[Effects of gantry position on the accuracy of lead block localization and relative quality control methods]

BACKGROUND & OBJECTIVE

To date, the low-melting point lead blocks are commonly used as a beam modifier for the accurate three-dimensional (3D) radiotherapy in China. However, care should be taken when using blocks even though they are verified on the specified simulator or linear accelerator before treatment. This is because that those verifications are only performed under the gantry angle of 0 degree. For the oblique gantry angles, the block may shift in the tray leading to the positioning error. In this study, we explored the influence of gantry position on the accuracy of lead block localization, and the relative quality control methods were also provided.

METHODS

After rotating the gantry to 90 degrees and setting the collimator angle to 0 degree on a Siemens linear accelerator, a crosshair was marked on the plastic base of the lead block fitted to the tray according to the crosshair of light field of the linear accelerator. Then the discrepancy between the base crosshair and the light crosshair was investigated for different gantry positions and collimator angles.

RESULTS

There was clearance between the plastic base of the lead block and the block tray. As for some collimator angles at oblique gantry positions, the lead block shifted in the tray due to the gravity. This led to the position error of blocked radiation fields from 0 up to 3.6 mm at the isocenter plane.

CONCLUSIONS

Accurately marking the crosshair on the plastic base of the lead block is an effective method of correctly fixing the lead block influenced by the plastic base as well as of double checking the accuracy of the localization of the lead block. The positioning error of oblique gantry positions can be minimized by limiting the collimator to some degrees which makes the block tray opening to the ceiling or parallel to the ground while the bisector of the restrictive angle points slanting to the ceiling (that is, the first baseline of the block tray points inclined or right to the ceiling).

Dosimetric characteristics of a newly designed grid block for megavoltage photon radiation and its therapeutic advantage using a linear quadratic model

Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, combined with conventional radiation therapy, has been demonstrated using a prototype GRID block [Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114-118 (1990)]. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radiation Product Design, Inc. (Albertive, MN). This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investigated for a Varian 21EX linear accelerator (Varian Associates, Palo Alto, CA). These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with alpha/beta ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented.

Perturbation factors for cylindrical ionization chambers in proton beams. Part I: corrections for gradients

An analytical model is presented to calculate the effective depth in water of cylindrical ionization chambers in clinical proton or heavy ion beams in the presence of modest and linear gradients. This model is compared with Monte Carlo simulations and recommendations in IAEA TRS-398 for all ionization chambers listed in that report. A refinement of the analytical model allows its application for depth-dose curves even in the nonlinear gradient region of the Bragg peak. Combined with information from the Monte Carlo simulations it also allows one to solve the inverse problem of deriving a depth-dose curve in homogeneous water from the depth-dose response obtained with a cylindrical ionization chamber. The results show that the effective depth in water calculated analytically, calculated by Monte Carlo, and derived from experimental literature are in good agreement. The agreement with the IAEA TRS-398 recommendation is good for most ionization chambers but can be wrong by up to 1.5 mm for some ionization chamber types. The agreement between the refined analytical model and Monte Carlo simulations of depth-dose response curves is shown to be good (even in extreme situations of large ionization chambers in low-energy beams). An indication is given of situations in which a simple shift of the entire depth-dose curve is sufficiently accurate. It is demonstrated that it is possible to solve the inverse problem with this method, even for rather noisy data. This is illustrated by successfully applying it to depth-dose measurements with cylindrical ionization chambers taken from the literature.

Peripheral dose from uniform dynamic multileaf collimation fields: implications for sliding window intensity-modulated radiotherapy

