Megavoltage computed tomography image-based low-dose rate intracavitary brachytherapy planning for cervical carcinoma.

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (D(B2cc) and D(R2cc), respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from D(R2cc) (p=0.749, mean difference was 24 cGy +/- 283 cGy). The ICRU bladder point dose was significantly lower than the D(B2cc) (p=0.024, mean difference was 291 cGy +/- 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm(3) and 2.0 cm(3), respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

Optical tracking technology in stereotactic radiation therapy.

The last decade has seen the introduction of advanced technologies that have enabled much more precise application of therapeutic radiation. These relatively new technologies include multileaf collimators, 3-dimensional conformal radiotherapy planning, and intensity modulated radiotherapy in radiotherapy. Therapeutic dose distributions have become more conformal to volumes of disease, sometimes utilizing sharp dose gradients to deliver high doses to target volumes while sparing nearby radiosensitive structures. Thus, accurate patient positioning has become even more important, so that the treatment delivered to the patient matches the virtual treatment plan in the computer treatment planning system. Optical and image-guided radiation therapy systems offer the potential to improve the precision of patient treatment by providing a more robust fiducial system than is typically used in conventional radiotherapy. The ability to accurately position internal targets relative to the linac isocenter and to provide real-time patient tracking theoretically enables significant reductions in the amount of normal tissue irradiated. This report reviews the concepts, technology, and clinical applications of optical tracking systems currently in use for stereotactic radiation therapy. Applications of radiotherapy optical tracking technology to respiratory gating and the monitoring of implanted fiducial markers are also discussed.

Evaluation of image-guidance protocols in the treatment of head and neck cancers.

PURPOSE: The aim of this study was to assess the residual setup error of different image-guidance (IG) protocols in the alignment of patients with head and neck cancer. The protocols differ in the percentage of treatment fractions that are associated with image guidance. Using data from patients who were treated with daily IG, the residual setup errors for several different protocols are retrospectively calculated. METHODS AND MATERIALS: Alignment data from 24 patients (802 fractions) treated with daily IG on a helical tomotherapy unit were analyzed. The difference between the daily setup correction and the setup correction that would have been made according to a specific protocol was used to calculate the residual setup errors for each protocol. RESULTS: The different protocols are generally effective in reducing systematic setup errors. Random setup errors are generally not reduced for fractions that are not image guided. As a consequence, if every other treatment is image guided, still about 11% of all treatments (IG and not IG) are subject to three-dimensional setup errors of at least 5 mm. This frequency increases to about 29% if setup errors >3 mm are scored. For various protocols that require 15% to 31% of the treatments to be image guided, from 50% to 60% and from 26% to 31% of all fractions are subject to setup errors >3 mm and >5 mm, respectively. CONCLUSION: Residual setup errors reduce with increasing frequency of IG during the course of external-beam radiotherapy for head-and-neck cancer patients. The inability to reduce random setup errors for fractions that are not image guided results in notable residual setup errors.

Characterization and use of EBT radiochromic film for IMRT dose verification.

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the film's dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The film's dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanner's OD readout. In addition, the film's large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.

Daily variations in delivered doses in patients treated with radiotherapy for localized prostate cancer.

PURPOSE: The aim of this work was to study the variations in delivered doses to the prostate, rectum, and bladder during a full course of image-guided external beam radiotherapy. METHODS AND MATERIALS: Ten patients with localized prostate cancer were treated with helical tomotherapy to 78 Gy at 2 Gy per fraction in 39 fractions. Daily target localization was performed using intraprostatic fiducials and daily megavoltage pelvic computed tomography (CT) scans, resulting in a total of 390 CT scans. The prostate, rectum, and bladder were manually contoured on each CT by a single physician. Daily dosimetric analysis was performed with dose recalculation. The study endpoints were D95 (dose to 95% of the prostate), rV2 (absolute rectal volume receiving 2 Gy), and bV2 (absolute bladder volume receiving 2 Gy). RESULTS: For the entire cohort, the average D95 (+/-SD) was 2.02 +/- 0.04 Gy (range, 1.79-2.20 Gy). The average rV2 (+/-SD) was 7.0 +/- 8.1 cc (range, 0.1-67.3 cc). The average bV2 (+/-SD) was 8.7 +/- 6.8 cc (range, 0.3-36.8 cc). Unlike doses for the prostate, there was significant daily variation in rectal and bladder doses, mostly because of variations in volume and shape of these organs. CONCLUSION: Large variations in delivered doses to the rectum and bladder can be documented with daily megavoltage CT scans. Image guidance for the targeting of the prostate, even with intraprostatic fiducials, does not take into account the variation in actual rectal and bladder doses. The clinical impact of techniques that take into account such dosimetric parameters in daily patient set-ups should be investigated.

