Intracranial Stereotactic Radiotherapy Research Articles

Initial Experience With Volumetric IMRT (RapidArc) for Intracranial Stereotactic Radiosurgery

Initial experience with delivering frameless stereotactic radiotherapy (SRT) using volumetric intensity-modulated radiation therapy (IMRT) delivered with RapidArc is presented. Treatment details for 12 patients (14 targets) with a mean clinical target volume (CTV) of 12.8 ± 4.0 cm(3) were examined. Dosimetric indices for conformality, homogeneity, and dose gradient were calculated and compared with published results for other frameless, intracranial SRT techniques, including CyberKnife, TomoTherapy, and static-beam IMRT. Statistics on setup and treatment times and per patient dose validations were examined. Dose indices compared favorably with other techniques. Mean conformality, gradient, and homogeneity index values were 1.10 ± 0.11, 64.9 ± 14.1, 1.083 ± 0.026, respectively. Median treatment times were 4.8 ± 1.7 min. SRT using volumetric IMRT is a viable alternative to other techniques and enables short treatment times. This is anticipated to have a positive impact on radiobiological effect and for facilitating wider use of SRT.

EVALUATION OF ROUTINE MRI FROM REFERRING HOSPITALS FOR INTRACRANIAL STEREOTACTIC RADIOTHERAPY; CT-MR REGISTRATION ERRORS IN CMS FOCAL

Purpose: The aim was to evaluate the possible use of routine MR brainscans from five referring hospitals in image registration for intracranial stereotactic radiotherapy planning. Registration errors and the chemical shifts were accessed. If the quality of the registration is sufficient additional MR scans for radiotherapy planning can be avoided Materials: Five referring hospitals participated in this study. A CosmanRobert-Wells frame based Radionics head phantom with four geometric structures was sent to each radiology department. Each department was asked to scan the phantom according to their standard brain protocol (under suspicion of brain metastasis). From two departments extra scans were requested with smaller slice thickness. The axial scans were imported in Focal (CMS ELEKTA) and registered with a reference CT and with a 13 degrees rotated CT. On each scan eight different points were identified and four structures were delineated from which the centre of mass was determined. Results: All but one hospital sent in 5 (or 5.5 mm) mm scans as a standard. For these scans registration errors were up to 5 mm. This improved with smaller slice thickness. (Mean and max: 0.8 and 1.3 mm for the 2 and 1.6 mm slices ; 1.0 and 1.7 mm for the 2.5 mm slices). T1 spin echo and T2 spin echo gave similar results. Also results in COM of structures and coordinates of individual points yielded comparable results. Other sequences showed (visual!) deformations. Registration errors increased with increasing difference in scan angle between the CT and the MRI. Increasing rotation from 1 to 13 degrees increased the registration error from 0.8 to 2.0 mm Conclusions: The standard MRI protocols of 5 mm slice thickness are not adequate for intracranial stereotactic radiotherapy. Up to 2.5 mm scans are acceptable for target definition. Angulation differences between CT and MRI of more than 10 degrees should be avoided. Only T1 SE and T2 SE can be used for coregistration.AcknowledgementsThe depts of Neurosurgery and Radiotherapy of UZ Gasthuisberg Leuven are acknowledged for kindly providing us with their CRW-frame based Radionics head phantom. Participating hospitals were: St. Elisabeth hospital, TweeSteden hospital, Jeroen Bosch hospital, Amphia hospital, and St. Franciscus hospital. Reference(1) Carter, et al. Accuracy of magnetic resonance imaging stereotactic coordinates with the Cosman-Roberts-Wells frame. Stereotact Funct Neurosurg 1999;72:3546.

SU-FF-T-535: Effects of Peripheral Dose Fall-Off On Biologically Equivalent Dose to Normal Brain for Intracranial Stereotactic Radiosurgery and Radiotherapy

Purpose: The peripheral dose volume such as 10‐Gy or 12‐Gy volume has been reported to correlate with incidence of radionecrosis for stereotactic radiosurgery(SRS) or radiotherapy (SRT). In this study, we investigated whether variations in peripheral dose fall‐off of individual treatments would significantly affect normal tissue sparing for SRS or SRT. Method and Materials: A power relationship for measuring general dose fall‐off near the target was developed for both isocentric and non‐isocentric intracranial radiosurgery modalities. For the study, we sampled the ranges of variations in the peripheral dose distributions from patients treated with Gamma Knife Perfexion, Cyberknife, or linac‐based delivery system. Equivalent uniform biological effective dose (EUBED) for the normal braintissue was formulated to study the effect of dose‐fall variations on treatments of different targets of varying a/b values. Functional relationship of normal brain EUBED with increasing number of fractions was derived for treating either fast growing tumors with a/b of 10–20) or abnormal tissues such as AVM with possible a/b of 2–5. Results: The average g‐index from the derived power formula was found to be −1.49±0.25 with a range of −1.19 to −1.79. Based on such range, EUBED of the normal brain was calculated and found to decrease with increasing number of fractions for targets with a/b of 10–20. This decrease was most pronounced for fractions fewer than 10. However, the EUBED was found to slightly increase with increasing number of fractions for targets with a/b of 2–5. Conclusion: In delivering SRS or SRT, normal tissue EUBED was found to favor hypofractionated treatments for fast growing tumors with a/b of 10–20, but single fraction treatments for abnormal tissues with a/b of 2–5. The result was found to be insensitive to variations in dose fall‐off from individual cases regardless being treated with Gamma Knife, Cyberknife or Linac‐based modality.