The increase in the number of monitor units in sliding window intensity-modulated radiotherapy, compared with conventional techniques for the same target dose, may lead to an increase in peripheral dose (PD). PD from a linear accelerator was measured for 6 MV X-ray using 0.6 cm3 ionization chamber inserted at 5 cm depth into a 35 cm x 35 cm x 105 cm plastic water phantom. Measurements were made for field sizes of 6 cm x 6 cm, 10 cm x 10 cm and 14 cm x 14 cm, shaped in both static and dynamic multileaf collimation (DMLC) mode, employing strip fields of fixed width 0.5 cm, 1.0 cm, 1.5 cm, and 2.0 cm, respectively. The effect of collimator rotation and depth of measurement on peripheral dose was investigated for 10 cm x 10 cm field. Dynamic fields require 2 to 14 times the number of monitor units than does a static open field for the same dose at the isocentre, depending on strip field width and field size. Peripheral dose resulting from dynamic fields manifests two distinct regions showing a crest and trough within 30 cm from the field edge and a steady exponential fall beyond 30 cm. All dynamic fields were found to deliver a higher PD compared with the corresponding static open fields, being highest for smallest strip field width and largest field size; also, the percentage increase observed was highest at the largest out-of-field distance. For 6 cm x 6 cm field, dynamic fields with 0.5 cm and 2 cm strip field width deliver PDs 8 and 2 times higher than that of the static open field. The corresponding factors for 14 cm x 14 cm field were 15 and 6, respectively. The factors by which PD for DMLC fields increase, relative to jaws-shaped static fields for out-of-field distance beyond 30 cm, are almost the same as the corresponding increases in the number of monitor units. Reductions of 20% and 40% in PD were observed when the measurements were done at a depth of 10 cm and 15 cm, respectively. When the multileaf collimator executes in-plane (collimator 90 degrees) motion, peripheral dose decreases by as much as a factor of 3 compared with cross-plane data. The knowledge of PD from DMLC field is necessary to estimate the increase in whole-body dose and the likelihood of radiation induced secondary malignancy.

Palliative chest irradiation in sitting position in patients with bulky advanced lung cancer

Some patients with bulky advanced lung cancer are not able to lie down because of dyspnea. We therefore designed a technique for irradiation in a sitting position by using a dedicated chair. The reproducibility of the sitting position was high and all ten patients preferred this position over lying down. In selected patients who are unable to lie down, palliative irradiation in a sitting position may be an option.

On the effective point of measurement in megavoltage photon beams

This paper presents a numerical investigation of the effective point of measurement of thimble ionization chambers in megavoltage photon beams using Monte Carlo simulations with the EGSNRC system. It is shown that the effective point of measurement for relative photon beam dosimetry depends on every detail of the chamber design, including the cavity length, the mass density of the wall material, and the size of the central electrode, in addition to the cavity radius. Moreover, the effective point of measurement also depends on the beam quality and the field size. The paper therefore argues that the upstream shift of 0.6 times the cavity radius, recommended in current dosimetry protocols, is inadequate for accurate relative photon beam dosimetry, particularly in the build-up region. On the other hand, once the effective point of measurement is selected appropriately, measured depth-ionization curves can be equated to measured depth-dose curves for all depths within +/- 0.5%.

Two-dimensional transmitted dose measurements using a scanning liquid ionization chamber EPID

The use of a scanning liquid ionization chamber electronic portal imaging device (SLIC-EPID) for two-dimensional transmitted dosimetry was investigated and a calibration method was developed using extended dose range (EDR2) film. In order to convert pixel value to dose, the acquired SLIC-EPID pixel values were calibrated using an ionization chamber on the central axis. The relationship between pixel values, dose rate and absorbed dose was identified for various linac output repetition rates. To correct EPIs for dosimetric purposes, the off-axis ratio of dose profiles measured by EPIDs and EDR2 film was used to derive correction factor matrices (CFMs) for a range of source-to-EPID distances (SEDs). The corrected relative dose maps acquired for different conditions, including open and wedged fields, measured using a SLIC-EPID were compared with EDR2 film images using a gamma function algorithm with distance to agreement (DTA) = 2.5 mm and dose difference (DeltaDmax) = 1% criteria. The results showed that (a) for two-dimensional dosimetric purposes, EPIDs must be calibrated using appropriate two-dimensional correction factors and (b) SLIC-EPIDs can be used to measure the transmitted dose with good accuracy.

