Commissioning of the discrete spot scanning proton beam delivery system at the University of Texas M.D. Anderson Cancer Center, Proton Therapy Center, Houston.

PURPOSE: To describe a summary of the clinical commissioning of the discrete spot scanning proton beam at the Proton Therapy Center, Houston (PTC-H). METHODS: Discrete spot scanning system is composed of a delivery system (Hitachi ProBeat), an electronic medical record (Mosaiq V 1.5), and a treatment planning system (TPS) (Eclipse V 8.1). Discrete proton pencil beams (spots) are used to deposit dose spot by spot and layer by layer for the proton distal ranges spanning from 4.0 to 30.6 g/cm2 and over a maximum scan area at the isocenter of 30 x 30 cm2. An arbitrarily chosen reference calibration condition has been selected to define the monitor units (MUs). Using radiochromic film and ion chambers, the authors have measured spot positions, the spot sizes in air, depth dose curves, and profiles for proton beams with various energies in water, and studied the linearity of the dose monitors. In addition to dosimetric measurements and TPS modeling, significant efforts were spent in testing information flow and recovery of the delivery system from treatment interruptions. RESULTS: The main dose monitors have been adjusted such that a specific amount of charge is collected in the monitor chamber corresponding to a single MU, following the IAEA TRS 398 protocol under a specific reference condition. The dose monitor calibration method is based on the absolute dose per MU, which is equivalent to the absolute dose per particle, the approach used by other scanning beam institutions. The full width at half maximum for the spot size in air varies from approximately 1.2 cm for 221.8 MeV to 3.4 cm for 72.5 MeV. The measured versus requested 90% depth dose in water agrees to within 1 mm over ranges of 4.0-30.6 cm. The beam delivery interlocks perform as expected, guarantying the safe and accurate delivery of the planned dose. CONCLUSIONS: The dosimetric parameters of the discrete spot scanning proton beam have been measured as part of the clinical commissioning program, and the machine is found to function in a safe manner, making it suitable for patient treatment.

Vision 20/20: proton therapy.

The first patients were treated with proton beams in 1955 at the Lawrence Berkeley Laboratory in California. In 1970, proton beams began to be used in research facilities to treat cancer patients using fractionated treatment regimens. It was not until 1990 that proton treatments were carried out in hospital-based facilities using technology and techniques that were comparable to those for modern photon therapy. Clinical data strongly support the conclusion that proton therapy is superior to conventional radiation therapy in a number of disease sites. Treatment planning studies have shown that proton dose distributions are superior to those for photons in a wide range of disease sites indicating that additional clinical gains can be achieved if these treatment plans can be reliably delivered to patients. Optimum proton dose distributions can be achieved with intensity modulated protons (IMPT), but very few patients have received this advanced form of treatment. It is anticipated widespread implementation of IMPT would provide additional improvements in clinical outcomes. Advances in the last decade have led to an increased interest in proton therapy. Currently, proton therapy is undergoing transitions that will move it into the mainstream of cancer treatment. For example, proton therapy is now reimbursed, there has been rapid development in proton therapy technology, and many new options are available for equipment, facility configuration, and financing. During the next decade, new developments will increase the efficiency and accuracy of proton therapy and enhance our ability to verify treatment planning calculations and perform quality assurance for proton therapy delivery. With the implementation of new multi-institution clinical studies and the routine availability of IMPT, it may be possible, within the next decade, to quantify the clinical gains obtained from optimized proton therapy. During this same period several new proton therapy facilities will be built and the cost of proton therapy is expected to decrease, making proton therapy routinely available to a larger population of cancer patients.

Feasibility studies of a passive scatter proton therapy nozzle without a range modulator wheel.