Daily variations in the position of the prostate bed in patients with prostate cancer receiving postoperative external beam radiation therapy.

PURPOSE: The aim of this study was to evaluate the extent of the variation in the position of the prostate bed with respect to the bony anatomy. METHODS AND MATERIALS: Four patients were treated to 70 Gy in 35 fractions. Before each fraction, a megavoltage computed tomography (CT) of the prostate bed was obtained, resulting in a total of 140 CT studies. Retrospectively, each CT scan was aligned to the simulation kilovoltage scan based on bony anatomy and the prostate bed. The difference between the 2 alignments was calculated for each scan. RESULTS: The average differences (+/-1 SD) between the two alignments were 0.06+/-0.37, 0.10+/-0.86, and 0.39+/-1.27 mm in the lateral, longitudinal (SI), and vertical (AP) directions, respectively. Laterally, there was no difference>or=3 mm. The cumulative frequency of SI differences were as follows; >or=3 mm: 3%, >or=4 mm: 1%, and >or=5 mm: 1% (maximum: 5 mm). The cumulative frequency of AP differences were as follows; >or=3 mm: 7%, and >or=4 mm: 3% (maximum: 4 mm). CONCLUSION: In patients with prostate cancer receiving postoperative radiotherapy, the prostate bed motion relative to the pelvic bony anatomy is of a relatively small magnitude. Significant motion (>or=3 mm) is infrequent. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed.

Evaluation of an infrared camera and X-ray system using implanted fiducials in patients with lung tumors for gated radiation therapy.

PURPOSE: To report on the initial clinical use of a commercially available system to deliver gated treatment using implanted fiducials, in-room kV X-rays, and an infrared camera tracking system. METHODS AND MATERIALS: ExacTrac Adaptive Gating from BrainLab is a localization system using infrared cameras and X-rays. Gating signals are the patient's breathing pattern obtained from infrared reflectors on the patient. kV X-rays of an implanted fiducial are synchronized to the breathing pattern. After localization and shift of the patient to isocenter, the breathing pattern is used to gate the radiation. Feasibility tests included localization accuracy, radiation output constancy, and dose distributions with gating. Clinical experience is reported on treatment of patients with small lung lesions. RESULTS: Localization accuracy of a moving target with gating was 1.7 mm. Dose constancy measurements showed insignificant change in output with gating. Improvements of dose distributions on moving targets improved with gating. Eleven patients with lung lesions were implanted with 20 mmx0.7 mm gold coil (Visicoil). The implanted fiducial was used to localize and treat the patients with gating. Treatment planning and repeat computed tomographic scans showed that the change in center of gross target volume (GTV) to implanted marker averaged 2.47 mm due in part to asymmetric tumor shrinkage. CONCLUSION: ExacTrac Adaptive Gating has been used to treat lung lesions. Initial system evaluation verified its accuracy and usability. Implanted fiducials are visible in X-rays and did not migrate.

Performance characterization of megavoltage computed tomography imaging on a helical tomotherapy unit.

Helical tomotherapy is an innovative means of delivering IGRT and IMRT using a device that combines features of a linear accelerator and a helical computed tomography (CT) scanner. The HI-ART II can generate CT images from the same megavoltage x-ray beam it uses for treatment. These megavoltage CT (MVCT) images offer verification of the patient position prior to and potentially during radiation therapy. Since the unit uses the actual treatment beam as the x-ray source for image acquisition, no surrogate telemetry systems are required to register image space to treatment space. The disadvantage to using the treatment beam for imaging, however, is that the physics of radiation interactions in the megavoltage energy range may force compromises between the dose delivered and the image quality in comparison to diagnostic CT scanners. The performance of the system is therefore characterized in terms of objective measures of noise, uniformity, contrast, and spatial resolution as a function of the dose delivered by the MVCT beam. The uniformity and spatial resolutions of MVCT images generated by the HI-ART II are comparable to that of diagnostic CT images. Furthermore, the MVCT scan contrast is linear with respect to the electron density of material imaged. MVCT images do not have the same performance characteristics as state-of-the art diagnostic CT scanners when one objectively examines noise and low-contrast resolution. These inferior results may be explained, at least partially, by the low doses delivered by our unit; the dose is 1.1 cGy in a 20 cm diameter cylindrical phantom. In spite of the poorer low-contrast resolution, these relatively low-dose MVCT scans provide sufficient contrast to delineate many soft-tissue structures. Hence, these images are useful not only for verifying the patient's position at the time of therapy, but they are also sufficient for delineating many anatomic structures. In conjunction with the ability to recalculate radiotherapy doses on these images, this enables dose guidance as well as image guidance of radiotherapy treatments.

