Preliminary investigations for the option to use fast uniform scanning with compensators on a gantry designed for IMPT.

PURPOSE: In this experimental study, the authors explored the possibility to deliver the dose for proton therapy with fast uniform scanning on a gantry primarily designed for the delivery of conformal beam scanning and IMPT. The uniform scanning submode has been realized without equipment modifications by using the same small pencil beam used for conformal scanning, resulting in reduced realization costs. Uniform scanning has recently been adopted in a few proton therapy centers, as a basic beam delivery solution, and as an alternative to the use of scattering foils. The option to use such a mode to mimic scattering on a full-fledged scanning gantry could be of interest for treating some specific indications and as a possible solution for treating moving targets. METHODS: Uniform iso-energy dose layers were painted by fast magnetic scanning alternated with fast energy changes with the gantry beam line. The layers were stacked and repainted appropriately to produce homogeneous three-dimensional dose distributions. A collimator∕compensator was used to adjust the dose to coincide laterally∕distally with the target volume. In addition, they applied volumetric repainting, since they are confident that this will further mitigate the effects of organ motion as compared with the presently used clinical scanning solutions. With the approach presented in this paper, they can profit from the higher flexibility of the scanning system to obtain additional advantages. For instance the shape of the energy layers can be adjusted to the projected target shape in order to reduce treatment time and neutrons produced in the collimator. The shape of the proximal layers can be shrunk, according to the cross section of the target at the corresponding range. This provides variable range modulation (proximal conformity) while standard scattering only provides fixed range modulation with unnecessary 100% dose proximal to the target. The field-specific hardware for a spherical target volume was mounted on the Gantry 2 nozzle. One field with proximal field size shrinking and one without, each of 1 Gy, were delivered. The dose distributions at different depths were recorded as CCD images of a scintillating screen. RESULTS: The time to scan the volume once was about 4 s and the total delivery time was approximately 30 s. For the field with proximal conformity, dose sparing of up to 25% was measured in the region proximal to the target. A repainting capability of 48 times was achieved on the most distal layer. The proximal layers were repainted more due to the contribution of the plateau dose from the deeper layers. CONCLUSIONS: The flexibility of a fast scanning gantry with very fast energy changes can easily provide beam delivery by uniform layer stacking with a significant degree of volumetric repainting and with the benefit of a dose reduction proximal to the target volume.

Special report: workshop on 4D-treatment planning in actively scanned particle therapy--recommendations, technical challenges, and future research directions.

This article reports on a 4D-treatment planning workshop (4DTPW), held on 7-8 December 2009 at the Paul Scherrer Institut (PSI) in Villigen, Switzerland. The participants were all members of institutions actively involved in particle therapy delivery and research. The purpose of the 4DTPW was to discuss current approaches, challenges, and future research directions in 4D-treatment planning in the context of actively scanned particle radiotherapy. Key aspects were addressed in plenary sessions, in which leaders of the field summarized the state-of-the-art. Each plenary session was followed by an extensive discussion. As a result, this article presents a summary of recommendations for the treatment of mobile targets (intrafractional changes) with actively scanned particles and a list of requirements to elaborate and apply these guidelines clinically.

More than 10 years experience of beam monitoring with the Gantry 1 spot scanning proton therapy facility at PSI.

PURPOSE: The beam monitoring equipments developed for the first PSI spot scanning proton therapy facility, Gantry 1, have been successfully used for more than 10 years. The purpose of this article is to summarize the author's experience in the beam monitoring technique for dynamic proton scanning. METHODS: The spot dose delivery and verification use two independent beam monitoring and computer systems. In this article, the detector construction, electronic system, dosimetry, and quality assurance results are described in detail. The beam flux monitor is calibrated with a Faraday cup. The beam position monitoring is realized by measuring the magnetic fields of deflection magnets with Hall probes before applying the spot and by checking the beam position and width with an ionization strip chamber after the spot delivery. RESULTS: The results of thimble ionization chamber dosimetry measurements are reproducible (with a mean deviation of less than 1% and a standard deviation of 1%). The resolution in the beam position measurement is of the order of a tenth of a millimeter. The tolerance of the beam position delivery and monitoring during scanning is less than 1.5 mm. CONCLUSIONS: The experiences gained with the successful operation of Gantry 1 represent a unique and solid background for the development of a new system, Gantry 2, in order to perform new advanced scanning techniques.

