Tamara LaCouture

Quantifying Rigid and Nonrigid Motion of Liver Tumors During Stereotactic Body Radiation Therapy

PURPOSEnTo quantify rigid and nonrigid motion of liver tumors using reconstructed 3-dimensional (3D) fiducials from stereo imaging during CyberKnife-based stereotactic body radiation therapy (SBRT).nnnMETHODS AND MATERIALSnTwenty-three liver patients treated with 3 fractions of SBRT were used in this study. After 2 orthogonal kilovoltage images were taken during treatment, the 3D locations of the fiducials were generated by the CyberKnife system and validated using geometric derivations. A total of 4824 pairs of kilovoltage images from start to end of treatment were analyzed. For rigid motion, the rotational angles and translational shifts were reported by aligning 3D fiducial groups from different image pairs, using least-squares fitting. For nonrigid motion, we quantified interfractional tumor volume variations by using the proportional volume derived from the fiducials, which correlates to the sum of interfiducial distances. The individual fiducial displacements were also reported (1) after rigid corrections and (2) without angle corrections.nnnRESULTSnThe proportional volume derived by the fiducials demonstrated a volume-increasing trend in the second (101.9% ± 3.6%) and third (101.0 ± 5.9%) fractions among most patients, possibly due to radiation-induced edema. For all patients, the translational shifts in left-right, anteroposterior, and superoinferior directions were 2.1 ± 2.3 mm, 2.9 ± 2.8 mm, and 6.4 ± 5.5 mm, respectively. The greatest translational shifts occurred in the superoinferior direction, likely due to respiratory motion from the diaphragm. The rotational angles in roll, pitch, and yaw were 1.2° ± 1.8°, 1.8° ± 2.4°, and 1.7° ± 2.1°, respectively. The 3D individual fiducial displacements with rigid corrections were 0.2 ± 0.2 mm and increased to 0.5 ± 0.4 mm without rotational corrections.nnnCONCLUSIONSnAccurate 3D locations of internal fiducials can be reconstructed from stereo imaging during treatment. As an effective surrogate to tumor motion, fiducials provide a close estimation of both rigid and nonrigid motion of liver tumors. The reported displacements could be further utilized for tumor margin definition and motion management in conventional linear accelerator-based liver SBRT.

Stereotactic body radiotherapy as an alternative to brachytherapy in gynecologic cancer.

Introduction. Brachytherapy plays a key role in the treatment of many gynecologic cancers. However, some patients are unable to tolerate brachytherapy for medical or other reasons. For these patients, stereotactic body radiotherapy (SBRT) offers an alternative form of treatment. Methods. Retrospective review of patients prospectively collected on SBRT database is conducted. A total of 11 gynecologic patients who could not have brachytherapy received SBRT for treatment of their malignancies. Five patients have been candidates for interstitial brachytherapy, and six have required tandem and ovoid brachytherapy. Median SBRT dose was 25u2009Gy in five fractions. Results. At last followup, eight patients were alive, and three patients had died of progressive disease. One patient had a local recurrence. Median followup for surviving patients was 420 days (median followup for all patients was 120 days). Two patients had acute toxicity (G2 dysuria and G2 GI), and one patient had late toxicity (G3 GI, rectal bleeding requiring cauterization). Conclusions. Our data show acceptable toxicity and outcome for gynecologic patients treated with SBRT who were unable to receive a brachytherapy boost. This treatment modality should be further evaluated in a phase II study.

Dose-volume effects on brainstem dose tolerance in radiosurgery

OBJECTnDose-volume data concerning the brainstem in stereotactic radiosurgery (SRS) for trigeminal neuralgia (TN) were analyzed in relation to associated complications. The authors present their set of data and compare it with currently cited information on brainstem dose tolerance associated with conventional fractionated radiation therapy and hypofractionated radiation treatment of other diseases.nnnMETHODSnStereotactic radiosurgery for TN delivers a much higher radiation dose to the brainstem in a single fraction than doses delivered by any other procedures. A literature survey of articles on radiosurgery for TN revealed no incidences of severe toxicity, unlike other high-dose procedures involving the brainstem. Published data on brainstem dose tolerance were investigated and compared with dose-volume data in TN radiosurgery. The authors also performed a biological modeling study of dose-volume data involving the brainstem in cases of TN treated with the Gamma Knife, CyberKnife, and linear accelerator-based systems.nnnRESULTSnThe brainstem may receive a maximum dose as high as 45 Gy during radiosurgery for TN. The major complication after TN radiosurgery is mild to moderate facial numbness, and few other severe toxic responses to radiation are observed. The biologically effective dose of 45 Gy in a single fraction is much higher than any brainstem dose tolerance currently cited in conventional fractionation or in single or hypofractionated radiation treatments. However, in TN radiosurgery, the dose falloff is so steep and the delivery so accurate that brainstem volumes of 0.1-0.5 cm(3) or larger receive lower planned and delivered doses than those in other radiation-related procedures. Current models are suggestive, but an extensive analysis of detailed dose-volume clinical data is needed.nnnCONCLUSIONSnPatients whose TN is treated with radiosurgery are a valuable population in which to demonstrate the dose-volume effects of an extreme hypofractionated radiation treatment on the brainstem. The result of TN radiosurgery suggests that a very small volume of the brainstem can tolerate a drastically high dose without suffering a severe clinical injury. The authors believe that the steep dose gradient in TN radiosurgery plays a key role in the low toxicity experienced by the brainstem.

