S Koren

4D patient dose reconstruction using online measured EPID cine images for lung SBRT treatment validation

PURPOSE This study aims to develop an EPID-guided 4D patient dose reconstruction framework and to investigate its feasibility for lung SBRT treatment validation. METHODS Both the beam apertures and tumor movements were detected based on the continuously acquired EPID images during the treatment. Instead of directly using the transit photon fluence measured by the EPID, this method reconstructed the entrance fluence with the measured beam apertures and the delivered MUs. The entrance fluence distributions were sorted into their corresponding phases based on the detected tumor motion pattern and then accumulated for each phase. Together with the in-room 4DCT taken before every treatment to consider the interfractional-motion, the entrance fluence was then used for the patient dose calculation. Deformable registration was performed to sum up the phase doses for final treatment assessment. The feasibility of using the transit EPID images for entrance fluence reconstruction was evaluated against EPID in-air measurements. The accuracy of 3D- and 4D-dose reconstruction was validated by experiments with a motor-driven cylindrical diode array for six clinical-SBRT plans. RESULTS The average difference between the measured and reconstructed fluence maps was within 0.16%. The reconstructed 3D-dose showed a less than 1.4% difference for the CAX-dose and at least a 98.3% gamma-passing-rate (2%∕2 mm) for the peripheral dose. Distorted dose distributions were observed in the measurement with the moving phantom. The comparison between the measured and the reconstructed 4D-dose without considering temporal information failed the gamma-evaluation for most cases. In contrast, when temporal information was considered, the dose distortion phenomena were successfully represented in the reconstructed dose (97.6%-99.7% gamma-passing rate). CONCLUSIONS The proposed method considered uncertainties of the beam delivery system, the interfractional- and intrafractional-motion, and the interplay effect. The experimental validation demonstrates that this method is practical and accurate for online or offline SBRT patient dose verification.

Measurement comparison and Monte Carlo analysis for volumetric-modulated arc therapy (VMAT) delivery verification using the ArcCHECK dosimetry system

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric‐modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in‐house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10×10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10×10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (<1.2% drop) for the 2%–3% dose and 2 mm–3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3 mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation based on the sensitivity to detecting misalignments, the clinical impact of the angular dependency, and the correlation between the dose agreements in the peripheral region and the central region. This work also demonstrated the importance of carrying out a thorough validation of both the TPS and the dosimetry system prior to utilizing it for QA, and the value of having an independent dose calculation tool, such as the MC method, in clinical practice. PACS number: 87.55.Qr

Investigation of pulsed low dose rate radiotherapy using dynamic arc delivery techniques

There has been no consensus standard of care to treat recurrent cancer patients who have previously been irradiated. Pulsed low dose rate (PLDR) external beam radiotherapy has the potential to reduce normal tissue toxicities while still providing significant tumor control for recurrent cancers. This work investigates the dosimetry feasibility of PLDR treatment using dynamic arc delivery techniques. Five treatment sites were investigated in this study including breast, pancreas, prostate, head and neck, and lung. Dynamic arc plans were generated using the Varian Eclipse system and the RapidArc delivery technique with 6 and 10 MV photon beams. Each RapidArc plan consisted of two full arcs and the plan was delivered five times to achieve a daily dose of 200 cGy. The dosimetry requirement was to deliver approximately 20 cGy/arc with a 3 min interval to achieve an effective dose rate of 6.7 cGy min⁻¹. Monte Carlo simulations were performed to calculate the actual dose delivered to the planning target volume (PTV) per arc taking into account beam attenuation/scattering and intensity modulation. The maximum, minimum and mean doses to the PTV were analyzed together with the dose volume histograms and isodose distributions. The dose delivery for the five plans was validated using solid water phantoms inserted with an ionization chamber and film, and a cylindrical detector array. Two intensity-modulated arcs were used to efficiently deliver the PLDR plans that provided conformal dose distributions for treating complex recurrent cancers. For the five treatment sites, the mean PTV dose ranged from 18.9 to 22.6 cGy/arc. For breast, the minimum and maximum PTV dose was 8.3 and 35.2 cGy/arc, respectively. The PTV dose varied between 12.9 and 27.5 cGy/arc for pancreas, 12.6 and 28.3 cGy/arc for prostate, 12.1 and 30.4 cGy/arc for H&N, and 16.2 and 27.6 cGy/arc for lung. Advanced radiation therapy can provide superior target coverage and normal tissue sparing for PLDR reirradiation of recurrent cancers, which can be delivered using dynamic arc delivery techniques with ten full arcs and an effective dose rate of 6.7 ± 4.0 cGy min⁻¹.

