A Sahgal

Image-Guided Robotic Stereotactic Body Radiotherapy for Benign Spinal Tumors: The University of California San Francisco Preliminary Experience

We evaluate our preliminary experience using the Cyberknife® Radiosurgery System in treating benign spinal tumors. A retrospective review of 16 consecutively treated patients, comprising 19 benign spinal tumors, was performed. Histologic types included neurofibroma [11], chordoma [4], hemangioma [2], and meningioma [2]. Three patients had Neurofibromatosis Type 1 (NF1). Only one tumor, recurrent chordoma, had been previously irradiated, and as such not considered in the local failure analysis. Local failure, for the remaining 18 tumors, was based clinically on symptom progression and/or tumor enlargement based on imaging. Indications for spine stereotactic body radiotherapy (SBRT) consisted of either adjuvant to subtotal resection (5/19), primary treatment alone (12/19), boost following external beam radiotherapy (1/19), and salvage following previous radiation (1/19). Median tumor follow-up is 25 months (2–37), and one patient (with NF1) died at 12 months from a stroke. The median total dose, number of fractions, and prescription isodose was 21 Gy (10–30 Gy), 3 fx (1–5 fx), 80% (42–87%). The median tumor volume was 7.6 cc (0.2–274.1 cc). The median V100 (volume V receiving 100% of the prescribed dose) and maximum tumor dose was 95% (77–100%) and 26.7 Gy (15.4–59.7 Gy), respectively. Three tumors progressed at 2, 4, and 36 months post-SR (n=18). Two tumors were neurofibromas (both in NF1 patients), and the third was an intramedullary hemangioblastoma. Based on imaging, two tumors had MRI documented progression, three had regressed, and 13 were unchanged (n=18). With short follow-up, local control following Cyberknife spine SBRT for benign spinal tumors appear acceptable.

Effects of residual target motion for image-tracked spine radiosurgery

A quality assurance method was developed to investigate the effects of residual target motion for hypofractionated spine radiosurgery. The residual target motion (target movement between successive image-guided corrections) was measured on-line via dual x-ray imagers for patients treated with CyberKnife (Accuray, Inc., Sunnyvale, CA), a robotic linear accelerator with intrafractional image-tracking capability. The six degree-of-freedom characteristics of the residual target motion were analyzed, the effects of such motion on patient treatment delivery were investigated by incorporating the probability distribution of the residual motion into the treatment planning dose calculations, and deviations of the doses from those originally planned were calculated. Measurements using a programmable motion phantom were also carried out and compared with the static treatment plan calculations. It was found that the residual target motions were patient specific and typically on the order of 2 mm. The measured dose distributions incorporating the residual target motion also exhibited 2.0 mm discrepancy at the prescription isodose level when compared with the static treatment plan calculations. For certain patients, residual errors introduced significant uncertainties (-1 Gy) for the dose delivered to the spinal cord, especially at the high dose levels covering a small volume of the spinal cord (e.g., 0.1 cc). In such cases, stringent cord constraints and frequent monitoring of the target position should be implemented.

SPLIT-VOLUME TREATMENT PLANNING OF MULTIPLE CONSECUTIVE VERTEBRAL BODY METASTASES FOR CYBERKNIFE IMAGE-GUIDED ROBOTIC RADIOSURGERY