Use of peripheral dose data from uniform dynamic multileaf collimation fields to estimate out-of-field organ dose in patients treated employing sliding window intensity-modulated radiotherapy

Peripheral doses (PD) from uniform dynamic multileaf collimation (DMLC) fields were measured for 6 MV x-rays on a Varian linear accelerator using a 0.6 cc ionization chamber inserted at 5 cm depth into a 35 x 35 x 105 cm3 plastic water phantom. PD measurements were also carried out under identical conditions for seven patients treated for head and neck and cervical cancer employing sliding window intensity-modulated radiotherapy (IMRT). The measured PD from these patient-specific intensity-modulated beams (IMBs) were compared with the corresponding data from uniform DMLC fields having similar jaws setting. The measured PD per monitor unit (PD/MU) decreases almost exponentially with out-of-field distance for all uniform DMLC and static fields. For the same strip field width of 1.2 cm, uniform DMLC fields with a larger size of 14 x 22 cm2 deliver an average of 3.51 (SD = 0.51) times higher PD/MU at all out-of-field distances compared to 6 x 6 cm2. Similar to uniform DMLC fields, PD/MU measured from different patient-specific IMBs was found to decrease almost exponentially with out-of-field distance and increase with increase in field dimension. PD per MU from uniform DMLC fields and patient-specific IMBs having similar jaws setting shows good agreement (+/-7%) except at the most proximal distance, where a variation of more than 10% (maximum 15%) was observed. Our study shows that PD data generated from uniform DMLC fields can be used as baseline data to estimate out-of-field critical organ or whole-body dose in patients treated employing sliding window IMRT if an appropriate correction factor for field dimension is applied. The whole-body dose information can be used to estimate the possible increase in risk of fatal secondary malignancy in patients treated employing sliding window IMRT.

Characterization of megavoltage electron beams delivered through a photon multi-leaf collimator (pMLC)

A study is presented that characterizes megavoltage electron beams delivered through an existing double-focused photon multi-leaf collimator (pMLC) using film measurements in a solid water phantom. Machine output stability and linearity were evaluated as well as the effect of source-to-surface distance (SSD) and field size on the penumbra for electron energies between 6 and 18 MeV over an SSD range of 60-100 cm. Penumbra variations as a function of field size, depth of measurement and the influence of the jaws were also studied. Field abutment, field flatness and target coverage for segmented beams were also addressed. The measured field size for electrons transported through the pMLC was the same as that for an x-ray beam up to SSDs of 70 cm. At larger SSD, the lower energy electron fields deviated from the projected field. Penumbra data indicated that 60 cm SSD was the most favourable treatment distance. Backprojection of P(20-80) penumbra data yielded a virtual source position located at 98.9 cm from the surface for 18 MeV electrons. For 6 MeV electrons, the virtual source position was at a distance of 82.6 cm. Penumbra values were smaller for small beam slits and reached a near-constant value for field widths larger than 5 cm. The influence of the jaws had a small effect on the penumbra. The R90 values ranged from 1.4 to 4.8 cm between 6 and 21 MeV as measured at 60 cm SSD for a 9 x 9 cm2 field. Uniformity and penumbra improvement could be demonstrated using weighted abutted fields especially useful for small segments. No detectable electron leakage through the pMLC was observed. Bremsstrahlung measurements taken at 60 cm SSD for a 9 x 9 cm2 field as shaped by the pMLC compared within 1% to bremsstrahlung measurements taken at 100 cm SSD for a 10 x 10 cm2 electron applicator field at 100 cm SSD.

GafChromic® RTQA film for routine quality assurance of high-energy photon beams

Self-developing film offers many advantages over conventional radiographic verification film for routine radiotherapy quality assurance (QA). This paper presents results from an initial evaluation of a beam measurement system using GafChromic RTQA film and a flatbed scanner. Variability and energy dependence of the film calibration and accuracy of scanner readout are investigated in the context of QA measurements. For exposures of film between 2 and 4 Gy, the system is adequate for measurement of beam dimensions, as in multi-leaf collimator (MLC) offsets and secondary jaw calibrations, where agreement with conventional film measurements is within 0.5 mm. However, the measurement of absolute dose is subject to errors of about 25 cGy.

