Florin Rosca

A clinical comparison of patient setup and intra-fraction motion using frame-based radiosurgery versus a frameless image-guided radiosurgery system for intracranial lesions

BACKGROUND AND PURPOSEnA comparison of patient positioning and intra-fraction motion using invasive frame-based radiosurgery with a frameless X-ray image-guided system utilizing a thermoplastic mask for immobilization.nnnMATERIALS AND METHODSnOverall system accuracy was determined using 57 hidden-target tests. Positioning agreement between invasive frame-based setup and image-guided (IG) setup, and intra-fraction displacement, was evaluated for 102 frame-based SRS treatments. Pre and post-treatment imaging was also acquired for 7 patients (110 treatments) immobilized with an aquaplast mask receiving fractionated IG treatment.nnnRESULTSnThe hidden-target tests demonstrated a mean error magnitude of 0.7mm (SD=0.3mm). For SRS treatments, mean deviation between frame-based and image-guided initial positioning was 1.0mm (SD=0.5mm). Fusion failures were observed among 3 patients resulting in aberrant predicted shifts. The image-guidance system detected frame slippage in one case. The mean intra-fraction shift magnitude observed for the BRW frame was 0.4mm (SD=0.3mm) compared to 0.7mm (SD=0.5mm) for the fractionated patients with the mask system.nnnCONCLUSIONSnThe overall system accuracy is similar to that reported for invasive frame-based SRS. The intra-fraction motion was larger with mask-immobilization, but remains within a range appropriate for stereotactic treatment. These results support clinical implementation of frameless radiosurgery using the Novalis Body Exac-Trac system.

An MLC-based linac QA procedure for the characterization of radiation isocenter and room lasers' position

We have designed and implemented a new stereotactic linac QA test with stereotactic precision. The test is used to characterize gantry sag, couch wobble, cone placement, MLC offsets, and room lasers positions relative to the radiation isocenter. Two MLC star patterns, a cone pattern, and the laser line patterns are recorded on the same imaging medium. Phosphor plates are used as imaging medium due to their sensitivity to red light. The red light of room lasers erases some of the irradiation information stored on the phosphor plates enabling accurate and direct measurements for the position of room lasers and radiation isocenter. Using film instead of the phosphor plate as imaging medium is possible, however, it is less practical. The QA method consists of irradiating four phosphor plates that record the gantry sag between the 0 degrees and 180 degrees gantry angles, the position and stability of couch rotational axis, the sag between the 90 degrees and 270 degrees gantry angles, the accuracy of cone placement on the collimator, the MLC offsets from the collimator rotational axis, and the position of laser lines relative to the radiation isocenter. The estimated accuracy of the method is +/- 0.2 mm. The observed reproducibility of the method is about +/- 0.1 mm. The total irradiation/ illumination time is about 10 min per image. Data analysis, including the phosphor plate scanning, takes less than 5 min for each image. The method characterizes the radiation isocenter geometry with the high accuracy required for the stereotactic radiosurgery. In this respect, it is similar to the standard ball test for stereotactic machines. However, due to the usage of the MLC instead of the cross-hair/ball, it does not depend on the cross-hair/ball placement errors with respect to the lasers and it provides more information on the mechanical integrity of the linac/couch/laser system. Alternatively, it can be used as a highly accurate QA procedure for the nonstereotactic machines. Noteworthy is its ability to characterize the MLC position accuracy, which is an important factor in IMRT delivery.

Determination of depth and field size dependence of multileaf collimator transmission in intensity-modulated radiation therapy beams

