Modulated Proton Beams Research Articles

TH‐C‐M100E‐10: A Correction Method for the MOSFET Energy Dependence Response to Therapeutic Proton Beams

The energy‐dependence response of the metal oxide semiconductor field‐effect transistor (MOSFET) dosimeter has been investigated with regard to therapeutic proton beams. The MOSFET configurations used include the commercial standard MOSFET (TN‐502RD), the microMOSFET (TN‐502RDM) dosimeters, and a prototype MOSFET with no encapsulation. Proton beams of non‐modulated pristine and modulated Spread‐Out Bragg Peak (SOBP) of 5 cm and 10 cm widths were used with beam ranges of 8.9cm, 15.9cm and 25.9cm in water. Each MOSFET dosimeter was calibrated at the center of the modulated 10 cm SOBP proton beam with conditions of 1 cGy per MU. The MOSFET energy‐dependence response was quantitatively evaluated by the ratio of measured doses between the MOSFET and an ionization chamber in a same condition. The three dosimeters showed a similar response for the pristine proton beams at various beam ranges. This indicates that the variation in dosimeter response is dominated by the change of the linear energy transfer (LET) for the used proton beam and not by the MOSFET encapsulation thickness. The observed MOSFET trends for various pristine proton beams have been modeled by an analytical function , where Rres is the residue range (distance to the distal 90% level dose), and A, B, C are constants. A similar modeling has been performed for modulated SOBP proton beams at different widths and for various beam ranges. The k (Rres) function has been used as a correction factor for patient dose measured by MOSFETs for corresponding residue ranges; this resulted in dose measurements with an uncertainty of less than 3.0% for various proton beams.

High-Dose proton beam radiotherapy of hepatocellular carcinoma: Preliminary results of a phase II trial

This phase II trial was undertaken to determine the efficacy and toxicity of proton beam radiotherapy for patients with locally unresectable hepatocellular carcinoma. Cirrhotic patients were eligible if they had a Child-Pugh score of 10 or less. Eligible patients included those with T 1 -T 3 hepatocellular carcinoma; selected T 4 patients also were eligible. Patients with lymph node or distant metastases were ineligible. Daily proton beam radiotherapy was directed to the liver tumor with an additional 1-2 cm margin. The total dose was 63 cobalt Gray equivalents, administered in 15 divided fractions over 3 weeks. Thirty-four patients have completed treatment and have been followed up for a minimum of 6 months, with a median follow-up period of 20 months. The mean age was 65 years, and average tumor size was 5.7 cm. Posttreatment toxicity included a small but significant decline in albumin levels and increased total bilirubin; 3 experienced duodenal or colonic bleeding when bowel was immediately adjacent to the treated tumor. Two-year actuarial data showed a 75% local tumor control rate and an overall survival rate of 55%. Of patients with an elevated pretreatment alpha-fetoprotein, 85% were found to have declining alpha-fetoprotein levels from a pretreatment mean of 1405 to 35 at 6 months after treatment. Six patients underwent liver transplantation between 6 and 16 months after completion of radiotherapy with 2 showing no evidence of residual carcinoma within the explanted liver. Overall the majority of patients responded to treatment, and the therapy was well tolerated.

A 62-MeV proton beam for the treatment of ocular melanoma at Laboratori Nazionali del Sud-INFN

At the Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) in Catania, Italy, the first Italian protontherapy facility, named Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) has been built in collaboration with the University of Catania. It is based on the use of the 62-MeV proton beam delivered by the K=800 Superconducting Cyclotron installed and working at INFN-LNS since 1995. The facility is mainly devoted to the treatment of ocular diseases like uveal melanoma. A beam treatment line in air has been assembled together with a dedicated positioning patient system. The facility has been in operation since the beginning of 2002 and 66 patients have been successfully treated up to now. The main features of CATANA together with the clinical and dosimetric features will be extensively described; particularly, the proton beam line, that has been entirely built at LNS, with all its elements, the experimental transversal and depth dose distributions of the 62-MeV proton beam obtained for a final collimator of 25-mm diameter and the experimental depth dose distributions of a modulated proton beam obtained for the same final collimator. Finally, the clinical results over 1 yr of treatments, describing the features of the treated diseases will be reported.

