I Rosen

FEASIBILITY OF POSTMASTECTOMY TREATMENT WITH HELICAL TOMOTHERAPY

PURPOSEnTo investigate the potential of helical tomotherapy for postmastectomy radiation therapy.nnnMETHODS AND MATERIALSnBy use of the TomoTherapy Hi-Art II treatment-planning system (TomoTherapy Inc., Madison, WI), helical tomotherapy dose plans were developed for 5 patients and compared with the mixed-beam (electron-photon) plans with which they had been treated. The TomoTherapy plans were evaluated by use of dose-volume quantities, tumor control probability, normal tissue complication probability (NTCP), and secondary cancer complication probability (SCCP).nnnRESULTSnThe TomoTherapy plans showed better dose homogeneity in the planning treatment volume containing the chest wall and internal mammary nodes (p = 0.001) and eliminated the need for abutting fields. For the normal tissues, the TomoTherapy plans showed a smaller fractional volume receiving 20 Gy or greater for the ipsilateral lung (p = 0.05), no change in NTCP for postradiation pneumonitis, increased SCCP for each lung and both lungs together (p < 0.02), no change in the volume of the heart receiving more than 15 Gy, no change in NTCP for excess cardiac mortality, and a larger mean dose and SCCP in the contralateral breast (p < 0.001). For nonspecific tissues, the volume receiving between 5 Gy and 25 Gy and SCCP were both larger for the TomoTherapy plans (p < 0.01). Total SCCP was larger for the TomoTherapy plans (p = 0.001).nnnCONCLUSIONSnOverall, the TomoTherapy plans had comparable tumor control probability and NTCP to the mixed-beam plans and increased SCCP. The TomoTherapy plans showed significantly greater dose homogeneity in the chest wall, which offers the potential for improved cosmesis after treatment. These factors have resulted in TomoTherapy often being the treatment of choice for postmastectomy radiation therapy in our clinic.

Accuracy of TomoTherapy treatments for superficial target volumes

Helical tomotherapy is a technique for delivering intensity modulated radiation therapy treatments using a continuously rotating linac. In this approach, fan beams exiting the linac are dynamically modulated in synchrony with the motion of the gantry and couch. Helical IMRT deliveries have been applied to treating surface lesions, and the purpose of this study was to evaluate the accuracy of dose calculated by the TomoTherapy HiArt treatment planning system for superficial planning target volumes (PTVs). TomoTherapy treatment plans were developed for three superficial PTVs (2-, 4-, and 6-cm deep radially by 90 degrees azimuthally by 4-cm longitudinally) contoured on a 27-cm diameter cylindrical white opaque, high-impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes that contained bare Kodak EDR2 films cut such that their edges matched the phantom surface (+/-0.05 cm). The phantom was aligned to the machines isocenter (+/-0.05 cm) and was irradiated according to the treatment plans. Films were scanned with a Vidar film digitizer, and optical densities were converted to dose using a calibration determined from a 6 MV perpendicular film exposure. This perpendicular calibration required that axial film doses (parallel irradiation) be scaled by 1.02 so that mid-arc depth doses matched those measured in the sagittal plane (perpendicular irradiation). All film readings were scaled by 0.935 to correct for over-response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Calculated doses overpredicted measured doses by as much as 9.5% of the prescribed dose at depths less than 1 cm. At depths greater than 1 cm, calculated dose distributions showed agreement to measurement within 5% in the high-dose region and within 0.2 cm distance-to-agreement in the dose falloff regions. In the low-dose region posterior to the PTVs (<10% of the prescribed dose), the dose algorithm underpredicted the dose by 1%-2% of the prescribed dose. Clinically, it is recommended that 1 cm of bolus be used on the surface to ensure that cancerous tissues less than 1 cm depth are not underdosed.