The purpose of this work was to determine the feasibility of producing a spread out Bragg peak (SOBP) without a range modulation wheel (RMW) using the passive scattering beam delivery technique. For this study, a comprehensive Monte Carlo model of a passive scattering treatment nozzle was used. The RMW was removed from the model leaving only the initial fixed scatterer (RMW-free configuration). Range modulation was achieved by directly changing the energy of the proton beam entering the nozzle. To produce a uniform SOBP, the number of protons injected into the nozzle at each beam energy was "dose weighted." To do so, the effective number of protons was calculated for the individual initial energies using an analytical dose-weighting function, and the resulting weighted Bragg curves were summed together to produce an SOBP of the desired width. We found that SOBPs calculated using the RMW-free nozzle configuration were in very good agreement to those calculated with the standard nozzle configuration containing the RMW for the 250, 180, and 100 MeV maximum beam energies. The depth of the distal 90% dose and the 80%-20% distal dose falloff of SOBPs calculated with the two different nozzle configurations agreed to within a millimeter for the three beam energy options considered in this study. In addition, the 80%-20% lateral penumbra for the cross-field dose profiles calculated with the RMW-free delivery method agreed with results calculated using the standard RMW technique to less than one millimeter. For an equal number of protons injected into the nozzle, an increase of up to 10% in the delivered dose and a significant reduction in both the in-air secondary neutron fluence and dose equivalent (H/D) were observed at the isocenter by removing the RMW from the treatment nozzle and modulating the initial proton beam energy. However, increases in delivery time of up to 70% were also estimated with this method. Our results suggest that it is feasible to deliver a passively scattered dose distribution with an RMW-free nozzle configuration with clinical characteristics comparable to those using standard methods.

Initial beam size study for passive scatter proton therapy. I. Monte Carlo verification.

The purpose of this work was to provide an initial validation of a Monte Carlo (MC) model of the passive scattering treatment nozzle at the University of Texas M. D. Anderson Cancer Center Proton Therapy Center. The MC model included a detailed definition of each beam-modifying element in the nozzle, and calculations accounted for interactions of the beam with the rotating modulator wheel used to create the spread out Bragg peak. In this work we show comparisons of calculated dose and fluence profiles with measured data from the nozzle for the 250 and 180 MeV beam energies used for patient treatments. Agreement to within 1.5 mm of measured data was observed for all MC calculations. The high level of agreement between the measurements and the MC model for the two beam energies studied provides validation for use of the model in a study of the dosimetric effects of the proton beam size and shape at the nozzle entrance.

Initial beam size study for passive scatter proton therapy. II. Changes in delivered depth dose profiles.

In passively scattered proton radiotherapy, a clinically useful treatment beam is produced by spreading a small proton "pencil beam" extracted from the accelerator to create both a uniform dose profile laterally and a uniform spread-out Bragg peak (SOBP) in depth. Lateral spreading and range modulation of the beam are accomplished using specially designed components within the treatment delivery nozzle. The purpose of this study was to determine how changes in the size of the initial proton pencil beam affect the delivery of dose with a passive scatter treatment nozzle. Monte Carlo calculations were used to study changes of the beam's in-air energy distribution at the exit of the nozzle and the central axis depth dose profiles in water resulting from changes in the incident beam size. Our results indicate that the width of the delivered SOBP decreases as the size of the initial beam increases.

Verification procedure for isocentric alignment of proton beams.

We present a technique--based on the Lutz, Winston, and Maleki test used in stereotactic linear accelerator radiosurgery--for verifying whether proton beams are being delivered within the required spatial coincidence with the gantry mechanical isocenter. Our procedure uses a proton beam that is collimated by a circular aperture at its central axis and is then intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for couch position assures precise positioning of the steel sphere at the mechanical isocenter of the gantry. A film-based radiation dosimetry technique, chosen for the good spatial resolution it achieves, records the proton dose distribution for optical image analysis. The optical image obtained presents a circular high-dose region surrounding a lower-dose area corresponding to the proton beam absorption by the steel sphere, thereby providing a measure of the beam alignment with the mechanical isocenter. We found the self-developing Gafchromic EBT film (International Specialty Products, Wayne, NJ) and commercial Epson 10000 XL flatbed scanner (Epson America, Long Beach, CA) to be accurate and efficient tools. The positions of the gantry mechanical and proton beam isocenters, as recorded on film, were clearly identifiable within the scanning resolution used for routine alignment testing (0.17 mm per pixel). The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22 +/- 0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.

Proton therapy.

