Robert S. Fields

Positron Emission Tomography―Computed Tomography Surveillance for the Node-Positive Neck After Chemoradiotherapy

To review our results with positron emission tomography and computed tomography fusion imaging (PET‐CT) surveillance of the postchemoradiotherapy neck in patients with advanced head and neck squamous cell carcinoma.

HELICAL TOMOTHERAPY FOR PAROTID GLAND TUMORS

PURPOSE To investigate helical tomotherapy (HT) intensity-modulated radiotherapy (IMRT) as a postoperative treatment for parotid gland tumors. METHODS AND MATERIALS Helical 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. RESULTS In 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. CONCLUSIONS Helical 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.

Accuracy of pencil‐beam redefinition algorithm dose calculations in patient‐like cylindrical phantoms for bolus electron conformal therapy

PURPOSE The purpose of this study was to document the improved accuracy of the pencil beam redefinition algorithm (PBRA) compared to the pencil beam algorithm (PBA) for bolus electron conformal therapy using cylindrical patient phantoms based on patient computed tomography (CT) scans of retromolar trigone and nose cancer. METHODS PBRA and PBA electron dose calculations were compared with measured dose in retromolar trigone and nose phantoms both with and without bolus. For the bolus treatment plans, a radiation oncologist outlined a planning target volume (PTV) on the central axis slice of the CT scan for each phantom. A bolus was designed using the planning.decimal(®) (p.d) software (.decimal, Inc., Sanford, FL) to conform the 90% dose line to the distal surface of the PTV. Dose measurements were taken with thermoluminescent dosimeters placed into predrilled holes. The Pinnacle(3) (Philips Healthcare, Andover, MD) treatment planning system was used to calculate PBA dose distributions. The PBRA dose distributions were calculated with an in-house C++ program. In order to accurately account for the phantom materials a table correlating CT number to relative electron stopping and scattering powers was compiled and used for both PBA and PBRA dose calculations. Accuracy was determined by comparing differences in measured and calculated dose, as well as distance to agreement for each measurement point. RESULTS The measured doses had an average precision of 0.9%. For the retromolar trigone phantom, the PBRA dose calculations had an average ± 1σ dose difference (calculated - measured) of -0.65% ± 1.62% without the bolus and -0.20% ± 1.54% with the bolus. The PBA dose calculation had an average dose difference of 0.19% ± 3.27% without the bolus and -0.05% ± 3.14% with the bolus. For the nose phantom, the PBRA dose calculations had an average dose difference of 0.50% ± 3.06% without bolus and -0.18% ± 1.22% with the bolus. The PBA dose calculations had an average dose difference of 0.65% ± 6.21% without bolus and 1.75% ± 5.94% with the bolus. From a clinical perspective an agreement of 5% or better between planned (calculated) and delivered (measured) dose is desired. Statistically, this was true for 99% (± 2σ) of the dose points for three of the four cases for the PBRA dose calculations, the exception being the nose without bolus for which this was true for 89% (± 1.6σ) of the dose points. For the retromolar trigone, with and without bolus, the PBA showed agreement of 5% or better for approximately 86% (± 1.5σ) of the dose points. For the nose, with and without bolus, the PBA showed agreement of 5% or better for only approximately 58% (± 0.8σ) of the dose points. CONCLUSIONS The measured data, whose high precision makes them useful for evaluation of the accuracy of electron dose algorithms, will be made publicly available. Based on the spread in dose differences, the PBRA has at least twice the accuracy of the PBA. From a clinical perspective the PBRA accuracy is acceptable in the retromolar trigone and nose for electron therapy with and without bolus.

Preliminary comp arison of helical tomotherapy and mixed beams of unmodulated electrons and intensity modulated radiation therapy for treating superficial cancers of the parotid gland and nasal cavity

Background and PurposeTo investigate combining unmodulated electron beams with intensity-modulated radiation therapy to improve dose distributions for superficial head and neck cancers, and to compare mixed beam plans with helical tomotherapy.Materials and methodsMixed beam and helical tomotherapy dose plans were developed for two patients with parotid gland tumors and two patients with nasal cavity tumors. Mixed beam plans consisted of various weightings of a enface electron beam and IMRT, which was optimized after calculation of the electron dose to compensate for heterogeneity in the electron dose distribution within the target volume.ResultsHelical tomotherapy plans showed dose conformity and homogeneity in the target volume that was equal to or better than the mixed beam plans. Electron-only plans tended to show the lowest doses to normal tissues, but with markedly worse dose conformity and homogeneity than in the other plans. However, adding a 20% IMRT dose fraction (i.e., IMRT:electron weighting = 1:4) to the electron plan restored target conformity and homogeneity to values comparable to helical tomotherapy plans, while maintaining lower normal tissue dose.ConclusionsMixed beam treatments offer some dosimetric advantages over IMRT or helical tomotherapy for target depths that do not exceed the useful range of the electron beam. Adding a small IMRT component (e.g., IMRT:electron weighting = 1:4) to electron beam plans markedly improved target dose homogeneity and conformity for the cases examined in this study.