Dosimetry using plane-parallel ionization chambers in a 75 MeV clinical proton beam

Reference ionization chamber dosimetry in clinical proton beams is generally performed with cylindrical ionization chambers. However, when the measurement is performed in the presence of a large depth dose gradient or in a narrow spread out Bragg peak (SOBP), it could be advisable to use a plane-parallel chamber. Few recommendations and studies have been devoted to this subject. In this paper, experimental information on perturbation correction factors for four plane-parallel ionization chamber types in proton beams is presented. The experiments were performed in 75 MeV modulated and non-modulated proton beams. Monte Carlo calculations have been performed to support the conclusions of the experimental work.Overall, we were not able to find experimental evidence for significant differences between the secondary electron perturbation correction factors for plane-parallel chambers and those for a cylindrical NE2571. We found experimental ratios of perturbation correction factors that did not differ by more than 0.6% from unity for a Roos and two NACP02 chambers, and by not more than 1.2% for a Calcam-2 and two Markus chambers. Monte Carlo simulations result in corrections that are limited to 0.6% in absolute value, but given the overall uncertainties of the measurements, the deviations of the correction factors from unity could not be resolved from the experimental results. The results of the simulations thus support the experimental conclusion that perturbation correction factors for the set of plane-parallel chambers in both proton beams (relative to NE2571) do not deviate from unity by more than 1.2%. This confirms, within the experimental uncertainties, the assumption that the overall perturbation correction factor for a plane-parallel chamber in a low-energy proton beam is unity, made in IAEA TRS-398 and other dosimetry protocols. Given the large uncertainties of the gradient correction factors to be applied when using a cylindrical ionization chamber in a narrow SOBP or in the presence of a strong depth dose gradient, the level of agreement between plane-parallel and cylindrical ionization chambers observed in this study shows that plane-parallel chambers are a reliable alternative for reference dosimetry in low-energy proton beams.

Preclinical biological assessment of proton and carbon ion beams at hyogo ion beam medical center

Purpose: To assess the biological effects of proton and carbon ion beams and to decide clinical RBE values before clinical use. Materials and methods: Cultured cells from human salivary gland cancer (HSG) were irradiated at 5 points along a 190MeV proton beam and along a 320MeV carbon ion beam, with its Bragg peak modulated to 6cm widths (P190/6-SOBP and C320/6-SOBP), and a linac 4MV X-ray (X4) was used as a reference. Survival curves were obtained by colony formation assay. RBE values at each point was calculated when D10 or D50 doses were given to the center of each SOBP beam. Cells were also irradiated in a cell-stack phantom to identify that presumed cell deaths were observed at predefined depth in a setting of single fractional irradiation. To represent in vivo biological effects, total body irradiation of C3H/He mice was performed at 4 points along both P190/6-SOBP and C320/6-SOBP. The number of regenerating crypts per jejunal section was compared between P190/6-SOBP or C320/6-SOBP and referential X4 to calculate intestinal RBE values. Mice right legs were also irradiated at 2 points along a C320/6-SOBP beam by 4-fractional treatment and the irradiated areas were followed-up for a month by means of skin reaction scoring. Results: RBE values for P190/6-SOBP calculated from cell survival curves ranged from 1.01 to 1.05 at D10 level and from 1.01 to 1.07 at D50 level, and the values for C320/6-SOBP ranged from 1.23 to 2.56 and from 1.21 to 3.47, respectively (Table). The biological doses in the SOBP portion were flat enough with the uniformity of }2.5% for P190/6-SOBP and }5.0% for C320/6-SOBP. The cell-stack phantom irradiation revealed the flatness of the biological doses of both SOBP beams with the same accuracy as anticipated. The intestinal RBE values ranged from 1.01 to 1.08 for P190/6-SOBP and from 1.15 to 1.18 for C320/6-SOBP at the dose level that the number of jejunal crypts decreases to be 10 per circumference. The skin RBE value calculated from 4-fractional treatment of mice right legs was 1.30 at 5 mm depth and 2.16 at the SOBP center. We adopted the clinical RBE value of 1.1 at the center of SOBP for modulated proton beams. A comparative biological study with the other 6cm-SOBP beam generated from a 290MeV carbon beam demonstrated that carbon beams at the two different facilities had the same biological effect when the dose-averaged LET was the same (46.6 keV/μm). Thus the clinical RBE value of 2.23 was assigned at the 46.6 keV/μm point in the modulated carbon ion beam (SOBP center of C320/6-SOBP) to share a clinical dose-response evaluation system between the two facilities. Conclusion: Expected biological depth-dose distributions were observed on both P190/6-SOBP and C320/6-SOBP beams. The clinical RBE value was decided as 1.1 at the center of SOBP for modulated proton beams and 2.23 at 46.6keV/μm point in modulated carbon ion beams. Tabled 1Biological systemEnd pointX4 dose levelRBE values of proton P190/6-SOBPRBE values of carbon ion C320/6-SOBPHSG cell10% survival5.23Gy1.01 – 1.051.23 – 2.56HSG cell50% survival2.33Gy1.01 – 1.071.21 – 3.46Mouse jejunal crypt10 crypts/section14.70Gy1.00 – 1.081.15 – 1.88Mouse skin reactionPeak score = 2.541.6GyNot assessed1.30 (entrance), 2.16 (SOBP center) Open table in a new tab

