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A Lomax

Publications and source records attributed to A Lomax.

At least 19 recordsLinked to original sources
PubMed2005

Experimental characterization and physical modelling of the dose distribution of scanned proton pencil beams.

In this paper we present the pencil beam dose model used for treatment planning at the PSI proton gantry, the only system presently applying proton therapy with a beam scanning technique. The scope of the paper is to give a general overview on the various components of the dose model, on the related measurements and on the practical parametrization of the results. The physical model estimates from first physical principles absolute dose normalized to the number of incident protons. The proton beam flux is measured in practice by plane-parallel ionization chambers (ICs) normalized to protons via Faraday-cup measurements. It is therefore possible to predict and deliver absolute dose directly from this model without other means. The dose predicted in this way agrees very well with the results obtained with ICs calibrated in a cobalt beam. Emphasis is given in this paper to the characterization of nuclear interaction effects, which play a significant role in the model and are the major source of uncertainty in the direct estimation of the absolute dose. Nuclear interactions attenuate the primary proton flux, they modify the shape of the depth-dose curve and produce a faint beam halo of secondary dose around the primary proton pencil beam in water. A very simple beam halo model has been developed and used at PSI to eliminate the systematic dependences of the dose observed as a function of the size of the target volume. We show typical results for the relative (using a CCD system) and absolute (using calibrated ICs) dosimetry, routinely applied for the verification of patient plans. With the dose model including the nuclear beam halo we can predict quite precisely the dose directly from treatment planning without renormalization measurements, independently of the dose, shape and size of the dose fields. This applies also to the complex non-homogeneous dose distributions required for the delivery of range-intensity-modulated proton therapy, a novel therapy technique developed at PSI.

Calibration↗
PubMed2001

Potential role of intensity modulated proton beams in prostate cancer radiotherapy.

PURPOSE: The present study was undertaken to assess the potential benefit of intensity modulated (IM) proton beams in optimizing the dose distribution to safely escalate the tumor dose in prostate cancer radiotherapy. METHODS AND MATERIALS: Four treatment plans were compared in a prostate cancer patient aiming to deliver 81 Gy to the target: 1) conformal 18 MV X-rays, 6-fields; 2) 214 MeV protons, 2-fields; 3) IM 15 MV X-rays, 5-fields; and 4) 177-200 Mev IM protons, 5-fields as in Plan 3. In addition, IM methods were used to further escalate the tumor dose to 99 Gy. Dose-volume histograms (DVH) were used to physically compare the treatment plans. DVH data were also used to obtain normal tissue complication probabilities (NTCP) for the rectum, bladder, femoral heads, and tumor control probabilities. RESULTS: Although the planning target volume dose distribution was satisfactory with the four treatment plans, the homogeneity was slightly reduced in both X-ray plans (IM and standard) and the low-to-medium doses delivered to all organs at risk, and other normal tissues were significantly reduced by both proton plans. For a prescribed dose of 81 Gy, only the IM X-ray and IM proton plans both succeeded in predicting an acceptably low NTCP for the rectum (<5%, Grade 3). The integral nontarget dose was significantly reduced with IM proton beams (i.e., 3.1, 1.3, and 1.7 times less than Plans 1, 2, and 3, respectively). When escalating the dose to 99 Gy, no additional improvement between IM protons and IM X-ray beams was observed. CONCLUSION: Both IM X-ray and proton beams were able to optimize the dose distribution and comply with the goal of delivering the highest dose to the target while reducing the risk of severe morbidity to acceptable levels. The main advantage compared to IM X-rays was that IM protons succeeded in significantly reducing the low-to-medium dose to the nontarget tissues and achieved a small improvement in planning target volume (PTV) dose heterogeneity.

Femur Head↗
PubMed2001

A treatment planning comparison of 3D conformal therapy, intensity modulated photon therapy and proton therapy for treatment of advanced head and neck tumours.

