T Liu

Initial clinical experience with helical tomotherapy for head and neck cancer

To report a single‐institutional experience with the use of helical tomotherapy (HT)‐based intensity‐modulated radiotherapy (IMRT) for head and neck cancer.

Comparison of Intensity-Modulated Radiotherapy Using Helical Tomotherapy and Segmental Multileaf Collimator-based Techniques for Nasopharyngeal Carcinoma: Dosimetric Analysis Incorporating Quality Assurance Guidelines from RTOG 0225

Intensity-modulated radiotherapy (IMRT) treatment plans generated by segmental multileaf collimator (SMLC) and helical tomotherapy (HT) techniques for patients with nasopharyngeal carcinoma were compared using standardized criteria proposed by Radiation Therapy Oncology Group (RTOG) protocol 0225. The goal was to deliver a prescribed dose of 70 Gy to at least 95% of the planning target volume (PTV) encompassing gross tumor, and 59.4 Gy and 50.4 Gy, respectively, to areas at high and low risk for microscopic disease, over 33 treatments while respecting constraints to organs at risk (OAR). HT-IMRT significantly reduced dose to the contralateral parotid gland and improved dose homogeneity to the PTVs. Mean doses to the inner and middle ears were also reduced by 18% and 24%, respectively, on the ipsilateral side, and 24%, and 35%, respectively, on the contralateral side using HT-IMRT compared to SMLC-IMRT. Additionally, HT-IMRT reduced mean doses to brainstem (p = 0.02), larynx (p = 0.03), and oral cavity (p = 0.03). These findings suggest that HT-IMRT may be of improve the therapeutic ratio in the radiotherapeutic treatment of nasopharyngeal carcinoma.

Intensity-modulated radiotherapy for nasopharyngeal carcinoma: improvement of the therapeutic ratio with helical tomotherapy vs segmental multileaf collimator-based techniques

OBJECTIVESnThe aim of the study was to compare differences in dosimetric, clinical and quality-of-life end points among patients treated with helical tomotherapy (HT) and segmental multileaf collimator (SMLC)-based intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma.nnnMETHODSnFrom June 2005 to August 2009, 30 consecutive patients were treated with IMRT for nasopharyngeal carcinoma to a dose of 70 Gy. 14 patients (47%) were treated using HT and 16 (53%) were treated using SMLC-based IMRT. 28 patients (93%) received concurrent chemotherapy. The patients were evenly balanced between the two radiotherapy groups with respect to clinical and pathological characteristics. Median follow-up was 30 months (range, 6-62 months).nnnRESULTSnThe 2-year estimates of overall survival, local-regional control and progression-free survival were 81%, 87% and 82%, respectively. There were no significant differences in any of these end points with respect to IMRT technique (p>0.05 for all). Dosimetric analysis revealed that patients treated by HT had significantly improved salivary sparing with respect to mean dose (27.3 vs 34.1 Gy, p=0.03) and volume receiving greater than or equal to 30 Gy (31.7% vs 47.3%, p=0.01) to the contralateral (spared) parotid gland. The incidence of Grade 3+ late xerostomia was 13 and 7% among patients treated with SMLC-based IMRT and HT, respectively (p=0.62). The corresponding proportion of patients who subjectively reported too little or no saliva at final follow-up was 38% and 7%, respectively (p=0.04).nnnCONCLUSIONnThe superior dosimetric outcome observed with HT appeared to translate into moderately improved clinical outcomes with respect to salivary sparing. Prospective trials are needed to validate this gain in the therapeutic ratio.

Vector Analysis of Prostate Patient Setup With Image-Guided Radiation Therapy via kV Cone Beam Computed Tomography

PURPOSEnTo analyze the daily setup variations in a cohort of intensity-modulated radiation therapy (IMRT) prostate cancer patients who had received daily image-guided RT without the use of fiducial markers to determine if daily image guidance is necessary.nnnMETHODS AND MATERIALSn2134 Kilovoltage (kV) cone beam computed tomography (CBCT) images were analyzed, with three shifts recorded for each image. The number of times that the vector of the combined shifts would have exceeded the planning tumor volume (PTV) margin was tallied. Then, the average scalar shift of the first five images was removed from all subsequent images for a given patient, and the number of days for which the shift vector was greater than the three-dimensional clinical tumor volume-PTV (3D CTV-PTV) margin (8 mm, created with rolling ball technique) was recorded. Additionally, the scalar shifts from every other fraction were studied to determine if the individual patients shift vector would be adequately sampled if CBCT was not performed daily, thus reducing patient imaging dose without compromising treatment quality.nnnRESULTSnThere were 297 cases where the vector shift was initially greater than the PTV margin. By correcting each patients data set by the average shift of their first five images the total was 248 cases. By considering only every other image of each patient data set (after correction for the first 5 days), only 137 days in which the CTV was outside the PTV would have been seen.nnnCONCLUSIONSnDaily imaging is recommended for prostate cancer IMRT patients in order to know the 3D (vector) position of the CTV and to ensure that it is always within the PTV margin. Correcting the data set by the average shift from the first 5 days reduces the overall number of outlier days but does not eliminate them completely.