164 oral Optimizing radiotherapy of orbital and paraorbital tumors: intensity modulated X-ray beams versus intensity modulated proton beams

164 oral Optimizing radiotherapy of orbital and paraorbital tumors: intensity modulated X-ray beams versus intensity modulated proton beams

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. Conclusions: 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.

Clinical Proton Dosimetry Part I: Beam Production, Beam Delivery and Measurement of Absorbed Dose (ICRU Report 59)

International Commission on Radiation Units and Measurements Bethesda, MD: ICRU (1998) 60pp, ISBN: 0-913394-58-0

Radiochromic film dosimetry of a low energy proton beam.

In this work some dosimetric characteristics of MD-55-2 GafChromic films were studied in a low energy proton beam (21.5 MeV) directly in a water phantom. The nonlinearity of the optical density was quantified by a factor P(lin). A correction factor P(en), that accounts for optical density dependence on the energy, was empirically determined. The effects of detector thickness in depth dose measurements and of the film orientation with respect to beam direction were investigated. The results show that the MD-55-2 films provide dose measurements with the films positioned perpendicularly to the proton beam. A dosimetric formalizm is proposed to determine the dose to water at depth d, with films oriented perpendicularly to the beam axis. This formalism uses a calibration factor of the radiochromic film determined directly on the proton beam at a reference depth in water, and the P(lin) factor, that takes into account the nonlinearity of the calibration curve and the P(en) factor that, in turn takes into account the change of proton beam energy in water. The MD-55-2 films with their high spatial resolution and the quasiwater equivalent material are attractive, positioned perpendicularly along the beam axis, for the absolute dose determination of very small beam sizes and modulated proton beams.

Radiobiological studies on the 65MeV therapeutic proton beam at Nice using human tumour cells

Purpose : To determine the relative biological effectiveness (RBE) for initial and delayed inactivation of cells by a modulated proton beam suitable for the treatment of tumours of the eye, within the spread-out Bragg peak and in its distal declining edge. Materials and methods : Human tumour SCC25 cells were irradiated with the 65 MeV proton beam at the Cyclotron Medicyc in Nice. Perspex plates of different thickness were used to simulate five positions along the beam line: 2 mm corresponding to the entrance beam; 15.6 and 25mm in the spread-out Bragg peak; 27.2 and 27.8mm for the distal edge. At each position clonogenic survival of the irradiated cells and of their progeny were determined at various dose values. 60 Co γ-rays were used as reference radiation. Results : RBE values evaluated at the survival level given by 2 Gy of γ-rays increased with increasing depth from close to 1.0 at the proximal to about 1.2 at the distal part of the peak. Within the declining edge it reached the value of about 1.4 at 27.2 and about 2 at 27.8 mm. For the progeny of irradiated cells, the RBE value ranged from 1.0 to 1.1 within the spread-out Bragg peak and then increased up to a value of 2.0 at the last position. The dose-effect curves for the progeny always had a larger shoulder than for the irradiated progenitors, their α parameters being lower by a factor of about 4 and their β parameters always being higher. The alpha/beta ratio was about 50Gy for the progenitors and about 6Gy for their progeny. The incidence of delayed eÚects increased with dose and with the depth within the beam. Conclusions : RBE values for the inactivation of cells irradiated in the spread-out Bragg peak are compatible with the value currently assumed in clinical applications. In the distal declining edge of the beam, the RBE values increased significantly to an extent that may be of concern when the region of the treatment volume is close to sensitive tissues. The yield of delayed reproductive cell death was significant at each position along the beam line.