Experiences at the Paul Scherrer Institute with a remote patient positioning procedure for high-throughput proton radiation therapy.

PURPOSE: To describe a remote positioning system for accurate and efficient proton radiotherapy treatments. METHODS AND MATERIALS: To minimize positioning time in the treatment room (and thereby maximize beam utility), we have adopted a method for remote patient positioning, with patients positioned and imaged outside the treatment room. Using a CT scanner, positioning is performed using orthogonal topograms with the measured differences to the reference images being used to define daily corrections to the patient table in the treatment room. Possible patient movements during transport and irradiation were analyzed through periodic acquisition of posttreatment topograms. Systematic and random errors were calculated for this daily positioning protocol and for two off-line protocols. The potential time advantage of remote positioning was assessed by computer simulation. RESULTS: Applying the daily correction protocol, systematic errors calculated over all patients (n = 94) were below 0.6 mm, whereas random errors were below 1.5 mm and 2.5 mm, respectively, for bite-block and for mask immobilization. Differences between pre- and posttreatment images were below 2.8 mm (SD) in abdominal/pelvic region, and below 2.4 mm (SD) in the head. Retrospective data analysis for a subset of patients revealed that off-line protocols would be significantly less accurate. Computer simulations showed that remote positioning can increase patient throughput up to 30%. CONCLUSIONS: The use of a daily imaging and correction protocol based on a "remote" CT could reduce positioning errors to below 2.5 mm and increase beam utility in the treatment room. Patient motion between imaging and treatment were not significant.

Postoperative spot-scanning proton radiation therapy for chordoma and chondrosarcoma in children and adolescents: initial experience at paul scherrer institute.

PURPOSE: To evaluate postoperative spot-scanning proton radiation therapy (PT) and intensity-modulated PT (IMPT) for chordoma and chondrosarcoma in pediatric patients. METHODS AND MATERIALS: Between 2000 and 2005, 10 patients (six male patients, four female patients; six chordomas, four chondrosarcomas), aged 10-20 years (median, 16 years), were treated at our institute. Tumor sites were in the brain (one case), skull base (five cases), cervical (three cases), and lumbar spine (one case). Three children had complete resections. In seven children, resection was incomplete, leaving residual tumor behind (range, 2.3-46.3 mL). PT was delivered using spot scanning, with (three patients) or without (seven patients) IMPT. Total dose was 74.0 cobalt Gray equivalents (CGE) for chordoma, and 63.2-68.0 CGE for chondrosarcoma (median, 66.0), depending on histopathological grading and whether the patient had concurrent chemotherapy. RESULTS: Median follow-up time was 36 months (range, 8-77 months). Radiation treatment was well tolerated. All patients remained failure-free at their last follow-up. Late adverse events were reported in three patients and were mild (neurosensory in one patient; alopecia and hypoaccusis in one patient) to moderate (one patient, Grade 2 pituitary insufficiency). CONCLUSIONS: Postoperative spot-scanning PT, delivered in combination with and without IMPT, for chordoma and chondrosarcoma in children and adolescents was tolerated without unacceptable adverse event and initial outcome is perfectly satisfactory in this small cohort. Longer follow-up time and larger cohort are needed to more fully assess tumor control, adverse events, as well as functional and cosmetic outcome.

Spot scanning proton therapy in the curative treatment of adult patients with sarcoma: the Paul Scherrer institute experience.