Comparison of daily versus nondaily image‐guided radiotherapy protocols for patients treated with intensity‐modulated radiotherapy for head and neck cancer

The purpose of this study was to determine the feasibility of nondaily image‐guided radiotherapy (RT) strategies with intensity‐modulated radiotherapy (IMRT) for head and neck cancer.

Utility of Megavoltage Fan-Beam CT for Treatment Planning in a Head-And-Neck Cancer Patient with Extensive Dental Fillings Undergoing Helical Tomotherapy

The purpose of this study was to demonstrate the potential utility of megavoltage fan-beam computed tomography (MV-FBCT) for treatment planning in a patient undergoing helical tomotherapy for nasopharyngeal carcinoma in the presence of extensive dental artifact. A 28-year-old female with locally advanced nasopharyngeal carcinoma presented for radiation therapy. Due to the extensiveness of the dental artifact present in the oral cavity kV-CT scan acquired at simulation, which made treatment planning impossible on tomotherapy planning system, MV-FBCT imaging was obtained using the HI-ART tomotherapy treatment machine, with the patient in the treatment position, and this information was registered with her original kV-CT scan for the purposes of structure delineation, dose calculation, and treatment planning. To validate the feasibility of the MV-FBCT-generated treatment plan, an electron density CT phantom (model 465, Gammex Inc., Middleton, WI) was scanned using MV-FBCT to obtain CT number to density table. Additionally, both a cheese phantom (which came with the tomotherapy treatment machine) with 2 inserted ion chambers and a generic phantom called Quasar phantom (Modus Medical Devices Inc., London, ON, Canada) with one inserted chamber were used to confirm dosimetric accuracy. The MV-FBCT could be used to clearly visualize anatomy in the region of the dental artifact and provide sufficient soft-tissue contrast to assist in the delineation of normal tissue structures and fat planes. With the elimination of the dental artifact, the MV-FBCT images allowed more accurate dose calculation by the tomotherapy system. It was confirmed that the phantom material density was determined correctly by the tomotherapy MV-FBCT number to density table. The ion chamber measurements agreed with the calculations from the MV-FBCT generated phantom plan within 2%. MV-FBCT may be useful in radiation treatment planning for nasopharyngeal cancer patients in the setting of extensive dental artifacts.

WE-D-224A-09: Proposed Pass/fail Criteria for IMRT Patient Specific QA

Purpose: To establish an acceptable quality assurance (QA) criteria for IMRT patient specific QA. There are no established criteria regarding agreement between patients planned dose distributions versus QA measurements. Method and Materials: A series of MLC QA test patterns were run to detect/correct MLC leaf positional inaccuracies for a Varian 2100C and Elekta Synergy (both with 80 leaf MLC). MapCheck, a 2D diode array system, was used to determine optimal correctional parameter, Dosimetric Leaf Gap (DLG), used in the Varian Eclipse/Helios treatment planning system (TPS) (accounts for effect of MLC rounded‐leaf end geometry). Patients IMRT plan was checked by resetting all fields to a fixed gantry angle (beam down), delivering summed dose of each beam, measuring with MapCheck, then comparing with TPS calculated dose distribution. QA results for a total of 48 segmented MLC (SMLC) IMRT cases (37 prostate, 9 head and neck, 1 pelvis, and 1 brain) were reviewed. Results:MLC position uncertainties were reduced from 0.3–0.4 mm to 0.1–0.2 mm by a careful calibration. An optimal DLG of 2 mm was determined for Elekta Synergy. Using criteria of ±3% dose agreement or ±3mm distance to agreement (DTA), measured absolute dose distributions agreed with planned dose distributions as follows: prostate: mean 98%, 2.3% S.D.; HN pelvis: mean 90.5%; brain: mean 91.1%; for the total 48 cases, mean 96%, S.D. 5.4%. Conclusion:MLC positional accuracy and having optimal correctional parameter within the TPS are two key factors to ensure IMRT delivered dose in good agreement with calculated dose distribution. Using MapCheck as described, we have implemented the following pass/fail criteria for patient specific QA measurement results: 85% of points within ±3% and ±3 mm DTA in absolute dose. Our clinical experience shows that this is achievable even with the most complicated H&N cases.

