Ileana Iftimia

Quality assurance methodology for Varian RapidArc treatment plans

With the commercial introduction of the Varian RapidArc, a new modality for treatment planning and delivery, the need has arisen for consistent and efficient techniques for performing patient‐specific quality assurance (QA) tests. In this paper we present our methodology for a RapidArc treatment plan QA procedure. For our measurements we used a 2D diode array (MapCHECK) embedded at 5 cm water equivalent depth in MapPHAN 5 phantom and an Exradin A16 ion chamber placed in six different positions in a cylindrical homogeneous phantom (QUASAR). We also checked the MUs for the RapidArc plans by using independent software (RadCalc). The agreement between Eclipse calculations and MapCHECK/MapPHAN 5 measurements was evaluated using both absolute distance‐to‐agreement (DTA) and gamma index with 10% dose threshold (TH), 3% dose difference (DD), and 3 mm DTA. The average agreement was 94.4% for the DTA approach and 96.3% for the gamma index approach. In high‐dose areas, the discrepancy between calculations and ion chamber measurements using the QUASAR phantom was within 4.5% for prostate cases. For the RadCalc calculations, we used the average SSD along the arc; however, for some patients the agreement for the MUs obtained with RadCalc versus Eclipse was inadequate (discrepancy>5%). In these cases, the plan was divided into partial arc plans so that RadCalc could perform a better estimation of the MUs. The discrepancy was further reduced to within ~4% using this approach. Regardless of the variation in prescribed dose and location of the treated areas, we obtained very good results for all patients studied in this paper. PACS number: 87.55.Qr

Treatment planning methodology for the Miami Multichannel Applicator following the American Brachytherapy Society recently published guidelines: the Lahey Clinic experience

The objective of this study was to develop a standardized procedure from simulation to treatment delivery for the multichannel Miami applicator, in order to increase planning consistency and reduce errors. A plan is generated prior to the 1st treatment using the CT images acquired with the applicator in place, and used for all 3 fractions. To confirm the application placement before each treatment fraction, an AP image is acquired and compared with the AP baseline image taken at simulation. A preplanning table is generated using the EBRT doses and is used to compute the maximum allowable D2cc for bladder, rectum, and sigmoid, and the mean allowable dose for the upper vaginal wall per HDR brachytherapy fraction. These data are used to establish the criteria for treatment planning dose optimization. A step‐by‐step treatment planning approach was developed to ensure appropriate coverage for the tumor (D90>100% prescribed dose of 700 cGy/fraction) and the uninvolved vaginal surface (dose for the entire treatment length > 600 cGy/fraction), while keeping the organs at risk below the tolerance doses. The equivalent dose 2 Gy (EQD2) tolerances for the critical structures are based on the American Brachytherapy Society (ABS) recently published guidelines. An independent second check is performed before the 1st treatment using an in‐house Excel spreadsheet. This methodology was successfully applied for our first few cases. For these patients: the cumulative tumor dose was 74–79 EQD2 Gy10 (ABS recommended range 70–85); tumor D90 was >100% of prescribed dose (range 101%–105%); cumulative D2cc for bladder, rectum, and sigmoid were lower than the tolerances of 90, 75, and 75 EQD2 Gy3, respectively; cumulative upper vaginal wall mean dose was below the tolerance of 120 EQD2 Gy3; the second check agreement was within 5%. By using a standardized procedure the planning consistency was increased and all dosimetric criteria were met. PACS numbers: 87.55‐x, 87.56 bg

Commissioning and quality assurance for the treatment delivery components of the AccuBoost system

