S Li

Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer

With the increased use of cone beam CT (CBCT) for daily patient setup, the accumulated dose from CBCT may be significantly higher than that from simulation CT or portal imaging. The objective of this work is to measure the dose from daily pelvic scans with fixed technical settings and collimations. CBCT scans were acquired in half-fan mode using a half bowtie and x-rays were delivered in pulsed-fluoro mode. The skin doses for seven prostate patients were measured on an IRB-approved protocol. TLD capsules were placed on the patients skin at the central axis of three beams: AP, left lateral (Lt Lat) and right lateral (Rt Lat). To avoid the ring artefacts centred in the prostate, the treatment couch was dropped 3 cm from the patients tattoo (central axis). The measured AP skin doses ranged 3-6 cGy for 20-33 cm separation. The larger the patient size the less the AP skin dose. Lateral doses did not change much with patient size. The Lt Lat dose was approximately 4.0 cGy, which was approximately 40% higher than the Rt Lat dose of approximately 2.6 cGy. To verify this dose asymmetry, surface doses on an IMRT QA phantom (oval shaped, 30 cm x 20 cm) were measured at the same three sites using TLD capsules with 3 cm table-drop. The dose asymmetry was due to: (1) kV source rotation which always starts from the patients Lt Lat and ends at Lt Lat. Gantry rotation gets much slower near the end of rotation but dose rate stays constant and (2) 370 degrees scan rotation (10 degrees scan overlap on the Lt Lat side). In vivo doses were measured inside a Rando pelvic heterogeneous phantom using TLDs. The left hip (femoral head and neck) received the highest doses of approximately 10-11 cGy while the right hip received approximately 6-7 cGy. The surface and in vivo doses were also measured for phantoms at the central-axis setup. The difference was less than approximately 12% to the table-drop setup.

Examining Margin Reduction and Its Impact on Dose Distribution for Prostate Cancer Patients Undergoing Daily Cone-Beam Computed Tomography

PURPOSE To examine the dosimetric impact of margin reduction and quantify residual error after three-dimensional (3D) image registration using daily cone-beam computed tomography (CBCT) for prostate cancer patients. METHODS AND MATERIALS One hundred forty CBCTs from 5 prostate cancer patients were examined. Two intensity-modulated radiotherapy plans were generated on CT simulation on the basis of two planning target volume (PTV) margins: 10 mm all around the prostate and seminal vesicles except 6 mm posteriorly (10/6) and 5 mm all around except 3 mm posteriorly (5/3). Daily CBCT using the Varian On-Board Imaging System was acquired. The 10/6 and 5/3 simulation plans were overlaid onto each CBCT, and each CBCT plan was calculated. To examine residual error, PlanCT/CBCT intensity-based 3D image registration was performed for prostate localization using center of mass and maximal border displacement. RESULTS Prostate coverage was within 2% between the 10/6 and 5/3 plans. Seminal vesicle coverage was reduced with the 5/3 plan compared with the 10/6 plan, with coverage difference within 7%. The 5/3 plan allowed 30-50% sparing of bladder and rectal high-dose regions. For residual error quantification, center of mass data show that 99%, 93%, and 96% of observations fall within 3 mm in the left-right, anterior-posterior, and superior-inferior directions, respectively. Maximal border displacement observations range from 79% to 99%, within 5 mm for all directions. CONCLUSION Cone-beam CT dosimetrically validated a 10/6 margin when soft-tissue localization is not used. Intensity-based 3D image registration has the potential to improve target localization and to provide guidelines for margin definition.

A technique of quantitatively monitoring both respiratory and nonrespiratory motion in patients using external body markers.

The purpose of this study was to develop a technique that could quantitatively monitor the nonrespiratory motion of a patient during stereotactic body radiotherapy (SBRT). Multiple infrared external markers were placed on the patients chest and abdominal surface to obtain patient motion signals. These motion signals contained both respiratory and nonrespiratory motion information. The respiratory motion usually has much larger amplitude on the abdominal surface than on the chest surface. Assuming that the nonrespiratory motion is a rigid body translation, we have developed a computer algorithm to derive both the respiratory and nonrespiratory motion signals instantly from two sets of motion signals. In first-order approximation, the respiratory motion was represented by the motion signal on the abdominal surface, and the nonrespiratory motion was represented by the motion signal on the chest surface subtracting its respiratory component. The algorithm was retrospectively tested on 24 patients whose motion signals were recorded during a gated-CT simulation procedure. The result showed that the respiratory noise in the nonrespiratory motion signal was reduced to less than 1 mm for almost all patients, demonstrating that the technique was able to detect nonrespiratory motion with a sensitivity of about 1 mm. It also showed that 50% of the patients had > or =2 mm, and 2 patients had > or =3 mm slow drift during the 15-25 min simulation procedure, suggesting that nonrespiratory motion could exist during prolonged treatment. This technique can potentially be used to control the nonrespiratory motion during SBRT. However, further validation is required for its clinical use.