SU-FF-T-532: Immobilization Accuracy of a Novel Re-Locatable Head Frame Investigated with a Real-Time Optical Tracking System

Purpose: To evaluate the immobilization accuracy of a novel vacuum bite block re‐locatable head frame (RHF) system in patients receiving intracranial stereotactic radiation therapy (SRT) by means of an infrared optical tracking system (OTS). Methods and Materials: The RHF was designed for multi‐session treatments on the Leksell GammaKnife Perfexion™. In the present REB approved study, the RHF was used for patients undergoing linac‐based SRT. A passive optical marker was placed on the bite block as a surrogate for patient motion. The OTS monitored patients continuously throughout each fraction. For each fraction, cone‐beam computed tomography(CBCT) was used to verify the patient position at three time points: after the initial setup, pre‐delivery following the automatic couch adjustments and post‐delivery. To investigate the reliability of the optical measurements, the OTS measurements were compared against the implemented couch shifts for patient setup. Intra‐fractional anatomical motion, as measured from the second and third CBCT scans, was compared with the difference in OTS marker positions. Real‐time OTS data throughout each fraction was assessed to determine if patients moved by more than 1 mm during treatments.Results: Currently, data from three patients have been analyzed. The couch motion measured from the OTS strongly correlated with the actual reported couch shift (Pearson correlation coefficient of 0.99). Mean differences between OTS and true couch motion were 0.01, 0.0, and 0.04 mm in the lateral, anterior‐posterior, and superior‐inferior directions, respectively. Mean differences between OTS and CBCT reported motion were 0.37, 0.07, and 0.01 mm respectively with a mean vector magnitude of 0.41 mm. The real‐time positional data indicated that there were few instances (3) of patient motion greater than 1mm. Conclusion: The OTS is a reliable tool for evaluation of intra‐fraction patient motion. The results of this study indicate that the RHF provides sufficient immobilization accuracy for SRT.

Evidence-based Treatment Margin for Intracranial Stereotactic Radiotherapy

Evidence-based Treatment Margin for Intracranial Stereotactic Radiotherapy

Cone Beam CT Image Guidance for Intracranial Stereotactic Treatments: Comparison With a Frame Guided Set-Up

An analysis is performed of the setup errors measured by a kV cone beam computed tomography (CBCT) for intracranial stereotactic radiotherapy (SRT) patients immobilized by a thermoplastic mask and a bite-block and positioned using stereotactic coordinates. We evaluated the overall positioning precision and accuracy of the immobilizing and localizing systems. The potential of image-guided radiotherapy to replace stereotactic methods is discussed. Fifty-seven patients received brain SRT. After a frame-guided setup, before each fraction (131 fractions), a CBCT was acquired and the detected displacements corrected online. Translational and rotational errors were analyzed calculating overall mean and standard deviation. A separate analysis was performed for bite-block (in conjunction with mask) and for simple thermoplastic mask. Interobserver variability for CBCT three-dimensional registration was assessed. The residual error after correction and intrafractional motion were calculated. The mean module of the three-dimensional displacement vector was 3.0 +/- 1.4 mm. Setup errors for bite block and mask were smaller (2.9 +/- 1.3 mm) than those for thermoplastic mask alone (3.2 +/- 1.5 mm), but statistical significance was not reached (p = 0.15). Interobserver variability was negligible. The maximum margin calculated for residual errors and intra fraction motion was small but not negligible (1.57 mm). Considering the detected setup errors, daily image guidance is essential for the efficacy of SRT treatments when mask immobilization is used, and even when a bite-block is used in conjunction. The frame setup is still used as a starting point for the opportunity of rotational corrections. Residual margins after on-line corrections must be evaluated.