PURPOSE: To assess the safety and efficacy of spot scanning proton beam therapy (PT) in the curative treatment of soft-tissue sarcoma (STS) in adults patients. PATIENTS AND METHODS: We identified 13 STS patients treated with PT between July 1998 and May 2005 in our institutional database. Tumor histology varied with the most common histologic subtypes including liposarcoma and peripheral nerve sheet tumor. All tumors were located in vicinity of critical structures, such as the spinal cord, optic apparatus, bowel, kidney, or bowel. Of the patients, 6 and 5 patients received PT either as adjuvant therapy for non-R0 resection or for recurrence, respectively. Two patients received radical PT for unresectable disease. The median prescribed dose was 69.4 CGE (CGE = proton Gy x 1.1)-Gy (range, 50.4-76.0) at 1.8 to 2 CGE-Gy (median, 1.9) per fraction. Pre-PT anthracycline-based chemotherapy was delivered to 3 patients only. No patient has been lost to follow-up (median 48.1 months, range, 19.1-100.7 months). RESULTS: Of the 13 patients, all but 2 patients were alive. Local recurrence developed in 3 (23%) patients. The administered dose to these patients was < or =60 Gy-CGE. Distant control was achieved in all but 2 patients (lung metastasis), 1 of whom presented with a concomitant local recurrence. The 4-year local control and metastasis-free survival rates were 74.1% and 84.6%, respectively. Late grade > or =2 toxicity was observed in only 2 patients. CONCLUSIONS: Spot scanning PT is an effective and safe treatment for patient with STS in critical locations. The observed toxicity rate was acceptable.

Extracranial chordoma: Outcome in patients treated with function-preserving surgery followed by spot-scanning proton beam irradiation.

PURPOSE: To evaluate the use of postoperative proton therapy (PT) in extracranial chordoma. PATIENTS AND METHODS: Twenty-six patients were treated. Gross total resection was achieved in 18 patients. Nine patients had cervical, 2 had thoracic, 8 had lumbar, and 7 had sacro-coccygeal chordomas. Thirteen patients had implants. PT was administered after function-preserving surgery, using a gantry and spot scanning, without or with intensity modulation (IMPT; 6 patients), and/or photon-based radiotherapy (RT, 6 patients). Median total dose was 72 cobalt Gray equivalent (CGE; range, 59.4-74.4), with means of 70.5 and 73.2 CGE for patients with and without implants. Median follow-up time was 35 months (range, 13-73 months). Adverse events were scored using the Common Terminology Criteria for Adverse Events grading system (version 3.0). RESULTS: At 3 years, actuarial overall survival (OS) and progression-free survival (PFS) rates were 84% and 77%, respectively. One patient each died of local failure (LF), distant failure (DF), suicide, and secondary tumor. We observed 5 LFs and 3 DFs; 3-year LF-free and DF-free survival rates were 86%. We observed four radiation-induced late adverse events (Grade 2 sensory neuropathy; Grade 3 subcutaneous necrosis, and osteonecrosis; and Grade 5 secondary cancer). In univariate analysis, implants were associated with LF (p = 0.034). Gross residual tumor above 30 mL was negatively associated with OS (p = 0.013) and PFS (p = 0.025). CONCLUSIONS: Postoperative PT for extracranial chordomas delivered with spot scanning offers high local control rates. Toxicity was acceptable. Implants were significantly associated with LF. Residual tumor above 30 mL impacted negatively on OS and PFS.

Spot-scanning proton therapy for malignant soft tissue tumors in childhood: First experiences at the Paul Scherrer Institute.

