Image-guided Proton Beam Therapy Research Articles

PO-1678 simulation of low-dose cone beam CT for paediatric image-guided proton beam therapy

PO-1678 simulation of low-dose cone beam CT for paediatric image-guided proton beam therapy

Preliminary Experience of Treating Children and Young Adults With Image-Guided Proton Beam Therapy in India.

PURPOSEProton beam therapy (PBT) has been a preferred modality in pediatric malignancies requiring radiotherapy. We report our preliminary experience of treating consecutive patients younger than 25 years with image-guided pencil beam scanning PBT from the first and only center on the Indian subcontinent.METHODSPatients were selected for PBT on the basis of a multidisciplinary tumor board decision. Patient demographic data, as well as tumor and treatment-related characteristics of the cohort, were captured. Patient and treatment-related factors and their association with acute toxicities were analyzed using univariable and multivariable analyses.RESULTSForty-seven patients (27 with CNS and 20 with non-CNS tumors) with a median age of 9 years (range, 2-25 years) were evaluated. Most common diagnoses were ependymoma, rhabdomyosarcoma, and glioma. Seventy-seven percent of patients traveled more than 500 km, and 70% of them lived in metropolitan cities. Forty-nine percent of patients had recurrent disease at presentation, and 15% had received a previous course of radiation. The median dose delivered was 54.8 cobalt gray equivalents (range, 40.0-70.4 cobalt gray equivalents) to a median clinical target volume of 175 mL (range, 18.7-3,083.0 mL), with 34% of patients requiring concurrent chemotherapy (CCT). Acute grade 2 and grade 3 dermatitis, mucositis, and hematologic toxicity was noted in 45% and 2%, 34% and 0%, and 38% and 30% of patients, respectively. Grade 2 fatigue was noted in 26% of patients. On multivariable analysis, for CNS tumors, both CCT and craniospinal irradiation were independently associated with ≥ 2 grade hematologic toxicity, whereas among non-CNS tumors, a clinical target volume > 150 mL was associated with ≥ 2 grade fatigue, head and neck irradiation was associated with ≥ 2 grade mucositis, and CCT was associated with grade ≥ 2 hematologic toxicity.CONCLUSIONThis study demonstrates safe implementation of a PBT program for children and young adults on the Indian subcontinent. Image-guided pencil beam scanning PBT in judiciously selected patients is feasible and can be delivered with acceptable acute toxicities.

Scatter-to-primary ratio in cone beam computed tomography with extended source to image-receptor distance for image-guided proton beam therapy system

PurposeThe number for clinical systems of proton beam dedicated to the tumor irradiation therapy has been increasing in recent years. In particular, these new systems are likely added with an imaging unit of cone beam CT (CBCT) for image guidance to reduce patient positioning error. In these CBCT systems, the x-ray source to image-receptor distance (SID) of the CBCT unit mounted on the proton beam gantry may be extensively longer than that of CBCT units on linear accelerators. To characterize the scattering effect for these CBCT units with a long SID, we used Monte Carlo computer simulation to calculate the quantity of scatter-to-primary ratio (SPR). Materials and methodsA general-purpose radiation transport code, PENELOPE, was used in our Monte Carlo computer simulation. Analytical uniform and non-uniform phantoms of different materials were used for SPR calculations, including adipose (density: 0.92 g/cm3, relative electron density: 0.49), water (1.00 g/cm3, 1), muscle (1.04 g/cm3, 1.043), and bone (1.85 g/cm3, 1.095). The phantom size was 50×50×20 cm3 in length, width, and height, respectively. The receiving area of flat-panel detector was 40×40 cm2. Cone angle was used to represent the maximum angle that the x-ray of cone beam can cover the entire receiving area of x-ray detector. Different SID geometries of 150, 220, and 300 cm with different cone beam x-ray energies were included in this work. ResultsIn the phantom study with different materials, SPR decreased as either the x-ray energy increased, density of material decreased, or SID increased. The cone angle was 7.59° for 150 cm of SID; 5.19° for 220 cm of SID; 3.81° for 300 cm of SID. A negatively linear relationship was characterized between the cone angle and SID. Our Monte Carlo calculations showed that SPR decreased as the cone angle decreased. A positive correlation was shown between the SPR and cone angle.. ConclusionsWe used different CBCT systems to determine SPRs and SPR decreased for the CBCT unit with a longer SID..