Improvements in Patient Alignment With Stereoscopic kVp Image Guidance During Linac-Based Intracranial Stereotactic Radiotherapy–A Prospective Study

Improvements in Patient Alignment With Stereoscopic kVp Image Guidance During Linac-Based Intracranial Stereotactic Radiotherapy–A Prospective Study

Cone Beam CT Image Guidance for Intra-Cranial Stereotactic Treatments: Comparison With a Frame Guided Set-up

High precision in target localization and treatment delivery is essential for effective intracranial stereotactic radiotherapy and radiosurgery. Patients are rigidly fixed to a stereotactic frame using both invasive methods or relocatable systems (thermoplastic masks and/or bite block immobilization). In this work, the accuracy of an immobilization and localization system, both for single and fractionated treatment courses, is assessed evaluating the set-up errors obtained by a cone beam CT prior to each treatment session. 43 patients with brain lesions were immobilized, by means of a thermoplastic mask and a bite block system, in a relocatable frame for stereotactic coordinate definition (3Dline). A CT scan was acquired for planning and the isocenter coordinates generated. Treatment was delivered using non-coplanar arcs (5–10) shaped with a dynamic mMLC (5 mm leaf width at isocenter) attached to an Elekta Synergy linac. The localization frame was used for isocenter set-up at the linac. Prior to each fraction (for a total of 95 fractions), a kV CBCT was acquired and registered with the reference planning CT aligning bony structures and air cavities: the displacements obtained respect to the frame isocenter were corrected on-line. We analyzed isocenter translations in all three dimensions, the absolute magnitude of isocenter dislocation and rotations about the three axes; for fractionated treatment courses systematic and random errors are estimated. For 3 patients (9 fractions) a post-treatment CBCT was acquired to estimate residual errors. Doses ranged from 22 to 30 Gy (1–4 fractions) at isocenter. The mean and median values of the set-up errors for all patients and all fractions are below 1 mm for the 3 axes. The standard deviations are 1.1 mm, 2.6 mm and 1.9 mm respectively for the lateral (X), caudo-cranial (Y) and anterior-posterior (Z) displacements. The mean and standard deviation for the magnitude of the sum vector are respectively 3.2 mm and 1.4 mm. The percentage of the observed set-up errors below 3 mm are 99%, 73%, and 87% along X, Y and Z respectively. Moreover, 90% of the observed total displacements (sum vector) is within 5 mm. Averaged values of rotational errors about X, Y and Z are all below 1 degree and the largest standard deviation is observed for rotations about the lateral axis (X) (1.5 degree). The SDs of the distributions of systematic errors in the patient group (Σ) along X, Y and Z are respectively 0.5, 1.0 and 0.9 mm; the corresponding average SDs of the distributions of the random errors (σ) are 0.95, 1.6 and 1.3 mm. Using the largest observed value of Σ and of σ and one of the recipes for determining CTV to PTV margin size (2.5Σ+0.7σ) a margin of 3.6 mm is obtained. Intra-fraction translation displacements were always inferior to 1 mm except one case (maximum 1.4 mm). The considered immobilization and localization devices provide an accurate and reproducible patient positioning. The larger errors observed along the caudo-cranial direction confirm that image guidance by CBCT performed at each fraction can be very useful for intracranial stereotactic treatments, reducing the necessity of CTV to PTV margins and making a frameless set-up possible. However, the isocenter position indicated by the frame is a good starting point, especially to avoid large rotational errors which are more difficult to correct.

Intracranial and extracranial stereotactic radiotherapy and radiosurgery

Intracranial and extracranial stereotactic radiotherapy and radiosurgery

1435 ORAL Continence and quality of life after salvage techniques to avoid colostomy: coloanal anastomosis versus perineal colostomy, in cases of very low rectal cancer

We evaluated coverage, dose homogeneity, dose conformity, and dose gradient in CyberKnife VSI treatment plans. Several dosimetric indices were calculated, and the results were compared with those of previous publications. The effect of target volume on the radiosurgical treatment indices selected was also investigated. The study population comprised the first 40 patients treated at our department from March 2011 to September 2012. Dosimetric indices were calculated and compared with published results for other frame-based and frameless intracranial stereotactic radiotherapy techniques. A comparison of the indices confirmed the ability of the CyberKnife VSI system to provide very high-quality dosing plans. The results were independent of target volume for coverage, homogeneity, and dose conformity. However, a dependence on target volume was observed for the dose-gradient indices analyzed. Based on the indices proposed, CyberKnife provides very good treatment plans and compares favorably with other techniques in most cases. However, greater consensus on the radiosurgery indices calculated would be desirable to facilitate comparison of the various techniques or the same techniques when applied by different users.