Biological implications of whole-brain radiotherapy versus stereotactic radiosurgery of multiple brain metastases

OBJECTnThe efficacy and safety of treatment with whole-brain radiotherapy (WBRT) or with stereotactic radiosurgery (SRS) for multiple brain metastases (> 10) are topics of ongoing debate. This study presents detailed dosimetric and biological information to investigate the possible clinical outcomes of these 2 modalities.nnnMETHODSnFive patients with multiple brain metastases (n = 11-23) underwent SRS. Whole-brain radiotherapy plans were retrospectively designed with the same MR image set and the same structure set for each patient, using the standard opposing lateral beams and fractionation (3 Gy × 10). Physical radiation doses and biologically effective doses (BEDs) in WBRT and SRS were calculated for each lesion target and for the normal brain tissues for comparison of the 2 modalities in the context of clinical efficacy and published toxicities.nnnRESULTSnThe BEDs targeted to the tumor were higher in SRS than in WBRT by factors ranging from 2.4- to 3.0- fold for the mean dose and from 3.2- to 5.3-fold for the maximum dose. In the 5 patients, mean BEDs in SRS (calculated as percentages of BEDs in WBRT) were 1.3%-34.3% for normal brain tissue, 0.7%-31.6% for the brainstem, 0.5%-5.7% for the chiasm, 0.2%-5.7% for optic nerves, and 0.6%-18.1% for the hippocampus.nnnCONCLUSIONSnThe dose-volume metrics presented in this study were essential to understanding the safety and efficacy of WBRT and SRS for multiple brain metastases. Whole-brain radiotherapy results in a higher incidence of radiation-related toxicities than SRS. Even in patients with > 10 brain metastases, the normal CNS tissues receive significantly lower doses in SRS. The mean normal brain dose in SRS correlated with the total volume of the lesions rather than with the number of lesions treated.

Validity of Current Stereotactic Body Radiation Therapy Dose Constraints for Aorta and Major Vessels

Understanding dose constraints for critical structures in stereotactic body radiation therapy (SBRT) is essential to generate a plan for optimal efficacy and safety. Published dose constraints are derived by a variety of methods, including crude statistics, actuarial analysis, modeling, and simple biologically effective dose (BED) conversion. Many dose constraints reported in the literature are not consistent with each other, secondary to differences in clinical and dosimetric parameters. Application of a dose constraint without discriminating the variation of all the factors involved may result in suboptimal treatment. This issue of Seminars in Radiation Oncology validates dose tolerance limits for 10 critical anatomic structures based on dose response modeling of clinical outcomes data to include detailed dose-volume metrics. This article presents a logistic dose-response model for aorta and major vessels based on 238 cases from the literature in addition to 387 cases from MD Anderson Cancer Center at Cooper University Hospital, for a total of 625 cases. The Radiation Therapy Oncology Group (RTOG) 0813 dose-tolerance limit of Dmax = 52.5Gy in 5 fractions was found to have a 1.2% risk of grade 3-5 toxicity, and the Timmerman 2008 limit of Dmax = 45Gy in 3 fractions had 2.3% risk. From the model, the 1% and 2% risk levels for D4cc, D1cc, and D0.5cc are also provided in 1-5 fractions, in the form of a dose-volume histogram (DVH) Risk Map.

