the course of treatments.Method and Materials: Five nasopharygeal cancer patients who received helical tomotherapy treatments were selected. At the end of the treatments, patients lost an average weight of 6.3 kg, or 8.4% relative to the weight at the beginning of the treatments. Daily megavoltage CT (MVCT) scans were performed prior to each treatment fraction, and MVCT images were registered with planning CTimages for patient setup corrections. Contours for spinal cord and parotid glands were drawn on MVCT images throughout the treatment course. Based on daily MVCT images, actual dose delivered to the critical structures were calculated using the Planned Adaptive application in the helical tomotherapy treatment planning system. The maximum dose to the spinal cord and the median doses to the parotid glands were analyzed. Results: The maximum dose to the spinal cord, after setup corrections, did not show significant variation as a result of patient weight loss. However, the median doses to the parotid glands showed significant increase due to facial tissue shrinkage during treatments. The average median dose to the parotid glands for the five patients were 83 cGy for the first treatment fraction, and it was 143 cGy per fraction for the last fraction. The average parotid gland volume decreased from 20.5 cc to 13.2 cc during treatments.Conclusion: Daily patient setup corrections based on MVCT image registrations prevented significant dose variation to the spinal cord due to setup uncertainty. During the course of radiotherapy, as a result of body weight loss, the parotid glands typically receive significantly higher dose compared to the dose dictated in the treatment plan.

Characterization and use of EBT radiochromic film for IMRT dose verification

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the film's dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The film's dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanner's OD readout. In addition, the film's large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.

MO‐E‐224C‐05: Comparison of TomoTherapy with Conventional Electron/X‐Ray Treatment Plans for Chest Wall

Purpose: Post‐mastectomy radiotherapy (PMRT) of the chest wall (CW) often will involve use of electron beams due to their rapid fall‐off of dose near the practical range of the electron energy. The TomoTherapy helical, fan‐beam delivery system, along with the inverse‐planning optimizer, has the potential to provide planning and delivery of 6 MV x‐rays to superficial target volumes conventionally treated with electrons. In the present study, five chest wall patients were planned for TomoTherapy and compared to their conventional plan that utilized electron beams. Analysis of breathing motion were not included. Methods and Materials: Five PMRT patients treated with electron beams and planned on the Pinnacle treatment planning system (TPS) were selected. A planning target volume (PTV) was generated to follow the isodose contour defined by 90% of the prescribed dose on the Pinnacle plan. Target dose homogeneity in the Tomotherapy TPS dose optimizer was relaxed to better achieve critical structure dose objectives when necessary. Normal tissue complication probabilities (NTCP) were calculated for the lung and the heart. A questionnaire form was provided for the radiation oncologist to evaluate each plan. Results: For all cases, the TomoTherapy plan was rated superior to the Pinnacle plan by the radiation oncologists. The TomoTherapy TPS produced a significantly more uniform dose distribution in the PTV. The average volume of ipsilateral lung receiving dose above 20 Gy was reduced from 21.5% to 17.7%. The average volume of heart receiving dose above 30 Gy was reduced from 2.2% to 1.3%. NTCP for the lung and heart were also reduced in the TomoTherapy plan. Conclusion: The results of this study demonstrated that TomoTherapy is able to deliver clinically‐acceptable dose distributions to a “static” model of PMRT patients conventionally treated with electron beams.Supported in part by a research agreement with TomoTherapy, Inc.

SU‐FF‐T‐194: Dosimetric Validation of Tomotherapy in Hetergeneous Media

Purpose: Helical Tomotherapy is a relatively new treatment modality that is being used to treat lesions that lie within and near low‐density organs. Accurate dose calculations are critical to the effective use of this modality. This study quantified the Tomotherapy treatment planning system (TPS) convolution‐superposition‐based dose calculation accuracy in heterogeneous media. Method and Materials: This evaluation required a custom fabricated dosimetry phantom with lung‐equivalent and water‐equivalent media. The phantom consisted of an 18×18×18 cm3 cuboid constructed from slabs of water‐equivalent and lung‐equivalent materials (LN300 Gammex RMI, Middleton, WI (ρ ∼0.3g/cc), and Balsa wood (ρ ∼0.1g/cc) and imaged on a CT scanner. Dose measurements were conducted using both film and ionization chambers using the same phantom geometry. Evaluations were conducted using an esophageal treatment plan, delivering 1.8 Gy/fraction, was superimposed onto the phantom CT‐datasets and computed the dose distributions using the Tomotherapy treatment planning system. Radiochromic film (EBT, International specialty Products, Wayne, NJ) sheets were inserted between slabs of virtual water and lung equivalent material LN‐300. Experiments were repeated with radiographic film (Kodak EDR2, Eastman Kodak, Rochester, NY) with balsa wood. Calibration curves for absolute dosimetry for both types of film were generated from additional film exposures and ion chamber measurements on the Tomotherapy unit. Ionization‐chamber measurements were performed to confirm the film dosimetry. Results: Doses measured inside the water‐equivalent plastic were within 2% of the computed doses by the Tomotherapy planning system. Measurements with radiochromic film in LN‐300 material verified that the planning system computed the doses within 5%. Similar results were observed with EDR2 film in Balsa wood. Conclusions: The dose calculation accuracy of TPS was measured to be within 5% in lung material and within 2% in water‐equivalent plastic. This work supported in part by Tomotherapy, Inc.