BACKGROUND AND PURPOSE: In this work, the potential benefits and limitations of different treatment techniques, based on mixed photon-electron beams, 3D conformal therapy, intensity modulated photons (IM) and protons (passively scattered and spot scanned), have been assessed using comparative treatment planning methods in a cohort of patients presenting with advanced head and neck tumours. MATERIAL AND METHODS: Plans for five patients were computed for all modalities using CT scans to delineate target volume (PTV) and organs at risk (OAR) and to predict dose distributions. The prescribed dose to the PTV was 54 Gy, whilst the spinal cord was constrained to a maximum dose of 40.5 Gy for all techniques. Dose volume histograms were used for physical and biological evaluation, which included equivalent uniform dose (EUD) calculations. RESULTS: Excluding the mixed photon-electron technique, PTV coverage was within the defined limits for all techniques, with protons providing significantly improved dose homogeneity, resulting in correspondingly higher EUD results. For the spinal cord, protons also provided the best sparing with maximum doses as low as 17 Gy. Whilst the IM plans were demonstrated to be significantly superior to non-modulated photon plans, they were found to be inferior to protons for both criteria. A similar result was found for the parotid glands. Although they are partially included in the treated volume there is a clear indication that protons, and to a lesser extent IM photons, could play an important role in preserving organ functionality with a consequent improvement of the patient's quality of life. CONCLUSIONS: For advanced head and neck tumours, we have demonstrated that the use of IM photons or protons both have the potential to reduce the possibility of spinal cord toxicity. In addition, a substantial reduction of dose to the parotid glands through the use of protons enhances the interest for such a treatment modality in cases of advanced head and neck tumours. However, in terms of target coverage, the use of 3D conformal therapy, although somewhat inferior in quality to protons or IM photons, has been shown to be a reasonable alternative to the more advanced techniques. In contrast, the conventional technique of mixed photon and electron fields has been shown to be inferior to all other techniques for both target coverage and OAR involvement.

Carcinoma, Squamous Cell↗
PubMed2001

New techniques in hadrontherapy: intensity modulated proton beams.

Inverse planning and intensity modulated (IM) X-ray beam treatment techniques can achieve significant improvements in dose distributions comparable to those obtained with forward planned proton beams. However, intensity modulation can also be applied to proton beams and further optimization in dose distribution can reasonably be expected. A comparative planning exercise between IM X-rays and IM proton beams was carried out on two different tumor cases: a pediatric rhabdomyosarcoma and a prostate cancer. Both IM X-rays and IM protons achieved equally homogenous coverage of the target volume in the two tumor sites. Predicted NTCPs were equally low for both treatment techniques. Nevertheless, a reduced low-to-medium dose to the organs at risk and a lesser integral non-target mean dose for IM protons in the two cases favored the use of IM proton beams.

Dose-Response Relationship, Radiation↗
PubMed2001

Comparative treatment planning using secondary cancer mortality calculations.

Calculations of mortality due to secondary cancer have been investigated for its use in comparative treatment planning. A patient with Hodgkin's disease has been chosen as an example and has been planned with different radiation treatment modalities using photons and protons. The ICRP calculation scheme has been used to calculate mortality from dose distributions. To this purpose target volumes as well as critical structures have been outlined in the CT set of a patient with Hodgkin's disease. Dose distributions have been calculated using conventional as well as intensity modulated treatment techniques using photon and proton radiation. From the mean doses of each organ the mortality has been derived. Our work suggests that calculations of mortality can be useful in comparative treatment planning. Such mortality calculations can be helpful to find decisions between radiotherapy treatment techniques (intensity modulated or conventional treatment) or between different types of radiation (photons, electrons, protons, neutrons).

Adult↗
PubMed2000

The role of proton therapy in the treatment of large irradiation volumes: a comparative planning study of pancreatic and biliary tumors.

PURPOSE: The purpose of this study was to examine the potential benefit of proton therapy for abdominal tumors. Extensive comparative planning was conducted investigating the most up-to-date photon and proton irradiation technologies. METHODS AND MATERIALS: A number of rival plans were generated for four patients: two inoperable pancreatic tumors, one inoperable and one postoperative biliary duct tumor. The dose prescription goal for these large targets was 50 Gy, followed by a boost dose up to 20 Gy to a smaller planning target volume (PTV). Photon plans were developed using "forward" planning of coplanar and noncoplanar conformal fields and "inverse" planning of intensity-modulated (IM) fields. Proton planning was simulated as administered using the so called spot-scanning technique. Plans were evaluated on the basis of normal tissues' dose-volume constraints (Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys 1990;21:109-122) and coverage of treatment volumes with prescribed doses. RESULTS: For all cases, none of the forward calculated photon plans was able to deliver 50 Gy to large PTVs at the same time respecting the dose-volume constraints on all critical organs. Nine evenly spaced IM fields achieved or nearly achieved all maximum dose constraints to critical structures for two out of three inoperable patients. IM plans also obtained good results for the postoperative patient, even though the dose to the liver was very close to the maximum allowed. In all cases, photon irradiation of large PTV1s to 50 Gy followed by a 20 Gy boost entailed a risk very close to or higher than 5% for serious complications to the kidneys, liver, or bowel. Simple arrangements of 2, 3, and 4 proton fields obtained better dose conformation to the target, allowing the delivery of planned doses including the boost to all patients, without excessive risk of morbidity. Dose homogeneity inside the targets was also superior with protons. CONCLUSION: For the irradiation of large PTVs located in the abdominal cavity, where multiple, parallel structured organs surround the target volumes, proton therapy, delivered with a sophisticated isocentric technique, has the potential to achieve superior dose distributions compared with state-of-the-art photon irradiation techniques. IM photon plans obtain better results in the postoperative case, because the reduced volume lessens the effect of the unavoidable increase of integral dose to surrounding tissues.