Intensity‐modulated radiation therapy (IMRT) plans for the treatment of large and complex volumes may contain a relatively large contribution from multileaf collimator (MLC) transmission. In such cases, comprehensive characterization of direct and scatter MLC transmission is important. We designed a set of tests (open beam, closed static MLC, and dynamic MLC gap) to determine dosimetric MLC properties as a function of field size and depth at the central axis. We developed a generalized model of MLC transmission to account for direct MLC transmission, MLC scatter, beam hardening, and leaf‐end transmission (dosimetric gap). The model is consistent with the beam model used in IMRT optimization. We tested the model for extreme asymmetric fields relevant for large targets and for split IMRT fields. We applied our MLC scatter estimation formula to clinically relevant cases and showed that MLC scatter is contributing an undesired background dose. This contribution is relatively large, especially in low‐dose regions. (For instance, a uniform extra dose may dramatically increase normal‐lung toxicity in thorax treatment.) For complex IMRT of large‐volume targets, we found direct MLC transmission dose to be as high as 30%, and MLC scatter, up to 10% within the target volume for the selected cases. We identified that the dose discrepancies between the IMRT planning system [Eclipse (Varian Medical Systems, Palo Alto, CA)] and ionization chamber measurements (inside and outside of the field) are attributable to an inadequate model of MLC transmission in the planning system (constant‐value model). In the present study, we measured MLC transmission properties for Varian 6EX (6 MV) and 21EXs (6 and 10 MV) linear accelerators; however, the experimental method and theoretical model are more general. PACS number: 87.53.‐j

Spatial dependence of MLC transmission in IMRT delivery

In complex intensity-modulated radiation therapy cases, a considerable amount of the total dose may be delivered through closed leaves. In such cases an accurate knowledge of spatial characteristics of multileaf collimator (MLC) transmission is crucial, especially for the treatment of large targets with split fields. Measurements with an ionization chamber, radiographic films (EDR2, EBT) and EPID are taken to characterize all relevant effects related to MLC transmission for various field sizes and depths. Here we present a phenomenological model to describe MLC transmission, whereby the main focus is the off-axis decrease of transmission for symmetric and asymmetric fields as well as on effects due to the tongue and groove design of the leaves, such as interleaf transmission and the tongue and groove effect. Data obtained with the four different methods are presented, and the utility of each measurement method to determine the necessary model parameters is discussed. With the developed model, it is possible to predict the relevant MLC effects at any point in the phantom for arbitrary jaw settings and depths.

An independent dose calculation algorithm for MLC-based radiotherapy including the spatial dependence of MLC transmission

An analytical dose calculation algorithm was developed and commissioned to calculate dose delivered with both static and dynamic multileaf collimator (MLC) in a homogenous phantom. The algorithm is general; however, it was designed specifically to accurately model dose for large and complex IMRT fields. For such fields the delivered dose may have a considerable contribution from MLC transmission, which is dependent upon spatial considerations. Specifically, the algorithm models different MLC effects, such as interleaf transmission, the tongue-and-groove effect, rounded leaf ends, MLC scatter, beam hardening and divergence of the beam, which results in a gradual MLC transmission fall-off with increasing off-axis distance. The calculated dose distributions were compared to measured dose using different methods (film, ionization chamber array, single ionization chamber), and the differences among the treatment planning system, the measurements and the developed algorithm were analysed for static MLC and dynamic IMRT fields. It was found that the calculated dose from the developed algorithm agrees very well with the measurements (mostly within 1.5%) and that a constant value for MLC transmission is insufficient to accurately predict dose for large targets and complex IMRT plans with many monitor units.

A hybrid electron and photon IMRT planning technique that lowers normal tissue integral patient dose using standard hardware