The objective for this work was to develop a commissioning methodology for the treatment delivery components of the AccuBoost system, as well as to establish a routine quality assurance program and appropriate guidance for clinical use based on the commissioning results. Various tests were developed: 1) assessment of the accuracy of the displayed separation value; 2) validation of the dwell positions within each applicator; 3) assessment of the accuracy and precision of the applicator localization system; 4) assessment of the combined dose profile of two opposed applicators to confirm that they are coaxial; 5) measurement of the absolute dose delivered with each applicator to confirm acceptable agreement with dose based on Monte Carlo modeling; 6) measurements of the skin‐to‐center dose ratio using optically stimulated luminescence dosimeters; and 7) assessment of the mammopad cushions effect on the center dose. We found that the difference between the measured and the actual paddle separation is <0.1 cm for the separation range of 3 cm to 7.5 cm. Radiochromic film measurements demonstrated that the number of dwell positions inside the applicators agree with the values from the vendor, for each applicator type and size. The shift needed for a good applicator‐grid alignment was within 0.2 cm. The dry‐run test using film demonstrated that the shift of the dosimetric center is within 0.15 cm. Dose measurements in water converted to polystyrene agreed within 5.0% with the Monte Carlo data in polystyrene for the same applicator type, size, and depth. A solid water‐to‐water (phantom) factor was obtained for each applicator, and all future annual quality assurance tests will be performed in solid water using an average value of 1.07 for the solid water‐to‐water factor. The skin‐to‐center dose ratio measurements support the Monte Carlo‐based values within 5.0% agreement. For the treatment separation range of 4 cm to 8 cm, the change in center dose would be <1.0% for all applicators when using a compressed pad of 0.2 cm to 0.3 cm. The tests performed ensured that all treatment components of the AccuBoost system are functional and that a treatment plan can be delivered with acceptable accuracy. Based on the commissioning results, a quality assurance manual and guidance documents for clinical use were developed. PACS numbers: 87.55.Qr, 87.56.Da, 87.90.+y

Use of customized intraoral mold high-dose-rate brachytherapy in the treatment of oral cavity cancer in an elderly patient

Head and neck (HN) cancers account for approximately 53,000 cases per year in the United States, with a median age of 60 years at diagnosis.1 Because of the growing age of the population, the incidence of oral cavity or pharyngeal cancers in adults ≥65 years is projected to increase from 19,000 in year 2010 to 31,000 in 2030. Treatment of elderly patients above age 85 can be challenging because of associated comorbidities, complications, and social conditions. According to a Surveillance, Epidemiology, and End Results database analysis of over 2500 patients with HN cancers, age was not an independent prognostic factor and there is no statistical difference in overall survival or disease-free survival after correction for stage.2 However, elderly patients may experience more toxicity from radiation treatment including dermatitis, mucositis, and xerostomia. Moreover, patients with comorbidities will have increased risks of posttreatment complications and selection and modification of treatment is important in this group of patients.3,4

Development of clinically relevant QA procedures for the BrainLab ExacTrac imaging system

Abstract Purpose The aim of this study was to develop Quality Assurance procedures for the BrainLab ExacTrac (ET) imaging system following the TG 142 recommendations for planar kV imaging systems. Materials and Methods A custom‐designed 3D printed holder was used to position the Standard Imaging QCkV‐1 phantom at isocenter, facing the ET X ray tubes. The linacs light field (collimator at 45⁰) was used to position the phantom holder. The ET images were exported to ARIA where geometric distortion was checked. The DICOM images were analyzed in the PIPSpro software. The following parameters were recorded (technique 80 kV/2mAs): spatial resolution (Modulated Transfer Function (MTF) F50/F40/F30), contrast‐to‐noise ratio (CNR), and noise. A baseline was generated for future image analysis. Beam quality and exposure were measured using the Unfors R/F detector. Using a rod holder, the detector was placed at isocenter, facing each ET X‐ray tube. The measurements were performed for all preset protocols ranging from cranial low (80 kV/6.3 mAs) to abdomen high (145 kV/25 mAs). The total exposure was converted to dose. Results and Discussion The image quality parameters were close for the two tubes. A common baseline was therefore generated. The average baseline values (both tubes, both images/tube) were 1.06/1.18/1.30, 1.32, and 67.3 for the MTF F50/F40/F30, noise, and CNR respectively. The procedure described here was used for another 24 sets. Using a positioning template and 3D printed phantom holder, experimental reproducibility has been acceptably high. The measured phantom dimensions were within 1 mm from the nominal values. The measured kV values were within 2% of the nominal values. The exposure values for the two tubes were comparable. The range of total measured dose was 0.099 mGy (cranial low) to 1.353 mGy (abdomen high). Conclusions A reliable process has been implemented for QA of the ET imaging system by characterizing the systems performance at isocenter, consistent with clinical conventions.