Algorithms↗
PubMed2000

The exchange of radiotherapy data as part of an electronic patient-referral system.

PURPOSE: To describe the implementation and use of an electronic patient-referral system as an aid to the efficient referral of patients to a remote and specialized treatment center. METHODS AND MATERIALS: A system for the exchange of radiotherapy data between different commercial planning systems and a specially developed planning system for proton therapy has been developed through the use of the PAPYRUS diagnostic image standard as an intermediate format. To ensure the cooperation of the different TPS manufacturers, the number of data sets defined for transfer has been restricted to the three core data sets of CT, VOIs, and three-dimensional dose distributions. As a complement to the exchange of data, network-wide application-sharing (video-conferencing) technologies have been adopted to provide methods for the interactive discussion and assessment of treatments plans with one or more partner clinics. RESULTS: Through the use of evaluation plans based on the exchanged data, referring clinics can accurately assess the advantages offered by proton therapy on a patient-by-patient basis, while the practicality or otherwise of the proposed treatments can simultaneously be assessed by the proton therapy center. Such a system, along with the interactive capabilities provided by video-conferencing methods, has been found to be an efficient solution to the problem of patient assessment and selection at a specialized treatment center, and is a necessary first step toward the full electronic integration of such centers with their remotely situated referral centers.

Computer Communication Networks↗
PubMed2000

Optimizing radiotherapy of orbital and paraorbital tumors: intensity-modulated X-ray beams vs. intensity-modulated proton beams.

PURPOSE: This study presents a dosimetric optimization effort aiming to compare intensity-modulated (IM) X-rays and IM protons in 4 different orbital and paraorbital tumors. These are most challenging targets for standard radiotherapy due to their close relationship with the eyes and related structures. METHODS AND MATERIALS: A primary orbital lymphoma, an optic nerve meningioma, a sphenoidal ridge meningioma protruding into the orbit, and a pediatric parameningeal paraorbital rhabdomyosarcoma were selected for the purpose of this study. Planning target volumes (PTVs) and organs at risk (OAR) were defined in each patient CT data set for each tumor site. IM X-ray and IM proton three-dimensional treatment plans were implemented. The following total tumor doses were prescribed: 30 Gy for the orbital lymphoma, 54 Gy for both meningiomas, and 50.4 Gy for the rhabdomyosarcoma case. Dose-volume histograms (DVHs) were obtained for all targets and OAR with both treatment techniques. DVHs were used to predict normal tissue complication probabilities (NTCPs) for the OAR in the vicinity of the tumor. RESULTS: The PTV coverage was optimal and equally homogeneous with both IM X-rays and IM proton plans in the 4 tumor sites. DVHs for most OAR were better with IM proton beams especially in the low- to mid-dose range region. The integral nontarget dose was lower with IM protons in every case (factor ranging from 1.5 to 1.9). However, predicted NTCPs (for severe late effects) were equally low for both treatment techniques in every tumor site. CONCLUSION: Although IM proton plans optimally decreased the dose to the OAR in all tumor sites, both optimized X-ray and proton beams equally succeeded to reduce severe-toxicity prediction risks to less than 5% while optimally treating the PTV.

Humans↗
PubMed2000

Performance of a fluorescent screen and CCD camera as a two-dimensional dosimetry system for dynamic treatment techniques.

A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating (fluorescent) screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device (CCD) camera. The observed light distribution at the screen is equivalent to the two-dimensional (2D)-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inhomogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the verification of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected (intended or calculated) dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams.

Algorithms↗
PubMed2000

Comparative risk assessment of secondary cancer incidence after treatment of Hodgkin's disease with photon and proton radiation.