Proton therapy has become a subject of considerable interest in the radiation oncology community and it is expected that there will be a substantial growth in proton treatment facilities during the next decade. I was asked to write a historical review of proton therapy based on my personal experiences, which have all occurred in the United States, so therefore I have a somewhat parochial point of view. Space requirements did not permit me to mention all of the existing proton therapy facilities or the names of all of those who have contributed to proton therapy.

Treatment planning with protons for pediatric retinoblastoma, medulloblastoma, and pelvic sarcoma: how do protons compare with other conformal techniques?

PURPOSE: To calculate treatment plans and compare the dose distributions and dose-volume histograms (DVHs) for photon three-dimensional conformal radiation therapy (3D-CRT), electron therapy, intensity-modulated radiation therapy (IMRT), and standard (nonintensity modulated) proton therapy in three pediatric disease sites. METHODS AND MATERIALS: The tumor volumes from 8 patients (3 retinoblastomas, 2 medulloblastomas, and 3 pelvic sarcomas) were studied retrospectively to compare DVHs from proton therapy with 3D-CRT, electron therapy, and IMRT. In retinoblastoma, several planning techniques were analyzed: A single electron appositional beam was compared with a single 3D-CRT lateral beam, a 3D-CRT anterior beam paired with a lateral beam, IMRT, and protons. In medulloblastoma, three posterior fossa irradiation techniques were analyzed: 3D-CRT, IMRT, and protons. Craniospinal irradiation (which consisted of composite plans of both the posterior fossa and craniospinal components) was also evaluated, primarily comparing spinal irradiation using 3D-CRT electrons, 3D-CRT photons, and protons. Lastly, in pelvic sarcoma, 3D-CRT, IMRT, and proton plans were assessed. RESULTS: In retinoblastoma, protons resulted in the best target coverage combined with the most orbital bone sparing (10% was the mean orbital bone volume irradiated at > or =5 Gy for protons vs. 25% for 3D-CRT electrons, 69% for IMRT, 41% for a single 3D lateral beam, 51% for a 3D anterolateral beam with a lens block, and 65% for a 3D anterolateral beam without a lens block). A single appositional electron field was the next best technique followed by other planning approaches. In medulloblastoma, for posterior fossa and craniospinal irradiation, protons resulted in the least dose to the cochlea (for only posterior fossa irradiation at > or =20 Gy, 34% was the mean cochlear volume irradiated for protons, 87% for IMRT, 89% for 3D-CRT) and hypothalamus-pituitary axis (for only posterior fossa irradiation at > or =10 Gy, 21% was the mean hypothalamus-pituitary volume irradiated for protons, 81% for IMRT, 91% for 3D-CRT); additional dose reductions to the optic chiasm, eyes, vertebrae, mandible, thyroid, lung, kidneys, heart, and liver were seen. Intensity-modulated radiotherapy appeared to be the second best technique for posterior fossa irradiation. For spinal irradiation 3D-CRT electrons were better than 3D-CRT photons in sparing dose to the thyroid, heart, lung, kidney, and liver. With pelvic sarcoma, protons were superior in eliminating any dose to the ovaries (0% of mean ovarian volume was irradiated at > or =2 Gy with protons) and to some extent, the pelvic bones and vertebrae. Intensity-modulated radiotherapy did show more bladder dose reduction than the other techniques in pelvic sarcoma irradiation. CONCLUSIONS: In the diseases studied, using various techniques of 3D-CRT, electrons, IMRT, and protons, protons are most optimal in treating retinoblastomas, medulloblastomas (posterior fossa and craniospinal), and pelvic sarcomas. Protons delivered superior target dose coverage and sparing of normal structures. As dose-volume parameters are expected to correlate with acute and late toxicity, proton therapy should receive serious consideration as the preferred technique for the treatment of pediatric tumors.

Proton radiation therapy for retinoblastoma: comparison of various intraocular tumor locations and beam arrangements.