PURPOSEnTo present a mixed electron and photon IMRT planning technique using electron beams with an energy range of 6-22 MeV and standard hardware that minimizes integral dose to patients for targets as deep as 7.5 cm.nnnMETHODSnTen brain cases, two lung, a thyroid, an abdominal, and a parotid case were planned using two planning techniques: a photon-only IMRT (IMRT) versus a mixed modality treatment (E+IMRT) that includes an enface electron beam and a photon IMRT portion that ensures a uniform target coverage. The electron beam is delivered using a regular cutout placed in an electron cone. The electron energy was chosen to provide a good trade-off between minimizing integral dose and generating a uniform, deliverable plan. The authors choose electron energies that cover the deepest part of PTV with the 65%-70% isodose line. The normal tissue integral dose, the dose for ring structures around the PTV, and the volumes of the 75%, 50%, and 25% isosurfaces were used to compare the dose distributions generated by the two planning techniques.nnnRESULTSnThe normal tissue integral dose was lowered by about 20% by the E+IMRT plans compared to the photon-only IMRT ones for most studied cases. With the exception of lungs, the dose reduction associated to the E+IMRT plans was more pronounced further away from the target. The average dose ratio delivered to the 0-2 cm and the 2-4 cm ring structures for brain patients for the two planning techniques were 89.6% and 70.8%, respectively. The enhanced dose sparing away from the target for the brain patients can also be observed in the ratio of the 75%, 50%, and 25% isodose line volumes for the two techniques, which decreases from 85.5% to 72.6% and further to 65.1%, respectively. For lungs, the lateral electron beams used in the E+IMRT plans were perpendicular to the mostly anterior/posterior photon beams, generating much more conformal plans.nnnCONCLUSIONSnThe authors proved that even using the existing electron delivery hardware, a mixed electron/photon planning technique (E+IMRT) can decrease the normal tissue integral dose compared to a photon-only IMRT plan. Different planning approaches can be enabled by the use of an electron beam directed toward organs at risk distal to the target, which are still spared due the rapid dose fall-off of the electron beam. Examples of such cases are the lateral electron beams in the thoracic region that do not irradiate the heart and contralateral lung, electron beams pointed toward kidneys in the abdominal region, or beams treating brain lesions pointed toward the brainstem or optical apparatus. For brain, electron vertex beams can also be used without irradiating the whole body. Since radiation retreatments become more and more common, minimizing the normal tissue integral dose and the dose delivered to tissues surrounding the target, as enabled by E+IMRT type techniques, should receive more attention.

An EPID response calculation algorithm using spatial beam characteristics of primary, head scattered and MLC transmitted radiation

We have developed an independent algorithm for the prediction of electronic portal imaging device (EPID) response. The algorithm uses a set of images [open beam, closed multileaf collimator (MLC), various fence and modified sweeping gap patterns] to separately characterize the primary and head-scatter contributions to EPID response. It also characterizes the relevant dosimetric properties of the MLC: Transmission, dosimetric gap, MLC scatter [P. Zygmansky et al., J. Appl. Clin. Med. Phys. 8(4) (2007)], inter-leaf leakage, and tongue and groove [F. Lorenz et al., Phys. Med. Biol. 52, 5985-5999 (2007)]. The primary radiation is modeled with a single Gaussian distribution defined at the target position, while the head-scatter radiation is modeled with a triple Gaussian distribution defined downstream of the target. The distances between the target and the head-scatter source, jaws, and MLC are model parameters. The scatter associated with the EPID is implicit in the model. Open beam images are predicted to within 1% of the maximum value across the image. Other MLC test patterns and intensity-modulated radiation therapy fluences are predicted to within 1.5% of the maximum value. The presented method was applied to the Varian aS500 EPID but is designed to work with any planar detector with sufficient spatial resolution.

SU‐FF‐T‐407: Testing the Accuracy and Usefulness of the Portal Vision Dosimetry System for Large‐Volume and Complex‐Geometry IMRT

Purpose: To test a commercial EPIDdosimetry system for the accuracy and usefulness for head&neck, whole‐pelvis, mesothelioma IMRT QA. Materials and Methods: Portal Vision (PV) dosimetry system was configured and experimentally used as intended by the manufacturer (Varian) for 6MV 2100Ex. Additional data analysis software was developed. Collimator angle=90deg and the smallest possible SDD=105cm and were selected to maximize the functional area of the EPID.EPID responses were calculated by Eclipse and compared to the experimentally determined responses in two ways: by comparing individual images and 3D‐response reconstructions for cumulative plans (home‐built software). To account for the PV arm sag during gantry rotation, and the need to shift the detector, raw PV images were automatically magnified and registered with calculated images. 3D‐response reconstructions for the measured and calculated images were performed by: backprojecting the images and applying attenuation and phantom scatter in a homogeneous virtual patient. Patient beam configuration and depths to isocenter were used. Results: PV dosimetry for large/complex targets is difficult and time‐consuming due to practical limitations (detector size, arm sag, manual shifting). Experimental response images show strong tongue&groove effects and elevated values outside of the field edges compared to Eclipse. Response discrepancies inside treatment fields cause (±2–3%) discrepancies in cumulative plan. Discrepancies outside of field edges cause systematic shift up to (5–7%) in cumulative plan, because fluences are split into 2–3 narrower subfields in the delivery. The observed discrepancies are consistent with but stronger than ion chamber measurements in solid water. The reason may be the small 1cm‐buildup and therefore larger PV sensitivity to MLC scatter and T&G. Conclusions: Neither MLC scatter nor T&G are modeled in Eclipse. Their contributions may be significant for large/complex IMRT due to the increased MLC blockage. PV dosimetry may capture these effects, but caution is indicated in interpretation.