Probabilities for secondary cancer incidence have been estimated for a patient with Hodgkin's disease for whom treatment has been planned with different radiation modalities using photons and protons. The ICRP calculation scheme has been used to calculate cancer incidence from dose distributions. For this purpose, target volumes as well as critical structures have been outlined in the CT set of a patient with Hodgkin's disease. Dose distributions have been calculated using conventional as well as intensity-modulated treatment techniques using photon and proton radiation. The cancer incidence has been derived from the mean doses for each organ. The results of this work are: (a) Intensity-modulated treatment of Hodgkin's disease using nine photon fields (15 MV) results in nearly the same cancer incidence as treating with two opposed photon fields (6 MV). (b) Intensity-modulated treatment using nine proton fields (maximum energy 177.25 MeV) results in nearly the same cancer incidence as treating with one proton field (160 MeV). (c) Irradiation with protons using the spot scanning technique decreases the avoidable cancer incidence compared to photon treatment by a factor of about two. This result is independent of the number of beams used. Our work suggests that there are radiotherapy indications in which intensity-modulated treatments will result in little or no reduction of cancer incidence compared to conventional treatments. However, proton treatment can result in a lower cancer incidence than photon treatment.

Adult↗
PubMed1999

Initial experience of using an active beam delivery technique at PSI.

At PSI a new proton therapy facility has been assembled and commissioned. The major features of the facility are the spot scanning technique and the very compact gantry. The operation of the facility was started in 1997 and the feasibility of the spot scanning technique has been demonstrated in practice with patient treatments. In this report we discuss the usual initial difficulties encountered in the commissioning of a new technology, the very positive preliminary experience with the system and the optimistic expectations for the future. The long range goal of this project is to parallel the recent developments regarding inverse planning for photons with a similar advanced technology optimized for a proton beam.

Humans↗
PubMed1999

Dose calculation models for proton treatment planning using a dynamic beam delivery system: an attempt to include density heterogeneity effects in the analytical dose calculation.

The gantry for proton radiotherapy at the Paul Scherrer Institute (PSI) is designed specifically for the spot-scanning technique. Use of this technique to its full potential requires dose calculation algorithms which are capable of precisely simulating each scanned beam individually. Different specialized analytical dose calculations have been developed, which attempt to model the effects of density heterogeneities in the patient's body on the dose. Their accuracy has been evaluated by a comparison with Monte Carlo calculated dose distributions in the case of a simple geometrical density interface parallel to the beam and typical anatomical situations. A specialized ray casting model which takes range dilution effects (broadening of the spectrum of proton ranges) into account has been found to produce results of good accuracy. This algorithm can easily be implemented in the iterative optimization procedure used for the calculation of the optimal contribution of each individual scanned pencil beam. In most cases an elemental pencil beam dose calculation has been found to be most accurate. Due to the long computing time, this model is currently used only after the optimization procedure as an alternative method of calculating the dose.

Algorithms↗
PubMed1999

Intensity modulation methods for proton radiotherapy.

The characteristic Bragg peak of protons or heavy ions provides a good localization of dose in three dimensions. Through their ability to deliver laterally and distally shaped homogenous fields, protons have been shown to be a precise and practical method for delivering highly conformal radiotherapy. However, in an analogous manner to intensity modulation for photons, protons can be used to construct dose distributions through the application of many individually inhomogeneous fields, but with the localization of dose in the Bragg peak providing the possibility of modulating intensity within each field in two or three dimensions. We describe four different methods of intensity modulation for protons and describe how these have been implemented in an existing proton planning system. As a preliminary evaluation of the efficacy of these methods, each has been applied to an example case using a variety of field combinations. Dose-volume histogram analysis of the resulting dose distributions shows that when large numbers of fields are used, all techniques exhibit both good target homogeneity and sparing of neighbouring critical structures, with little difference between the four techniques being discerned. As the number of fields is decreased, however, only a full 3D modulation of individual Bragg peaks can preserve both target coverage and sparing of normal tissues. We conclude that the 3D method provides the greatest flexibility for constructing conformal doses in challenging situations, but that when large numbers of beam ports are available, little advantage may be gained from the additional modulation of intensity in depth.

Computing Methodologies↗
PubMed1997

Potential role of proton therapy in the treatment of pediatric medulloblastoma/primitive neuro-ectodermal tumors: spinal theca irradiation.

