M Huq

Single magnetic resonance imaging vs magnetic resonance imaging/computed tomography planning in cervical cancer brachytherapy.

AIMS To compare differences in dose to the target volume and organs at risk (OARs) for ring and tandem brachytherapy using individualised magnetic resonance imaging (MRI)/computed tomography-based three-dimensional treatment plans for each application vs plans based on a single scan for all fractions. MATERIALS AND METHODS The study was carried out in 10 patients with carcinoma of the uterine cervix, treated with external beam radiotherapy and five fractions of high dose rate brachytherapy. Planning was carried out using MRI for the first fraction and computed tomography for each of the four subsequent fractions. The MRI-based plan was taken as the reference and the single-plan procedure was calculated by using the weights from the reference plan to calculate the dose distribution for each subsequent computed tomography-based plan. The high-risk clinical target volume (HRCTV) and OARs were delineated as per GEC-ESTRO guidelines. Total doses from external beam radiotherapy and brachytherapy were summated and normalised to a 2 Gy fraction size. RESULTS The mean D(90) for the HRCTV was 81.9 Gy when using one plan and 84 Gy when using individual treatment plans. Similarly, the mean D(2 cc) was 75.68 Gy vs 74.99 Gy for the bladder, 55.84 Gy vs 56.56 Gy for the rectum and 64.8 Gy vs 65.5 Gy for the sigmoid. Ring rotation was identified in three patients, resulting in a change in dwell positions, which otherwise could have led to either a high bladder dose or suboptimal coverage of the HRCTV. CONCLUSIONS Our study has shown that a single-plan procedure achieved acceptable dosimetry in most patients. However, the individualised plan improved dosimetry by accounting for variations in applicator geometry and the position of critical organs.

Four-dimensional computed tomography-based respiratory-gated whole-abdominal intensity-modulated radiation therapy for ovarian cancer: a feasibility study.

This study assesses the feasibility and implementation of respiratory-gated whole-abdominal intensity-modulated radiation therapy (RG-WAIMRT). Three patients were treated with RG-WAIMRT. The planning target volume (PTV1) included the entire peritoneal cavity and a pelvic boost field was created (PTV2). The dose prescribed was 30 Gy to PTV1 and 14.4 Gy to PTV2. For comparison, a conventional three-dimensional (3D) plan was generated for each patient. In the WAIMRT plan, an average of 90% of PTV1 received 30 Gy compared to 70% for the conventional 3D plan. The percent volume receiving 30 Gy (V30) for liver averaged 54% (WAIMRT) vs 43% (3D). The percent volume receiving 20 Gy (V20) for kidneys averaged 19% vs 0%, and the mean V20 for bone marrow was 74% vs 83%, respectively. Major acute toxicities were anemia (grade 2: 1/3), leukopenia (grade 3: 2/3 patients), and thrombocytopenia (grade 2: 1/3 patients, grade 3: 1/3 patients). One patient could not complete the whole-abdomen field after 19.5 Gy because of persistent nausea. No major subacute toxicity has been reported. WAIMRT demonstrated superior target coverage and reduced dose to bone marrow, with a slightly increased dose to liver and kidneys. WAIMRT is a novel and feasible technique for ovarian cancer treatment.

Evaluation of Set-up Uncertainties with Daily Kilovoltage Image Guidance in External Beam Radiation Therapy for Gynaecological Cancers

AIMS To assess the set-up uncertainty for gynaecological cancer patients treated with external beam radiation therapy using daily kilovoltage image guidance and to estimate set-up margins for treatment and factors that would predict higher set-up uncertainty. MATERIALS AND METHODS Alignment data from daily two-dimensional kilovoltage planar images and three-dimensional kilovoltage cone beam images for 52 patients treated on a Varian 2300iX linear accelerator with On Board Imaging (OBI; version 1.4) capability were analysed. The mean displacements of translational shifts, population systematic errors and random errors were calculated. Using van Herks formula, the clinical target volume (CTV) to planning target volume (PTV) margins for set-up uncertainties were calculated. The differences in set-up error were calculated with respect to the type of cancer, imaging type and body mass index (BMI). RESULTS Population systematic and random errors were 1.1 mm, 2.3 mm, 2.3 mm and 3.9 mm, 5.0 mm, 3.5 mm in the anterior-posterior (AP), medial-lateral (ML) and superior-inferior (SI) directions, respectively, for the entire patient population. Using van Herks formula, the CTV to PTV margins for set-up uncertainties were found to be 5.5, 9.1 and 8.3 mm in the AP, ML and SI directions respectively. The mean displacements in the AP, ML and SI directions for BMI ≥ 30 (28 patients) versus <30 (24 patients) were -0.1 mm, 0.9 mm and 1.0 mm versus -0.1 mm, 0.1 mm and 0.4 mm, respectively, (P = 0.02). CONCLUSIONS Daily imaging helps to assess set-up uncertainty. The set-up margin for CTV to PTV was larger for patients with BMI ≥ 30 without image guidance and these patients would benefit more from daily image guidance.

