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Radiation therapy requires accurate dose delivery to targets often identifiable only on computed tomography (CT) images. Translation between the isocenter localized on CT and laser setup for radiation treatment, and interfractional head repositioning are frequent sources of positioning error. The objective was to design a simple, accurate apparatus to eliminate these sources of error. System accuracy was confirmed with phantom and in vivo measurements. A head repositioner that fixates the maxilla via dental mold with fiducial marker Z-plates attached was fabricated to facilitate the connection between the isocenter on CT and laser treatment setup. A phantom study targeting steel balls randomly located within the head repositioner was performed. The center of each ball was marked on a transverse CT slice on which six points of the Z-plate were also visible. Based on the relative position of the six Z-plate points and the ball center, the laser setup position on each Z-plate and a top plate was calculated. Based on these setup marks, orthogonal port films, directed toward each target, were evaluated for accuracy without regard to visual setup. A similar procedure was followed to confirm accuracy of in vivo treatment setups in four dogs using implanted gold seeds. Sequential port films of three dogs were made to confirm interfractional accuracy. Phantom and in vivo measurements confirmed accuracy of 2 mm between isocenter on CT and the center of the treatment dose distribution. Port films confirmed similar accuracy for interfractional treatments. The system reliably connects CT target localization to accurate initial and interfractional radiation treatment setup.

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We present a block design that facilitates split-field tests, a technique frequently employed to assess several potential misalignments in linear accelerators. Two tests are possible with this device using: (1) opposed collimator angles and (2) opposed gantry angles. The first test is sensitive to jaw symmetry only. In the second test, the alignment of one field edge indicates gantry sag, while the other field edge is sensitive to the combined effects of jaw asymmetry, misalignment of the beam spot, and misalignment of the gantry and collimator rotation axes. The block arrangement consists of a pattern of three polygons cast in lead or Cerrobend. The assembly mounts in the accessory tray of the accelerator and allows all necessary exposures to be taken in a single room entry. Any field size up to 24 cm can be tested with the assembly constructed here. A simultaneous test of light field-radiation field coincidence can also be accommodated. Exposed films with a prototype device show that offsets of +/-1 mm are readily visible. The block design facilitates the required measurements for split-field tests and makes this a practical technique for routine quality assurance on a linear accelerator.

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PURPOSE: To design and implement a noninvasive stereotactic immobilization technique with daily CT image-guided positioning to treat patients with paraspinal lesions accurately and to quantify the systematic and random patient setup errors occurring with this method. METHODS AND MATERIALS: A stereotactic body frame (SBF) was developed for "rigid" immobilization of paraspinal patients. The inherent accuracy of this system for stereotactic CT-guided treatment was evaluated with phantom studies. Seven patients with thoracic and lumbar spine lesions were immobilized with the SBF and positioned for 33 treatment fractions using daily CT scans. For all 7 patients, the daily setup errors, as assessed from the daily CT scans, were corrected at each treatment fraction. A retrospective analysis was also performed to assess what the impact on patient treatment would have been without the CT-based corrections (i.e., if patient setup had been performed only with the SBF). RESULTS: The average magnitude of systematic and random errors from uncorrected patient setups using the SBF was approximately 2 mm and 1.5 mm (1 SD), respectively. For fixed phantom targets, the system accuracy for the SBF localization and treatment was shown to be within 1 mm (1 SD) in any direction. Dose-volume histograms incorporating these uncertainties for an intensity-modulated radiotherapy plan for lumbar spine lesions were generated, and the effects on the dose-volume histograms were studied. CONCLUSION: We demonstrated a very accurate and precise method of patient immobilization and treatment delivery based on a noninvasive SBF and daily image guidance for paraspinal lesions. The SBF provides excellent immobilization for paraspinal targets, with setup accuracy better than 2 mm (1 SD). However, for highly conformal paraspinal treatments, uncorrected systematic and random errors of 2 mm in magnitude can result in a significantly greater (>100%) dose to the spinal cord than planned, even though the planned target coverage may not change substantially. With daily CT guidance using the SBF, we showed that the maximal spinal cord dose is ensured to be within 10-15% of the planned value.

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PURPOSE: Delivering high dose to prostate with external beam radiation has been shown to improve local tumor control. However, it has to be carefully performed to avoid partial target miss and delivering excessive dose to surrounding normal tissues. One way to achieve safe dose escalation is to precisely localize prostate immediately before daily treatment. Therefore, the radiation can be accurately delivered to the target. Once the prostate position is determined with high confidence, planning target volume (PTV) safety margin might be reduced for further reduction of rectal toxicity. A rapid computed tomography (CT)-based online prostate localization method is presented for this purpose. METHODS AND MATERIALS: Immediately before each treatment session, the patient is immobilized and undergoes a CT scan in the treatment position using a CT scanner situated in the treatment room. At the CT console, posterior, anterior, left, and right extents of the prostate are manually identified on each axial slice. The translational prostate displacements relative to the planned position are estimated by simultaneously fitting these identified extents from this CT scan to a template created from the finely sliced planning CT scan. A total of 106 serial CT scans from 8 prostate cancer patients were performed immediately before treatments and used to retrospectively evaluate the precision of this daily prostate targeting method. The three-dimensional displacement of the prostate with respect to its planned position was estimated. RESULTS: Five axial slices from each treatment CT scan were sufficient to produce a reliable correction when compared with prostate center of gravity (CoG) displacements calculated from physician-drawn contours. The differences (mean +/- SD) between these two correction schemes in the right-left (R/L), posterior-anterior (P/A), and superior-inferior (S/I) directions are 0.0 +/- 0.4 mm, 0.0 +/- 0.7 mm, and -0.4 +/- 1.9 mm, respectively. With daily CT extent-fitting correction, 97% of the scans showed that the entire posterior prostate gland was covered by PTV given a margin of 6 mm at the rectum-prostate interface and 10 mm elsewhere. In comparison, only 74% and 65% could be achieved by the corrections based on daily and weekly bony matching on portal images, respectively. CONCLUSIONS: Results show that daily CT extent fitting provides a precise correction of prostate position in terms of CoG. Identifying prostate extents on five axial CT slices at the CT console is less time-consuming compared with daily contouring of the prostate on many slices. Taking advantage of the prostate curvature in the longitudinal direction, this method also eliminates the necessity of identifying prostate base and apex. Therefore, it is clinically feasible and should provide an accelerated localization of the prostate immediately before daily treatment.

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