M. Manning

SU-E-T-197: Comparing Quality Assurance Strategies for Single Isocenter Treatments of Multiple Brain Metastases

Purpose: To demonstrate a new quality assurance (QA) technique for single isocenter volumetric modulated arc therapy (VMAT) plans for stereotactic radiosurgery of multiple brain metastases and to compare quantitative QA results using this method with results using a cylindrical diode array and full field coronal films. Methods: This study employs a phantom built of 28 thin slabs to accommodate radiochromic film insertions near all planning target volume (PTV) locations, providing target specific data for individual PTVs. Single isocenter VMAT plans were designed for two patients with 5 and 11 intra-cranial metastases. Both plans were modified three times to create plans that overdosed a target by 10%, and underdosed a target by 10 % and 20%. Gamma analyses were performed for full field coronal films, the diode array measurements and for individual PTVs with the proposed film stack method using square ROIs centered on the PTVs, measuring twice the length of the PTV’s equivalent sphere diameter. Passing gamma criteria was 95% of points passing at 5%/1mm and a threshold of 10% of the prescription dose. Results: Of the 6 introduced errors, 1 was detected by the diode array gamma test (93.1% pass rate). Pass rates for the other 5 ranged from 95.4–98.8%. None of the errors were detected using the full field film gamma test (pass rates: 98.7–99.9%). Using the proposed film stack method, 5 of 6 errors were detected by failed gamma tests (pass rates: 51, 53, 71, 87.3, 93.9 and 97.8%). The two high pass rates were due to attempts to underdose a PTV that measured too high in the original plan. Conclusion: The results suggest that current methods may be lacking quantitative value and rely heavily on challenging qualitative evaluation, while the proposed method offers a much needed practical method for meaningful quantitative results.

Do we need a new CT scan for retreatment of intracranial SRS patients

Abstract Purpose To determine if the treatment planning computed tomography scan (CT) from an initial intracranial stereotactic radiosurgery (SRS) treatment can be used for repeat courses of SRS. Methods and materials Twenty‐five patients with 40 brain metastases that received multiple courses of SRS were retrospectively studied. Magnetic resonance scans from repeat SRS (rMR) courses were registered to CT scans from the initial SRS (iCT) and repeat SRS (rCT). The CT scans were then registered to find the displacement of the rMR between iCT and rCT registrations. The distance from each target to proximal skull surface was measured in 16 directions on each CT scan after registration. The mutual information (MI) coefficients from the registration process were used to evaluate image set similarity. Targets and plans from the rCTs were transferred to the iCTs, and doses were recalculated on the iCT for repeat plans. The two dose distributions were compared through 3D gamma analysis. Results The magnitude of the mean linear translations from the MR registrations was 0.6 ± 0.3 mm. The mean differences in distance from target to skull on a per target basis were 0.3 ± 0.2 mm. The MI was 0.582 ± 0.042. Registration between a comparison group of 30 CT scans that had the same data resampled and 30 scans that were intercompared with different patients gave MI = 0.721 ± 0.055 and MI = 0.359 ± 0.031, respectively. The mean gamma passing rates were 0.997 ± 0.007 for 1 mm/1% criteria. Conclusions The rMR can be aligned to the iCT to accurately define targets. The skull shows minimal change between scans so the iCT can be used for set‐up at repeat treatments. The dosimetry provided by the iCT dose calculation is adequate for repeat SRS. Treatment based on iCT is feasible.

SU-F-T-510: Single Isocenter Radiosurgery for the Simultaneous Treatment of Multiple Brain Metastases: Volumetric Modulated Arc Therapy Or Dynamic Conformal Arc?

