Akshay Nanduri

Emission guided radiation therapy for lung and prostate cancers: a feasibility study on a digital patient.

PURPOSE Accurate tumor tracking remains a challenge in current radiation therapy. Many strategies including image guided radiation therapy alleviate the problem to certain extents. The authors propose a new modality called emission guided radiation therapy (EGRT) to accurately and directly track the tumor based on its biological signature. This work is to demonstrate the feasibility of EGRT under two clinical scenarios using a 4D digital patient model. METHODS EGRT uses lines of response (LORs) from positron emission events to direct beamlets of therapeutic radiation through the emission sites inside a tumor. This is accomplished by a radiation delivery system consisting of a Linac and positron emission tomography (PET) detectors on a fast rotating closed-ring gantry. During the treatment of radiotracer-administrated cancer patients, PET detectors collect LORs from tumor uptake sites and the Linac responds in nearly real-time with beamlets of radiation along the same LOR paths. Moving tumors are therefore treated with a high targeting accuracy. Based on the EGRT concept, the authors design a treatment method with additional modulation algorithms including attenuation correction and an integrated boost scheme. Performance is evaluated using simulations of a lung tumor case with 3D motion and a prostate tumor case with setup errors. The emission process is simulated by Geant4 Application for Tomographic Emission package (GATE) and Linac dose delivery is simulated using a voxel-based Monte Carlo algorithm (VMC++). RESULTS In the lung case with attenuation correction, compared to a conventional helical treatment, EGRT achieves a 41% relative increase in dose to 95% of the gross tumor volume (GTV) and a 55% increase to 50% of the GTV. All dose distributions are normalized for the same dose to the lung. In the prostate case with the integrated boost and no setup error, EGRT yields a 19% and 55% relative dose increase to 95% and 50% of the GTV, respectively, when all methods are normalized for the same dose to the rectum. In the prostate case with integrated boost where setup error is present, EGRT contributes a 21% and 52% relative dose increase to 95% and 50% of the GTV, respectively. CONCLUSIONS As a new radiation therapy modality with inherent tumor tracking, EGRT has the potential to substantially improve targeting in radiation therapy in the presence of intrafractional and interfractional motion.

Emission-Guided Radiation Therapy: Biologic Targeting and Adaptive Treatment

a i r m i P s s h d he past decade has witnessed sigificant growth in applications of tereotactic body radiation therapy SBRT), which involves delivering umoricidal doses of radiation in as ew as 1 to 5 fractions to tumors in he head and neck, lung, liver, panreas, spine, and prostate, with very igh (80%-98%) local control rates eported in a variety of studies. The ncrease in SBRT procedures can be argely attributed to the advent of n-board localization technologies hat include single and stereoscoic x-ray imaging, kilovoltage and egavoltage CT imaging, implantble fiducial transponders, ultraound imaging, and others. Onoard MRI radiotherapy systems re also emerging so that the covted soft tissue contrast of MRI ay be exploited for localization. n addition to target localization, n-board MRI has the potential to ore accurately estimate the dose elivered to sensitive organs during ach treatment. Stereotactic body radiation therpy requires precise localization beause of the very high doses (6-30 y) delivered for each fraction. merican Association of Physicists n Medicine Task Group 101 reently published a report on its recmmendations for SBRT practice 1]. In this report, it was emphaized that SBRT “requires a high evel of confidence in the accuracy f the entire treatment delivery proess.” Therefore, it is essential to

Toward a planning scheme for emission guided radiation therapy (EGRT): FDG based tumor tracking in a metastatic breast cancer patient.