Comparison of similarity measures for rigid-body CT/dual X-ray image registrations

A set of experiments were conducted to evaluate six similarity measures for intensity-based rigid-body 3D/2D image registration. Similarity measure is an index that measures the similarity between a digitally reconstructed radiograph (DRR) and an x-ray planar image. The registration is accomplished by maximizing the sum of the similarity measures between biplane x-ray images and the corresponding DRRs in an iterative fashion. We have evaluated the accuracy and attraction ranges of the registrations using six different similarity measures on phantom experiments for head, thorax, and pelvis. The images were acquired using Varian Medial System On-Board Imager. Our results indicated that normalized cross correlation and entropy of difference showed a wide attraction range (62 deg and 83 mm mean attraction range, ωmean), but the worst accuracy (4.2 mm maximum error, emax). The gradient-based similarity measures, gradient correlation and gradient difference, and the pattern intensity showed sub-millimeter accuracy, but narrow attraction ranges (ωmean=29 deg, 31 mm). Mutual information was in-between of these two groups (emax=2.5 mm, ωmean= 48 deg, 52 mm). On the data of 120 x-ray pairs from eight IRB approved prostate patients, the gradient difference showed the best accuracy. In the clinical applications, registrations starting with the mutual information followed by the gradient difference may provide the best accuracy and the most robustness.

Image-Guided Target Localization for Stereotactic Radiosurgery:Accuracy of 6D versus 3D Image Fusion

Purpose: This study was aimed to demonstrate the accuracy of an image-guided target localization system with 6D image fusion in a dedicated linear accelerator radiosurgery unit, and compared to that

SU‐FF‐T‐60: A Simplified Frame Work Using Deep Inspiration Breath‐Hold (DIBH) for the Treatment of Left Breast Cancer with Improved Heart Sparing

Purpose: To develop a simplified frame work using deep inspiration breath‐hold (DIBH) for left breast treatment. Materials and Methods: The current version of Varians RPM system was rarely used in amplitude gating mode, especially with breath hold. The major reason is that the breathing amplitude is much less reproducible than breathing phase. Further, the same signal captured by the infrared camera in simulation room and that in treatment room could be different in amplitude. In this study, we presented a simplified frame work to improve the reproducibility of patients breathing amplitude. First, an aqua‐plastic body mask of 1.0–1.5 in wide was made right before patients simulation while the patient is in DIBH. The body mask was set at umbilicus right superior to the marker box. It will then be used to guide the patient herself for DIBH. The DIBH signal is also displayed on a computer monitor set close to patient, which is a duplicate display of the DIBH signal in the RPM computer. The patient can see her own signal and can therefore guide her breath such that relatively constant amplitude can be achieved. Results: The frame work was tested by a few volunteers and all agree that the system is feasible for left breast treatment. The DIBH can last 15–35s with good constant amplitude. In case the captured amplitude is different in treatment room, the two gating threshold lines set in simulation can be adjusted overlay to the DIBH signal before treatment. Conclusion: The system is feasible for the treatment of left breast cancer with DIBH. Further improvement can be made by wiring the gating cable through patient using two electrodes; one on patients body and the other on the guiding mask so that the amplitude‐gated CT scans and treatment can be actively controlled by patient herself.

TH-D-VaIB-02: Skin and Body Dose Measurements for Varian Cone-Beam CT (CBCT) During IMRT for Prostate Cancer

Purpose: With the increased use of CBCT for daily patient setup, kV dose delivered to patient should be investigated. This study is to measure skin and body dose from Varian daily CBCT for prostate patients. Methods and Materials: CBCT scans were acquired in half‐fan and pulsed‐fluoro mode with a half bow‐tie mounted. A technical setting of 125kV, 80mA and 25ms was used. Skin and body doses were first measured for a Rando pelvic and an IMRT QA phantom, set centrally, with TLD and a cylindrical chamber. Then skin dose for 7 prostate patients undergoing daily CBCT was measured. To avoid the ring artifacts centered at prostate, the treatment couch was dropped 3cm from patients tattoo. TLD capsules were placed on patients skin at 3 sites: AP, Lt Lat and Rt Lat. Phantom measurement was also made for this setup. The absorbed dose was determined by the air‐kerma‐based calibration method recommended by TG61. Results: For phantoms set centrally, measured skin dose was ∼6 cGy, ∼5.6 cGy, ∼3.7cGy at AP, Lt Lat, and Rt Lat, respectively. Body dose at the center was ∼3–4 cGy. With table dropping (TD), only AP skin dose was increased ∼12%. Patient AP skin dose varied with separation, ranging 4–6 cGy for thicker patients (AP 23 – 33 cm) and 6 – 8 cGy for thinner patients. Minimum changes were observed on lateral dose for patients with different size. Lt Lat skin (4cGy) and bone (9cGy) doses were higher than Rt Lat skin (3cGy) and bone dose (6cGy) Conclusions: Daily CBCT provides better patient setup but it increases skin and body dose. The dose can range from 120 – 330 cGy for skin and 120 – 380 cGy for body during the 42 daily fractions delivered for IMRT prostate patients.