The case for particle therapy

Among the most important decisions facing the British Government regarding the treatment of cancer in the National Health Service (NHS) is the purchase of charged particle therapy (CPT) centres. CPT is different from conventional radiotherapy: the dose is deposited far more selectively in Bragg Peaks by either protons or "heavy" ions, such as carbon. In this way, it is possible to "dose paint" targets, voxel by voxel, with far less dose to surrounding tissues than with X-ray techniques. At present the UK possesses a 62 MeV cyclotron proton facility at Clatterbridge (Wirral), which provides therapy for intraocular cancers such as melanoma; for deeper situated cancers in the pelvis, chest etc., much higher energies, over 200 MeV are required from a synchrotron facility. There is an impressive expansion in particle beam therapy (PBT) centres worldwide, since they offer good prospects of improved quality of life with enhanced cancer cures in situations where conventional therapy is limited due to radioresistance or by the close proximity of critical normal tissues. There is a threat to UK Oncology, since it is anticipated that several thousand British patients may require referral abroad for therapy; this would severely disrupt their multidisciplinary management and require demanding logistical support.

Evaluation of the EDR-2 film for relative dosimetry of high-energy photon and electron beams

A sensitometric study of Kodak XV and EDR-2 radiographic films (Eastman Kodak Company, Rochester, NY) was performed using photons ranging from 75 kV to 18 MV and electrons ranging from 6 to 20 MeV. To investigate the applicability of the EDR-2 film for clinical radiation dosimetry, percentage depth-doses, profiles and distributions in open and dynamically wedged fields were measured using film and compared with data from a linear diode. Moreover, conventional quality assurance dose parameters were measured, including open-field dose profiles to determine flatness and symmetry of photon and electron beams. Finally, film was employed to validate dose distributions produced by complex computerised treatment planning techniques. Our conclusion is that the EDR-2 film is an effective tool for relative dosimetry of photon and electron beams.

Charged particle equilibrium of small field in clinical proton beams

It is expected that proton beam radiotherapy will become an effective treatment for tumors. For an organ for which a correct dose prescription is required, a proton beam has the ability to provide the dose most suitable for the specific purpose. We measured the charged particle equilibrium factor of proton beams in water using a plane parallel ionization chamber. The maximum energy of the proton beam used in this study was 70 MeV, produced from an isochronous cyclotron. We assume that the charged particle equilibrium factor can be separated into longitudinal and lateral components, that is, the factor E (Z, R) is dependent on depth, Z, and field radius, R; such that E (Z, R) =E (Z) E (R). The E (Z) -factor of primary protons was considered in order to investigate the influence of secondary charged particles. From the results, the charged particle equilibrium factor for the longitudinal component does not remain sharp with the decrease of water depth, and the lateral component is not maintained with the decrease of field size. However, the longitudinal components of primary protons at shallow depth were in equilibrium.

Dosimetric characteristics of the Elekta Beam Modulator™

The dosimetric characteristics of a production pilot multi-leaf collimator (Elekta Beam Modulator, Elekta Oncology Systems, Crawley, UK) having a 4 mm leaf width (at isocentre) have been investigated. Characteristics explored included leaf bank set-up, penumbra width (80-20%) as a function of leaf position, leaf positioning reproducibility, interleaf leakage and leaf transmission. The penumbra values for leaf ends were measured to be between 4.2 and 4.8 mm for various large rectangular fields studied using Kodak X-omat V film at isocentre (1.5 cm deep). Similar films were taken with a standard 1 cm width multi-leaf collimator (MLC) and the penumbra for leaf ends was found to range from 4.3 to 5.2 mm. Other results showed that the rounded leaf tip provided tight control of the penumbra across the leaves' full range of travel. The positioning of the leaves was within a 0.5 mm range when approaching from the same direction. The maximum interleaf leakage was found to be 1.7% and the average leaf transmission less than 1.0%. No major differences were observed in leakage and transmission with changing gantry angle.