Feasibility of dynamic adaptive passive scattering proton therapy with computed tomography image guidance in the lung.

Hypo-fractionated proton beam therapy (PBT) is an approach that has been increasingly explored over the past decade. It requires high geometric accuracy for targeting of the PBT beams. However, image-guided PBT is currently commonly performed with kV X-ray images of bony anatomy. A dynamic adaptive passive scattering PBT system using computed tomography-based three-dimensional image guidance was developed, and its effectiveness was then evaluated retrospectively in patients with nonsmall cell lung cancer (NSCLC). The dynamic adaptive PBT system consisted of computed tomography-based image registration and proton dose calculation using a simplified Monte Carlo algorithm, with a range adaptation system that could adjust the range shifter thickness to alter the dose distribution. Three patients were retrospectively analyzed. All plans, which each had a total dose of 60 Gy (relative biological effectiveness; RBE), were generated using two fields (Gantry angles: 270 degree and 180 degree) in a passive scattering method. Three dose distributions were generated for each patient according to the following different registrations: bone registration, tumor registration, and tumor registration with range adaptation. The following dosimetric parameters were compared with the original plan: target dose coverage at D95% for the clinical target volume (CTV), homogeneity of D5% to D95% for the CTV, and dose distributions in normal tissue (Dmax of Spinal cord and V20 Gy of lung). For the bone registration method, the average D95% and D5% to D95% for the CTV showed average differences from the original plan of -3.7 ± 4.1 Gy (mean ± 1SD; RBE) and 3.6 ± 3.9 Gy (RBE) respectively. The tumor registration method achieved better coverage than the bone registration method, although the dosimetric parameters for coverage and homogeneity still showed average differences in -2.0 ± 2.3 Gy (RBE) and 1.9 ± 2.2 Gy (RBE) respectively. The range adaptive plan showed comparable coverage and homogeneity [D95%: -1.0 ± 1.3 Gy (RBE) and D5% to D95%: 0.9 ± 1.0 Gy (RBE) on average] to the original plan, as well as demonstrating similar normal tissue sparing. The approach could be completed in less than 10 min, including CT acquisition, image registration, dose recalculation with range optimization, and the operator's visual verification. The tumor dose coverage in patients with NSCLC may deteriorate as a result of respiratory or body movement if daily proton range adaptation is not performed. Our approach may provide higher geometric accuracy for localization of the tumor, and the dynamic range adaptation enables us to achieve the planned dose distribution for hypo-fractionated PBT in the lung.

Realization of the Cone Beam CT by FPDs That Mounted on the Spot-Scanning Dedicated Proton Beam Gantry