Cyberknife treatment planning of multiple consecutive vertebral body metastases is challenging due to large target volumes adjacent to critical normal tissues. A split-volume treatment planning technique was developed to improve the treatment plan quality of such lesions. Treatment plans were generated for 1 to 5 consecutive thoracic vertebral bodies (CVBM) prescribing a total dose of 24 Gy in 3 fractions. The planning target volume (PTV) consisted of the entire vertebral body(ies). Treatment plans were generated considering both the de novo clinical scenario (no prior radiation), imposing a dose limit of 8 Gy to 1 cc of spinal cord, and the retreatment scenario (prior radiation) with a dose limit of 3 Gy to 1 cc of spinal cord. The split-volume planning technique was compared with the standard full-volume technique only for targets ranging from 2 to 5 CVBM in length. The primary endpoint was to obtain best PTV coverage by the 24 Gy prescription isodose line. A total of 18 treatment plans were generated (10 standard and 8 split-volume). PTV coverage by the 24-Gy isodose line worsened consistently as the number of CVBM increased for both the de novo and retreatment scenario. Split-volume planning was achieved by introducing a 0.5-cm gap, splitting the standard full-volume PTV into 2 equal length PTVs. In every case, split-volume planning resulted in improved PTV coverage by the 24-Gy isodose line ranging from 4% to 12% for the de novo scenario and, 8% to 17% for the retreatment scenario. We did not observe a significant trend for increased monitor units required, or higher doses to spinal cord or esophagus, with split-volume planning. Split-volume treatment planning significantly improves Cyberknife treatment plan quality for CVBM, as compared to the standard technique. This technique may be of particular importance in clinical situations where stringent spinal cord dose limits are required.

Functional Relationship between the Volume of a Near-Target Peripheral Isodose Line and Its Isodose Value for Gamma Knife® Radiosurgery

Aims: To study how the volume of a peripheral isodose line would relate to its isodose value for Gamma Knife® (GK) radiosurgery. Method: A theoretical model was developed to derive the relationship between the volume of a near-target peripheral isodose line and its isodose value. The formula was tested based on cases with uniformly matched target volumes as well as random target volumes ranging from 0.1 to 37 cc. Results: To the second order approximation, the volume of a near-target peripheral isodose line (V) relates to its dose value (D) by the following formula: V/V₀ = (D/D₀)–3/2, where D₀ is the prescription dose and V₀ is the prescription isodose volume. Applied to patient data, we found V/V₀ = (D/D₀)Γ, where Γ = ––1.50 ± 0.16 at the 95% of the confidence level, and showed excellent agreement with the theoretical prediction. Conclusion: For GK radiosurgery, the near-target peripheral isodose volume relates to its isodose value in a similar fashion as to the way in which the volume of a sphere relates to the inverse of its surface area. Such a predictable relationship justifies the use of a single near-target peripheral isodose volume, as a sufficient measure of the dose to the normal brain.

Comparisons of Novalis and CyberKnife® Spinal Stereotactic Body Radiotherapy Treatment Planning Based on Physical and Biological Modeling Parameters

Aims: To compare treatment planning quality of CyberKnife® and Novalis systems for vertebral body stereotactic body radiotherapy. Methods: Treatme

SU‐FF‐T‐13: A Generalized Biologically Equivalent Dose (gBED) Model and Its Application for Radiosurgery and Hypofractionated Body Radiotherapy

Purpose: Vastly different dose fractionation and dose prescription schemes exist for radiosurgery and body radiotherapy of intracranial and extracranial tumors. A generalized biological equivalent dose (gBED) model has been developed to account for variations in dose fractionations and non‐uniform dose distributions for both targets and critical structures of such treatments. Method and Materials: Assuming cell survival fraction (S) can be expressed as S = e −α gBED , we derived gBED = ∑ i w i BED i where w i = v i S(d i / ∑ i S(d i ), BED i = nd i [1+d i /(α/β)] of the linear‐quadratic model, and vi is the ith voxel receiving the dose of di as in the calculation of the dose volume histogram. In the above gBED formula, wi is the probability‐weighted volume unit which is analogous to the voxel mass (density×volume). As a result, we derived a new histogram by plotting accumulative BEDi versus accumulative wi and we called it Dose Unit Histogram (DUH). To study DUH model, analyses were applied to a group of >20 radiosurgery and body radiotherapy cases of varying fractionations and dose volume histograms. The dependence of gBED on α/β values (ranging from 2–20) and other physical dose parameters were also investigated. Results: The normalized DUH fell consistently below the normalized DVH curves regardless of α/β values. This indicates a decreased effect to target and increased tolerance of the normal structure. For high α/β values such as 20, gBED approached to the conventional BED calculated from the mean dose of a volume. For low values of α/β such as 2, gBED was significantly different, particularly for non‐uniform target dose distributions. Conclusion: A generalized BED formula was developed and used for radiosurgery and body radiotherapy planning analysis. The formula yielded different histogram plots from conventional DVH by accounting for the variations in dose fractionation and non‐uniformity of the dose distributions.