SU‐FF‐J‐124: The Hounsfield Unit (HU) Accuracy in Varian's Cone‐Beam CT (CBCT) and Its Effect On Dosimetric Verification

Purpose: To evaluate the HU accuracy of Varians on board CBCT and its effect on the accuracy of dose calculation for dosimetric verification. Methods and Materials: A mini CT QC phantom (15cm diameter, 2cm thickness) with different inserts (2cm diameter) of known electron densities was embedded into an IMRT QA phantom to form a body phantom and scanned using CBCT. The scan was acquired in half‐fan and pulsed‐fluoro mode with a half bowtie mounted. A technical setting of 125kV, 80mA and 25ms was used. The HU for each insert was measured and the HU‐ED curve for CBCT was obtained. After that, a Rando pelvic phantom was scanned with both CBCT and SIM‐CT using nearly the same KV. The two sets of CT were fused so that SSD at any beam direction agree to 1mm. In this way, the structures drawn in SIM‐CT (to simulate prostate treatment) can be exactly transferred to CBCT. Without inhomgeneity correction; the two sets of CT generate exactly the same plan. With inhomogeneity correction, the dosimetric difference was mainly from the HU difference. Results: The average HU difference between CBCT and SIM‐CT is ∼50 but the standard deviation of HU in CBCT is 3–4 times higher. Due to higher beam hardening effect in CBCT, the HU at phantom center is 60–80 higher than that at edges. There is also a ring artifact of 20cm diameter and 1.5cm broad in which the HU is 200 lower. Even though, the dosimetric difference with inhomogeneity correction is relatively small. The minimum dose, maximum dose and mean dose etc. for any structure generally agrees within ∼2–5% between the CBCT plan and the SIM‐CT plan. The CBCT plan is ∼2% hotter at the phantom center. Conclusions: Dosimetric difference between CBCT and SIM‐CT is ∼2–5% due to the inaccurate HU in CBCT.

Cord Dose Specification and Validation for Stereotactic Body Radiosurgery of Spine

Effective dose to a portion of the spinal cord in treatment segment, rather than the maximum point dose in the cord surface, was set as the dose limit in stereotactic-body radiosurgery (SBRS) of spine. Such a cord dose specification is sensitive to the volume size and position errors. Thus, we used stereotactic image guidance to minimize phantom positioning errors and compared the results of a 0.6-cm(3) Farmer ionization chamber and a 0.01-cm(3) compact ionization chamber to determine the detector size effect on 9 SBRS cases. The experimental errors ranging from 2% to 7% were estimated by the deviation of the mean dose in plans to the chamber with spatial displacements of 0.5 mm. The mean and measured doses for the large chamber to individual cases were significantly (approximately 17%) higher than the doses with the compact chamber placed at the same point. Our experimental results shown that the mean doses to the volume of interest could represent the measured cord doses. For the 9 patients, the mean doses to 10% of the cord were about 10 Gy, while the maximum cord doses varied from 11.6 to 17.6 Gy. The mean dose, possibly correlated with the cord complication, provided us an alternative and reliable cord dose specification in SBRS of spine.

SU‐FF‐J‐79: Implementation of Four Different Image‐Guided Radiotherapy (IGRT) Systems in a Radiotherapy Department

Purposes: to implement and compare four newly developed image‐guidedradiotherapy systems (Varians Cone‐Beam CT, BrainLAB ExacTrac, Restitu Ultrasound (U/S)‐Sim and Guide, and in‐house stereovision) in one department. Methods and Materials: The cone‐beam CT(CBCT) and the ultrasound(US) systems provide volumetric images of the target at daily setup. The ExacTrac system acquires the biplanar radiographs at patient setup. Both the US and ExacTrac systems are integrated with infrared‐tracking systems for patient‐couch positioning. The in‐house stereovision system captures 3D surface images of the patient at the instants of daily patient setup and during individual beam irradiation. All of four IGRT systems have used treatment planning volumetric imaging information for target position verification and adjustment. Electronic portal images are routinely used for patient position verification. External markers and possible internal markers such as seeds or small cysts or calcifications can be localized and used for additional verification. Results: Emerging data from several institutional IRB‐approved clinical trials demonstrate that the target reposition error and dose delivery uncertainties can be significantly reduced by using such image‐guided systems, each of which may be most useful in specific clinical situations. Conclusions: Our customized stereovision system, which, like US, involves no radiation exposure, is extremely efficient (<2 minutes) and accurate (<2 millimeters) for superficial sites, such as breast cancer. The ExacTrac system appears ideal for lesions associated with bony structures, such as spine and skull. The US and CBCT may be most useful for deformable internal structures, such as prostate cancer. Special methods for dealing with imaging artifacts, such as ring patterns in CBCT, shadow casts and multiple reflections in stereovision and US, and patient motion in ExacTrac and stereovision will be presented.