PURPOSE: To study the optimization of proton beam arrangements for various intraocular tumor locations; and to correlate isodose distributions with various target and nontarget structures. METHODS AND MATERIALS: We considered posterior-central, nasal, and temporal tumor locations, with straight, intrarotated, or extrarotated eye positions. Doses of 46 cobalt grey equivalent (CGE) to gross tumor volume (GTV) and 40 CGE to clinical target volume (CTV) (2 CGE per fraction) were assumed. Using three-dimensional planning, we compared isodose distributions for lateral, anterolateral oblique, and anteromedial oblique beams and dose-volume histograms of CTVs, GTVs, lens, lacrimal gland, bony orbit, and soft tissues. RESULTS: All beam arrangements fully covered GTVs and CTVs with optimal lens sparing. Only 15% of orbital bone received doses > or =20 CGE with a lateral beam, with 20-26 CGE delivered to two of three growth centers. The anterolateral oblique approach with an intrarotated eye resulted in additional reduction of bony volume and exposure of only one growth center. No appreciable dose was delivered to the contralateral eye, brain tissue, or pituitary gland. CONCLUSIONS: Proton therapy achieved homogeneous target coverage with true lens sparing. Doses to orbit structures, including bony growth centers, were minimized with different beam arrangements and eye positions. Proton therapy could reduce the risks of second malignancy and cosmetic and functional sequelae.

Benchmarking analytical calculations of proton doses in heterogeneous matter.

A proton dose computational algorithm, performing an analytical superposition of infinitely narrow proton beamlets (ASPB) is introduced. The algorithm uses the standard pencil beam technique of laterally distributing the central axis broad beam doses according to the Moliere scattering theory extended to slablike varying density media. The purpose of this study was to determine the accuracy of our computational tool by comparing it with experimental and Monte Carlo (MC) simulation data as benchmarks. In the tests, parallel wide beams of protons were scattered in water phantoms containing embedded air and bone materials with simple geometrical forms and spatial dimensions of a few centimeters. For homogeneous water and bone phantoms, the proton doses we calculated with the ASPB algorithm were found very comparable to experimental and MC data. For layered bone slab inhomogeneity in water, the comparison between our analytical calculation and the MC simulation showed reasonable agreement, even when the inhomogeneity was placed at the Bragg peak depth. There also was reasonable agreement for the parallelepiped bone block inhomogeneity placed at various depths, except for cases in which the bone was located in the region of the Bragg peak, when discrepancies were as large as more than 10%. When the inhomogeneity was in the form of abutting air-bone slabs, discrepancies of as much as 8% occurred in the lateral dose profiles on the air cavity side of the phantom. Additionally, the analytical depth-dose calculations disagreed with the MC calculations within 3% of the Bragg peak dose, at the entry and midway depths in the phantom. The distal depth-dose 20%-80% fall-off widths and ranges calculated with our algorithm and the MC simulation were generally within 0.1 cm of agreement. The analytical lateral-dose profile calculations showed smaller (by less than 0.1 cm) 20%-80% penumbra widths and shorter fall-off tails than did those calculated by the MC simulations. Overall, this work validates the usefulness of our ASPB algorithm as a reasonably fast and accurate tool for quality assurance in planning wide beam proton therapy treatment of clinical sites either composed of homogeneous materials or containing laterally extended inhomogeneities that are comparable in density and located away from the Bragg peak depths.

Proton beam dosimetry for radiosurgery: implementation of the ICRU Report 59 at the Harvard Cyclotron Laboratory.

Recent proton dosimetry intercomparisons have demonstrated that the adoption of a common protocol, e.g. ICRU Report 59, can lead to improved consistency in absorbed dose determinations. We compared absorbed dose values, measured in the 160 MeV proton radiosurgery beamline at the Harvard Cyclotron Laboratory, based on ionization chamber methods with those from a Faraday cup technique. The Faraday cup method is based on a proton fluence determination that allows the estimation of absorbed dose with the CEMA approximation, under which the dose is equal to the fluence times the mean mass stopping power. The ionization chamber technique employs an air-kerma calibration coefficient for 60Co radiation and a calculated correction in order to take into account the differences in response to 60Co and proton beam radiations. The absorbed dose to water, based on a diode measurement calibrated with a Faraday cup technique, is approximately 2% higher than was obtained from an ionization chamber measurement. At the Bragg peak depth, the techniques agree to within their respective uncertainties, which are both approximately 4% (1 standard deviation). The ionization chamber technique exhibited superior reproducibility and was adopted in our standard clinical practice for radiosurgery.