Neutrons from fragmentation of light nuclei in tissue-like media: a study with the GEANT4 toolkit

We study energy deposition by light nuclei in tissue-like media taking into account nuclear fragmentation reactions, in particular, production of secondary neutrons. The calculations are carried out within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit. Experimental data on depth-dose distributions for 135-400 A MeV (12)C and (18)O beams are described very well without any adjustment of the model parameters. This gives confidence in successful use of the GEANT4 toolkit for MC simulations of cancer therapy with beams of light nuclei. The energy deposition due to secondary neutrons produced by (12)C and (20)Ne beams in a (40-50 cm)(3) water phantom is estimated to be 1-2% of the total dose, that is only slightly above the neutron contribution (approximately 1%) induced by a 200 MeV proton beam.

[Organization of work in the radiotherapy department in Burdenko MMCH]

Radiotherapy is one of the main methods of malignant tumor treatment. The radiotherapy department in Burdenko Main Military-and-clinical Hospital as a part of a radiological center has been working for 40 years. During vhis period it has constantly been developing its technical base, quality of irradiation and professionalism of its staff. And nowadays at the modern stage there exists a real perspective of future development of radiological treatment for servicemen and their families' members basing on the radiotherapy department in Burdenko MMCH.

Monte Carlo simulations of a nozzle for the treatment of ocular tumours with high-energy proton beams

By the end of 2002, 33 398 patients worldwide had been treated with proton radiotherapy, 10 829 for eye diseases. The dose prediction algorithms used today for ocular proton therapy treatment planning rely on parameterizations of measured proton dose distributions, i.e., broad-beam and pencil-beam techniques, whose predictive capabilities are inherently limited by severe approximations and simplifications in modelling the radiation transport physics. In contrast, the Monte Carlo radiation transport technique can, in principle, provide accurate predictions of the proton treatment beams by taking into account all the physical processes involved, including coulombic energy loss, energy straggling, multiple Coulomb scattering, elastic and nonelastic nuclear interactions, and the transport of secondary particles. It has not been shown, however, whether it is possible to commission a proton treatment planning system by using data exclusively from Monte Carlo simulations of the treatment apparatus and a phantom. In this work, we made benchmark comparisons between Monte Carlo predictions and measurements of an ocular proton treatment beamline. The maximum differences between absorbed dose profiles from simulations and measurements were 6% and 0.6 mm, while typical differences were less than 2% and 0.2 mm. The computation time for the entire virtual commissioning process is less than one day. The study revealed that, after a significant development effort, a Monte Carlo model of a proton therapy apparatus is sufficiently accurate and fast for commissioning a treatment planning system.

A new extended-length parallel-plate ionization chamber

A special parallel-plate ionization chamber was developed. The motivation for the construction of this new chamber was mainly to fulfil the need of a reference system for computed tomography standard beams in the Calibration Laboratory of IPEN. However, the chamber was tested also in standard radiation beams of mammography and conventional diagnostic radiology. The chamber was manufactured at the institute workshop, as simply and cheaply as possible. Its design differs from the common ionization chambers used in dosimetric procedures of computed tomography equipment, because it is a parallel-plate chamber instead of a cylindrical chamber. However, its dimensions and sensitive volume are very similar to those of a commercial pencil ionization chamber. The new ionization chamber was submitted to several characterization and quality control tests, showing its very good performance.

Monte Carlo simulation of a protontherapy platform devoted to ocular melanoma

Patients with ocular melanoma have been treated since June 1991 at the medical cyclotron of the Centre Antoine Lacassagne (CAL). Positions and sizes of the ocular nozzle elements were initially defined based on experimental work, taking as a pattern functional existing facilities. Nowadays Monte Carlo (MC) calculation offers a tool to refine this geometry by adjusting size and place of beam modeling devices. Moreover, the MC tool is a useful way to calculate the dose and to evaluate the impact of secondary particles in the field of radiotherapy or radiation protection. Both LINAC and cyclotron producing x rays, electrons, protons, and neutrons are available in CAL, which suggests choosing MCNPX for its particle versatility. As a first step, the existing installation was input in MCNPX to check its aptitude to reproduce experimentally measured depth-dose profile, lateral profile, output-factor (OF), and absolute dose. The geometry was defined precisely and described from the last achromatic bending magnet of our proton beam line to the position of treated eyes. Relative comparisons of percentage depth-dose and lateral profiles, performed between measured data and simulations, show an agreement of the order of 2% in dose and 0.1 mm in range accuracy. These comparisons, carried out with and without beam-modifying device, yield results compatible to the required precision in ocular melanoma treatments, as long as adequate choices are made on MCNPX input decks for physics card. Absolute dose and OF issued from calculations and measurements were also compared. Results obtained for these two kinds of data, carried out in the simplified situation of an unmodulated beam, indicate that MC calculation could effectively complement measurements. These encouraging results are a large source of motivation to promote further studies, first in a new design of the ocular nozzle, and second in the analysis of the influence of beam-modifying devices attached to the final patient collimator, such as wedge or compensators, on dose values.