Robotic radiosurgery system patient-specific QA for extracranial treatments using the planar ion chamber array and the cylindrical diode array

Robotic radiosurgery system has been increasingly employed for extracranial treatments. This work is aimed to study the feasibility of a cylindrical diode array and a planar ion chamber array for patient‐specific QA with this robotic radiosurgery system and compare their performance. Fiducial markers were implanted in both systems to enable image‐based setup. An in‐house program was developed to postprocess the movie file of the measurements and apply the beam‐by‐beam angular corrections for both systems. The impact of noncoplanar delivery was then assessed by evaluating the angles created by the incident beams with respect to the two detector arrangements and cross‐comparing the planned dose distribution to the measured ones with/without the angular corrections. The sensitivity of detecting the translational (1–3 mm) and the rotational (1°–3°) delivery errors were also evaluated for both systems. Six extracranial patient plans (PTV 7–137 cm3) were measured with these two systems and compared with the calculated doses. The plan dose distributions were calculated with ray‐tracing and the Monte Carlo (MC) method, respectively. With 0.8 by 0.8 mm2 diodes, the output factors measured with the cylindrical diode array agree better with the commissioning data. The maximum angular correction for a given beam is 8.2% for the planar ion chamber array and 2.4% for the cylindrical diode array. The two systems demonstrate a comparable sensitivity of detecting the translational targeting errors, while the cylindrical diode array is more sensitive to the rotational targeting error. The MC method is necessary for dose calculations in the cylindrical diode array phantom because the ray‐tracing algorithm fails to handle the high‐Z diodes and the acrylic phantom. For all the patient plans, the cylindrical diode array/ planar ion chamber array demonstrate 100%/>;92%(3%/3 mm) passing rates. The feasibility of using both systems for robotic radiosurgery system patient‐specific QA has been demonstrated. For gamma evaluation, 2%/2 mm criteria for cylindrical diode array and 3%/3 mm criteria for planar ion chamber array are suggested. The customized angular correction is necessary as proven by the improved passing rate, especially with the planar ion chamber array system. PACS number: 29.40.‐n

WE‐A‐BRB‐06: 3D In‐Patient Dose Reconstruction from the PET‐CT Imaging of Y‐90 Microspheres for Metastatic Cancer to the Liver

Purpose: The development of radioembolization with SIR‐Spheres represents a significant advance in the treatment of patients with metastatic disease to the liver. This technique has evolved to use a formula for dose calculation that relies on body surface area as the main determinant of dose. However, what is prescribed is not dose, but rather activity. It has been traditionally thought that the tracer 90Y is a pure electron emitter. However, the decay of 90Y has also a minor s+ branch to the first excited state of 90Zr, with 34 ppm branching ratio. While the positron emission is rare, it can be observed in a PET scan. Therefore the main objective of this work is to develop a new method for 3D dose reconstruction based on the PETimaging of a patient injected with SIR‐Spheres. Methods: Using Fluka Monte Carlo code, the voxel dose kernel (VDK) was calculated for 90Y source of size equal to that of the PET scanner voxel size. Subsequently, the convolution of the VDK with measured PET data was performed to recover the absorbed dose. The absolute dose calibration was done by taking the ratio of the measured positron activity to the known activity due to electrons Results: The developed model was used to reconstruct the dose distribution for two patients treated with 90Y microspheres. The 3D dose calculation was subsequently superimposed on the patients CT information to provide a complete description of the absorbed dose to the liver,tumor and adjacent structures. Conclusions: The proposed method offers significant improvement in characterization of the dose deposited by SIR‐Spheres with the hope that this may help answer some of the clinical questions concerning the influence of the dose distribution on the response rate, progression‐free or overall survival.