Long‐term stability of the Leksell Gamma Knife® Perfexion™ patient positioning system (PPS)

PURPOSE To assess the long-term mechanical stability and accuracy of the patient positioning system (PPS) of the Leksell Gamma Knife(®) Perfexion™ (LGK PFX). METHODS The mechanical stability of the PPS of the LGK PFX was evaluated using measurements obtained between September 2007 and June 2011. Three methods were employed to measure the deviation of the coincidence of the radiological focus point (RFP) and the PPS calibration center point (CCP). In the first method, the onsite diode test tool with single diode detector was used together with the 4 mm collimator on a daily basis. In the second method, a service diode test tool with three diode detectors was used biannually at the time of the routine preventive maintenance. The test performed with the service diode test tool measured the deviations for all three collimators 4, 8, and 16 mm and also for three different positions of the PPS. The third method employed the conventional film pin-prick method. This test was performed annually for the 4 mm collimator at the time of the routine annual QA. To estimate the effect of the patient weight on the performance of the PPS, the focus precision tests were also conducted with varying weights on the PPS using a set of lead bricks. RESULTS The average deviations measured from the 641 daily focus precision tests were 0.1 ± 0.1, 0.0 ± 0.0, and 0.0 ± 0.0 mm, respectively, for the 4 mm collimator in the X (left/right of the patient), Y (anterior/posterior of the patient), and Z (superior/inferior of the patient) directions. The average of the total radial deviations as measured during ten semiannual measurements with the service diode test tool were 0.070 ± 0.029, 0.060 ± 0.022, and 0.103 ± 0.028 mm, respectively for the central, long, and short diodes for the 4 mm collimator. Similarly, the average total radial deviations measured during the semiannual measurements for the 4, 8, and 16 mm collimators and using the central diode were 0.070 ± 0.029, 0.097 ± 0.025, 0.159 ± 0.028 mm, respectively. The average values of the deviations as obtained from the five annual film pin-prick tests for the 4 mm collimator were 0.10 ± 0.06, 0.06 ± 0.09, and 0.03 ± 0.03 mm for the X, Y, Z stereotactic directions, respectively. Only a minor change was observed in the total radial deviations of the PPS as a function of the simulated patient weight up to 202 kg on the PPS. CONCLUSIONS Excellent long-term mechanical stability and high accuracy was observed for the PPS of the LGK PFX. No PPS recalibration or any adjustment in the PPS was needed during the monitored period of time. Similarly, the weight on the PPS did not cause any significant disturbance in the performance of the PPS for up to 202 kg simulated patient weight.

SU‐EE‐A2‐02: Present Status of IAEA/AAPM Recommendations on Small and Composite Field Dosimetry

Purpose: An international IAEA/AAPM working group is preparing recommendations for reference dosimetry in small fields and composite fields. A proposed formalism for dosimetry in small and composite fields was published that introduces machine specific reference (msr) fields for static small fields and plan‐class specific reference (pcsr) fields for composite fields such as an IMRT sequence. The status of present activities, based on the proposed formalism, will be reviewed. Method and Materials: For msr fields a literature review is conducted and for some modalities experiments and Monte Carlo simulations are performed. For composite fields various routes are explored to arrive at suitable pcsr fields. Some of these start from clinical plans and try to distil most representative reference deliveries, while others start from treatment unit specific features to arrive at relevant modulated plans. Experiments and Monte Carlo simulations are performed as well to determine correction factors for ionisation chambers in pcsr fields and to evaluate their suitability to represent a class of clinical plans. One idea being explored is that for a composite field where full charged particle equilibrium is established, the overall correction factor equals the reciprocal of the gradient correction factor in the conventional reference field.Results: State‐of‐the‐art results on the correction factors for msr and pcsr fields will be reviewed. Experimental work performed on pcsr fields in TomoTherapy and step‐and‐shoot prostate plans indicates that correction factors for suitably sized ion chambers are not more than 1% different from unity. Monte Carlo simulations confirm this. Studies are underway to test the sensitivity of the correction factors to characteristics of the pcsr field. Conclusion: Results indicate that reference dosimetry in msr and pcsr fields is feasible with acceptably small corrections. A working document on static small field dosimetry is aimed for by the end of this year.