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 10×10 cm 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 10×10 cm 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.

Clinical evaluation of an algorithm for inhomogeneity corrections of electron beam dosimetry

An algorithm for inhomogeneity correction of electron beam dose distributions has been developed at U.T. M. D. Anderson Hospital and Tumor Institute. The enhanced perturbation of the electron beam requires a more rigorous and exact analysis compared to megavoltage photon beams. The algorithm uses a pencil beam calculation model to correct for air gap, tissue inhomogeneity and field shape. Cross sectional computer tomography data provide surface contour and tissue inhomogeneity information. The accuracy of the calculation model has been verified by TLD measurements in tissue substitute phantoms reconstructed from CT patient scans.

SU‐GG‐T‐477: Influence of Pitch and Jaw Width On Helical Tomotherapy Head and Neck Treatment Planning

Purpose: To investigate the influence of pitch and jaw width selection on helical tomotherapy treatment plans for head and neck. Method and Materials: Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for three patients previously treated for head and neck cancer. Three jaw widths (5, 2.5 and 1 cm) and six pitches (0.86, 0.7, 0.43, 0.34, 0.287 and 0.215) were used with a (maximum) modulation factor setting of 4. The 18 plans per patient were generated using an identical optimization procedure (e.g., number of iterations, objective weights and penalties, etc.), based on recommendations from TomoTherapy. The plans were compared using isodose plots, dose volume histograms, dose homogeneity indices, conformity indices, radiobiological models, and treatment times. Results: Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. The conformity index had a maximum for pitches between 0.287 and 0.43. The tumor control probabilities were greater than 99% for the 2.5‐ and 1‐cm jaw widths for all pitches except 0.86. The treatment time increased inversely proportional to the jaw width. Conclusion: Our study indicates that the smaller jaw widths (2.5 and 1 cm) and pitches near the range of 0.3–0.4 produce better plans for helical tomotherapy head and neck treatment. However, because of the large increase in treatment time with only slight improvement in sparing of the critical structures for the 1‐cm jaw width, the 2.5‐cm jaw width, currently used in our clinic, remains our preference. Supported in part by a research agreement with TomoTherapy, Inc.

TH‐D‐M100E‐04: Evaluation of MVCT Images Containing Lead Alloy Masks for Electron Beam Treatment Planning

Purpose: To evaluate the accuracy of electron beam dose calculations in MVCT images containing lead alloy masks. Method and Materials: A phantom consisting of two 30×30×5‐cm3 slabs of CIRS plastic water® was imaged using kVCT (GE Lightspeed‐RT) and MVCT (TomoTherapy Hi⋅Art). The MVCT scans were taken with nine square masks of Cerrobend® (density = 9.4gcm−3) on top of the phantom. The masks contained square openings of 3×3cm2, 6×6cm2 and 10×10cm2 and had thicknesses of 6mm, 8mm and 10mm. The same collimation was simulated in the kVCT images by creating regions‐of‐interest (ROI) duplicating the sizes, shapes, and density of the masks. Using the Philips Pinnacle3treatment planning system, twelve treatment plans were created using electron energies of 6, 9, 12, and 16 MeV for each opening size. For each plan, the mask thickness appropriate for the electron energy was used and the dose distributions calculated using the kVCT and MVCT images were compared. In uniform dose regions (doses above 90% of maximum) dose differences were calculated; in high‐dose gradient regions (doses below 90% of maximum) distances‐to‐agreement (DTA) were determined. Results: In the uniform dose region, the maximum difference between the doses in the MVCT images and the doses in the kVCT image was greater than or equal to ±5% for all opening and energy combinations. In the high‐dose gradient region, almost half of the maximum DTA values exceeded 2mm. Analysis of the MVCT images showed that DTA differences were largely due to distortions in the phantom CT numbers caused by the masks. Conclusion: Although Cerrobend® produces dramatically less distortion in MVCT images compared to kVCT images,image distortion is still too great for accurate electron beam dose calculations. Supported in part by a research agreement with TomoTherapy, Inc.