Gamma analysis dependence on specified low-dose thresholds for VMAT QA.

The American Association of Physicists in Medicine Task Group 119 instructed institutions to use a low-dose threshold of 10% or a region of interest determined by the jaw setting when they collected gamma analysis quality assurance (QA) data for the planar dose distribution. However, there are no clinical data to quantitatively demonstrate the impact of the low-dose threshold on the gamma index. Therefore, we performed a gamma analysis with various low-dose thresholds in the range of 0% to 15% according to both global and local normalization and different acceptance criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm). A total of 30 treatment plans--10 head and neck, 10 brain, and 10 prostate cancer cases--were randomly selected from the Varian Eclipse treatment planning system (TPS). For the gamma analysis, a calculated portal image was acquired through a portal dose calculation algorithm in the Eclipse TPS, and a measured portal image was obtained using an electronic portal-imaging device. Then, the gamma analysis was performed using the Portal Dosimetry software (Varian Medical Systems, Palo Alto, CA). The gamma passing rate (%GP) for the global normalization decreased as the low-dose threshold increased, and all low-dose thresholds led to %GP values above 95% for both the 3%/3 mm and 2%/2 mm criteria. However, for the local normalization, %GP for a low-dose threshold of 10% was 7.47%, 10.23%, and 6.71% greater than the low-dose threshold of 0% for head and neck, brain, and prostate for the 3%/3 mm criteria, respectively. The results indicate that applying the low-dose threshold to global normalization does not have a critical impact on patient-specific QA results. However, in the local normalization, the low-dose threshold level should be carefully selected because the excluded low-dose points could cause the average %GP to increase rapidly.

Effect of jaw size in megavoltage CT on image quality and dose.

PURPOSE: Recently, the jaw size for the TomoTherapy Hi-Art II(®) (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue. METHODS: MVCT image quality (image noise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements. RESULTS: The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69-2.32 cGy (peripheral: 0.83-2.49 cGy) at J4 to 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) at J1. The measured lens dose increased from 0.92-3.36 cGy at J4 to 1.06-3.91 cGy at J1. CONCLUSIONS: The change in jaw size improved longitudinal resolution. The MVCT imaging dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.

Development of a deformable lung phantom for the evaluation of deformable registration.

The deformable lung phantom was developed to account for the patient breathing motion, and to evaluate for a deformable image registration algorithm. The phantom consisted of an acryl cylinder filled with water and a latex balloon located in the inner space of the cylinder. A silicon membrane was attached to the inferior end of the phantom. This silicon membrane was designed to simulate a real lung diaphragm and to reduce motor workload. This specific design was able to reduce the metal use which may prevent infrared sensing of the real position management (RPM) gating system on 4D CT image acquisition. Verification of intensity based 3D demon deformable registration was based on peak exhale and peak inhale breathing phases. The registration differences ranged from 0.85 mm to 1.47 mm, and accuracy was determined according to inner target deformation. This phantom was able to simulate the features and deformation of real human lung and has the potential for wide application in 4D radiation treatment planning.

Metal artifacts with dental implants: Evaluation using a dedicated CT/MR oral phantom with registration of the CT and MR images.

The aims of this study were to develop a computed tomography/magnetic resonance (CT/MR) oral phantom with insertable dental implants and to register CT/MR images to generate artifact-free MR images for patients undergoing teeth restorations. All measurements were done using a human MR scanner with spin echo (SE) and gradient echo (GRE) sequences image scan together with CT image. The metal regions and normal teeth parts are extracted with a suitable threshold from an initial image reconstructed with artifact from the CT images. Corrected metal projection regions of MR images and CT images are fused to produce artifact-free MR image that include dental restorations. After CT/MR registration, artifact size presented differences on the x- (SE, 12.0 mm; GRE, 18.0 mm) and y- (SE, 24.0 mm; GRE, 36.6 mm). When comparing the dental restoration with normal teeth, the structural similarity index metric (SSIM) of GRE 50 was lower than for the GRE 8 sequence and the SSIM of SE 145 shown higher than for the SE 490 sequence. The dedicated phantom provides a useful tool in head and neck research for multi-modality images. Therefore, CT/MR image-based approach for ground truth and registration offers visualization in diagnostic system and radiation treatment planning system.

Recurrent traumatic urethral strictures near the external sphincter: treatment with a covered, retrievable, expandable nitinol stent--initial results.

PURPOSE: To investigate the clinical effectiveness of a covered, retrievable, expandable nitinol stent in treating traumatic urethral strictures near the external sphincter. MATERIALS AND METHODS: The stent was 10 mm in diameter when fully expanded and 40-50 mm long. To make it removable, two nylon drawstrings were attached to its lower inner margin. Twelve consecutive men (19-67 years; mean age, 47 years) with traumatic urethral strictures near the external sphincter that were refractory to endoscopic urethrotomy were treated. With fluoroscopic guidance, a stent was placed to completely bridge the external sphincter. The stent was electively removed with a retrieval hook wire 2 months after placement. Stent placement and removal were successful in all patients, with no procedural complications. RESULTS: Mean maximum urine flow rate was 5 mL/sec (range, 3-7 mL/sec) before stent placement and 27 mL/sec (range, 16-40 mL/sec) at 1 week after placement. During the mean follow-up of 20 months (range, 2-37 months) after the first stent removal, strictures recurred in eight of 12 patients. For the eight patients with recurrence, a second stent was placed and 4 months later was removed from seven of the eight patients. During the mean follow-up of 18 months (range, 4-32 months) after the second stent removal, strictures recurred in two patients, in whom a third stent was placed and then removed 4 months later, with good results. Four (33%) of 12 patients, five (62%) of eight patients, and two (100%) of two patients were successfully treated with placement of the first stent, the second stent, and the third stent, respectively. In the remaining patient, the second stent remains in place. CONCLUSION: Placement of a covered, retrievable, expandable nitinol stent seems to be effective in treating urethral strictures near the external sphincter and warrants further investigation.