Low-Dose Computed Tomography Scanning Protocols for Online Adaptive Proton Therapy of Head-and-Neck Cancers.

PURPOSE: To evaluate the suitability of low-dose CT protocols for online plan adaptation of head-and-neck patients. METHODS: We acquired CT scans of a head phantom with protocols corresponding to CT dose index volume CTDIvol in the range of 4.2-165.9 mGy. The highest value corresponds to the standard protocol used for CT simulations of 10 head-and-neck patients included in the study. The minimum value corresponds to the lowest achievable tube current of the GE Discovery RT scanner used for the study. For each patient and each low-dose protocol, the noise relative to the standard protocol, derived from phantom images, was applied to a virtual CT (vCT). The vCT was obtained from a daily CBCT scan corresponding to the fraction with the largest anatomical changes. We ran an established adaptive workflow twice for each low-dose protocol using a high-quality daily vCT and the corresponding low-dose synthetic vCT. For a relative comparison of the adaptation efficacy, two adapted plans were recalculated in the high-quality vCT and evaluated with the contours obtained through deformable registration of the planning CT. We also evaluated the accuracy of dose calculation in low-dose CT volumes using the standard CT protocol as reference. RESULTS: The maximum differences in D98 between low-dose protocols and the standard protocol for the high-risk and low-risk CTV were found to be 0.6% and 0.3%, respectively. The difference in OAR sparing was up to 3%. The Dice similarity coefficient between propagated contours obtained with low-dose and standard protocols was above 0.982. The mean 2%/2 mm gamma pass rate for the lowest-dose image, using the standard protocol as reference, was found to be 99.99%. CONCLUSION: The differences between low-dose protocols and the standard scanning protocol were marginal. Thus, low-dose CT protocols are suitable for online adaptive proton therapy of head-and-neck cancers. As such, considering scanning protocols used in our clinic, the imaging dose associated with online adaption of head-and-neck cancers treated with protons can be reduced by a factor of 40.

A comparative study using time-driven activity-based costing in single-fraction breast high-dose rate brachytherapy: An integrated brachytherapy suite vs. decentralized workflow.

INTRODUCTION: Precision breast intraoperative radiation therapy (PB-IORT) is a novel approach to adjuvant radiation therapy for early-stage breast cancer performed as part of a phase II clinical trial at two institutions. One institution performs the entire procedure in an integrated brachytherapy suite which contains a CT-on-rails imaging unit and full anesthesia capabilities. At the other, breast conserving surgery and radiation therapy take place in two separate locations. Here, we utilize time-driven activity-based costing (TDABC) to compare these two models for the delivery of PB-IORT. METHODS: Process maps were created to describe each step required to deliver PB-IORT at each institution, including personnel, equipment, and supplies. Time investment was estimated for each step. The capacity cost rate was determined for each resource, and total costs of care were then calculated by multiplying the capacity cost rates by the time estimate for the process step and adding any additional product costs. RESULTS: PB-IORT costs less to deliver at a distributed facility, as is more commonly available, than an integrated brachytherapy suite ($3,262.22 vs. $3,996.01). The largest source of costs in both settings ($2,400) was consumable supplies, including the brachytherapy balloon applicator. The difference in costs for the two facility types was driven by personnel costs ($1,263.41 vs. $764.89). In the integrated facility, increased time required by radiation oncology nursing and the anesthesia attending translated to the greatest increases in cost. Equipment costs were also slightly higher in the integrated suite setting ($332.60 vs. $97.33). CONCLUSIONS: The overall cost of care is higher when utilizing an integrated brachytherapy suite to deliver PB-IORT. This was primarily driven by additional personnel costs from nursing and anesthesia, although the greatest cost of delivery in both settings was the disposable brachytherapy applicator. These differences in cost must be balanced against the potential impact on patient experience with these approaches.

CT-on-Rails Versus In-Room CBCT for Online Daily Adaptive Proton Therapy of Head-and-Neck Cancers.