Independent calculation of dose from a helical TomoTherapy unit

A new calculation algorithm has been developed for independently verifying doses calculated by the TomoTherapy® Hi·Art® treatment planning system (TPS). The algorithm is designed to confi rm the dose to a point in a high dose, low dose‐gradient region. Patient data used by the algorithm include the radiological depth to the point for each projection angle and the treatment sinogram file controlling the leaf opening time for each projection. The algorithm uses common dosimetric functions [tissue phantom ratio (TPR) and output factor (Scp)] for the central axis combined with lateral and longitudinal beam profile data to quantify the off‐axis dose dependence. Machine data for the dosimetric functions were measured on the Hi·Art machine and simulated using the TPS. Point dose calculations were made for several test phantoms and for 97 patient treatment plans using the simulated machine data. Comparisons with TPS‐predicted point doses for the phantom treatment plans demonstrated agreement within 2% for both on‐axis and off‐axis planning target volumes (PTVs). Comparisons with TPS‐predicted point doses for the patient treatment plans also showed good agreement. For calculations at sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Calculations within lung and superficial PTVs overestimated the dose by an average of 3.1% (σ=2.4%) and 3.2% (σ=2.2%), respectively. Systematic errors within lung are probably due to the weakness of the algorithm in correcting for missing tissue and/or tissue density heterogeneities. Errors encountered within superficial PTVs probably result from the algorithm overestimating the scatter dose within the patient. Our results demonstrate that for the majority of cases, the algorithm could be used without further refinement to independently verify patient treatment plans. PACS number(s): 87.53.Bn, 87.53.Dq, 87.53.Xd

HELICAL TOMOTHERAPY FOR PAROTID GLAND TUMORS

PURPOSEnTo investigate helical tomotherapy (HT) intensity-modulated radiotherapy (IMRT) as a postoperative treatment for parotid gland tumors.nnnMETHODS AND MATERIALSnHelical tomotherapy plans were developed for 4 patients previously treated with segmental multileaf collimator (SMLC) IMRT. A primary planning target volume (PTV64) and two secondary PTVs (PTV60, PTV54) were defined. The clinical goals from the SMLC plans were applied as closely as possible to the HT planning. The SMLC plans included bolus, whereas HT plans did not.nnnRESULTSnIn general, the HT plans showed better target coverage and target dose homogeneity. The minimum doses to the desired coverage volume were greater, on average, in the HT plans for all the targets. Minimum PTV doses were larger, on average, in the HT plans by 4.6 Gy (p = 0.03), 4.8 Gy (p = 0.06), and 4.9 Gy (p = 0.06) for PTV64, PTV60, and PTV54, respectively. Maximum PTV doses were smaller, on average, by 2.9 Gy (p = 0.23), 3.2 Gy (p = 0.02), and 3.6 Gy (p = 0.03) for PTV64, PTV60, and PTV54, respectively. Average dose homogeneity index was statistically smaller in the HT plans, and conformity index was larger for PTV64 in 3 patients. Tumor control probabilities were higher for 3 of the 4 patients. Sparing of normal structures was comparable for the two techniques. There were no significant differences between the normal tissue complication probabilities for the HT and SMLC plans.nnnCONCLUSIONSnHelical tomotherapy treatment plans were comparable to or slightly better than SMLC plans. Helical tomotherapy is an effective alternative to SMLC IMRT for treatment of parotid tumors.

Investigation of pitch and jaw width to decrease delivery time of helical tomotherapy treatments for head and neck cancer.

Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for 3 patients previously treated for head and neck cancer. Three jaw widths (5, 2.5, and 1 cm) and 4 pitches (0.86, 0.43, 0.287, and 0.215) were used with a (maximum) modulation factor setting of 4. Twelve plans were generated for each patient using an identical optimization procedure (e.g., number of iterations, objective weights, and penalties, etc.), based on recommendations from TomoTherapy (Madison, WI). The plans were compared using isodose plots, dose volume histograms, dose homogeneity indexes, conformity indexes, radiobiological models, and treatment times. Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. However, the treatment time increased inversely proportional to the jaw width, resulting in delivery times of 24 ± 1.9 min for the 1-cm jaw width. Although treatment plans produced with the 2.5-cm jaw were dosimetrically superior to plans produced with the 5-cm jaw, subsequent calculations of tumor control probabilities and normal tissue complication probabilities suggest that these differences may not be radiobiologically meaningful. Because treatment plans produced with the 5-cm jaw can be delivered in approximately half the time of plans produced with the 2.5-cm jaw (5.1 ± 0.6 min vs. 9.5 ± 1.1 min), use of the 5-cm jaw in routine treatment planning may be a viable approach to decreasing treatment delivery times from helical tomotherapy units.