SU-FF-T-75: Accuracy Assessment of a Non-Invasive Image Guided System for Intra-Cranial Linac Based Stereotactic Radiosurgery

Purpose: To assess the total system accuracy of a non‐invasive image guided technique for intra‐cranial radiosurgery.Method and Materials: To test the system three fiducials were placed in a Rando head phantom in cerebellar, mid‐brain and lateral‐frontal locations. A stereotactic mask system by Brainlab was used for immobilization. Positioning was based on the Novalis Body system consisting of two kV x‐ray cameras with amorphous silicondetectors and an IR tracking system calibrated to the treatment isocenter. CT scans were acquired using a slice thickness of 1.25 mm. For each scan the phantom was positioned in the immobilizer and an attachment with IR‐reflective spheres was added. Using the Brainscan 5.31 treatment planningsystem, the IR markers were localized and an isocenter placed on each fiducial. Plan information was exported to the Novalis Body computer. The phantom was initially positioned using the IR tracking system. X‐ray images were obtained and an automatic 6‐D bony fusion performed. Shifts calculated by the fusion were performed under guidance of the IR system. Port films were taken and the deviation between the center of the fiducial and the treatment isocenter was evaluated. Results: A total of 57 phantom setups were performed (19 for each anatomical location). The measured mean total system error magnitude was 0.73 mm with a standard deviation of 0.29 mm. The positioning accuracy for the lateral frontal fiducial was found to be slightly inferior to the other two with a mean error magnitude of 0.96 mm compared to 0.60 mm and 0.64 mm for cerebellar and mid‐brain respectively. Conclusion: In all cases the radiosurgery accuracy requirements specified in AAPM Report #54 were met or exceeded. This system provides equivalent accuracy to conventional invasive frame based radiosurgery and can substantially improve both patient comfort and treatment planning workflow.

SU‐FF‐T‐159: Determination of the MLC Scatter, MLC Transmission and Dosimetric Gap in Dynamic IMRT as a Function of Field Size, Depth and Beam Energy

Purpose: To develop a dynamic MLC test for the determination of the MLC scatter, transmission and dosimetric gap for large field size and complex geometry IMRT.Materials and Methods: A series of dynamic MLC tests was designed and performed with ionization chamber in a solid water phantom as a function of field size, depth, MLC gap size for 6,10MV 2100Ex: open beam (OB), closed MLC (cMLC), and dynamic sweeping gap (dMLCgap). Based on a generalized fluence model, MLC scatter, direct MLC transmission (no scatter) and dosimetric gap (due to rounded leaves) were determined. IMRT planning system predictions and measurement doses were compared at the central axis and outside of the field edges. Dose errors were corrected using the generalized fluence model. Results:MLC scatter is responsible for field size dependence of cMLC‐to‐OB dose ratio (1.45% for size=5cm×5cm, 1.8% for size=14cm×30cm, 6MV). MLC scatter is rather uniform within and outside of the field edges, and decreases only slightly with depth. Direct MLC transmission (no MLC scatter) changes with depth up to 10% (6MV), 5% (10MV) due to beam hardening. In dynamic MLC delivery, MLC scatter is significant for large field sizes (14cm×30cm) and low average fluence 〈φdMLC〉: MLC scatter=1%–5% for clinically realistic 〈φdMLC〉 =30%–10%, (〈φOB〉=100%, 〈φcMLC〉 ≈1.5% by definition). Dose errors of 1%–8% for large sizes and sweeping gaps 1.0cm–0.1cm were corrected when the modified fluence model (including MLC scatter) was used instead of the Eclipse fluence. Conclusions: Many commercial IMRT planning systems do not account for the MLC scatter. It is suggested that the OB‐cMLCd‐MLCgap test be used for commissioning of the IMRT when field sizes are large and average fluence 〈φdMLC〉 is low. MLC parameters in IMRT planning system may need to be adjusted separately for each IMRT class depending on field size and fluence complexity (or 〈φdMLC〉).