PURPOSE: Conventional postoperative photon-beam radiotherapy to the spine in children with medulloblastoma/PNET is associated with severe late effects. This morbidity (growth and developmental) is related to the exit dose of the beams and is particularly severe in young children. With the purpose of reducing this toxicity, a dosimetric study was undertaken in which proton therapy was compared to standard megavoltage photon treatment. METHODS AND MATERIALS: The results of a comparative dosimetric study are presented in such a way that the dose distribution achievable with a posterior modulated 100 MeV proton beam (spot scanning method) is compared with that of a standard set of posterior 6 MV x-ray fields. The potential improvements with protons are evaluated, using dose-volume histograms to examine the coverage of the target as well as the dose to the vertebral bodies (growth plates), lungs, heart, and liver. RESULTS: The target (i.e., the spinal dural sac) received the full prescribed dose in both treatment plans. However, the proportions of the vertebral body volume receiving > or = 50% of the prescribed dose were 100 and 20% for 6 MV x-rays and protons, respectively. For 6 MV x-rays > 60% of the dose prescribed to the target was delivered to 44% of the heart volume, while the proton beam was able to completely avoid the heart, the liver, and in all likelihood the thyroid and gonads as well. CONCLUSION: The present study demonstrates a potential role of proton therapy in decreasing the dose (and toxicity) to the critical structures in the irradiation of the spinal neuraxis in medulloblastoma/PNET. The potential bone marrow and growth arrest sparing effects make this approach specially attractive for intensive chemotherapy protocols and for very young children. Sparing the thyroid gland, the posterior heart wall, and the gonads may be additional advantages in assuring a long-term posttreatment morbidity-free survival.

Brain Neoplasms↗
PubMed1997

Potential role of proton therapy in the treatment of pediatric medulloblastoma/primitive neuroectodermal tumors: reduction of the supratentorial target volume.

PURPOSE: One of the components of radiotherapy (RT) in medulloblastoma/primitive neuroectodermal tumors is the prophylactic irradiation of the whole brain (WBI). With the aim of reducing late neuropsychologic morbidity a CT-scan-based dosimetric study was undertaken in which treatment was confined mainly or exclusively to supratentorial sites considered at high risk for disease recurrence. METHODS AND MATERIALS: A comparative dosimetric study is presented in which a three field (two laterals and one posterior) proton plan (spot scanning method) is compared with a two-field conventional WBI 6 MV x-ray plan, to a 6-field "hand-made" 6 MV x-ray plan, and to a computer-optimized 9-field "inverse" 15 MV x-ray plan. For favorable patients, 30 Gy were delivered to the ventricles and main cisterns, the subfrontal and subtemporal regions, and the posterior fossa. For the unfavorable patients, 10 Gy WBI preceeded a boost to 30 Gy to the same treatment volume chosen for favorable patients. The dose distribution was evaluated with dose-volume histograms to examine the coverage of the targets as well as the dose to the nontarget brain and optical structures. In addition, the risks of radiation-related late neuropsychologic effects after WBI were collected from the literature and used to predict normal tissue complication probabilities (NTCPs) for an intelligence quotient deficit after treatment with photon or proton beams. RESULTS: Proton beams succeeded better in reducing the dose to the brain hemispheres and eye than any of the photon plans. A 25.1% risk of an IQ score <90 was predicted after 30 Gy WBI. Almost a 10% drop in the predicted risk was observed when using proton beams in both favorable and unfavorable patients. However, predicted NTCPs for both optimized photon plans ("hand made" and "inverse") were only slightly higher (0.3-2.5%) than those of proton beams. An age-modifying factor was introduced in the predictive NTCP model to assess for IQ differences in relation with age at irradiation. Children with ages between age 4 to 8 benefitted most from the dose reduction in this exercise (similar NTCP predictions for both proton and "inverse" plans). CONCLUSION: Modulated proton beams may help to significantly reduce the irradiation of normal brain while optimally treating the supratentorial subsites at higher risk for relapse. A decrease in morbidity can be expected from protons and both optimized proton plans compared to WBI.

Brain Neoplasms↗
PubMed1996

Proton dosimetry intercomparison.

BACKGROUND AND PURPOSE: Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols. MATERIALS AND METHODS: Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV). RESULTS: The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%. CONCLUSION: The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Calibration↗
PubMed1996

Characterization of dose distribution in radiation therapy plans.

As a method of considering only significant radiation doses to different tissues, the ICRU Report 50 recommends taking the dose given to a significant tissue volume (minimum diameter greater then 15 mm) instead of choosing a single, potentially insignificant, voxel value. In order to find this significant volume, we have adapted an emission imaging analysis method to radiation therapy planning. The resulting method finds and characterizes the dose distribution in the volumes of interest in a way that includes spatial arrangement. The data can be used to signal significant hot or cold volumes in the dose plan and to score the plans based on significant dose to the tissues.

Humans↗