J Flanz

Proton vs carbon ion beams in the definitive radiation treatment of cancer patients.

BACKGROUND AND PURPOSE Relative to X-ray beams, proton [(1)H] and carbon ion [(12)C] beams provide superior distributions due primarily to their finite range. The principal differences are LET, low for (1)H and high for (12)C, and a narrower penumbra of (12)C beams. Were (12)C to yield a higher TCP for a defined NTCP than (1)H therapy, would LET, fractionation or penumbra width be the basis? METHODS Critical factors of physics, radiation biology of (1)H and (12)C ion beams, neutron therapy and selected reports of TCP and NTCP from (1)H and (12)C irradiation of nine tumor categories are reviewed. RESULTS Outcome results are based on low dose per fraction (1)H and high dose per fraction (12)C therapy. Assessment of the role of LET and dose distribution vs dose fractionation is not now feasible. Available data indicate that TCP increases with BED with (1)H and (12)C TCPs overlaps. Frequencies of GIII NTCPs were higher after (1)H than (12)C treatment. CONCLUSIONS Assessment of the efficacy of (1)H vs(12)C therapy is not feasible, principally due to the dose fractionation differences. Further, there is no accepted policy for defining the CTV-GTV margin nor definition of TCP. Overlaps of (1)H and (12)C ion TCPs at defined BED ranges indicate that TCPs are determined in large measure by dose, BED. Late GIII NTCP was higher in (1)H than (12)C patients, indicating LET as a significant factor. We recommend trials of (1)H vs(12)C with one variable, i.e. LET. The resultant TCP vs NTCP relationship will indicate which beam yields higher TCP for a specified NTCP at a defined dose fractionation.

PET/CT imaging for treatment verification after proton therapy: a study with plastic phantoms and metallic implants.

The feasibility of off-line positron emission tomography/computed tomography (PET/CT) for routine three dimensional in-vivo treatment verification of proton radiation therapy is currently under investigation at Massachusetts General Hospital in Boston. In preparation for clinical trials, phantom experiments were carried out to investigate the sensitivity and accuracy of the method depending on irradiation and imaging parameters. Furthermore, they addressed the feasibility of PET/CT as a robust verification tool in the presence of metallic implants. These produce x-ray CT artifacts and fluence perturbations which may compromise the accuracy of treatment planning algorithms. Spread-out Bragg peak proton fields were delivered to different phantoms consisting of polymethylmethacrylate (PMMA), PMMA stacked with lung and bone equivalent materials, and PMMA with titanium rods to mimic implants in patients. PET data were acquired in list mode starting within 20 min after irradiation at a commercial luthetium-oxyorthosilicate (LSO)-based PET/CT scanner. The amount and spatial distribution of the measured activity could be well reproduced by calculations based on the GEANT4 and FLUKA Monte Carlo codes. This phantom study supports the potential of millimeter accuracy for range monitoring and lateral field position verification even after low therapeutic dose exposures of 2 Gy, despite the delay between irradiation and imaging. It also indicates the value of PET for treatment verification in the presence of metallic implants, demonstrating a higher sensitivity to fluence perturbations in comparison to a commercial analytical treatment planning system. Finally, it addresses the suitability of LSO-based PET detectors for hadron therapy monitoring. This unconventional application of PET involves countrates which are orders of magnitude lower than in diagnostic tracer imaging, i.e., the signal of interest is comparable to the noise originating from the intrinsic radioactivity of the detector itself. In addition to PET alone, PET/CT imaging provides accurate information on the position of the imaged object and may assess possible anatomical changes during fractionated radiotherapy in clinical applications.

The prediction of output factors for spread-out proton Bragg peak fields in clinical practice

The reliable prediction of output factors for spread-out proton Bragg peak (SOBP) fields in clinical practice remained unrealized due to a lack of a consistent theoretical framework and the great number of variables introduced by the mechanical devices necessary for the production of such fields. These limitations necessitated an almost exclusive reliance on manual calibration for individual fields and empirical, ad hoc, models. We recently reported on a theoretical framework for the prediction of output factors for such fields. In this work, we describe the implementation of this framework in our clinical practice. In our practice, we use a treatment delivery nozzle that uses a limited, and constant, set of mechanical devices to produce SOBP fields over the full extent of clinical penetration depths, or ranges, and modulation widths. This use of a limited set of mechanical devices allows us to unfold the physical effects that affect the output factor. We describe these effects and their incorporation into the theoretical framework. We describe the calibration and protocol for SOBP fields, the effects of apertures and range-compensators and the use of output factors in the treatment planning process.

A systematic error analysis of robotic manipulators: application to a high performance medical robot

A systematic methodology to calculate the end-effector position and orientation errors of a robotic manipulator is presented. The method treats the physical error sources in a unified manner during the systems design so that the effect they have on the end-effector positioning accuracy can be compared and the dominant sources identified. Based on this methodology, a computer program has been developed that can perform the error analysis on any serial link manipulator. This methodology and the software are applied here to the error analysis of a six degree of freedom high performance medical manipulator, for positioning in proton therapy.