SU‐FF‐T‐213: Evaluation of Dose From TomoTherapy Irradiation of Superficial PTVs

Purpose: To evaluate the accuracy of dose calculated by the TomoTherapy HI⋅ART treatment planning system for superficial planning target volumes (PTVs). Methods and Materials: TomoTherapy treatment plans were developed for three superficial PTVs (2−, 4−, and 6−cm deep radially by 90° azimuthally by 4−cm longitudinally) contoured on a 27−cm diameter cylindrical white opaque, high‐impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes. Kodak EDR2 films were cut using templates to match the phantom surface (±0.5mm). The phantom was aligned to the machines isocenter (±0.5mm), and films were irradiated according to the treatment plans. Films were then scanned with a Vidar film digitizer and converted to dose using a calibration determined from a 6 MV perpendicular film exposure. Hence, axial film doses were scaled by 1.027 so that mid‐arc depth doses matched those measured in the sagittal plane. All film readings were scaled by 0.934 to correct for over response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Results: For all PTVs, at depths less than 10mm calculated overestimated measured doses by as much as 8%. At depths greater than 10mm, the calculated and measured doses agreed to within 5%. In the dose falloff region, measured and calculated dose distributions agreed to within ±2mm. The calculated dose in the low dose region distal to the PTV was lower by as much as 2%. Each of these differences is related to an assumption in the dose model. Conclusion: Calculated dose distributions showed good agreement with measurement for depths greater than 10mm. For more superficial depths, the dose model should be carefully evaluated or one should use bolus of at least 10mm to ensure accurate calculated dose to superficial PTVs. Supported in part by a research agreement with TomoTherapy, Inc.

Evaluation of MVCT images with skin collimation for electron beam treatment planning.

This study assessed the potential of using megavoltage CT (MVCT) images taken with high density skin collimation in place for electron beam treatment planning. MVCT images were taken using the TomoTherapy Hi‐Art system (TomoTherapy Inc., Madison, WI), and the CT numbers were converted to density by calibrating the Hi‐Art system using an electron density phantom. Doses were computed using MVCT images and kVCT images and compared by calculating dose differences in the uniform dose region (>90%, excluding buildup region) and calculating distance‐to‐agreement (DTA) in high dose‐gradient regions (penumbra and distal falloff, 90%–10%). For 9 and 16 MeV electron beams of 10×10u2009cm calculated on a homogeneous CIRS Plastic Water (Computerized Imaging Research Systems Inc., Norfolk, VA) phantom without skin collimation, the maximum dose differences were 2.3% and the maximum DTAs were 2.0 mm for both beams. The same phantom was then MVCT scanned nine times with square skin collimators of Cerrobend on its surface ‐ field sizes of 3×3, 6×6, and 10×10u2009cm and thicknesses of 6, 8, and 10 mm. Using the Philips Pinnacle 3 treatment planning system (Philips Medical Systems, N.A., Bothwell, WA), a treatment plan was created for combinations of electron energies of 6, 9, 12, and 16 MeV and each field size. The same treatment plans were calculated using kVCT images of the phantom with regions‐of‐interest (ROI) manually drawn to duplicate the sizes, shapes, and density of the skin collimators. With few exceptions, the maximum dose differences exceeded ±5% and the DTAs exceeded 2 mm. We determined that the dose differences were due to small distortions in the MVCT images created by the high density material and manifested as errors in the phantom CT numbers and in the shape of the skin collimator edges. These results suggest that MVCT images without skin collimation have potential for use in patient electron beam treatment planning. However, the small distortion in images with skin collimation makes them unsuitable for clinical use. PACS: 87.53.Tf, 87.59.Fm, 87.53.Fs

SU‐FF‐T‐122: Comparison of Helical Tomotherapy to SMLC IMRT for Treatment of Parotid Gland Tumors

Purpose: To investigate the quality of helical tomotherapy intensity modulated radiotherapy treatment plans for parotid gland tumors by comparing them to step‐and‐shoot MLC (SMLC) IMRT plans Method and Materials: Helical tomotherapy (TomoTherapy Hi⋅Art System) plans were generated for five patients previously planned using Pinnacle3 and treated using SMLC IMRT with bolus. One primary and two elective planning target volumes (PTVs) and anatomic structures that had been outlined in Pinnacle3 were imported into Hi⋅Art. Hi⋅Art plans were generated without bolus. Doses from the Hi⋅Art plans were transferred to Pinnacle3 for analysis and plan comparisons. All dose‐volume histogram (DVH) calculations were done in Pinnacle3. PTV doses were compared using cumulative DVH, conformity index (CI), and tumor control probability (TCP). Doses to the critical structures were compared using maximum dose, mean dose, and normal tissue complication probability (NTCP). Results: PTV doses were generally higher for the Hi⋅Art plans. Hi⋅Art plans also had steeper dose gradients at the edges of PTVs, leading to greater minimum doses. In addition, the maximum target doses were generally smaller, suggesting greater PTV dose homogeneity for the Hi⋅Art plans. The conformity index was generally higher for the Hi⋅Art plans. TCPs were 100% for both techniques, except for one case. Hi⋅Art surface doses without bolus were comparable to SMLC IMRT ones with bolus. Mean contralateral parotid dose was lower for all of the Hi⋅Art plans, substantially so in three of the cases. Doses to eyes, optic nerve, and spinal cord were similar or lower for Hi⋅Art plans. Similar or decreased NTCPs were found for all the OARs. Conclusion: Hi⋅Art plans generally gave higher and more uniform target doses. Both SMLC IMRT and Hi⋅Art plans were excellent for sparing critical structures, except for the contralateral parotid, where Hi⋅Art plans were better. Supported in part by a research agreement with TomoTherapy, Inc.