PURPOSE: Radiotherapy plays a major role in the treatment strategy of childhood sarcomas. Consequences of treatment are likely to affect the survivor's quality of life significantly. We investigated the feasibility of spot-scanning proton therapy (PT) for soft tissue tumors in childhood. METHODS AND MATERIALS: Sixteen children with soft tissue sarcomas were included. Median age at PT was 3.3 years. In 10 children the tumor histology was embryonal rhabdomyosarcoma. All tumors were located in the head or neck, parameningeal, or paraspinal, or pelvic region. In the majority of children, the tumor was initially unresectable (Intergroup Rhabdomyosarcoma Study [IRS] Group III in 75%). In 50% of children the tumors exceeded 5 cm. Fourteen children had chemotherapy before and during PT. Median total dose of radiotherapy was 50 cobalt Gray equivalent (CGE). All 16 children were treated with spot-scanning proton therapy at the Paul Scherrer Institute, and in 3 children the PT was intensity-modulated (IMPT). RESULTS: After median follow-up of 1.5 years, local control was achieved in 12 children. Four children failed locally, 1 at the border of the radiation field and 3 within the field. All 4 children died of tumor recurrence. All 4 showed unfavorable characteristic either of site or histopathology of the tumor. Acute toxicity was low, with Grade 3 or 4 side effects according to Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer (RTOG/EORTC) criteria occurring in the bone marrow only. CONCLUSIONS: Proton therapy was feasible and well tolerated. Early local control rates are comparable to those being achieved after conventional radiotherapy. For investigations on late effect, longer follow-up is needed.

Relative biologic effectiveness determination in mouse intestine for scanning proton beam at Paul Scherrer Institute, Switzerland. Influence of motion.

PURPOSE: To determine the relative biologic effectiveness (RBE) of the Paul Scherrer Institute (PSI) scanning proton beam in reference conditions and to evaluate the influence of intestine motion on the proton dose homogeneity. METHODS AND MATERIALS: First, RBE was determined for crypt regeneration in mice after irradiation in a single fraction. Irradiation was performed at the middle of a 7-cm spread out Bragg peak (SOBP; reference position), as well as in the proximal part of the plateau and at the distal end of the SOBP. Control gamma-irradiation was randomized with proton irradiation and performed simultaneously. Second, motion of mouse intestine was determined by radiographs after copper wire markers had been placed on the jejunum and intestinal wall. RESULTS: Proton RBE (reference (60)Co gamma) was equal to 1.16 for irradiation at the middle of the SOBP and to 1.11 and 1.21 for irradiation in the initial plateau and end of the SOBP, respectively. The confidence intervals for these RBE values were much larger than those obtained in the other proton beams we have tested so far. They exceeded +/-0.20 (compared with the usual value of +/-0.07), which resulted from the unusually large dispersion of the individual proton data. The instantaneous positions of the mice intestines varied by +/-2 mm in the course of irradiation. CONCLUSION: The results of this study have shown that the RBE of the PSI proton beam is in total accordance with the RBE obtained at the other centers. This experiment has corroborated that proton RBE at the middle of the SOBP is slightly larger than the generic value of 1.10 and that there is a slight tendency for the RBE to increase close to the end of the SOBP. Also, excessive dispersion of individual proton data may be considered to result from intestine motion, taking into account that irradiation at the PSI is delivered dynamically by scanning the target volume with a pencil proton beam ("spot scanning"). Because 2-mm movements resulted in significant variations in local dose depositions, this should be considered for moving targets. Strategies to reduce this effect for the spot scanning technique have been developed at the PSI for radiotherapy of humans.

Results of spot-scanning proton radiation therapy for chordoma and chondrosarcoma of the skull base: the Paul Scherrer Institut experience.