Factors that may determine the targeting accuracy of image-guided radiosurgery

PURPOSEnThe AAPM TG-135 report is a landmark recommendation for the quality assurance (QA) of image-guided robotic radiosurgery. The purpose of this paper is to present results pertaining to intentionally offsetting the phantom as recommended by TG-135 and to present data on targeting algorithm accuracy as a function of imager parameters in less than ideal circumstances, which had not been available at the time of publication of TG-135.nnnMETHODSnAll tests in this study were performed at the Cooper University Hospital CyberKnife Center in Mt. Laurel, NJ. For intentional offsets, initial tests were performed on the Accuray-supplied anthropomorphic head and neck phantom, whereas for subsequent tests, the Accuray-supplied alignment quality assurance (AQA) phantom was used. To simulate the effects of imager parameters for larger patients, slabs of Blue Water (Standard Imaging, Inc., Middleton, WI) were added to attenuate the x-ray images in some of the tests. In conjunction with attenuated x-ray tests, the number of fiducials was varied by systematically deselecting them one at a time at the CyberKnife console.nnnRESULTSnTests using the AQA phantom verified that submillimeter alignments were consistently achieved even with intentional shifts and rotations of up to 10.0 mm and 1.0°, respectively. An analysis of 17 months of daily QA alignment tests showed that submillimeter alignments were achieved more than 99% of the time even with such intentional shifts and rotations of the phantom. When additional slabs of Blue Water were added to simulate patient attenuation of the x-ray images, targeting errors could be induced depending on imager parameters and the amount of Blue Water used. A series of consecutive tests showed that two helpful variables to ensure good accuracy of the system were (1) the fiducial extraction confidence level (FECL) system parameter and (2) the number of targeted fiducials. When fewer than four fiducials were used, the FECL reported by the CyberKnife was sometimes high even when a false lock occurred, so using multiple fiducials helped to ensure reliable targeting.nnnCONCLUSIONSnRadiosurgery requires the highest degree of targeting accuracy, and in our experience, the CyberKnife has been able to maintain submillimeter accuracy consistently. It has been verified that our CyberKnife can correct for phantom shifts of up to 10.0 mm and rotations of up to 1.0°. It has also been discovered that false locks are more likely to occur with a single fiducial than with multiple fiducials. Although targeting accuracy can only be measured on a phantom, the insight gained from analyzing the QA tests can help us in devising better strategies for achieving the best treatment for our patients.

Preoperative Radiosurgery for Soft Tissue Sarcoma.

Objectives: Preoperative radiation followed by surgical resection is a standard treatment for soft tissue sarcomas (STSs). The conventional method of radiation is 5 weeks to approximately 50 Gy. We report on our initial experience and phase II single-arm study assessing 5 fractions of stereotactic body radiotherapy (SBRT), followed by surgical resection for STS. Methods: Thirteen patients and 14 tumors were treated with preoperative SBRT; tumors were mostly poorly differentiated (5) or myxoid (5) and were located on the leg (10), arm (2) or groin (2). The median tumor size in greatest dimension was 7.6 cm (maximum 16 cm). Twelve patients received 35 Gy in 5 fractions; for 2 deeper tumors the dose was 40 Gy in 5 fractions. Ten patients were administered 0.5 cm bolus to improve the dose. Gross tumor volume was expanded 0.5 cm radially and 3 cm along the tissue plane. Treatment was to an isodose line (median 81%) and was delivered every other day. Maximum dose to the skin was 46 Gy (median 41 Gy). Results: The median follow-up period was 279 days. Surgical resection occurred a median of 37 days after completion of SBRT. Four patients had acute toxicity consisting of 2 grade 2 and 2 grade 3 skin reactions; all cases of skin toxicity resolved by the time of surgery. Percent tumor necrosis ranged from 10% to 95% (median 60%). All patients had negative margins. Planned vacuum-assisted wound closure was used in 4 patients; there were no other major wound complications. There was 1 local recurrence and 7 distant recurrences. Conclusion: This is the initial experience of radiosurgery for preoperative treatment of STSs. We have found this to be well tolerated, convenient for the patients, and a much shorter treatment course, allowing patients to undergo surgery and subsequent chemotherapy quicker. Surgical complications and control rates are satisfactory. The initial results are encouraging for further investigation.

The dosimetric impact of the prescription isodose line (IDL) on the quality of robotic stereotactic radiosurgery (SRS) plans

Purpose: There is no consensus on the optimal prescription isodose line (IDL) in CyberKnife (CK) SRS. We designed a strategy to search for optimal CK plans at different levels of IDLs and investigated the dosimetric impact on the quality of CK plans. Methods and materials: The retrospective study consisted of 13 CK patients with 16 brain tumors. The mean volume and size of the tumors was 9.7 ± 10.4 cc and 30.3 ± 10.9 mm, respectively. Four shells were created at distances of 2–3 mm to 60 mm from the target. The constraint dose of the innermost shell (D1) was the primary optimization parameter. For isolated brain tumors, D1 started from the prescription dose and gradually reduced after optimization started over. The optimal plans were reached when the coverage started to degrade and the desired IDL was achieved. For eight tumors abutting an OAR, both the D1 and constraint dose to the OAR were gradually pushed until an optimal plan was reached for the desired IDL. Results: For the isolated tumors, the V5 Gy, V10 Gy, V15 Gy, V20 Gy, and V25 Gy of low IDL (49.6 ± 2.1%) plans were on average 23.6%, 28.6%, 33.8%, 26.2%, and 10.6% lower, respectively, comparing to the high IDL (88.6 ± 1.3%) plans. The Conformality Index (CI) of the low IDL plans outperformed the high IDL plans (mean: 1.15 vs. 1.24), except for a lesion under 0.5 cc. The quality of the middle IDL plans (69.6 ± 1.5%) was close to the low IDL plans. Similar results were observed for tumors abutting an OAR. Conclusions: Low IDL plans outperformed high IDL plans for all metrics in tumors > 0.5 cc. The lower dose exposure of normal brain tissue and better CI could potentially reduce radiation necrosis while the higher maximum dose could improve local control.