SU‐FF‐J‐08: Accuracy of Cranial Co‐Planar Beam Therapy with BrainLab ExacTrac Image Guidance

Purpose: To evaluate the positional accuracy (displacement between planned and delivereddose distributions) of cranial co‐planar beam treatments for image guided stereotactic radiation therapy with Novalis. Methods and Materials: Positional accuracy was investigated using a CIRS anthropomorphic head phantom loaded with 6.35cm × 6.35cm sections of EDR2 film oriented to measure dose in the three principal planes. BrainScan was used to develop a treatment plan consisting of seven equally spaced coplanar mMLC beams that conformed to irradiate a 2‐cm diameter by 2‐cm long PTV. Prior to delivery phantom misalignments were imposed in combinations of ±8‐mm offsets in one or more of the principal directions. ExacTrac X‐ray corrections were applied 1–3 times until the reported alignment was within 0.4mm/0.4° in all 6 degrees of freedom based on X‐ray to DRRimage fusion prior to treatmentdelivery. Phantom positions were tracked by the Novalis IR system. The delivereddose distribution was measured with a precision of ±0.3mm. Measured and calculated dose distributions were registered using 4 fiducial rods in the phantom. Results: Based on the 70% dose contour, the displacements of the delivered from the planned dose distributions ranged from 0.7 to 1.3mm, −0.4 to 0mm, and −0.6mm to 0.5mm in Posterior‐Anterior, Right‐Left and Inferior‐Superior directions respectively. For the 80% dose contour, the displacements of the delivered from the planned dose distributions ranged from 0.8 to 1.8mm, 0.1 to 0.8mm, and 0.1 to 1.1mm, respectively. Final displacements were independent of initial misalignments. Conclusions: Using recommended Novalis calibration procedures, the ExacTrac X‐ray image‐guidance system used in our clinic can deliver cranial dose distributions within 1.3mm of the planned dose distributions when initial misalignments are within ±8mm in the three principal directions. 80% isodose contours can have up to 1.8‐mm errors. This work is part of a research agreement with BrainLab, Inc.

SU‐GG‐J‐198: Phantom Evaluation of Implanted Coil Localization Accuracy of the BrainLab ExacTrac Gating System and 4DCT

Purpose: The purpose of this study was to evaluate the accuracy of measured target motion using 4DCT scanning and BrainLab ExacTrac (ET) Gating system for gated therapy. Motions of an implantable coil as a function of respiratory phase were compared from the two systems to the known motion for a commercial gating phantom. Method and Materials: A Quasar respiratory motion phantom containing an implantable coil as a surrogate target was used. Phantom motion was sinusoidal with a 4‐second period and amplitudes of 5–25 mm. 4DCT datasets were sorted in 10 distinct respiratory phases, reconstructed, and imported into the Pinnacle TPS. Coil endpoints were identified on each phase‐sorted CT datasets to measure coil distortion. To compare coil localization accuracy, both systems imaged the implanted coil at the same respiratory phases and measured overall relative coil displacement as a function of respiratory phase. Results: Coil length errors measured on the 4DCT were <0.8 mm at end inhale/exhale phases, but 8.1 mm at mid‐inhalation. Maximum localization error from the expected position for all motion profiles was 5.5 mm for both coil tips in 4DCT. but only 0.8 mm for the ET Gating system. Even at the greatest coil velocity, ExacTrac coil localization agrees with calculated coil motion within 1 mm. 4DCT showed problems resolving a coil during large respiratory‐induced velocities, but accurately resolved the coil length within 1 mm of actual coil length at end expiration/inhalation. Conclusion: 4DCT can provide accurate representation of the phantom at end‐respiration for treatment planning. ExacTrac can accurately localize the coil to determine target motion over all phases. Good agreement of the techniques will allow minimization of internal motion margins in gated radiotherapy.Conflict of Interest: This work was supported in part by a research agreement with BrainLab, Inc.