SU‐FF‐T‐122: Characterization and Use of EBT Radiochromic Film for IMRT Dose Verification

Purpose: To quantify and evaluate the use of the new EBT GafChromic film for its implementation in routine IMRT QA. Method and Materials: The new film was examined using a characterized flat‐bed scanner for several properties including polarization effects with scan angle and delivered dose, dose sensitivity, dose uniformity, OD time evolution post exposure, and dose response in comparison to water. The film high elasticity and relative insensitivity to water immersion allowed for its non conventional use in verifying dose distributions on curved surfaces. The film was wrapped into a cylindrical geometry inside an in‐house built cylindrical phantom and then immersed in water. Calculated dose distributions from the TomoTherapy treatment planning system (TPS) were then compared to the film's measured IMRT deliveries for 10 IMRT cases (5 Head and Neck, 5 prostate). Results: The film's intrinsic polarization was shown to be a function of delivered dose and could significantly affect the scanner's OD output readout. Film uniformity was shown to improve with delivered dose from ±5% for 50 cGy to nearly ±1.5% at 200 cGy. In addition, the film's OD was shown to saturate within 2 hours post exposure to a 2Gy dose, making it the fastest and least noisy RCF film so far. An excellent agreement was found between scanned water and film PDDs indicating the EBT energy response with depth is similar to water. Excellent agreement between predicted and measured isodose distributions and dose profiles were seen using the EBT film based on 10 IMRT deliveries. Conclusions: This work indicates that this new RCF film possesses unique and improved characteristics which allows for its use in routine patient‐specific IMRT QA. These encouraging results have motivated us to design an in‐house cylindrical phantom to verify dose beyond 2D planar surfaces and in water phantoms.

SU‐FF‐T‐447: Utility of Film Dosimetry for Assessing TomoTherapy Treatments of Superficial PTVs

Purpose: To assess the utility of film dosimetry in a cylindrical polystyrene phantom for evaluating the accuracy of dose calculated by the TomoTherapy HI⋅ART treatment planning system for superficial planning target volumes (PTVs). Method and Materials: A TomoTherapy treatment plan was developed for a superficial PTV (1‐cm deep radially by 90° azimuthally by 4.2‐cm longitudinally) contoured on a 27‐cm diameter by 37.4‐cm long cylindrical white opaque polystyrene phantom. The phantom included two removable planar film cassettes, one perpendicular to and one including the cylinder's axis. Kodak EDR2 film was cut using templates, resulting in film edges coinciding with the phantom surface (±0.25‐mm). The axial film was irradiated according to the TomoTherapy plan then the phantom was translated longitudinally and the sagittal film irradiated. The films were scanned with a Vidar film digitizer, converted to dose, and the common depth‐dose curves compared. Measured axial and sagittal dose distributions were compared with those calculated. Results: The common depth‐dose of the axial and sagittal films agreed well, although the axial film had a lower relative dose by 2.5%. Comparisons of measured and calculated dose on the axial film showed agreements to within 5% at depths greater than 3‐mm. At shallower depths, doses showed larger differences but distance‐to‐agreement values were small (&lt;2mm). These differences could be due to planned to delivered dose alignment techniques and/or the film cutting process, which is under investigation. Conclusion: Orthogonal films irradiated in a cylindrical polystyrene phantom produced common depth‐dose curves that agreed well. Future comparisons with thermoluminescent dosimetry should confirm the utility of EDR2 film for a comprehensive study of the accuracy of the TomoTherapy treatment planning system for planning treatments of superficial PTVs normally treated with electrons.Supported in part by a research agreement with TomoTherapy, Inc.