PURPOSE This study evaluates a novel algorithm that can be used with any treatment planning system for simple and rapid generation of stereotactic radiosurgery (SRS) plans for treating multiple brain metastases using a single isocenter dynamic conformal arc (DCA) approach. This technique is compared with a single isocenter volumetric modulated arc therapy (VMAT) technique in terms of delivery time, conformity, low dose spread and delivery accuracy. METHODS Five patients, with a total of 37 (5 - 11) targets were planned using a previously published method for generating optimal VMAT plans and using the proposed DCA algorithm. All planning target volumes (PTVs) were planned to 20 Gy, meeting a minimum 99% coverage and maximum 135 % hot spot for both techniques. Quality assurance was performed using radiochromic film, with films placed in the high dose regions of each PTV. Normal tissue volumes receiving 12 Gy and 6 Gy (V12 and V6) were computed for each plan. Conformity index (CI) and gamma evaluations (95% of points passing 4%/0.5mm) were computed for each PTV. RESULTS Delivery times, including beam on and table rotation times, were comparable: 17 - 22 minutes for all deliveries. V12s for DCA plans were (18.5±15.2 cc) vs. VMAT (19.7±14.4 cc). V6s were significantly lower for DCA (69.0±52.0 cc) compared with VMAT (154.0±91.0 cc) (p <<0.05). CIs for VMAT targets were (1.38±0.50) vs. DCA (1.61±0.41). 36 of 37 DCA planned targets passed gamma tests, while 29 of 37 VMAT planned targets passed. CONCLUSION Single isocenter DCA plans were easily achieved. The evaluation suggests that DCA may represent a favorable technique compared with VMAT for multiple target SRS by reducing dose to normal tissue and more accurately depicting deliverable dose.

SU-F-T-626: Intracranial SRS Re-Treatment Without Acquisition of New CT Images

PURPOSE Linear accelerator based stereotactic radiosurgery (SRS) for multiple intracranial lesions with frequent surveillance is becoming a popular treatment option. This strategy leads to retreatment with SRS as new lesions arise. Currently, each course of treatment uses magnetic resonance (MR) and computed tomography (CT) images for treatment planning. We propose that new MR images, with course 1 CT images, may be used for future treatment plans with negligible loss of dosimetric accuracy. METHODS Ten patients that received multiple courses of SRS were retrospectively reviewed. The treatment plans and contours from non-initial courses were copied to the initial CTs and recalculated. Doses metrics for the plans calculated on the initial CTs and later CTs were compared. All CT scans were acquired on a Philips CT scanner with a 600 mm field of view and 1 mm slice thickness (Philips Healthcare, Andover, MA). All targets were planned to 20 Gy and calculated in Eclipse V. 13.6 (Varian, Palo Alto, CA) using analytic anisotropic algorithm with 1 mm calculation grid. RESULTS Sixteen lesions were evaluated. The mean time between courses was 250 +/- 215 days (range 103-979). The mean target volume was 2.0 +/- 2.9 cc (range 0.1-10.1). The average difference in mean target dose between the two calculations was 0.2 +/- 0.3 Gy (range 0.0 - 1.0). The mean conformity index (CI) was 1.28 +/- 0.14 (range 1.07 - 1.82). The average difference in CI was 0.03 +/- 0.16 (range 0.00 - 0.44). Targets volumes < 0.5 cc showed the largest changes in both metrics. CONCLUSION Continued treatment based on initial CT images is feasible. Dose calculation on the initial CT for future treatments provides reasonable dosimetric accuracy. Changes in dose metrics are largest for small volumes, and are likely dominated by partial volume effects in target definition.

SU-E-T-177: Clinical Experience with Spirometer Guided Breath Hold Lung SBRT

Purpose: Tumor motion in lung SBRT is typically managed by creating an internal target volume (ITV) based on 4D-CT information. Another option, which may reduce lung dose and imaging artifact, is to use a breath hold (BH) during simulation and delivery. Here we evaluate the reproducibility of tumor position at repeated BH using a newly released spirometry system. Methods: Three patients underwent multiple BH CT’s at simulation. All patients underwent a BH cone beam CT (CBCT) prior to each treatment. All image sets were registered to a patient’s first simulation CT based on local bony anatomy. The gross tumor volume (GTV), and the diaphragm or the apex of the lung were contoured on the first image set and expanded in 1 mm increments until the GTVs and diaphragms on all image sets were included inside an expanded structure. The GTV and diaphragm margins necessary to encompass the structures were recorded. Results: The first patient underwent 2 BH CT’s and fluoroscopy at simulation, the remaining patients underwent 3 BH CT’s at simulation. In all cases the GTV’s remained within 1 mm expansions and the diaphragms remained within 2 mm expansions on repeat scans. Each patient underwent 3 daily BH CBCT’s. In all cases the GTV’s remained within a 2 mm expansions, and the diaphragms (or lung apex in one case) remained within 2 mm expansions at daily BH imaging. Conclusions: These case studies demonstrate spirometry as an effective tool for limiting tumor motion (and imaging artifact) and facilitating reproducible tumor positioning over multiple set-ups and BH’s. This work was partially supported by Qfix.