SU‐F‐T‐21: A Novel Beam‐Light View, Collimating Applicator for HDR Ocular Conjunctiva Brachytherapy‐Treatments

PURPOSE We propose the use of a HDR X-ray source collimator to apply a conformal, relatively small, radiation suitable for a single fraction with short delivery time. In addition, this technique can be applied using a radioactive source. METHODS We have built a stainless steel 1.5 mm thick applicator, to accommodate the needle applicator of the Intra-Beam X-ray source. Additional cavity is created in the applicator to allow the hosting/nesting/positioning of a LED diode. This LED is allowing a pre-irradiation beam marking on the tissue. The visible light emitted from the opening of the collimated applicator will delineate/verify the aperture of the kV beam to be applied, as well as serve as distance indicator and will assist in the determination of dose to be delivered. For the evaluation of the collimated spatial dose distribution we have performed water tank measurements using (IBA Dosimetry) with a 0.4 cc ion chamber (IBA Dosimetry). We have scanned a two dimensional array with 1mm pitch in depth and 0.3 mm step size laterally. Additional verifications were conducted using Gaf-Chromic film for PDD measurements and Optical Stimulated Luminescence Dosimetry (OSLD, Landauer inc.) for absolute dosimetry. RESULTS The collimated applicator enables a conformal irradiated cross-section of about 3 mm square at the applicator surface was used in this study. A 180 seconds of 50 kVp delivery yielded 29 Gy, 20.6 Gy and 14.5 Gy at 5, 10 and 15 mm depths respectively. These results are in good agreement with the needle applicator depth dose curve published data. CONCLUSION We have demonstrated the feasibility of focal HDR brachytherapy for conjunctival and ocular tumors, using the Intra-Beam needle applicator with in-house developed collimator. The delivery time was found to be several minutes- suitable for an intra-operative procedure and will allow dose fractionation deliveries.

MO‐F‐108‐12: Evaluation of the Planar Ion Chamber and the Cylindrical Diode Arrays for Extracranial Cyberknife Patient Specific QA

PURPOSE The robotic-radiosurgery-system (Cyberknife) has been increasingly employed for extracranial treatments. The recent development of the MLC-equipped - Cyberknife emphasizes this application more, especially for larger treatment volumes. This work is aimed to study the feasibility of the ArcCHECK and the MatriXX systems for CyberKnife patient-specific QA and co mpare their performance. METHODS Fiducial markers were implanted in both systems to enable image-based setup. The output factors measured by them were co mpared with co mmissioning data. The impact of angular dependency on the Cyberknife measurements and the sensitivity of detecting the translational (1-3mm) and the rotational (1-3 degree) delivery errors were also evaluated for both systems. Single-beam-plans, isocentric-plans and ten extracranial Cyberknife patient plans (PTV 5-136cm3 ) were measured and co mpared with calculated QA plans. The plan dose distributions were calculated with Ray-tracing and the Monte Carlo (MC) method, respectively. The gamma-passing-rates with different criteria were evaluated and co mpared. RESULTS With the 0.8×0.8 mm2 diodes, the output factors measured with the ArcCHECK co mpare better with co mmissioning data. The maximum angular-co rrection for a given beam is 8.2% for the MatriXX and 2.4% for the ArcCHECK. Both systems demonstrate a co mparable sensitivity of detecting the translational targeting errors while the ArcCHECK is more sensitive to the rotational targeting error. MC calculations are necessary for the ArcCHECK calculations since the Ray-tracing-algorithm fails to handle the heterogeneity. For all the studied Cyberknife patient plans, the ArcCHECK system demonstrates 100% (3%/3mm) and >96% (2%/2mm) passing-rates. In co ntrast, the MatriXX system demonstrates >92% (3%/3mm) and ∼80% (2%/2mm) passing-rates and the passing-rates decrease with PTV/co ne sizes. Co nclusions: The feasibility of using the ArcCHECK or the MatriXX systems for Cyberknife patient-specific QA has been demonstrated. The gamma-passing-rate is higher for the ArcCHECK system. The custom angular co rrection is necessary as proven by the improved passing-rate, especially with the MatriXX system.

SU-E-T-53: 3D Dose Measurements Using the Planned Dose Perturbation Technique (PDP) for the Evaluation of Head and Neck VMAT Treatment