PURPOSE: To assess the clinical results of spot scanning proton beam radiation therapy (PT) in the treatment of skull base chordomas and low-grade chondrosarcomas (CS). METHODS AND MATERIALS: Between October 1998 and October 2003, 29 patients (median age, 39 years) with chordomas (n = 18) and CS (n = 11) were treated at the Paul Scherrer Institut (PSI) with protons using the main 510-MeV cyclotron. Tumor conformal application of proton beams was realized by spot scanning technology. The median chordoma and CS dose was 74 and 68 cobalt Gy equivalent, respectively (cobalt Gy equivalent = proton Gy x 1.1). Median gross tumor volumes (GTV) were 16.4 mL (range, 1.8-48.1 mL) and 15.2 mL (range, 2.3-57.3 mL) for chordoma and CS, respectively. Late toxicity was assessed using the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE, v3.0) grading system. The median follow-up time was 29 months (range, 6-68 months). RESULTS: Actuarial 3-year local control rates were 87.5% and 100% for chordoma and CS, respectively. We observed one surgical pathway and one marginal failure in patients with chordomas. No regional failure or distant metastasis was observed. At 3 years, actuarial PFS and OS for the entire cohort was 90% and 93.8%, respectively. Actuarial 3-year complication-free survival was 82.2%. Radiation-induced pituitary dysfunction was observed in 4 (14%) patients (CTCAE Grade 2). No patient presented with post-PT brainstem or optic pathways necrosis or dysfunction. In univariate analysis, age < or =40 years at the time of PT affected favorably on PFS (p = 0.09). CONCLUSION: Spot-scanning PT offers high tumor control rates of skull base chordoma and CS. These results compare favorably to other combined proton-photon or carbon ion irradiation series. Observed toxicity was acceptable. Younger age (< or =40 years) was a favorable prognostic factor of PFS. These preliminary results are encouraging but should be confirmed during a longer follow-up.

Patient specific optimization of the relation between CT-hounsfield units and proton stopping power with proton radiography.

The purpose of this work is to show the feasibility of using in vivo proton radiography of a radiotherapy patient for the patient individual optimization of the calibration from CT-Hounsfield units to relative proton stopping power. Water equivalent tissue (WET) calibrated proton radiographs of a dog patient treated for a nasal tumor were used as baseline in comparison with integrated proton stopping power through the calibrated CT of the dog. In an optimization procedure starting with a stoichiometric calibration curve, the calibration was modified randomly. The result of this iteration is an optimized calibration curve which was used to recalculate the dose distribution of the patient. One result of this experiment was that the mean value of the deviations between WET calculations based on the stoichiometric calibration curve and the measurements was shifted systematically away from zero. The calibration produced by the optimization procedure reduced this shift to around 0.4 mm. Another result was that the precision of the calibration, reflected as the standard deviation of the normally distributed deviations between WET calculation and measurement, could be reduced from 7.9 to 6.7 mm with the optimized calibration. The dose distributions based on the two calibration curves showed major deviations at the distal end of the target volume.

Secondary neutron dose during proton therapy using spot scanning.

PURPOSE: During proton radiotherapy, secondary neutrons are produced by nuclear interactions in the material in the beam line before and after entering the patient. The dose equivalent deposited by these neutrons is usually not considered in routine treatment planning. In this study, we estimated the neutron dose in patients from a spot scanning beam line by performing measurements and Monte Carlo simulations. METHODS AND MATERIALS: Measurements of the secondary neutron dose were performed during irradiation of a water phantom with 177-MeV protons using a Bonner sphere and CR39 etch detectors. Additionally, Monte Carlo simulations were performed using the FLUKA code. RESULTS: A comparison of our measurements with measurements taken at a beam line using the scatter foil technique shows a dose advantage of at least 10 for the spot scanning technique. In the region of the Bragg peak, the neutron dose equivalent can reach for a medium-sized target volume approximately 1% of the treatment dose. Neutron doses expected in healthy tissues of the patient (in the not-treated volume) are for large and medium target volumes, approximately 0.004 Sv and 0.002 Sv per treatment Gy, respectively. CONCLUSIONS: We conclude from the measurements and simulations that the dose deposited by secondary neutrons during proton radiotherapy using the spot scanning technique can be neglected in the treatment region. In the healthy tissue, the dose coming from neutrons (0.002 Sv per treatment Gy) is approximately a factor of two larger than during photon treatment (0.001 Sv). These contributions to the integral dose from neutrons are still very low when compared to the dose sparing that can be achieved by using a proton beam instead of photons.

Spot-scanning proton radiation therapy for recurrent, residual or untreated intracranial meningiomas.