SU-F-T-636: Comprehensive Approach to Motion Assessment for Liver and Pancreas SBRT Patients

PURPOSE Our past practice for liver and pancreas SBRT consisted of free breathing (FB) with gated treatment delivery using a 30-70% phase window. We have recently adopted an assessment method leading to individualized motion management to minimize target motion. We present our results from 47 patients treated with this new approach. METHODS We perform an initial patient coaching and assessment session in our conventional simulator suite to observe the motion of the implanted fiducials with FB anterior and lateral 20-second cine acquisitions. The physician decides whether to attempt inhale or exhale breath-hold (BH). The patient is coached while observing with cine to ascertain their ability to achieve the desired BH mode for long periods as needed for treatment delivery. If the patient cannot comply, a FB approach is adopted using gating or simple ITV method (for patients without fiducials). After achieving a patient-specific motion management mode, we perform CT-simulation using the Varian RPM system to reproduce the chosen mode and record a reference session for treatment delivery. For pre-treatment imaging, the fiducials are observed under fluoro while coaching the patient. RESULTS Of 47 SBRT cases analyzed, 32 were liver and 15 were pancreas. The chosen techniques were: 32 exhale BH (12 with abdominal compression), 7 FB gated, 4 inhale BH, and 4 FB ITV. Maximum fiducial motion amplitude was 5 mm for the FB gated patients, and less than 5 mm for all BH patients with most able to achieve a maximum amplitude of 3 mm. CONCLUSION This study showed that an individualized motion management approach can reduce the target volume and, therefore, the volume of irradiated healthy tissue from liver or pancreas SBRT. Effective coaching is essential in achieving consistent BH with 3 mm amplitude. The fluoro/cine session is helpful in establishing the right coaching approach for each patient.