Potential role of intensity modulated proton beams in prostate cancer radiotherapy

Potential role of intensity modulated proton beams in prostate cancer radiotherapy

Proton dosimetry intercomparison based on the ICRU report 59 protocol

Background and purpose: A new protocol for calibration of proton beams was established by the ICRU in report 59 on proton dosimetry. In this paper we report the results of an international proton dosimetry intercomparison, which was held at Loma Linda University Medical Center. The goals of the intercomparison were, first, to estimate the level of consistency in absorbed dose delivered to patients if proton beams at various clinics were calibrated with the new ICRU protocol, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration factors. Materials and methods: Eleven institutions participated in the intercomparison. Measurements were performed in a polystyrene phantom at a depth of 10.27 cm water equivalent thickness in a 6-cm modulated proton beam with an accelerator energy of 155 MeV and an incident energy of approximately 135 MeV. Most participants used ionization chambers calibrated in terms of exposure or air kerma. Four ionization chambers had 60Co -based calibration in terms of absorbed dose-to-water. Two chambers were calibrated in a 60Co beam at the NIST both in terms of air kerma and absorbed dose-to-water to provide a comparison of ionization chambers with different calibrations. Results: The intercomparison showed that use of the ICRU report 59 protocol would result in absorbed doses being delivered to patients at their participating institutions to within ±0.9% (one standard deviation). The maximum difference between doses determined by the participants was found to be 2.9%. Differences between proton doses derived from the measurements with ionization chambers with N K-, or N W - calibration type depended on chamber type. Conclusions: Using ionization chambers with 60Co calibration factors traceable to standard laboratories and the ICRU report 59 protocol, a distribution of stated proton absorbed dose is achieved with a difference less than 3%. The ICRU protocol should be adopted for clinical proton beam calibration. A comparison of proton doses derived from measurements with different chambers indicates that the difference in results cannot be explained only by differences in 60Co calibration factors.

Comparison of radiobiological effective depths in 65-MeV modulated proton beams.

To assess the achievement of uniformity of radiobiological effectiveness at different depths in the proton spread-out Bragg peak (SOBP), Chinese hamster ovary (CHO) cells were exposed to 65-MeV modulated proton beams at the Research Center for Nuclear Physics (RCNP) of Osaka University. We selected four different irradiation positions: 2 mm depth, corresponding to the entrance, and 10, 18 and 23 mm depths, corresponding to different positions in the SOBP. Cell survival curves were generated with the in vitro colony formation method and fitted to the linear-quadratic model. With 137Cs gamma-rays as the reference irradiation, the relative biological effectiveness (RBE) values for a surviving fraction (SF) level of 0.1 are 1.05, 1.10, 1.12 and 1.19 for depths of 2, 10, 18 and 23 mm respectively. A significant difference was found between the survival curves at 10 and 23 mm (P < 0.05), but not between 18 and 10 mm or between 18 and 23 mm. There was a significant dependence of RBE on depths in modulated proton beams at the 0.1 surviving fraction level (P < 0.05). Moreover, the rise of RBEs significantly depended on increasing SF level or decreased approximately in correspondence with irradiation dose (P = 0.0001). To maintain uniformity of radiobiological effectiveness for the target volume, careful attention should be paid to the influence of depth of beam and irradiation dose.

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

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. 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. 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). 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.

Alanine dosimetry of proton therapy beams.