BACKGROUND AND PURPOSE: To assess the safety and efficacy of spot scanning proton beam radiation therapy (PRT) in the treatment of intracranial meningiomas. PATIENTS AND METHODS: Sixteen patients with intracranial meningioma (histopathologically proven in 13/16 cases) were treated with PRT between July 1997 and July 2002. Eight patients had skull base lesions. Thirteen patients received PRT after surgery either as adjuvant therapy for incomplete resection (eight patients) or for recurrence (five patients). Three patients received radical PRT after presumptive diagnosis based on imaging. The median prescribed dose was 56 CGE (range, 52.2-64, CGE=proton Gy X 1.1) at 1.8-2.0 CGE (median, 2.0) per fraction. Gross tumor volume and planning target volume ranged from 0.8 to 87.6 cc (median, 17.5) and 4.6-208.1 cc (median 107.7), respectively. Late ophthalmologic and non-ophthalmologic toxicity was assessed using the Subjective, Objective, Management and Analytic scale (SOMA) of the Late Effects of Normal Tissue scoring system and National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE, v3.0) grading system, respectively. The median follow-up time was 34.1 months (range, 6.5-67.8). RESULTS: Cumulative 3-year local control, progression-free survival and overall survival were 91.7, 91.7 and 92.7%, respectively. No patient died from recurrent meningioma. One patient progressed locally after PRT. Radiographic follow-up (median, 34 months) revealed an objective response in three patients and stable disease in 12 patients. Cumulative 3-year toxicity free survival was 76.2%. One patient presented with radiation induced optic neuropathy (SOMA Grade 3) and retinopathy (SOMA Grade 2) 8.8 and 30.4 months after treatment, respectively. These patients with ophthalmologic toxicity received doses higher than those allowed for the optic/ocular structures. Another patient developed a symptomatic brain necrosis (CTCAE Grade 4) 7.2 months after treatment. No radiation-induced hypothalamic/pituitary dysfunction was observed. CONCLUSIONS: Spot-scanning PRT is an effective treatment for patient with untreated, recurrent or incompletely resected intracranial meningiomas. It offers highly conformal irradiation for complex-shaped intracranial meningiomas, while delivering minimal non-target dose. Observed ophthalmologic toxicity is dose-related.

First proton radiography of an animal patient.

The purpose of this work is to show the feasibility of proton radiography in terms of radiation dose, imaging speed, image quality (density and spatial resolution), and image content under clinical conditions. Protons with 214 MeV energy can penetrate through most patients and were used for imaging. The measured residual range (or energy) of the protons behind the patient was subtracted from the range without an object in the beam path and used to create a projected image. The image content is therefore proportional to the range that protons have lost in the patient. We took proton images of the head of a dog after it received proton radiotherapy treatment of a nasal tumor. The spatial resolution by measuring for each proton separately its coordinate in front of and behind the patient was approximately 1 mm. The acquisition time was on the order of several seconds and was limited by the patient table movement. The range sensitivity of the images was approximately 0.6 mm, which is good enough to use the images for therapy range verification. The dose that the dog received during exposure was 0.03 mGy, which is approximately a factor 50-100 smaller than for a comparable x-ray image. The potential to obtain quantitative images of proton ranges with satisfying spatial and range resolution and low dose to the patient suggests that proton radiography should be applied to patients who are under proton radiotherapy treatment.

The water equivalence of solid materials used for dosimetry with small proton beams.