PURPOSE: To compare the efficacy of CT-on-rails versus in-room CBCT for daily adaptive proton therapy. METHODS: We analyzed a cohort of ten head-and-neck patients with daily CBCT and corresponding virtual CT images. The necessity of moving the patient after a CT scan is the most significant difference in the adaptation workflow, leading to an increased treatment execution uncertainty σ. It is a combination of the isocenter-matching σi and random patient movements induced by the couch motion σm. The former is assumed to never exceed 1 mm. For the latter, we studied three different scenarios with σm = 1, 2, and 3 mm. Accordingly, to mimic the adaptation workflow with CT-on-rails, we introduced random offsets after Monte-Carlo-based adaptation but before delivery of the adapted plan. RESULTS: There were no significant differences in accumulated dose-volume histograms and dose distributions for σm = 1 and 2 mm. Offsets with σm = 3 mm resulted in underdosage to CTV and hot spots of considerable volume. CONCLUSION: Since σm typically does not exceed 2 mm for in-room CT, there is no clinically significant dosimetric difference between the two modalities for online adaptive therapy of head-and-neck patients. Therefore, in-room CT-on-rails can be considered a good alternative to CBCT for adaptive proton therapy.

Dosimetric evaluation of image-guided radiation therapy for prostate cancer.

The aim of this study was to investigate the dosimetric accuracy of imaged-guided radiation therapy for prostate patients using the in-room computed tomography (CT) target localization technique. A Siemens CT-on-rails system was used for patient setup and target localization for intensity-modulated radiation therapy (IMRT) of prostate cancer. Fifteen previously treated prostate patients were included in this retrospective study. CT-on-Rails scans were performed before and after the IMRT treatment under local IRB approval. A total of 15 original simulation CT scans and 98 post-treatment CT scans were contoured by the same oncologist to delineate the prostate target, bladder, and rectum. IMRT plans were generated on the original simulation CTs and the same MUs and leaf sequences were used to compute the dose distributions using post-treatment CTs. Varian Velocity deformable registration was used for the summation of the fractional doses and the cumulative doses were compared with the planned doses. For the 15 patients investigated, the mean isocenter shift was up to 4.0 mm in the left-right direction, 5.9 mm in the anterior-posterior direction and 5.6 mm in the superior-inferior direction due to interfractional organ motion. The mean rectal volume varied from 0.6 to 1.73 times and the mean bladder volume varied from 0.59 to 3.65 times between simulation and the end of treatment. The prescription dose to 95% of the PTV, Dp, was set to 76 Gy for all treatment plans. The dose to 95% of the clinical treatment volume (CTV), D95, was 74.0 to 77.6 Gy and the minimum CTV dose, Dmin, was 61.0 to 71.6 Gy, respectively, in the cumulative dose distributions. Detailed analyses showed that 7.1% of the treatment fractions had cold spots (< 85% of Dp) in the peripheral CTV, leading to Dmin < 64 Gy in the cumulative dose distributions for 4 patients. The rectal dose-volume constraints were violated in 35.7% of the treatment fractions while the bladder dose was much improved in 82.7% of the treatment fractions. The current IGRT procedure for patient setup and target localization using rigid-body registration based on contour/anatomy matching is effective for population-based PTV margins. For a small group of patients, specific PTV margins and/or real-time target monitoring/tracking will be necessary due to significant prostate deformation/rotation caused by inter- and intrafractional bladder and rectal volume variation.

CyberKnife with integrated CT-on-rails: System description and first clinical application for pancreas SBRT.