Calculations of neutron dose equivalent exposures from range-modulated proton therapy beams

Passive beam spreading techniques have been used for most proton therapy treatments worldwide. This delivery method employs static scattering foils to spread the beam laterally and a range modulating wheel or ridge filter to spread the high dose region in depth to provide a uniform radiation dose to the treatment volume. Neutrons produced by interactions of the treatment beam with nozzle components, such as the range modulation wheel, can account for a large portion of the secondary dose delivered to healthy tissue outside the treatment volume. Despite this fact, little is known about the effects of range modulation on the secondary neutron exposures around passively scattered proton treatment nozzles. In this work, the neutron dose equivalent spectra per incident proton (H(E)/p) and total neutron dose equivalent per therapeutic absorbed dose (H/D) were studied using Monte Carlo techniques for various values of range modulation at 54 locations around a passive scattering proton therapy treatment nozzle. As the range modulator wheel step thickness increased from 1.0 to 11.5 cm, the peak values of H(E)/p decreased from approximately 1 x 10(-17) mSv Gy(-1) to approximately 2 x 10(-18) mSv Gy(-1) at 50 cm from isocentre along the beam's central axis. In general, H/D increased with increasing range modulation at all locations studied, and the maximum H/D exposures shifted away from isocentre.

Feasibility of cranio-spinal axis radiation with the Hi-Art tomotherapy system

BACKGROUND AND PURPOSE

Helical tomotherapy can eliminate the need for junction lines. The goal of this study is to evaluate tomotherapy in the delivery of CSA radiation and measurement of plan quality using physical parameters in comparing conventional (CSA-RT) and helical tomotherapy (CSA-TOMO) plans.

PATIENTS AND METHODS

CSA-TOMO and CSA-RT plans were created for dosimetric comparison. Integral dose values were calculated. The ratios D50% (dose received by 50% of the organ at risk's volume) and D10% (dose received by 10% of the organ at risk's volume) were calculated representing large volumes and small volumes of organs at risk receiving significant dose.

RESULTS

When considering D50% and D10%, CSA-TOMO has a dosimetric advantage over CSA-RT for most organs at risk. The body integral dose was higher for the CSA-TOMO plan by approximately 6.5%.

CONCLUSIONS

Tomotherapy is a feasible alternative for treatment of CSA. Analysis shows that tomotherapy improves dose ratios over conventional radiation for most organs at risk. The impact of a small increase in whole body integral dose is unknown. Long-term follow-up will be needed to answer this question as others have argued of the possibility of increased risk of secondary malignancies due to delivery of radiotherapy with IMRT.

Ion recombination correction for very high dose-per-pulse high-energy electron beams