SU‐E‐T‐677: The Effects of Respiratory Motion in RapidArc Spinal Stereotactic Radiosurgery

PURPOSE Spinal radiosurgery is planned to strict tolerances. Unaccounted for dosimetric changes resulting from respiratory motion might lead to delivery of a sub-optimal plan. We examine these effects for RapidArc spine radiosurgery. METHODS Plans were created for three patients that received 4D-CT scans in a spinal radiosurgery set-up. A target covering a single vertebral body and pedicles was defined and the spinal canal was contoured for each patient. Plans that delivered 18 Gy to the targets were created on the free breathing scans (FB). The maximum inspiration and expiration 4D-CT phases were fused to the FB based on target and spinal canal position. The FB plans were copied onto these images and doses were calculated. Point doses, DVHs, and plan subtractions were used to evaluate plan differences. Motion was measured at fixed points on the 4D-CTs. RESULTS The target for case 1 was defined at T4, case two at T7, and case three at T9. For all cases the target and the spinal canal moved < 1 mm, while the skin surface moved 1-2.5 mm. For the T4 plan the aortic arch moved about 6 mm. For the T7 and T9 cases the diaphragm moved 5 mm and 12.3 mm, respectively. Point dose changes in the targets between the maximum inspiration and exhalation plans were 8.1 cGy, 69.0 cGy, and 58.1 cGy for the T4, T7, and T9 plans. Point changes in the spinal canal were 13.8 cGy, 34.0 cGy, and 34.1 cGy for the T4, T7, and T9 plans. Spinal canal DVHs changes were negligible in all cases. Changes in target coverage at doses >= 18 Gy were approximately 5% for the T7 and T9 plans. CONCLUSION Respiratory motion may alter the dosimetry of spinal radiosurgery plans. The clinical impact of these changes appears to be small.

SU‐E‐T‐568: A Comparison of Apparent Tumor Volume on Treatment Planning Scans and Daily Volumetric Imaging Scans for Lung SBRT

Purpose: The effectiveness of stereotactic body radiation therapy(SBRT) depends on proper alignment of the patient and tumor. The objective of this study is to analyze the apparent tumor volume measured on daily volumetric imaging scans compared to the volume measured on various simulation and planning computed tomography(CT)images. Methods: Gross tumor volumes (GTVs) were contoured retrospectively in MIMVista for 10 lungcancer patients who underwent SBRT. Daily pretreatment megavoltage CT (MVCT) GTVs were compared to the treatment plan internal target volume (ITV), a contour derived solely from four‐dimensional CT (4DCT) cine in MIMVista, the maximum intensity projection (MIP) of the 4DCT data, and the average (phase‐binned) intensity projection (AIP). Results: On average, the ITV was 2.1 +/− 1.08 times larger than the MVCT GTV. Similarly, the MIP volume was 1.85 +/− 0.84 and the AIP volume was 1.07 +/− 0.37 times larger. Minimum and maximum values ranged from 0.95 to 6.93 times larger for the ITV and 0.92 to 5.73 for the MIP. Results for the AIP were more centrally distributed, ranging from 0.54 to 2.97 Conclusions: For lungcancer patients, daily MVCT scans show significantly smaller tumor volumes than treatment planningCT scans. For 9 out of 10 patients, all MVCTs showed smaller tumor volumes than the ITV and MIP. Because lungtumors are not stationary, this could cause uneven target dose and greater dose to healthy tissue. MVCT GTVs were closest in size to the AIP, but the minimum and maximum deviations were still significant. The largest tumor displayed relatively small deviations from the ITV and MIP volume, but there was no clear relationship between tumor volume and the magnitude of the deviations. This effect was less pronounced for the AIP. Further investigation will determine the effect this volume difference has on daily target registration.