PURPOSE Emission guided radiation therapy (EGRT) is a new modality that uses PET emissions in real-time for direct tumor tracking during radiation delivery. Radiation beamlets are delivered along positron emission tomography (PET) lines of response (LORs) by a fast rotating ring therapy unit consisting of a linear accelerator (Linac) and PET detectors. The feasibility of tumor tracking and a primitive modulation method to compensate for attenuation have been demonstrated using a 4D digital phantom in our prior work. However, the essential capability of achieving dose modulation as in conventional intensity modulated radiation therapy (IMRT) treatments remains absent. In this work, the authors develop a planning scheme for EGRT to accomplish sophisticated intensity modulation based on an IMRT plan while preserving tumor tracking. METHODS The planning scheme utilizes a precomputed LOR response probability distribution to achieve desired IMRT planning modulation with effects of inhomogeneous attenuation and nonuniform background activity distribution accounted for. Evaluation studies are performed on a 4D digital patient with a simulated lung tumor and a clinical patient who has a moving breast cancer metastasis in the lung. The Linac dose delivery is simulated using a voxel-based Monte Carlo algorithm. The IMRT plan is optimized for a planning target volume (PTV) that encompasses the tumor motion using the MOSEK package and a Pinnacle3™ workstation (Philips Healthcare, Fitchburg, WI) for digital and clinical patients, respectively. To obtain the emission data for both patients, the Geant4 application for tomographic emission (GATE) package and a commercial PET scanner are used. As a comparison, 3D and helical IMRT treatments covering the same PTV based on the same IMRT plan are simulated. RESULTS 3D and helical IMRT treatments show similar dose distribution. In the digital patient case, compared with the 3D IMRT treatment, EGRT achieves a 15.1% relative increase in dose to 95% of the gross tumor volume (GTV) and a 31.8% increase to 50% of the GTV. In the patient case, EGRT yields a 15.2% relative increase in dose to 95% of the GTV and a 20.7% increase to 50% of the GTV. The organs at risk (OARs) doses are kept similar or lower for EGRT in both cases. Tumor tracking is observed in the presence of planning modulation in all EGRT treatments. CONCLUSIONS As compared to conventional IMRT treatments, the proposed EGRT planning scheme allows an escalated target dose while keeping dose to the OARs within the same planning limits. With the capabilities of incorporating planning modulation and accurate tumor tracking, EGRT has the potential to greatly improve targeting in radiation therapy and enable a practical and effective implementation of 4D radiation therapy for planning and delivery.

SU-E-J-154: Free Breathing Motion Tracking in Emission Guided Radiation Therapy

Purpose: To evaluate an emission guided radiation therapy (EGRT) systems ability to deliverdose to a moving PET‐avid target. Methods: We are developing a treatmentsystem that will simultaneously deliverradiation during PET acquisition. Due to PETs slow imaging time, a method to compensate for motion will be to deliverradiation beam‐lets along individual PET lines‐of‐response (LORs) as they are detected. The EGRT system involves rotating a linac and PET detectors on a closed ring gantry while dynamically controlling a binary multi‐leaf collimator for helical delivery. A cylindrical phantom and six spherical inserts were filled with FDG to achieve an 8:1 target‐to‐background ratio. The phantom was mounted onto a motion stage programmed with a free breathing trajectory acquired from a human subject using an external marker. This trajectory was translated into a purely superior/inferior (z) motion path for the stage and used as ground truth. A planning target volume (PTV) was defined around the largest spheres motion path. LOR data were acquired in list‐mode from a GE Discovery system and used as input to a voxel monte carlo simulation of radiationdosedelivered by the EGRT system with an average lag time of 250 ms. The EGRT method was compared to uniform irradiation of the PTV. Results: The EGRT approach exhibited a non‐uniform dose distribution to the tumor. However, for this specific case there was a 30% relative increase in dose to 50% of the tumor volume for the EGRT approach with both methods normalized to have the same integral dose to the phantom. Conclusions: We have shown that a target exhibiting free breathing motion can be tracked and treated using individual PET emissions to guide delivery. There is dose peaking at the center of a uniform PET‐avid volume. This is the first EGRT feasibility demonstration with experimental data. SRM and ASN are stockholders of RefleXion Medical.

TU-A-BRA-05: Lung Cancer Patient Feasibility Study for Emission Guided Radiation Therapy

Purpose: Emission guided radiation therapy (EGRT) is a new concept that allows for online biological targeting with radioactive tracers. The concept was previously demonstrated in phantom experiments involving free breathing trajectories. This study involves the first patient imaging data to assess feasibility and estimate performance in a more realistic context. Methods: A proposed EGRT geometry involves rotating two PET detector arcs with a linear accelerator and binary multi‐leaf collimator on a CT gantry to deliver beamlets of radiation dynamically along detected PET emission paths. A lungcancer patient underwent PET‐CT as part of radiotherapy planning. PET list‐mode data were retrospectively used as an input to simulate the EGRT systems response and Monte Carlo simulations were used to calculate the dose to the patient. The gross target volume (GTV) was contoured based on the PET‐CT images and the planning volume (PTV) was defined as a 10 mm extension of the GTV in all directions. The EGRT method was compared to uniformly irradiating the same PTV (IMRT), with both methods normalized for the same integral dose to the chest wall. Physiologic motion was ignored during the dose calculations as the tumor exhibited < 2 mm motion. Results: The dose peaks towards the center of the GTV with the EGRT method. However, even in the presence of this inhomogeneity, the EGRT method resulted in 18% relative increase in dose to 95% of the GTV, and 45% increase in dose to 50% of the GTV, compared to the IMRT method. Both methods were normalized for the same integral dose to the chest wall. Conclusions: The feasibility of EGRT has been demonstrated with PET data from a lungcancer patient. Future work will analyze the ability of EGRT to compensate for motion in PET‐CT patient studies which will include breathing traces to estimate ground truth. RefleXion Medical is a company commercializing PET‐guided radiotherapy. SRM and ASN have financial interest in the company.