Varian HDR surface applicators — commissioning and clinical implementation

The purpose of this study was to validate the dosimetric performance of Varian surface applicators with the source vertically positioned and develop procedures for clinical implementation. The Varian surface applicators with the source vertically positioned provide a wide range of apertures making them clinically advantageous, though the steep dose gradient in the region of 3‐4 mm prescription depth presents multiple challenges. The following commissioning tests were performed: 1) verification of functional integrity and physical dimensions; and 2) dosimetric measurements to validate data provided by Varian as well as data obtained using the Acuros algorithm for heterogeneity corrected dose calculation. A solid water (SW) phantom was scanned and the Acuros algorithm was used to compute the dose at 5 mm depth and at surface for all applicators. Two sets of reference dose measurements were performed, with the source positioned at (i) −10 mm and (ii) −15 mm from the center of the first nominal dwell position. Measurements were taken at 5 mm depth in a SW phantom and in air at the applicator surface. The results were then compared to the vendors data and to the Acuros calculated dose. Relative dose measurements using Gafchromic films were taken at a depth of 4 mm in SW. Percent depth ionization (PDI) measurements using ion chamber were performed in SW. The profiles generated from film measurements and the PDI plots were compared with those computed using the Acuros algorithm and vendors data, when available. Preliminary leakage tests were performed using optically stimulated luminescence dosimeters (OSLDs) and the results were compared with Acuros predictions. All applicators were found to be functional with physical dimensions within 1 mm of specifications. For scenario (ii) measurements taken in SW at 5 mm depth and in air at the surface of each applicator were within 10% and 4% agreement with vendors data, respectively. Compared with Acuros predictions, these measurements were within 6% and 5%, respectively. Measurements taken for scenario (i) showed reduced agreement with both the vendors data as well as the Acuros calculations, especially when using the 10 mm applicator. The full widths of the measured dose profiles were within 2 mm of those predicted by Acuros at the 90% dose level. The PDI plots and measured leakage dose were in good agreement with vendors data and Acuros predictions. Based on the dosimetric results, a quality assurance program and procedures for clinical implementation were developed. Treatment planning will be performed using scenario (ii). The 10 mm applicator will not be released for clinical use. A prescription depth of 4 mm is recommended, to ensure full coverage at 3 mm and a minimum dose of 90% of prescribed dose at 4 mm depth. PACS number(s): 87.55 Qr, 87.56 Da, 87.90 +yThe purpose of this study was to validate the dosimetric performance of Varian surface applicators with the source vertically positioned and develop procedures for clinical implementation. The Varian surface applicators with the source vertically positioned provide a wide range of apertures making them clinically advantageous, though the steep dose gradient in the region of 3-4 mm prescription depth presents multiple challenges. The following commissioning tests were performed: 1) verification of functional integrity and physical dimensions; and 2) dosimetric measurements to validate data provided by Varian as well as data obtained using the Acuros algorithm for heterogeneity corrected dose calculation. A solid water (SW) phantom was scanned and the Acuros algorithm was used to compute the dose at 5 mm depth and at surface for all applicators. Two sets of reference dose measurements were performed, with the source positioned at (i) -10 mm and (ii) -15 mm from the center of the first nominal dwell position. Measurements were taken at 5 mm depth in a SW phantom and in air at the applicator surface. The results were then compared to the vendors data and to the Acuros calculated dose. Relative dose measurements using Gafchromic films were taken at a depth of 4 mm in SW. Percent depth ionization (PDI) measurements using ion chamber were performed in SW. The profiles generated from film measurements and the PDI plots were compared with those computed using the Acuros algorithm and vendors data, when available. Preliminary leakage tests were performed using optically stimulated luminescence dosimeters (OSLDs) and the results were compared with Acuros predictions. All applicators were found to be functional with physical dimensions within 1 mm of specifications. For scenario (ii) measurements taken in SW at 5 mm depth and in air at the surface of each applicator were within 10% and 4% agreement with vendors data, respectively. Compared with Acuros predictions, these measurements were within 6% and 5%, respectively. Measurements taken for scenario (i) showed reduced agreement with both the vendors data as well as the Acuros calculations, especially when using the 10 mm applicator. The full widths of the measured dose profiles were within 2 mm of those predicted by Acuros at the 90% dose level. The PDI plots and measured leakage dose were in good agreement with vendors data and Acuros predictions. Based on the dosimetric results, a quality assurance program and procedures for clinical implementation were developed. Treatment planning will be performed using scenario (ii). The 10 mm applicator will not be released for clinical use. A prescription depth of 4 mm is recommended, to ensure full coverage at 3 mm and a minimum dose of 90% of prescribed dose at 4 mm depth. PACS number(s): 87.55 Qr, 87.56 Da, 87.90 +y.

SU‐E‐T‐152: A Practical Solution to the Recommendations of TG‐142 for a Respiratory Gating Program

PURPOSE To develop a practical solution to the recommendations of TG-142 for a Respiratory Gating Program Methods: A phantom including an insert with a solid tumor and drilled for a chamber was placed on a motion platform. A 4DCT was performed. Two groups of images were generated. Group 1 was used to simulate the process of ITV generation and static treatment. Group 2 was used to simulate the gated treatment process. A measurement of the tumor motion noted on the CT data set was compared to the known motion in both cases. Base line output and ratio measurements were performed using a solid water phantom and 2 chambers placed at different depths in the phantom in both stationary and gated delivery. The Group 1 and Group 2 plans were then delivered using the moving platform with the intended treatment delivery technique. As an additional check IMRT QA was performed on the Group 2 plan. RESULTS Tumor motion was confirmed. Baseline output measurements for stationary versus gated delivery were 101.1 cGy and 101.4 cGy respectively. The ratio of measurements at two depths was the same for both gated and static delivery. Central axis measurement of the Group 1 plan was 2123 cGy compared to the planned 2127 cGy. Central axis measurement of the Group 2 (gated) plan was 2142 cGy compared to the planned 2094 cGy. IMRT QA of the gated delivery showed 99% agreement for 3mm DTA and 3% absolute dose. CONCLUSION A baseline test with stationary and gated delivery coupled with an additional phantom end to end test as described can serve as a practical solution to the recommendations of TG-142 for a Respiratory Gating Program. It is anticipated that the complete testing from simulation to treatment can be completed in approximately 4 hours with experience.