SU-E-T-724: Testing the Limitations of a Photon Dose Algorithm in Commissioning

Purpose: This work is to test the limitations of a photon dose algorithm during the commissioning of a treatment planning system. Methods: Commissioning tests for a model-based dose calculation algorithm typically involve the verification of PDD, output and dose profiles, but often those tests are performed at one SSD, one depth and one nominal gantry position. Those tests are useful to validate the modeling of radiation beams, but they may not be sufficient to detect the limitations of an algorithm. Additional tests were performed at several SSDs, various depths, oblique gantry angles and off-axis conditions to demonstrate the accuracy and limitations of the dose calculation algorithm. Dose profiles were measured by in-water scanning with a three-dimensional scanning system and calculated in the TPS using a virtual water phantom. Results: Dose profiles from both the measurement and the calculation were plotted for comparison. Also plotted were the difference in absolute dose between the measurement and the calculation. For the off-axis MLC field tests, larger discrepancies were observed at the open portion and the far edge of the field, which became greater with short SSD and deep depth. In addition, the shape of the profile appeared different for the open portion of the field. For the oblique MLC field tests, the largest difference was seen around the high-dose shoulder of the profile. Differences in some point doses were observed to be greater than 10% of the maximum field dose. Conclusion: We have learned the limitations of the superposition dose algorithm in the XiO planning system through some specific tests, which are related to the parallel kernel approximation and the simplified energy spectra employed by the algorithm to calculate field-size dependent fluence.

Assessment of Brain Tumor Displacements after Skull-based Registration: A CT/MRI Fusion Study

Purpose: To assess brain tumor displacements between skull based and soft-tissue based matching during CTMRI fusion for a total of 35 brain lesions. nMethods: Twenty-five patients who underwent CT and MRI scans in the same day were retrospectively recruited into the study. Semi-automatic skull based fusion was first performed and reviewed on a Treatment Planning System (TPS). A secondary fine-tuning of the fusion was then performed, if mismatch was observed in the tumor or neighboring soft-tissue, using nearby visible soft-tissue, such as gyri, sulci, and fissures. Two physicists fine-tuned the secondary fusion until the best match could be agreed upon. The resulting rotations and translations between the two fusions were recorded, which indicated local displacements between skull based and soft-tissue based matching. We further created a PTV by expanding a 2 mm margin around the GTV after skull-based fusion, and then evaluated the coverage of the GTV within the PTV after fine tuning with soft-tissue based fusion. nResults: In 29 of the 35 lesions, minor to no mismatch was found between the soft-tissue and skull based fusions. The corresponding translational and rotational shifts were 0.05 ± 0.63 mm (LR), 0.01 ± 0.79 mm (AP), 0.37 ± 1.01 mm (SI); -0.15 ± 0.67° (pitch), -0.19 ± 0.34° (yaw), and -0.12 ± 0.49° (roll). Thus the GTV, after soft-tissue based fusion, was 100% covered by the PTV. However, in the remaining 6 lesions in the study, noticeable displacements were observed between the skull and soft-tissue based fusions. Excluding an outlier lesion, the mean translational and rotational shifts for 5 of the 6 remaining lesions were 0.90 ± 2.15 mm (LR), 1.50 ± 2.27 mm (AP), -1.01 ± 1.83 mm (SI); -1.42 ± 3.12°(pitch), 0.02 ± 0.83°(yaw), and -0.17 ± 0.68°(roll). For the outlier lesion, the GTV was nearly missed by the PTV, and for the rest of the 5 lesions, the mean coverage of the GTV was 98.9% within the PTV. nConclusion: In a small portion of lesions, our study showed noticeable brain tumor displacement with typical patient setup in CT and MRI scans when using skull based fusion in comparison with soft-tissue fusion. Careful review of the skull based fusion is recommended by examining the match with nearby soft-tissue and/or tumors. If fusion deviations are found, it is also recommended to consider adding a margin to the GTV to account for such variations, since such variations could potentially affect target localization accuracy at the time of treatment.