PURPOSE: This article reports on the integration of a sliding-gantry CT-on-rails with a robotic linear accelerator. METHODS: The system consists of a SOMATOM Definition AS CT scanner (Siemens Healthcare, Forchheim, Germany) and a CyberKnife M6 FIM (Accuray, Inc., Sunnyvale, CA, USA). Additional movement programs were implemented in the robotic treatment table (RoboCouch, Accuray Inc.) to move between CT and treatment position. Acceptance testing was performed on the CT scanner according to AAPM83 guidelines, as well as safety tests for collision avoidance and electromagnetic (EM) compatibility. For the first clinical application of the system, daily dose was evaluated in five pancreas SBRT patients. A second envisioned use is the optimal alignment of the treatment beams to soft-tissue targets without the use of implanted fiducials. To this end, an offset vector feature has been implemented, which shifts the treatment center according to the daily position of the tumor relative to the spine (established by a CT scan). This offset can be applied by either moving the treatment couch (physical couch shift) or by moving the CyberKnife robot (virtual couch shift). An End-to-End (E2E) test was specifically designed to evaluate the accuracy of this feature using the Xsight Lung Tracking Phantom (Computerized Imaging Reference Systems, Inc., Norfolk, VA, USA). The position of the tumor with respect to the spine was varied by moving the insert inside the phantom and a CT scan was made for each position. The treatment plan was subsequently delivered to the phantom employing spine tracking. The test was repeated four times for a physical couch shift and four times for a virtual couch shift. RESULTS: All acceptance, safety and EM compatibility testing was successful. For the first pancreas SBRT patients treated using daily CT imaging, the volume of stomach, duodenum, or small bowel receiving >35 Gy was found to increase or remain constant during treatment; however, the clinical constraint of 5 cc was not violated. For the offset vector E2E test, the reference accuracy (without any tumor shift) was (0.74, -0.61, -0.33) mm in the inferior, left, and anterior direction respectively. The difference in deviation with respect to the reference was (-0.1 ± 0.15, 0.01 ± 0.16, -0.17 ± 0.25) mm, when applying a physical couch shift. With a virtual couch shift, the deviations were (0.02 ± 0.15, 0.06 ± 0.23, -0.4 ± 0.31) mm. CONCLUSIONS: The first combination of a CyberKnife treatment unit with a sliding-gantry CT scanner is operational in our department enabling future developments toward image-guided online-adaptive SBRT supported by diagnostic-quality CT imaging.

Prospective validation of treatment accuracy using implanted fiducial markers for spinal stereotactic body radiation therapy.

In order to accurately assess positioning errors in spinal SBRT, many institutions employ bony-fusion based imaging techniques, such as the ExacTrac™ (Brain Lab) system, in conjunction with 3D verification (performed via CT-on-rails in our practice). We hypothesized that the use of implanted gold fiducial markers could improve the accuracy of patient positioning over bony fusion alone. We addressed this question prospectively, enrolling patients on an IRB-approved protocol. Gold seeds were implanted in the vertebral pedicles flanking the target level. At treatment, setup error was calculated using two methods-standard kV image fusion, and geometric fiducial-based projection, with independent CT-on-rails verification. Analyses of residual set-up error showed that fiducial-based setup agreed with fusion-based determination, but did not significantly reduce error. Offline 6D fusion of the treatment and planning CT illustrated residual rotational error using standard or fiducial based setup. We conclude that the ExacTrac and CT-on-rails platform yields highly accurate results for spinal SBRT setup, with reduced residual error than previously reported. While the addition of fiducials did not further reduce error, the bony fusion approach is now prospectively validated in comparison to implanted fiducials. Both bony fusion and fiducial marker methods are associated with residual rotational error, thus 3D verification remains an important component of spinal SBRT treatment.

Implementing MRI-based target delineation for cervical cancer treatment within a rapid workflow environment for image-guided brachytherapy: A practical approach for centers without in-room MRI.

PURPOSE: Magnetic resonance imaging (MRI)-based intracavitary brachytherapy offers several advantages over computed tomography (CT)-based brachytherapy, but many centers are unable to offer it at the time of brachytherapy because of logistic and/or financial considerations. We have implemented a method of integrating MRI into a CT-guided, high-dose-rate intracavitary brachytherapy workflow in clinics that do not have immediately available MRI capability. METHODS: At our institution, patients receiving high-dose-rate intracavitary brachytherapy as a component of the definitive treatment of cervical cancer have a Smit sleeve placed during the first brachytherapy fraction in a dedicated suite with in-room CT-on-rails. After the first fraction of brachytherapy, an MRI is obtained with the Smit sleeve, but no applicator, in place. For each subsequent fraction, CT scans are coregistered to the MRI scan by the Smit sleeve. The gross target volume is defined by MRI and overlaid on the CT images for each brachytherapy treatment for dose optimization. RESULTS: This MRI-integrated workflow adds <5 minutes to the brachytherapy session for image fusion. Our initial clinical experience suggests that this approach is feasible and results in target volume reductions compared with CT-alone brachytherapy. CONCLUSIONS: Our proposed combination MRI and/or CT workflow is a feasible compromise to preserve an efficient workflow while integrating MRI target delineation, and it provides many of the advantages of both MRI- and CT-based brachytherapy. The future collection and analysis of clinical data will serve to compare the proposed approach to non-MRI containing techniques.