For the integrated image guided proton beam therapy (IGPT), Cone-Beam Computed Tomography (CBCT) is considered to be one of the most promising candidates to reduce intra- and inter- fractional setup errors. To realize the IGPT, obtaining precise images within a patient, including tumors and soft tissues around the tumor as well as just before the beam delivery, we have developed a CBCT system using FPDs mounted on the spot-scanning dedicated proton beam therapy (SSPT) system. We have been developing a SSPT system with real-time tumor monitoring and beam gating function to treat large tumors that have motion caused by respiration or other internal reasons. This system has two sets of orthogonal X-ray tubes and FPDs that are mounted besides the nozzle within the gantry for the usage of real-time tumor/fiducial monitoring. CBCT images can be obtained from the acquired source images through X-ray systems with the gantry rotation. This system is equipped with a full rotating gantry in 2.5m bore diameter, its rotating speed is 1rpm at maximum and the rotation accuracy is within 1mm at the isocenter. Each FPD size is 30 x 30cm; maximum frame rate is 30Hz. These FPDs are situated beside the nozzle at 2.1m in source to imager distance. FPDs are designed to be offsettable from the X ray axis through the isocenter when there is a need to obtain large field of view. Two sets of the X-ray and FPDs are designed to be operated in synchronization with the therapeutic beam. The source images for constructing the CBCT images can be acquired independently from each FPD as well. The CBCT images are successfully constructed through the X-ray tubes and FPDs mounted on the proton beam gantry. Acquisition time for obtaining the source image is 40 seconds approximately, under the image acquisition condition of 110kv, 33mA, 20ms, 10fps for a lung phantom study. Lung and Pelvic images from the phantom study showed sufficient quality in clinical usage to distinguish the soft tissue structures surrounded by the bony structure. In addition, the offset function of the FPDs from the X-ray axis enabled expanding the FOV from 20x20cm to 40x40cm at the isocenter. Field size of view can be expanded enough to recognize the surface of the patient’s body to check whether the beam path length has been changed or not in comparison with the planned one. The development and construction of the spot-scanning dedicated proton beam therapy system has been accomplished and CBCT images can be obtained successfully.

2514: Biological Lung Phantom for Image-Guided Proton Therapy Validation

To evaluate the feasibility of a biological lung phantom to validate image-guided proton beam therapy. Human lung tissue is a complex, dynamic system consisting of lung parenchyma, blood vessels, and airways, each having different electron densities. Furthermore, this system deforms and moves in a variable manner during normal respiration. In contrast, conventional radiological phantoms (e.g., the Rando® phantom) assume a static, uniform-density lung field. A more realistic phantom system that takes regional density variations and ventilation-induced motion and deformation into account is needed to verify the ability to deliver proton treatment plans to well-defined penetration depths. To this end, swine lung preserved to retain vital mechanical properties was obtained and compared to human lung for suitability as a phantom. The phantom was ventilated and imaged on a GE Lightspeed CT scanner at various lung filling states. Volume rendering of the CT image data using BioImage (SCIRun BioPSE software) was performed to visualize and determine coordinates of airway bifurcations. To demonstrate the residual range variation in proton track length in a dynamic, heterogeneous lung tissue system, a virtual lung tissue model based on lung histology and matched to measured swine lung phantom CT density at different points in the ventilatory cycle was generated. A 100 MeV proton pencil beam traversing the 10 cm thick tissue model was then simulated with the proton beam terminating in a water beamstop to calculate residual range. Preserved swine lung was found to be equivalent to human lung in terms of radiological and physical properties, lobar structure, airway architecture, volume and overall mass, and has been determined to be histologically comparable to human lung by a board-certified pathologist. Analysis of volume rendered CT images yielded a conservative minimum of 31 reproducible anatomic landmarks (airway bifurcations) evenly distributed throughout the lung. Residual ranges varying from 3 and 76 mm in the post-model water beamstop were calculated (mean of 39 mm, standard deviation of 30 mm), dependent on the breathing phase of the lung phantom. The preserved swine lung lasted approximately 8 weeks before degrading, which may be extended through improved preservation techniques. Simulation results suggest that respiratory motion not only compromises the therapeutic dose delivery to a moving target, but can result in significant excess dose delivery to critical structures downstream of the target volume (e.g., spinal cord, esophagus) through variations in residual range. Advantages of the dynamically ventilated preserved swine lung phantom for proton depth-dose studies with a proton range telescope include realistic geometry, motion, deformation, and the ability to implant radio-opaque clips and tumor surrogates. Disadvantages include compromises in true lung deformation due to external ventilation instead of physiological respiration, as well as the inevitable degradation of the system. We believe that this dynamic biological lung phantom will be useful as a verification tool for image-guided proton therapy, in that it will allow for the planning, scanning, and delivery of proton dose to a reproducible, complex, spatially and temporally variable system.