Various solid materials are used instead of water for absolute dosimetry with small proton beams. This may result in a dose measurement different to that in water, even when the range of protons in the phantom material is considered correctly. This dose difference is caused by the diverse cross sections for inelastic nuclear scattering in water and in the phantom materials respectively. To estimate the magnitude of this effect, flux and dose measurements with a 177 MeV proton pencil beam having a width of 0.6 cm (FWHM) were performed. The proton flux and the deposited dose in the beam path were determined behind water, lucite, polyethylene, teflon, and aluminum of diverse thicknesses. The number of out-scattered protons due to inelastic nuclear scattering was determined for water and the different materials. The ratios of the number of scattered protons in the materials relative to that in water were found to be 1.20 for lucite, 1.16 for polyethylene, 1.22 for teflon, and 1.03 for aluminum. The difference between the deposited dose in water and in the phantom materials taken in the center of the proton pencil beam, was estimated from the flux measurements, always taking the different ranges of protons in the materials into account. The estimated dose difference relative to water in 15 cm water equivalent thickness was -2.3% for lucite, -1.7% for polyethylene, -2.5% for teflon, and -0.4% for aluminum. The dose deviation was verified by a measurement using an ionization chamber. It should be noted that the dose error is larger when the effective point of measurement in the material is deeper or when the energy is higher.

Treatment planning and verification of proton therapy using spot scanning: initial experiences.

Since the end of 1996, we have treated more than 160 patients at PSI using spot-scanned protons. The range of indications treated has been quite wide and includes, in the head region, base-of-skull sarcomas, low-grade gliomas, meningiomas, and para-nasal sinus tumors. In addition, we have treated bone sarcomas in the neck and trunk--mainly in the sacral area--as well as prostate cases and some soft tissue sarcomas. PTV volumes for our treated cases are in the range 20-4500 ml, indicating the flexibility of the spot scanning system for treating lesions of all types and sizes. The number of fields per applied plan ranges from between 1 and 4, with a mean of just under 3 beams per plan, and the number of fluence modulated Bragg peaks delivered per field has ranged from 200 to 45 000. With the current delivery rate of roughly 3000 Bragg peaks per minute, this translates into delivery times per field of between a few seconds to 20-25 min. Bragg peak weight analysis of these spots has shown that over all fields, only about 10% of delivered spots have a weight of more than 10% of the maximum in any given field, indicating that there is some scope for optimizing the number of spots delivered per field. Field specific dosimetry shows that these treatments can be delivered accurately and precisely to within +/-1 mm (1 SD) orthogonal to the field direction and to within 1.5 mm in range. With our current delivery system the mean widths of delivered pencil beams at the Bragg peak is about 8 mm (sigma) for all energies, indicating that this is an area where some improvements can be made. In addition, an analysis of the spot weights and energies of individual Bragg peaks shows a relatively broad spread of low and high weighted Bragg peaks over all energy steps, indicating that there is at best only a limited relationship between pencil beam weighting and depth of penetration. This latter observation may have some consequences when considering strategies for fast re-scanning on second generation scanning gantries.

Development of an inorganic scintillating mixture for proton beam verification dosimetry.

The availability at the Paul Scherrer Institute (PSI) of a spot-scanning technique with an isocentric beam delivery system (gantry) allows the realization of intensity-modulated proton therapy (IMPT). The development of 3D dosimetry is an important tool for the verification of IMPT therapy plans based on inhomogeneous 3D conformal dose distributions. For that purpose new dosimeters are being developed. The concept is to use a system of many millimetre sized scintillating volumes distributed in a polyethylene block, which are read on a CCD camera over a bundle of optical fibres and which can be irradiated from any direction orthogonal to the fibre axis. The purpose of this work is to investigate the composition of such small sensitive volumes. A mixture of inorganic phosphors and optical cement allows an optimal coupling between the scintillating volume and the optical fibre. Five different inorganic phosphors, available as powder, have been examined by considering their response along the Bragg curve. In particular, two phosphors have shown interesting behaviours: Gd2O2S:Tb and (Zn, Cd)S:Ag. Both phosphors have a high emission efficiency but contrasting behaviour in the Bragg peak region. The efficiency of Gd2O2S:Tb decreases with increasing stopping power (quenching of luminescence) while that of (Zn, Cd)S:Ag increases. Because of these contrasting behaviours it is possible to prepare a mixture of the two scintillating powders in a certain ratio in order to modulate the height of the measured Bragg peak relative to the entrance value so that it is in agreement with the ionization chamber measurements. We propose to use a mixture for the sensitive volume consisting of the following weight fractions: 48% Gd2O2S:Tb, 12% (Zn, Cd)S:Ag and 40% optical cement.