Retrospective review of Contura HDR breast cases to improve our standardized procedure

To retrospectively review our first 20 Contura high dose rate breast cases to improve and refine our standardized procedure and checklists. We prepared in advance checklists for all steps, developed an in-house Excel spreadsheet for second checking the plan, and generated a procedure for efficient contouring and a set of optimization constraints to meet the dose volume histogram criteria. Templates were created in our treatment planning system for structures, isodose levels, optimization constraints, and plan report. This study reviews our first 20 high dose rate Contura breast treatment plans. We followed our standardized procedure for contouring, planning, and second checking. The established dose volume histogram criteria were successfully met for all plans. For the cases studied here, the balloon-skin and balloon-ribs distances ranged between 5 and 43 mm and 1 and 33 mm, respectively; air_seroma volume/PTV_Eval volume≤5.5% (allowed≤10%); asymmetry<1.2mm (goal≤2 mm); PTV_Eval V90%≥97.6%; PTV_Eval V95%≥94.9%; skin max dose≤98%Rx; ribs max dose≤137%Rx; V150%≤29.8 cc; V200%≤7.8 cc; the total dwell time range was 225.4 to 401.9 seconds; and the second check agreement was within 3%. Based on this analysis, more appropriate ranges for the total dwell time and balloon diameter tolerance were found. Three major problems were encountered: balloon migration toward the skin for small balloon-to-skin distances, lumen obstruction, and length change for the flexible balloon. Solutions were found for these issues and our standardized procedure and checklists were updated accordingly. Based on our review of these cases, the use of checklists resulted in consistent results, indicating good coverage for the target without sacrificing the critical structures. This review helped us to refine our standardized procedure and update our checklists.

SU‐E‐T‐138: Quantification of Dwell Position Inaccuracy in Varian GammaMed HDR Titanium Ring Applicators

PURPOSE To quantify the dwell position inaccuracy in Titanium ring applicators and develop a test to be performed quarterly, after source exchange. METHODS All three rings from our Titanium kit (30, 45, 60 deg.) were used for this study. EDR2 film was placed on the Simulator table and a ring was taped to the film, with a solid water slab as buildup. A 1-cm spacing dummy wire was inserted into the ring. The film was exposed using 135 kV, 80 mA, 400 mAs. An HDR treatment was then delivered using the even source dwell positions from 2 to 16, with a 5 mm step size, nominal dwell time 0.4 sec/position. The procedure was repeated three times for each ring. The films were scanned and analyzed with the RIT software. The distance between the center of each source position to the adjacent dummy dots was measured for each ring on all three films. An average shift (AS) was obtained for each ring.New films were exposed with a treatment offset equal and in the opposite direction relative to the AS for the ring used. The films were visually inspected to assess if the source positions are centered in between two adjacent dummy dots, and also scanned and analyzed with the RIT software. This test will be performed quarterly to verify if the shifts remain stable. RESULTS The average shift was 2.5, 2.4, and 2.4 mm distally for the 30, 45, and 60 deg. rings, respectively. The offset for the quarterly test was set to 2 mm proximally, to take into account the 1 mm tolerance for the source position. CONCLUSIONS The dwell position inaccuracy in Titanium ring applicators was quantified and the quarterly test was successfully performed for two quarters. Work is started to assess the dosimetric implications of this shift.