SU‐FF‐T‐281: Leakage Radiation of Elekta Synergy‐S LINAC Machine

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy‐S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy‐S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x‐ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulatortreatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy‐S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.

SU‐FF‐T‐242: Feasibility of Using a 2D Diode Array System for Clinical Electron Beam Measurements

Purpose: To investigate the feasibility of using a 2D diode array system for clinical electron beam measurements Method and Materials:Dose distributions were measured for electron beams (EBs) generated on an Elekta Synergy LINAC (Elekta Ltd., UK). Beam data were measured using the Blue Phantom and converted on OmniPro‐Accept (Scanditronix Wellhofer, Bartlett, TN) according to AAPM TG 51. These data were used to commission Eclipse TPS (Varian, Mountain View, CA). MapCheck Model 1175 (Sun Nuclear, Melbourne, FL), a 2D diode array system, was exposed using largest applicator size 20cm×20cm at SSD 123.5cm for array calibration. Central axis dose was also calibrated. Plans were generated and measured by MapCheck for several different geometries. Exported plans were compared with measured dose map using comparison criteria of ±3% difference and ±3mm distance‐to‐agreement (DTA) within 10% isodose‐line threshold. Set‐ups include different cone sizes (field sizes) and energies at different SSDs, irregular surface, and different depths. Direct comparisons between MapCheck and ion chamber results were also performed. Results: Output factors measured with MapCheck and ion chamber agree within 1.3%. Comparison of measured and planned electron beamdose maps for 9MeV EB with 14cm×14cm applicator, SSD=100cm, depth=2cm showed 99.7% passing rate for stated criteria. Central axis dose differed by 1.6%. Passing rates and central axis dose differences for two electron applicators (10×10 and 14×14) at different SSDs (95.4, 100, and 104.4) were also summarized. Overall, >90% passing rates and <3% central axis dose differences can be achieved. Most of the failed points are at the edge. A stepped phantom was also tested and is under further investigation. Conclusions: Preliminary results show that MapCheck can be used to perform quick and relatively accurate electron beamdose map comparison. It may also prove useful for electron beam intensity‐modulated radiotherapy (EB‐IMRT) measurements in the future.

SU‐FF‐T‐176: Rectal and Bladder Dose in Relationship to PTV Percentage Coverage in Prostate IMRT

Purpose: To study the relationship between percentage coverage of planning target volume (PTV) in prostate cancerIMRT and dose received by normal critical structures Method and Materials: Five patients with early stage prostate cancer were selected for this retrospective planning study. For 4 of 5 patients, PTV contains clinical target volume (CTV), which is the prostate gland, plus 0.8 cm uniform margin. For the fifth patient, PTV is formed by expanding CTV (prostate plus 1 cm proximal seminal vesicle) with 0.8 cm uniform margin. The prescription is 74 Gy for the first four patients and 79.2 Gy (last patient) prescribed to 95% iso dose line and dose normalization is to isocenter. For each patient, seven different plans were generated using Varian Eclipse treatment planning system version 8.1 for 18 MV photon beams from Varian 2100C: one 6‐field conformal plan and six 7‐field IMRT plans with PTV volume coverage raging from 99.5% to 95%. IMRT plans were optimized by iterations to reach the targeted coverage of PTV. Results: A linear correlation between the volume receiving 70 Gy in percentage or cc and PTV volume coverage from 99.5% to 95% has been found for both balder and rectum with R2 better than 0.95 for bladder and 0.91 for rectum for the first four patients. A generalized relationship can be written as V 70 ( cc )= k * PTV (%)+ b (1). All patients have a similar linear slope for bladder (average slope is 0.867±0.083 (SD)) and a slightly different slope for rectum (average slope is 0.545±0.230 (SD)). In equation (1), interception b is dependant of structure volume (bladder and rectum). Conclusions: Volume received 70 Gy by bladder and rectum in IMRT plans for prostate as CTV may have a linear relationship with PTV coverage ranging from 99.5% to 95%. One may use this relationship to guide in treatment planning process.