Purpose: To evaluate the 3D dose distributions using the PDP technique for VMAT H&N treatments. Methods: The novel PDP algorithm uses the patient structures, TPS dose calculation and plan as a base line, then applies the ARC delivery time dependent ArcCheck (Sun Nuclear, Inc.) measurement with the TPS phantom dose to derive patient dose. Five VMAT H&N plans were generated on a Rando phantom with PTV‐to‐skin distances of 0,1,2,3, and 5 mm, using the Eclipse TPS (Varian, Inc.). Treatments were then delivered on a Varian iX linear accelerator. We compared the measured to calculated data by using 3D gamma analysis, and examined the mean and maximum dose of the PTV DVH. Results: By using a recommended 2 mm3 calculation voxel the 3D gamma analysis passed 99.6 to 99.9% for a 3% global dose difference and 3mm DTA with a 5% dose threshold. The PTV organanalysis hot‐to‐cold dose failing point ratio was about 33.8, 21.7 and 22.2, for the 5, 3, and 2 mm PTV‐to‐surface distance respectively. For the 1 mm distance case, the ratio was about 0.45 and for the 0 mm distance the ratio was found to be 0.37. With a PTV‐to‐surface distance decrease, the hot spot was found to increase, and the target coverage and homogeneity were degraded. Conclusions: For the recommended 5 mm PTV‐to‐surface distance the DVH analysis indicated a lower measured target coverage and homogeneity than the planned. This indication is more pronounced as the PTV‐to‐surface distance decreases. The failing points grew colder as the PTV moves closer to the skin, indicating a TPS over estimation of the surface dose, which agrees with TLD skin measurement published data.

WE‐G‐BRB‐09: CyberKnife Patient Specific QA Using a 4D Cylindrical Diode Array System

PURPOSE To examine and facilitate the feasibility of the ArcCheck cylindrical diode array system as a patient specific QA device for CyberKnife radiosurgery delivery. METHODS There is an obvious necessity for CyberKnife robotic radiosurgery patient QA procedures for hypofractionated treatment of larger planned treatment volumes (PTV), e.g. prostate. This need will increase when the future CyberKnife MLC is introduced. The small unflattened CyberKnife fields, along with the variation of beam-to-detector spatial angles, pose a significant detection challenge for dosimetric systems. The feasibility of the ArcCheck (Sun Nuclear Inc.) cylindrical diode array system for patient-specific QA on the CyberKnife is demonstrated using a beam-to-diode specific angular correction that was developed and has been applied. For localization and tracking, four gold seed fiducial markers were embedded in the systems central plug. We used a Monte Carlo 1% uncertainty for the dose calculation. RESULTS By disabling the Linac based corrections and applying the custom CyberKnife correction that we developed, the passing rate increased from 39.6% to 99.8% using a 3%3mm gamma criteria for a given lung case. An additional lung case passed 98.5%. In both cases, a 10% dose threshold was used. In addition, brain, trigeminal nerve and lung cases with synchrony tracking are being investigated. CONCLUSIONS We demonstrated the ArcCheck feasibility for CyberKnife patient specific QA performance. The custom CK angular correction that we developed and applied showed a high passing rate for the lung cases. A verification of the polar angle response should be conducted, in addition to the azimuthal angle that was verified for Linacs. Any data that is being retrieved is additional data to the current chamber point measurement procedures.

SU‐E‐T‐594: Dosimetric Evaluation of Different Treatment Techniques for Prostate Cancer

PURPOSE To evaluate the Volumetric Modulated Arc Therapy (VMAT, RapidArc) and IMRT plan quality for prostate deliveries conducted by two different treatment planning systems: Oncentra Masterplan (Nucletron inc.) and Eclipse (Varian inc.). METHODS We investigated ten prostate treatment delivery plans. For a given case studied we created a RapidArc plan (Eclipse), a VMAT and an IMRT plan (Oncentra) by using both treatment planning systems. The rotational therapy plans consisted of 2 to 3 arcs and the IMRT fields consisted of 7 to 9 fields. The prescription dose was 200 cGy X 40 fx using a Varian Trilogy with 10 MV beams. The treatment parameters were used to evaluate the plan quality: the minimal, mean and maximal doses to the target (PTV) and the volumes received 65Gy and 40 Gy, respectively, for the rectum and bladder, V65 and V40. In addition, we calculated the conformity index (CI) and the heterogeneity index (HI) for each delivery type. RESULTS No significant difference was found between RapidArc, VMAT and IMRT, regarding the minimal and average PTV dose value. The rectum and bladder constraints showed no significant variation as well. The PTV hot spot was significantly higher for the VMAT plan compared to the RapidArc plan (p=0.007). The target CI for VMAT (0.55±0.05) and IMRT (0.71±0.08) was found to be smaller than the RapidArc (0.82±0.04) and the difference is statistically significant (p=0). The HI, value was found to have no significant difference between RapidArc, VMAT and IMRT plan deliveries. CONCLUSIONS Two TPS are capable of producing high-quality treatment plans for prostate cancer. The quality is associated with the degree of intensity modulation and the number of incident angles. Overall, the RapidArc plans with 2-3 arcs showed better dosimetric qualities than the VMAT and IMRT plans.