An analysis of some of the properties of the ESR-alanine dosimetry in therapeutic proton beams is reported. Thin alanine-based detectors (1 and 2 mm thick pellets and 0.25 mm thick films) were tested in a clinical 62 MeV proton beam. The response of the alanine detectors in unmodulated and modulated proton beams was studied in tissue equivalent phantoms. The dose assessed by alanine was compared to the dose provided by a Markus parallel plate ionization chamber, used for reference dosimetry. Experiments in the 5-250 Gy dose range showed linearity of alanine dose response and no significant differences in the alanine response per unit dose to gammas and protons. Agreement within the experimental uncertainties was found between the alanine and the Markus chamber depth dose curves, including the Bragg peak region.

Calculation of relative biological effectiveness for proton beams using biological weighting functions

Purpose : The microdosimetric weighting function approach is used widely for beam comparison studies. The suitability of this model to predict the relative biological effectiveness (RBE) of therapeutic proton beams was studied. The RBE α (i.e., linear approximation) dependence on the type of biological end point, initial proton energy, energy spread of the input proton beam, and depth of beam penetration was investigated. Methods and Materials : Proton transport calculations for a proton energy range from 80 to 250 MeV were performed to obtain proton energy spectra at a given depth. The corresponding microdosimetric distributions of lineal energy were calculated. To these distributions the biological response function approach was applied to calculate RBE α the biological effectiveness based on a linear dose-response relationship. The early intestinal tolerance assessed by crypt regeneration in mice and the inactivation of V7 cells were taken as biological and points. Results : The RBF α values approach about 1 in the plateau region and gradually increase with the proton penetration depth. In the center of the Bragg peak, at the maximum dose delivery, the values of RBE α range from 1.1 (250-MeV) beam, early intestinal tolerance in mice) to 1.9 (70-MeV beam, Chinese hamster V79 cells in G 1/S phase). Distal to the Bragg peak, where only a small fraction of dose is delivered, the RBE α was found to be even higher. For modulated proton beams we found an increasing RBE α with depth in the spread-out Bragg peak (SOBP). Values up to 1.37 at the distal end of the SOBP plateau (155-MeV beams, SOBP between 5.3 and 13.2 cm) were obtained. Conclusion : More experimental work on the determination of microdosimetric weighting functions is needed. The results of the presented calculations indicate that for therapy planning it may be necessary to account for a depth dependence on proton RBE, especially for lower energy.

Substructure in the cell survival response at low radiation dose: effect of different subpopulations

In order to obtain more accurate measurements of cell survival after low doses of radiation, we have used the cell sorter assay, in which a cell sorter is used to accurately count out the number of cells plated for colony formation. This method, combined with data averaging, permits measurements of survival with superior precision, which have revealed that there is substructure in the radiation response of asynchronously dividing Chinese hamster cells. The substructure, observed at doses of a few Gy, has features of a 2-component response, consistent with the presence of subpopulations of cells of different cell-cyclerelated radiosensitivity. The absence of any substructure in the radiation response of homogeneous (tightly synchronized) cell populations lends strong support to this subpopulation explanation of the substructure. This assay has also been used on a variety of human tumour cell lines, most of which exhibited substructure similar to that of Chinese hamster cells. This paper outlines the application of the cell sorter assay to three different problems: (i) radiosensitizer mechanisms- etanidazole and RB 6145 are shown to enhance primarily the beta term and alpha term, respectively, of tumour cell kill, indicating that sensitizer efficacy may be tumour-specific and predictable from tumour response parameters; (ii) accurate measurement of Relative Biological Effectiveness (RBE) in a modulated clinical proton beam shows that the RBE is both dose- and depth- dependent; and (iii) measurements at lower doses clearly demonstrate a second order of substructure, termed the hypersensitive response, at doses 1 Gy.