A geometrically based method of step and shoot stereotactic radiosurgery with a miniature multileaf collimator.

Conventional methods of inverse planning for intensity-modulated radiotherapy (IMRT) and intensity-modulated radiosurgery (IMRS) are generally based upon optimizing a set of beam fluence profiles according to a set of dose-volume constraints specified by a human planner. This optimization is generally carried out through an iterative approach that relies upon the optimization of a score, driving the plan's ability to satisfy the user-provided constraints. Following optimization of the fluence distribution, the non-trivial problem of converting the fluence distribution into a set of deliverable, intensity-modulated beams must be solved. A novel approach to solving this IMRS total inverse problem is presented in this paper. The proposed method uses a class solution that provides an optimized dose gradient and a method of designing a conformal plan based on an existing geometrically based optimization algorithm. After developing an optimal fluence distribution, the process then arranges the fluence into a set of simple and efficient MLC beam delivery sequences. The algorithm presented here offers several potential advantages for the application of intensity modulation to radiosurgery treatment planning. The geometrically based optimization process' simplicity requires far less human user input and decision making in the specification of dose and dose-volume constraints than do conventional inverse planning algorithms. This simplicity allows the optimization process to be completed much faster than conventional inverse-planning algorithms, literally seconds compared with at least several minutes. Likewise, the fluence conversion step is a simplified process (compared to conventional IMRT planning), which takes advantage of some simplifications uniquely appropriate to the problem at hand (IMRS). The converted, deliverable IMRS beams allow superior conformity and dose gradient relative to conventional IMRS planning or 3DCRT radiosurgery planning. Another benefit is that the number of beam intensity levels is greatly reduced, from hundreds to as few as a half-dozen intensity levels. Finally, since the treatment plan optimization process is based upon proven principles applicable to optimizing radiosurgery (rather than the general problem of optimizing fractionated radiotherapy plans), the plans generated and deliverable with this method of IMRS are potentially superior to those produced by conventional inverse-planning methods of IMRT/IMRS.

Serial megavoltage CT imaging during external beam radiotherapy for non-small-cell lung cancer: observations on tumor regression during treatment.

PURPOSE: The ability to obtain soft-tissue imaging in the treatment room, such as with megavoltage CT imaging, enables the observation of tumor regression during a course of external beam radiation therapy. In this current study, we report on the most extensive study looking at the rate of regression of non-small-cell lung cancers during a course of external beam radiotherapy by analyzing serial megavoltage CT images obtained on 10 patients. METHODS AND MATERIALS: The analysis is performed on 10 patients treated with the Helical Tomotherapy Hi*Art device. All 10 patients had non-small-cell lung cancer. A total of 274 megavoltage CT sets were obtained on the 10 patients (average, 27 scans per patient; range, 9-35). All patients had at least a scan at beginning and at the end of treatment. The frequency of scanning was determined by the treating physician. The treatment was subsequently delivered with the Tomotherapy Hi*Art system. The gross tumor volumes (GTVs) were later contoured on each megavoltage CT scan, and tumor volumes were calculated. Although some patients were treated to draining nodal areas in addition to the primary tumor, only the primary GTVs were tracked. Response to treatment was quantified by the relative decrease in tumor volume over time, i.e., elapsed days from the first day of therapy. The individual GTVs ranged from 5.9 to 737.2 cc in volume at the start of treatment. In 6 of the 10 patients, dose recalculations were also performed to document potential variations in delivered doses within the tumors. The megavoltage CT scans were used, and the planned treatment was recalculated on the daily images. The hypothesis was that dose deposited in the target would increase throughout the course of radiotherapy because of tumor shrinkage and subsequent decreasing attenuation. Specifically, the dose received by 95% of the GTV (D95) was monitored over time for each of the 6 patients treated at M. D. Anderson Cancer Center Orlando. RESULTS: Regression of all 10 lung tumors could be observed on the serial megavoltage CT scans. The decrease in volume was observed at a relatively constant rate throughout the treatments, with no obvious initial or final plateaus. For all 10 tumors, the average decrease in volume was 1.2% per day. However, individual tumor regression rates were observed with a range of 0.6% to 2.3% per day. The lowest rate of shrinkage was observed for the smallest lesion, and the highest rate was observed in the largest lesion. Of the 6 cases in which dose recalculations were performed, 5 demonstrated a small but noticeable gradual increase in deposited doses within the tumor, with the D95 increases ranging from 0.02% to 0.1% per day. CONCLUSION: With the advent of in-room soft-tissue imaging techniques such as megavoltage CT imaging with a helical tomotherapy unit, daily documentation of the status of a grossly visible targeted tumor becomes possible. The current study demonstrated that tumor regression can be documented for patients with non-small-cell lung cancer treated with helical tomotherapy. Clinical correlations between the observations made during the course of treatment and ultimate outcomes, e.g. local control, should be investigated.