SU-D-16A-06: Modeling Biological Effects of Residual Uncertainties For Stereotactic Radiosurgery.

PURPOSE Residual uncertainties on the order of 1-2 mm are frequently observed when delivering stereotactic radiosurgery via on-line imaging guidance with a relocatable frame. In this study, a predictive model was developed to evalute potentiral late radiation effects associated with such uncertainties. METHODS A mathematical model was first developed to correlate the peripherial isodose volume with the internal and/or setup margins for a radiosurgical target. Such a model was then integrated with a previoulsy published logistic regression normal tissue complication model for determining the symptomatic radiation necrosis rate at various target sizes and prescription dose levels. The model was tested on a cohort of 15 brain tumor and tumor resection cavity patient cases and model predicted results were compared with the clinical results reported in the literature. RESULTS A normalized target diameter (D0 ) in term of D0 = 6V/S, where V is the volume of a radiosurgical target and S is the surface of the target, was found to correlate excellently with the peripheral isodose volume for a radiosurgical delivery (logarithmic regression R2 > 0.99). The peripheral isodose volumes were found increase rapidly with increasing uncertainties levels. In general, a 1-mm residual uncertainties as calculated to result in approximately 0.5%, 1%, and 3% increases in the symptomatic radiation necrosis rate for D0 = 1 cm, 2 cm, and 3 cm based on the prescription guideline of RTOG 9005, i.e., 21 Gy to a lesion of 1 cm in diameter, 18 Gy to a lesion 2 cm in diameter, and 15 Gy to a lesion 3 cm in diameter respectively. CONCLUSION The results of study suggest more stringent criteria on residual uncertainties are needed when treating a large target such as D0 ≤ 3 cm with stereotactic radiosurgery. Dr. Ma and Dr. Sahgal are currently serving on the board of international society of stereotactic radiosurgery (ISRS).

SU-E-T-568: Improving Normal Brain Sparing with Increasing Number of Arc Beams for Volume Modulated Arc Beam Radiosurgery of Multiple Brain Metastases

PURPOSE Intensity modulated arc beams have been newly reported for treating multiple brain metastases. The purpose of this study was to determine the variations in the normal brain doses with increasing number of arc beams for multiple brain metastases treatments via the TrueBeam Rapidarc system (Varian Oncology, Palo Alto, CA). METHODS A patient case with 12 metastatic brain lesions previously treated on the Leksell Gamma Knife Perfexion (GK) was used for the study. All lesions and organs at risk were contoured by a senior radiation oncologist and treatment plans for a subset of 3, 6, 9 and all 12 targets were developed for the TrueBeam Rapidarc system via 3 to 7 intensity modulated arc-beams with each target covered by at least 99% of the prescribed dose of 20 Gy. The peripheral normal brain isodose volumes as well as the total beam-on time were analyzed with increasing number of arc beams for these targets. RESULTS All intensisty modulated arc-beam plans produced efficient treatment delivery with the beam-on time averaging 0.6-1.5 min per lesion at an output of 1200 MU/min. With increasing number of arc beams, the peripheral normal brain isodose volumes such as the 12-Gy isodose line enclosed normal brain tissue volumes were on average decreased by 6%, 11%, 18%, and 28% for the 3-, 6-, 9-, 12-target treatment plans respectively. The lowest normal brain isodose volumes were consistently found for the 7-arc treatment plans for all the cases. CONCLUSION With nearly identical beam-on times, the peripheral normal brain dose was notably decreased when the total number of intensity modulated arc beams was increased when treating multiple brain metastases. Dr Sahgal and Dr Ma are currently serving on the board of international society of stereotactic radiosurgery.