The parallel-plate ionization chamber is the recommended tool for the absorbed dose measurement in pulsed high-energy electron beams. Typically, the electron beams used in radiotherapy have a dose-per-pulse value less then 0.1 cGy/pulse. In this range the factor to correct the response of an ionization chamber for the lack of complete charge collection due to ion recombination (ksat) can be properly evaluated with the standard "two voltage" method proposed by the international dosimetric reports. Very high dose-per-pulse electron beams are employed in some special Linac dedicated to the Intra-Operatory-Radiation-Therapy (IORT). The high dose-per-pulse values (3-13 cGy/pulse) characterizing the IORT electron beams allow to deliver the therapeutic dose (10-20 Gy) in less than a minute. This considerably reduces the IORT procedure time, but some dosimetric problems arise because the standard method to evaluate ksat overestimates its value by 20%. Moreover, if the dose-per-pulse value >1 cGy/pulse, the dependence of ksat on the dose-per-pulse value cannot be neglected for relative dosimetry. In this work the dependence of ksat on the dose-per-pulse value is derived, based on the general equation that describes the ion recombination in the Boag theory. A new equation for ksat, depending on known or measurable quantities, is presented. The new ksat equation is experimentally tested by comparing the absorbed doses to water measured with parallel-plate ionization chambers (Roos and Markus) to that measured using dose-per-pulse independent dosimeters, such as radiochromic films and chemical Fricke dosimeters. These measurements are performed in the high dose-per-pulse (3-13 cGy/pulse) electron beams of the IORT dedicated Linac Hitesys Novac7 (Aprilia-Latina, Italy). The dose measurements made using the parallel-plate chambers and those made using the dose-per-pulse independent dosimeters are in good agreement (<3%). This demonstrates the possibility of using the parallel-plate ionization chambers also for the very high dose-per-pulse (> 1 cGy/pulse) electron-beam dosimetry.

Beam modeling and verification of a photon beam multisource model

Dose calculations for treatment planning of photon beam radiotherapy require a model of the beam to drive the dose calculation models. The beam shaping process involves scattering and filtering that yield radiation components which vary with collimator settings. The necessity to model these components has motivated the development of multisource beam models. We describe and evaluate clinical photon beam modeling based on multisource models, including lateral beam quality variations. The evaluation is based on user data for a pencil kernel algorithm and a point kernel algorithm (collapsed cone) used in the clinical treatment planning systems Helax-TMS and Nucletron-Oncentra. The pencil kernel implementations treat the beam spectrum as lateral invariant while the collapsed cone involves off axis softening of the spectrum. Both algorithms include modeling of head scatter components. The parameters of the beam model are derived from measured beam data in a semiautomatic process called RDH (radiation data handling) that, in sequential steps, minimizes the deviations in calculated dose versus the measured data. The RDH procedure is reviewed and the results of processing data from a large number of treatment units are analyzed for the two dose calculation algorithms. The results for both algorithms are similar, with slightly better results for the collapsed cone implementations. For open beams, 87% of the machines have maximum errors less than 2.5%. For wedged beams the errors were found to increase with increasing wedge angle. Internal, motorized wedges did yield slightly larger errors than external wedges. These results reflect the increased complexity, both experimentally and computationally, when wedges are used compared to open beams.

Development of high quantum efficiency, flat panel, thick detectors for megavoltage x-ray imaging: a novel direct-conversion design and its feasibility

Most electronic portal imaging devices (EPIDs) developed to date, including recently developed flat panel systems, have low x-ray absorption, i.e., low quantum efficiency (QE) of 2%-4% as compared to the theoretical limit of 100%. A significant increase of QE is desirable for applications such as a megavoltage cone-beam computed tomography (MVCT) and megavoltage fluoroscopy. However, the spatial resolution of an imaging system usually decreases significantly with an increase of QE. The key to the success in the design of a high QE detector is therefore to maintain the spatial resolution. Recently, we demonstrated theoretically that it is possible to design a portal imaging detector with both high QE and high resolution [see Pang and Rowlands, Med. Phys. 29, 2274 (2002)]. In this paper, we introduce such a novel design consisting of a large number of microstructured plates (made by, e.g., photolithographic patterning of evaporated or electroplated layers) packed together and aligned with the incident x rays. On each plate, microstrip charge collectors are focused toward the x-ray source to collect charges generated in the ionization medium (e.g., air or gas) surrounded by high-density materials that act as x-ray converters. The collected charges represent the x-ray image and can be read out by various means, including a two-dimensional (2-D) active readout matrix. The QE, spatial resolution, and sensitivity of the detector have been calculated. It has been shown that the new design will have a QE of more than an order of magnitude higher and a spatial resolution equivalent to that of flat panel systems currently used for portal imaging. The new design is also quantum noise limited down to very low doses (approximately 1-2 radiation pulses of the linear accelerator).