Optically guided patient positioning techniques.

Optical tracking determines an object's position by measuring light either emitted or reflected from the object. The hallmark of optical tracking systems is their high spatial resolution and measurement in real time; such systems can resolve the position of a point source within a fraction of a millimeter and report at a rate of 10 Hz or faster. Several systems have been developed for radiation therapy, all of which track infrared markers attached to the patient's external surface. The positions of the optical markers relative to the target volume, together with the desired marker positions relative to treatment isocenter, are determined during computed tomography simulation. In the treatment room, the real marker positions are measured relative to isocenter; rigid-body mathematics then determine marker displacements from their desired positions and hence target displacement from isocenter. Real-time feedback allows one to correct the patient's position. The first systems were used for intracranial stereotaxis radiotherapy; rigid arrays of optical markers were attached to the patient via a biteplate linkage. Subsequent systems for extracranial radiotherapy tracked external markers to determine patient position and/or gate the radiation beam based on patient motion. Lastly, optical tracking has been integrated with ultrasound or stereoscopic x-ray imaging to determine the position of internal anatomy targets relative to isocenter.

Geometrically based optimization for extracranial radiosurgery.

For static beam conformal intracranial radiosurgery, geometry of the beam arrangement dominates overall dose distribution. Maximizing beam separation in three dimensions decreases beam overlap, thus maximizing dose conformality and gradient outside of the target volume. Webb proposed arrangements of isotropically convergent beams that could be used as the starting point for a radiotherapy optimization process. We have developed an extracranial radiosurgery optimization method by extending Webb's isotropic beam arrangements to deliverable beam arrangements. This method uses an arrangement of N maximally separated converging vectors within the space available for beam delivery. Each bouquet of isotropic beam vectors is generated by a random sampling process that iteratively maximizes beam separation. Next, beam arrangement is optimized for critical structure avoidance while maintaining minimal overlap between beam entrance and exit pathways. This geometrically optimized beam set can then be used as a template for either conformal beam or intensity modulated extracranial radiosurgery. Preliminary results suggest that using this technique with conformal beam planning provides high plan conformality, a steep dose gradient outside of the tumour volume and acceptable critical structure avoidance in the majority of clinical cases.

A simple and reliable index for scoring rival stereotactic radiosurgery plans.

PURPOSE: A simple and robust index for ranking rival stereotactic radiosurgery plans is presented. METHODS: The radiosurgery plan score index, CGI (Conformity/Gradient Index), is an average of a conformity score and a gradient score. Computation of the CGI score is simple, requiring only three pieces of data: (1) the total volume irradiated to the prescription isodose level, (2) the volume of the target, and (3) the total volume irradiated at half of the prescription isodose level. The overall CGI Index is a simple function of these three pieces of data. RESULTS: When multiple sets of rival stereotactic radiosurgery plans were ranked with respect to this single score index, the resulting plan rankings closely matched the plan rankings according to biologic indices (calculated nontarget brain normal tissue complication probabilities). CONCLUSIONS: The CGI is a simple and fast plan evaluation tool that can assist the radiosurgery planner in evaluating and optimizing multiple candidate radiosurgery plans.