504 Verification and accuracy of doses during intracranial conformal stereotactic radiotherapy

504 Verification and accuracy of doses during intracranial conformal stereotactic radiotherapy

Stereotactic Body Radiation Therapy

Stereotactic body radiation therapy constitutes an emerging therapeutic paradigm. These treatments are unique relative to the large body of experience with conventional fractionated radiotherapy. On the basis of the treatment principles of intracranial stereotactic radiation combined with enhancements associated with immobilization and imaging, the role of extracranial treatment continues to evolve. However, most clinical reports on extracranial treatments suffer from short or incomplete follow-up, making final assessments of benefit and toxicity, particularly late toxicity, problematic. These techniques are centered on a very basic understanding of the use of ionizing radiation for the treatment of cancer. Nonetheless, they do require a unique and special understanding of radiobiologic and physics principles. It is hoped that using high-dose, single-fraction treatment or a few fractions of treatment, the therapeutic ratio is improved, thus potentially changing the way some cancers are treated. Ideally, all patients receiving such treatments would be enrolled in formal protocols. As data accrue and understanding of these techniques improve, it will be possible to better define the indications for stereotactic body radiation therapy. At that time, appropriate applications can be submitted for permanent billing codes that will describe a process of care that embraces this technology without vendor favoritism. This review summarizes the state of stereotactic body radiation in 2005.

Comparison of a micro-multileaf collimator with a 5-mm-leaf-width collimator for intracranial stereotactic radiotherapy

Purpose To dosimetrically compare a micro-multileaf collimator (minimum leaf width of 3 mm) with the 5-mm-leaf multileaf collimator (MLC) of a standard linear accelerator for stereotactic conformal radiotherapy treatment of intracranial lesions. Methods and materials Fourteen patients previously treated for a variety of irregularly shaped intracranial lesions using BrainLAB's micro-MLC were retrospectively replanned using the Varian Millennium MLC (5 mm leaf width). All planning was performed with the BrainSCAN v 5.1 software. The same fixed, noncoplanar beam arrangement was used for both plans, and identical target coverage was achieved by adjusting the MLC shape around the planning target volume (PTV). The isodose distributions and dose–volume histograms (DVH) were computed and plans were compared in terms of conformity of the prescription isodose to the PTV and dose received by surrounding normal tissue. Results Equivalent PTV coverage was achieved using the 5-mm collimator by adjusting the MLC shape around the target in every case. There was a statistically significant increase in the conformity index for the Varian MLC compared with the micro-MLC ( p < 0.001), indicating a worse conformity of the prescription isodose to the PTV, but this parameter was within our (and Radiation Therapy Oncology Group) clinical criterion in all cases. There was no statistically significant difference in the maximum dose to critical structures, but DVH curves demonstrated an increased volume of normal tissue irradiated to the lower isodose levels. The mean increase in the volume of critical structure enclosed within the 50% and 70% isodose surfaces was 5.7% and 4.9%, respectively. Conclusions The micro-MLC consistently improves both PTV conformity and surrounding tissue sparing when compared to that of a standard linear accelerator. However, when viewed quantitatively, the improvements are small enough that individual centers may question their choice of equipment when outfitting a stereotactic radiotherapy service.

129 oral Comparison of a micro-multileaf collimator with a 5 mm leaf width collimator for intracranial conformal stereotactic radiotherapy

129 oral Comparison of a micro-multileaf collimator with a 5 mm leaf width collimator for intracranial conformal stereotactic radiotherapy

Investigations of a minimally invasive method for treatment of spinal malignancies with LINAC stereotactic radiation therapy: accuracy and animal studies

Purpose: A new method for stereotactic irradiation of spinal malignancies is presented, with evaluations of the theoretic and practical limitations of localization accuracy and the implementation of the method in swine.Methods and Materials: In a percutaneous procedure, a minimum of three small (1.7-mm-diameter) titanium markers are permanently affixed to a vertebra. Markers are localized on biplanar radiographs while isocenter positions are determined on CT. An external fiducial frame defines a three-dimensional coordinate system through the patient. Radiographs coupled with a rigid body rotation algorithm account for daily differences in patient position. Phantom studies were used to verify theoretic uncertainty calculations from a simulation program. A swine model was used to evaluate the difficulty and duration of the implant technique, the suitability of the vertebral process as an implant site, vertebral motion due to normal respiration, and the ability to target one vertebra with markers in an adjacent vertebra.Results: Theoretic accuracy studies confirmed that localization accuracy is a function of marker separation. Phantom studies involving 296 measurements showed that individual implants could be localized within ±0.25 mm. The largest targeting error observed in 3,600 measurements of 100 implant configurations was 1.17 mm. The implant procedure took 5–10 minutes per site. No significant migration of implants was observed up to 35 days postimplantation, and respiratory motion had no detectable influence on vertebral position. Adjacent vertebrae may be useful for targeting one another with a small sacrifice in localization accuracy.Conclusions: The use of implanted markers for localization of spinal malignancies has potential for applications in stereotactic radiotherapy. Phantom measurements suggest that localization accuracy similar to intracranial stereotactic radiotherapy techniques is achievable. Swine studies suggest that the implant technique is expedient and feasible for tumor targeting purposes.