Response analysis of TLD-300 dosimeters in heavy-particle beams

In vivo dosimetry is recommended as part of the quality control procedure for treatment verification in radiation therapy. Using thermoluminescence, such controls are planned in the p(65)+Be neutron and 85 MeV proton beams produced at the cyclotron at Louvain-La-Neuve and dedicated to therapy applications. A preliminary study of the peak 3 (C) and peak 5 (C) response of :Tm (TLD-300) to neutron and proton beams aimed to analyse the effect of different radiation qualities on the dosimetric behaviour of the detector irradiated in phantom. To broaden the range of investigation, the study was extended to an experimental C-12 heavy ion beam (95 MeV/nucleon). The peak 3 and 5 sensitivities in the neutron beam, compared to Co-60, varied little with depth. A major change of peak 5 sensitivity was observed for samples positioned under five leaves of the multi-leaf collimator. While peak 3 sensitivity was constant with depth in the unmodulated proton beam, peak 5 sensitivity increased by 15%. Near the Bragg peak, peak 3 showed the highest decrease of sensitivity. In the modulated proton beam, the sensitivity values were not significantly smaller than those measured in the unmodulated beam far from the Bragg peak region. The ratio of the heights of peak 3 and peak 5 decreased by 70% from the Co-60 reference radiation to the C-12 heavy-ion beam. This parameter was strongly correlated with the change of radiation quality.

Evaluation of a pencil-beam dose calculation technique for charged particle radiotherapy

Purpose : The purpose of this article is to evaluate a pencil-beam dose calculation algorithm for protons and heavier charged particles in complex patient geometries defined by computed tomography (CT) data and to compare isodose distributions calculated with the new technique to those calculated with conventional algorithms in selected patients with skull-base tumors. Methods and Materials : Monte Carlo calculations were performed to evaluate the pencil-beam algorithm in patient geometries for a modulated 150-MeV proton beam. A modified version of a Mont Carlo code described in a previous publication (18) was used for these comparisons. Tissue densities were inferred from patient CT data on a voxel-by-voxel basis, and calculations were peformed with an without tissue compensators. A dose calculation module using the new algorithm was written, and treatment plans using the new algorithm were compared to plans using standard ay-tracing techniques for 10 patients with clival chordoma and three patients with nasopharyngeal carcinoma who were treated with helium ions at Lawrence Berkeley National Laboratory (LBL). Results : Pencil beam calculations agreed well with Monte Carlo calculations in the patient geometries. The pencil-beam algorithm predicted several multiple-scattering effects that are not modeled by conventional ray-tracing calculations. These includes (a) the widening of the penumbra as a function of beam penetration, (b) the degradation in the sharpness of the dose gradient at the end of the particle range in hgihly heterogeneous regions, and (c) the appearance of hot and cold dose regions in the shadow of cmplex heterogeneities. In particular, pencil-beam calculations indicated that the dose distribution within the target was not a homogeneous as expected on the basis of ray-tracing calculations. on average, for the 13 patients considered, only about 72% of the cone down target volume received at least 99% of the prescribed dose, whereas, 93% of the conedwn volume was contained within the 95% isodose surface. This may e significant because in standard charged particle dose calculations, the dose across the spread-Bragg peak is assumed to be uniform and equal to the maximum or prescribed dose. Conclusions : Dose distributions computed with the pencil-beam model are more accurate than ray-tracing calculations, providing additional information to clinicians, which may influence the doses they prescribe. In particular, these calculations indicate that for some patients with skull-base tumors, it may be advantageous to prescribe proton doses to a lower isodose level than is commonly done.

Fiber-optic proton beam intensity monitor

The thermally induced change of the birefringence of a high-birefringence fused-silica optical fiber, when exposed to a proton beam (2 MeV, 0.5–44 nA, 5-mm spot diameter), was measured with a fiber-optic polarimetric sensor. A short length (5 cm) of 125-μ-diam, single-mode, polarization-maintaining optical fiber was put orthogonally to the beam in vacuo and the linear polarization of a HeNe laser beam launched into the fiber was analyzed by a polarizer and recorded by a photodetector. The response of the sensor both to a continuous and to a modulated proton beam was studied. The photodetector output, which varies with the cosine of the induced relative phase retardance between the fiber eigenmodes, provides a dc or an ac (10 Hz test modulation frequency) measurement of the beam current. This device acts as a proton beam intensity monitor, with minimum beam perturbation and a sensitivity of 0.1 nA. Its possible use also as a beam position monitor or as a wire-scanner beam profiler is suggested.