SU‐E‐T‐407: Dosimetric Influence of Setup Errors On RapidArc‐Based SRS for Simultaneous Irradiation of Multiple Intracranial Targets

PURPOSE To evaluate the dosimetric influence of setup errors on RapidArc-based SRS for simultaneous irradiation of multiple intracranial targets. METHODS Eight patients previously treated with RapidArc™ technique for multiple intracranial lesions were included in this study. A RapidArc plan was designed to irradiate multiple targets simultaneously with one isocenter and 4 non-coplanar arcs. 1 mm margin was added to generate PTVs from GTVs. BrainLAB Novalis system was used to position the targets with 6D corrections and monitor patient position during treatment. CBCT was acquired for verification before irradiation. Velocity AI was used for image registration and dose mapping for CBCT, planning CT, contours and dose matrix. The DVHs of targets and critical structures and dose distributions were compared with the planned results and dosimetric influence from setup errors was analyzed. RESULTS We found that the translational errors were less than 1 mm in the three directions and rotational errors were less than 0.6 degree for all the patients. The overall PTV coverage decreases of ∼10 percent on average while the overall GTV coverage slightly influenced. Although prescribed dosed still covered most of GTVs, there were still some targets with a GTV coverage drop greater than 5 percent even with a 1 mm margin. Most of influenced targets were small targets and relatively far from isocenter. Our study also showed that the dosimetric influence on critical structures was negligible. CONCLUSIONS Although RapidArc technique can generate good plans for effectively treating multiple intracranial lesions simultaneously using stereotactic radiosurgery, this technique is more susceptible to the setup errors, especially rotational errors. 0.5 degree rotational error may Result in non-ignorable drop in target coverage. Our study show that a 1 mm margin for PTV and 6D positioning are necessary for a successful treatment with this technique.

SU‐E‐T‐556: Interplay Effect Between Dynamic MLC and Moving Target for Lung SBRT with IMAT Technique Delivered by Flattening Filter Free Beam of True Beam Machine

Purpose: To investigate the dosimetric impact of the interplay effect between dynamic MLC and moving target on lungSBRT with intensity‐ modulated arc therapy (IMAT) technique delivered by flattening filter free (FFF) beam of True‐Beam machine. Methods: 6 lungcancer patients with 0.5–1.0 cm tumor motions were investigated in this study. All patients underwent 4DCT scan using audio coaching. For each patient, a 2‐arc IMAT plan was retrospectively generated using Varian RapidArc planning system. Based on the total MU, dose rate and respiratory cycle, the planned MLC control points of IMAT plans were assigned into different respiratory phases. Afterward, 10 new IMAT plans related to different respiratory phases were generated and imported back into Eclipse planning system to calculate the radiationdose on the CTimages of different respiratory phases. In‐house 4D dose calculation program with deformable registration capacity was used to calculate the cumulative doses from all respiratory phases. Following parameters were used to evaluate the dosimetric impacts: dose error (DE), i.e., the difference between the 3D dose of original IMAT plan and the calculated 4‐D dose, and the percentage of volume receiving 100% of the prescribed dose (V100).Results: For all patients, the average maximum DE of PTV and CTV are 32.6% and 4.8%. The DE is larger than 5% for 22% of PTV and 0.3% of CTV volume, and the DE is less than 3% for over 88% of the CTV volume. Compared to the original IMAT plans, the V100 of PTV are lower by 15%. However, the changes on V100 of CTV are less than 1%. Conclusions: For lung IMAT treatment delivered by FFF beam of True‐Beam machine, the interplay effect between dynamic MLC and moving target could change the absolute doses within the target, but its impact on CTV dose coverage is insignificant.

SU‐GG‐T‐450: Measurement of Relative Output Factors for the Leksell Gamma Knife PERFEXION by Film Dosimetry