Overview of the MGH-Northeast Proton Therapy Center plans and progress

Abstract The Northeast Proton Therapy Center (NPTC) is currently being designed and is scheduled for completion in 1998. The goal of the project is to provide the northeast region of the United States with a first class proton therapy facility which has the capabilities needed for the conduct of innovative research, and proven treatments using proton therapy. The NPTC will be built on the Massachusetts General Hospital (MGH) campus. MGH has contracted Bechtel Corporation to coordinate the design and building of the civil construction. Ion Beam Applications (IBA) who is teamed with General Atomics, is responsible for the equipment. The specifications for the facility are written in terms of the clinical performance requirements and will be presented. Aspects of the facility design, status and plans will also be presented.

Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy

This study is aimed at identifying the potential benefits of using a patient-specific aperture in proton beam scanning. For this purpose, an accurate Monte Carlo model of the pencil beam scanning (PBS) proton therapy (PT) treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy. The code was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. For the conditions explored, the penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10 cm from the field edge, the doses in PBS appear to be lower than those observed for DS. We found that placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. In conclusion, using a patient-specific aperture has the potential to further improve the normal tissue sparing capabilities of PBS.

High performance medical robot requirements and accuracy analysis

Abstract The treatment of disease using particle beams requires highly accurate patient positioning. Patients must be well immobilized and precisely aligned with the treatment beam to take full advantage of the dose localization potential. Robots can be used as high accuracy patient positioning systems. In this paper, the first such implementation using robotics techniques for patient positioning will be discussed. This robot is being developed for the Northeast Proton Therapy Center at the Massachusetts General Hospital. The unique requirements and design characteristics of the patient positioning system are presented. Of special interest is the system’s patient positioning accuracy. A systematic methodology to perform the error analysis of serial link manipulators and its application to the PPS is described. Experimental measurements that verified the validity of the method are shown.

Evolution of Technology to Optimize the Delivery of Proton Therapy: The Third Generation

The evolution of proton therapy technology will lead to a new generation of systems that allow for greater accuracy and precision of the dose delivery and will be more compact. We envision that over the next 10-15 years, the quality of deliverable proton dose distributions in the patient will be pushed nearly toward the physical limit of proton therapy. Those future proton therapy systems will fit into treatment rooms of similar size as todays conventional radiation treatment rooms. At the same time, due to technological advancements, the cost of proton therapy will come down to the cost of advanced photon therapy. We discuss some of the technologies that will put these speculative improvements within reach.

Numerical solutions of the γ-index in two and three dimensions.

The γ-index is used routinely to establish correspondence between two dose distributions. The definition of the γ-index can be written with a single equation but solving this equation at millions of points is computationally expensive, especially in three dimensions. Our goal is to extend the vector-equation method in Bakai et al (2003 Phys. Med. Biol.48 3543-53) to higher order for better accuracy and, as important, to determine the magnitude of accuracy in a higher order solution. We construct a numerical framework for calculating the γ-index in two and three dimensions and present an efficient method for calculating the γ-index with zeroth-, first- and second-order methods using tricubic spline interpolation. For an intensity-modulated radiation therapy example with 1.78 × 10⁶ voxels, the zeroth-order, first-order, first-order iterations and semi-second-order methods calculate the three-dimensional γ-index in 1.5, 4.7, 34.7 and 35.6 s with 36.7%, 1.1%, 0.2% and 0.8% accuracy, respectively. The accuracy of linear interpolation with this example is 1.0%. We present efficient numerical methods for calculating the three-dimensional γ-index with tricubic spline interpolation. The first-order method with iterations is the most accurate and fastest choice of the numerical methods if the dose distributions may have large second-order gradients. Furthermore, the difference between iterations can be used to determine the accuracy of the method.

Direct absorbed dose to water determination based on water calorimetry in scanning proton beam delivery

PURPOSE The aim of this manuscript is to describe the direct measurement of absolute absorbed dose to water in a scanned proton radiotherapy beam using a water calorimeter primary standard. METHODS The McGill water calorimeter, which has been validated in photon and electron beams as well as in HDR 192Ir brachytherapy, was used to measure the absorbed dose to water in double scattering and scanning proton irradiations. The measurements were made at the Massachusetts General Hospital proton radiotherapy facility. The correction factors in water calorimetry were numerically calculated and various parameters affecting their magnitude and uncertainty were studied. The absorbed dose to water was compared to that obtained using an Exradin T1 Chamber based on the IAEA TRS-398 protocol. RESULTS The overall 1-sigma uncertainty on absorbed dose to water amounts to 0.4% and 0.6% in scattered and scanned proton water calorimetry, respectively. This compares to an overall uncertainty of 1.9% for currently accepted IAEA TRS-398 reference absorbed dose measurement protocol. The absorbed dose from water calorimetry agrees with the results from TRS-398 well to within 1-sigma uncertainty. CONCLUSIONS This work demonstrates that a primary absorbed dose standard based on water calorimetry is feasible in scattered and scanned proton beams.