SU‐E‐T‐669: Dose Interplay Effects in Stereotactic Radiosurgery (SRS) of Multiple Brain Lesions

PURPOSE Volumetric modulated arc therapy (VMAT) has enabled rapid treatments of multiple brain tumors with a single or few isocenters. We investigated inter-lesion dose interplay effects for such a treatment and compared against standard multi-isocenteric Gamma Knife (GK) or dynamic-conformal-arc (DCA) SRS deliveries. METHODS A patient case with 12 intracranial targets and simulated cases with 2-60 targets in the brain parenchyma were used for the study. For the patient case, all targets and organs-at-risk were contoured by a senior clinician. A subset of 3, 6, 9 and 12 targets were then planned at different institutions for GK, DCA and VMAT SRS. Identical dose-volume constraints to the targets and critical structures were applied. Each target was prescribed with 20 Gy covering at least 99% of the target volume. Relationships between the mean 4-Gy to 12-Gy isodose volumes per lesion versus increasing number of lesions were analyzed for each modality. RESULTS For all the cases, 12-Gy isodose volumes per lesion exhibited negligible dependence with the increasing number of targets for GK SRS and DCA deliveries. However, for VMAT delivery, a strong statistically significant dependence with increasing number of targets was found at all levels of isodose volumes. For example, the increase in the 12-Gy volumes of the patient case was 0.068+/-0.016 cc/lesion (p=0.05) for the VMAT delivery in contrast to 0.0+/-0.0 cc/lesion (p < 0.0001) for both GK and DCA deliveries. The increase in the 4-Gy isodose volumes was 2.79+/-0.40 cc/lesion (p=0.02) for the VMAT delivery, and 2.13+/-0.15 cc/lesion (p=0.005) for the DCA delivery in contrast to 0.08+/-0.29 cc/lesion (p=0.10) for the GK delivery. CONCLUSION Significant dose interplay effects were found for single-or few-isocenter VMAT SRS of multiple lesions, somewhat for multi-isocenteric DCA SRS, but nearly negligible for GK SRS treatments.

SU-E-T-162: Prospective Treatment Plan Modeling To Guide Patient-Specific Treatment Setups for Hypofractionated Intracranial and Spinal Radiosurgery

PURPOSE Residual patient setup shifts such as 1 mm or 1 degree are commonly observed during online treatment setups of hypofractionated radiosurgery (HRS) of spine and intracranial lesions. To effectively guide patient-specific setups, a prospective treatment planning approach was developed for such treatments. METHODS For a cohort of intracranial and spinal HRS treatments, individualized setup guidelines were prospectively determined via the following approach: (1) systematically shift and rotate 3D patient image data in incremental small steps. (2) For each step such as 0.1 mm or 0.1 degree, dose distributions from original treatment plan were recalculated. (3) Each resulting dose distributions were then iteratively checked against a set of preset dosimetric criteria for compliance. (4) From step 3, maximum tolerable shift levels along the major axes were then determined. Finally, we tested such an approached on a cohort of treatment cases performed at our institutions. RESULTS For studied treatment sessions (n=63), the mean maximum tolerable shift levels including rotations projected unto the major axes were determined to be 2.0±0.7mm, 2.1±0.9 mm and 1.9±0.8 mm along the lateral, longitudinal, and vertical axis, respectively. Such tolerance levels were significantly (p=0.02) larger than the machine preset tolerance level of 1.0 mm that commonly enforced for all treatment sessions. However, more than 50% of the treatment sessions exhibited sub-millimeter tolerance level along certain directions to meet critical structure constraints. For example, a 0.6-mm anterior maximum shift could incur 36% increase in the maximum dose to the brainstem for one patient case. CONCLUSION Patient specific online setup guidelines for HRS has been demonstrated to prevent potential deleterious effects from residual shifts while facilitating online setups by providing more room of larger acceptable shifts for a majority of treatment cases.