Purpose: To measure relative output factors (ROFs) for 4 and 8 mm collimators for the Leksell Gamma Knife PERFEXION unit. Effective ROFs calculated by Monte Carlo (MC) by ELEKTA and used in the Gamma Plan 8.0 treatment planning system are 0.805, 0.924 for 4 and 8 mm collimators, respectively. Method and Materials: Three types of films, Kodak EDR2, gafchromic EBT and MD‐V2‐55, were used within the spherical polystyrene phantom of diameter of 160 mm. To obtain calibration curve 10 films were exposed to doses of 0–400, 0–800, 0–8000 cGy for EDR2, EBT, MD‐V2‐55 films, respectively. Three samples of each film type were then exposed in identical setup for 4, 8 and 16 mm collimators. Films were scanned by EPSON EXPRESSION 10000 XL scanner with 200 dpi resolution in 16‐bit grayscale for radiographic and 48‐bit color for gafchromic film and imported in red channel. Films were evaluated by FilmQA version 2.0.1215 software. Fifth degree polynomial fit for calibration curve was used and background corrections applied for all films. Doses delivered to the center of each of studied films by 4, 8, 16 mm collimators were obtained and ROFs calculated and corrected for transit dose. Results: Measured ROFs and deviations compared to MC calculated value for 8 mm collimator were 0.899 (−2.7%), 0.917 (−0.8%), and 0.920 (−0.4%) for EDR2, EBT, and MD‐V2‐55 films, respectively. Measured ROFs and deviations compared to MC calculated value for 4 mm collimator were 0.778 (−3.4%), 0.818 (1.6%), and 0.804 (−0.1%) for EDR2, EBT, and MD‐V2‐55 films, respectively. Conclusion: MD‐V2‐55 film seems to be an ideal dosimeter for small stereotactic field ROFs measurements. All results obtained in this study were in close agreement with MC values. Largest deviations noted from the MC were for EDR 2 films. The closest agreement with MC values was observed for MD‐V2‐55 films.

SU‐GG‐J‐84: Evaluation of Comparing Daily Ultrasound Images with a Reference Ultrasound Image for Prostate Localization

Purpose: To evaluate if acquiring an ultrasoundimage at the time of CT simulation for comparison with daily ultrasoundimages improves daily localization of prostates. Method and Materials: Resonant Medicals® ultrasound localization system was installed and implemented in our clinic. The technique relies on acquiring a 3D ultrasoundimage at the time of CT simulation for daily comparison whereas other ultrasound localization techniques compare daily ultrasoundimages to the original CTimage.Treatment planning is done on the CT. DRRs are also constructed from the CT, and fiducials implanted in the prostate are outlined on the DRRs. Each day a 3D ultrasoundimage was acquired and compared to the ultrasound that was acquired at the time of CT simulation. Daily, if the ultrasoundimage was approved by the physician, the couch was shifted to align the current prostate location with its location at the time of simulation. After the ultrasound, ports were taken as often as prescribed by the physician. The fiducial locations as seen in the ports were compared to their locations on the DRRs. Any necessary shifts were made to align the fiducials. Following the treatments, an analysis was made of the ultrasound localization as compared to the fiducial localization. 22 patients had 7 or more days in which both ultrasound and ports of fiducials were acquired and are included in this analysis. Results: The measured average difference between the ultrasound localization and the localization based on ports of fiducials is 7.2 mm. This is comparable to what is reported in literature for other ultrasound localization techniques. Conclusion: Using a 3D ultrasoundimage acquired at the time of CT simulation does not improve ultrasound localization accuracy as compared to techniques that compare daily ultrasoundimages to the simulation CT for localization.

SU‐FF‐J‐56: Patient Dose From Kilo‐Voltage Cone Beam Computed Tomography (kV‐CBCT) Imaging

Purpose: To investigate patient dose from on‐board imager‐based kV‐CBCT. Method and Materials: Radiation doses from kV‐CBCT were measured using TLDs at different locations in three anthropomorphic‐phantoms (H&N, chest and pelvis) and patients retrospectively. kV‐CBCT scans were performed in standard settings (125 kV, 80 mA and 25 ms) using a Varian Trilogy linear accelerator. Both full‐fan (FOV=24 cm) and half‐fan (FOV=40 cm) modes were evaluated for H&N case while only half‐fan (FOV=45 cm) technique was studied for chest and pelvic cases. The skin dose in both phantoms and patients were measured at 4 locations: anterior, posterior, Rt‐Lat, and Lt‐Lat. Doses measured in the phantoms included different critical organs. The dosimeters used were high sensitivity TLD‐100H and only those with standard‐deviations less than 3% and sensitivity within ±3% were selected for this study. Each TLD was individually calibrated using an ion chamber under the irradiation condition. Phantom data was averaged from 3 separate measurements and patient data was averaged from 5 measurements in each category. Results: The skin dose for H&N cases were 9–10cGy for half‐fan mode in both the phantom and patients. The dose for brain and brainstem were 7.1cGy and 7.6cGy, respectively. The doses in same locations were 2–3cGy lower if the full‐fan mode is used. The skin dose for chest cases was 8–10cGy and were same for the phantom and patient measurements. Measured mean lung dose was 8.5cGy and spinal cord dose was 6.2cGy. For pelvis, measured skin dose was 2.9–4.2cGy and the prostate and rectum dose were 2.9cGy. Conclusions: For pelvic cases, kV‐CBCT dose was comparable or less than that from portal imaging. For chest and H&N cases the dose can be two times higher than that for the pelvis cases. Daily CBCT may lead to extra 400cGy to skin and 250cGy to spinal cord in 40 fractions.