The clinical potential of intensity modulated proton therapy.

Intensity Modulated Proton Therapy (IMPT) differs from conventional proton therapy in its ability to deliver depth-shifted, arbitrarily complex proton fluence maps from each incident field direction. As the individual Bragg peaks delivered from any field can be distributed in three-dimensions throughout the target volume, IMPT provides many more degrees of freedom for designing dose distributions than IMRT or conventional proton therapy techniques. So how can the flexibility of IMPT best be exploited? Here we argue that IMPT has two main advantages over photon IMRT and conventional proton therapy: the ability to better 'sculpt' the dose to the target and around neighbouring critical structures, and the ability to find clinically acceptable solutions whilst simultaneously reducing the sensitivity of the treatments to potential delivery errors. The concept of IMPT as a tool for generating 'safer' plans opens an interesting new avenue of research from the point of view of plan optimisation, the potential of which is only just beginning to be explored.

The PSI Gantry 2: a second generation proton scanning gantry.

PSI is still the only location in which proton therapy is applied using a dynamic beam scanning technique on a very compact gantry. Recently, this system is also being used for the application of intensity-modulated proton therapy (IMPT). This novel technical development and the success of the proton therapy project altogether have led PSI in Year 2000 to further expand the activities in this field by launching the project PROSCAN. The first step is the installation of a dedicated commercial superconducting cyclotron of a novel type. The second step is the development of a new gantry, Gantry 2. For Gantry 2 we have chosen an iso-centric compact gantry layout. The diameter of the gantry is limited to 7.5 m, less than in other gantry systems (approximately 10-12 m). The space in the treatment room is comfortably large, and the access on a fixed floor is possible any time around the patient table. Through the availability of a faster scanning system, it will be possible to treat the target volume repeatedly in the same session. For this purpose, the dynamic control of the beam intensity at the ion source and the dynamic variation of the beam energy will be used directly for the shaping of the dose.

Spatial resolution of proton tomography: Methods, initial phase space and object thickness.

PURPOSE: Proton radiography and tomography was investigated since the early 1970s because of its low radiation dose, high density resolution and ability to image directly proton stopping power. However, spatial resolution is still a limiting factor and as a consequence experimental methods and image reconstruction should be optimized to improve position resolution. METHODS: Spatial resolution of proton radiography and tomography is given by multiple Coloumb scattering (MCS) of the protons in the patient. In this paper we employ an improved MCS model to study the impact of various proton tomographic set-ups on the spatial resolution, such as different combinations of entrance and exit coordinate and angle measurements, respectively, initial particle energy and angular confusion of the incident proton field. RESULTS: It was found that best spatial resolution is obtained by measuring in addition to the entrance and exit coordinates also the entrance and exit angles. However, by applying partial backprojection and by using a perfect proton fan beam a sufficient spatial resolution can be achieved with less experimental complexity (measuring only exit angles). It was also shown that it is essential to use the most probable proton trajectory to improve spatial resolution. A simple straight line connection for image reconstruction results in a spatial resolution which is not clinically sufficient. The percentage deterioration of spatial resolution due to the angular confusion of the incident proton field is less than the phase space in mrad. A clinically realistic proton beam with 10 mrad angular confusion results in a less than 10% loss of spatial resolution. CONCLUSIONS: Clinically sufficient spatial resolution can be either achieved with a full measurement of entrance and exit coordinates and angles, but also by using a fan beam with small angular confusion and an exit angle measurement. It is necessary to use the most probable proton path for image reconstruction. A simple straight line connection is in general not sufficient. Increasing proton energy improves spatial resolution of an object of constant size. This should be considered in the design of proton therapy facilities.