Determination of consensus k Q values for megavoltage photon beams for the update of IAEA TRS-398.

The IAEA is currently coordinating a multi-year project to update the TRS-398 Code of Practice for the dosimetry of external beam radiotherapy based on standards of absorbed dose to water. One major aspect of the project is the determination of new beam quality correction factors, k Q , for megavoltage photon beams consistent with developments in radiotherapy dosimetry and technology since the publication of TRS-398 in 2000. Specifically, all values must be based on, or consistent with, the key data of ICRU Report 90. Data sets obtained from Monte Carlo (MC) calculations by advanced users and measurements at primary standards laboratories have been compiled for 23 cylindrical ionization chamber types, consisting of 725 MC-calculated and 179 experimental data points. These have been used to derive consensus k Q values as a function of the beam quality index TPR20,10 with a combined standard uncertainty of 0.6%. Mean values of MC-derived chamber-specific [Formula: see text] factors for cylindrical and plane-parallel chamber types in 60Co beams have also been obtained with an estimated uncertainty of 0.4%.

Small field dosimetry correction factors for circular and MLC shaped fields with the CyberKnife M6 System: evaluation of the PTW 60023 microSilicon detector.

The PTW 60023 microSilicon is a new unshielded diode detector for small-field photon dosimetry. It provides improved water equivalence and a slightly larger sensitive region diameter in comparison to previous diode detectors in this range. In this study we evaluated the correction factors relevant to commissioning a CyberKnife System with this detector by Monte Carlo simulation and verified this data by multi-detector measurement comparison. The correction factors required for output factor determination were substantially closer to unity at small field sizes than for previous diode versions (e.g. [Formula: see text] = 0.981 at 5 mm field size which compares with corrections of 5%-6% with other stereotactic diodes). Because of these differences we recommend that corrections to small field output factor measurements generated specifically for the microSilicon detector rather than generic data taken from other diode types should be used with this new detector. For depth-dose measurements the microSilicon is consistent with a microDiamond detector to <1% (global), except at depths <10 mm where the diode gives a significantly lower measurement, by 6%-8% at the surface. For profile measurements, the microSilicon requires negligible corrections except in the low dose region outside the beam, where it underestimates off-axis-ratio (OAR) for small fields and overestimates for large fields. Where this effect is most noticeable at the largest field size and depth (115 mm × 100 mm and 300 mm depth) the microSilicon overestimates OAR by 2.3% (global) in the profile tail. This is consistent with other unshielded diodes.

The impact of inter-unit variations on small field dosimetry correction factors, with application to the CyberKnife system.

Small field dosimetry correction factors are usually determined from calculations or measurements using one specific example of a treatment system. The sensitivity of the corrections to inter-unit variation is therefore not evaluated. We propose two methods for this evaluation that could be applied to any system. We use them to assess the variability in [Formula: see text] for the CyberKnife System caused by design changes between pre-M6 and M6 versions, and to the variability in [Formula: see text] and [Formula: see text] resulting from measured beam-data variations across 139 units. We also perform measurements to investigate the differences in [Formula: see text] reported for microchambers in a CyberKnife-specific study versus TRS-483. The results show that [Formula: see text] is smaller for the M6 version than pre-M6 versions by 0.4% for a Farmer chamber, and 0.1% for shorter chambers. The presence or absence of a lead filter within the treatment head had no significant impact on [Formula: see text]. The beam-data analysis showed inter-unit variations in [Formula: see text] of ±0.8% (2 s.d.) for Farmer chambers and ⩽ ±0.5% for shorter cavities (<10 mm) pre-M6, reducing to 0.4% and 0.2% respectively with M6. Inter-unit [Formula: see text] variations for microDiamond and microchambers were ⩽ ±1% at 5 mm field size, except for microchambers with axis perpendicular to the beam where this was > ±2%. Differences of up to 9% were confirmed between Output Factors measured using a microchamber and corrected using TRS-483 [Formula: see text], and a consensus dataset for the same treatment unit determined using multiple detectors and Monte Carlo simulation. A set of practical recommendations for small field dosimetry with the CyberKnife System is derived from these results.

Monte Carlo simulated corrections for beam commissioning measurements with circular and MLC shaped fields on the CyberKnife M6 System: a study including diode, microchamber, point scintillator, and synthetic microdiamond detectors.

Monte Carlo simulation was used to calculate correction factors for output factor (OF), percentage depth-dose (PDD), and off-axis ratio (OAR) measurements with the CyberKnife M6 System. These include the first such data for the InCise MLC. Simulated detectors include diodes, air-filled microchambers, a synthetic microdiamond detector, and point scintillator. Individual perturbation factors were also evaluated. OF corrections show similar trends to previous studies. With a 5 mm fixed collimator the diode correction to convert a measured OF to the corresponding point dose ratio varies between -6.1% and -3.5% for the diode models evaluated, while in a 7.6 mm × 7.7 mm MLC field these are -4.5% to -1.8%. The corresponding microchamber corrections are +9.9% to +10.7% and +3.5% to +4.0%. The microdiamond corrections have a maximum of -1.4% for the 7.5 mm and 10 mm collimators. The scintillator corrections are <1% in all beams. Measured OF showed uncorrected inter-detector differences >15%, reducing to <3% after correction. PDD corrections at d > d max were <2% for all detectors except IBA Razor where a maximum 4% correction was observed at 300 mm depth. OAR corrections were smaller inside the field than outside. At the beam edge microchamber OAR corrections were up to 15%, mainly caused by density perturbations, which blurs the measured penumbra. With larger beams and depths, PTW and IBA diode corrections outside the beam were up to 20% while the Edge detector needed smaller corrections although these did vary with orientation. These effects are most noticeable for large field size and depth, where they are dominated by fluence and stopping power perturbations. The microdiamond OAR corrections were <3% outside the beam. This paper provides OF corrections that can be used for commissioning new CyberKnife M6 Systems and retrospectively checking estimated corrections used previously. We recommend the PDD and OAR corrections are used to guide detector selection and inform the evaluation of results rather than to explicitly correct measurements.

Use of novel fibre-coupled radioluminescence and RADPOS dosimetry systems for total scatter factor measurements in small fields.

A dosimetry system based on Al2O3:C radioluminescence (RL), and RADPOS, a novel 4D dosimetry system using microMOSFETs, were used to measure total scatter factors, (S(c,p))(f(clin))(det), for the CyberKnife robotic radiosugery system. New Monte Carlo calculated correction factors are presented and applied for the RL detector response for the 5, 7.5 and 10 mm collimators in order to correct for the detector geometry and increased photoelectric cross section of Al2O3:C relative to water. For comparison, measurements were also carried out using a micro MOSFET, PTW60012 diode and GAFCHROMIC(®) film (EBT and EBT2). Uncorrected (S(c,p))(f(clin))(det) were obtained by taking the ratio of the detector response for each collimator to that for the 60 mm diameter reference field. Published Monte Carlo calculated correction factors were applied to the RADPOS, microMOSFET and diode detector measurements to yield corrected field factors, Ω(f(clin),f(msr))(Q(clin),Q(msr)), following the terminology of a recent formalism introduced for small and composite field relative dosimetry. With corrections, the RL measured Ω(f(clin),f(msr))(Q(clin),Q(msr)) were 0.656 ± 0.002, 0.815 ± 0.002 and 0.865 ± 0.003 for the 5, 7.5 and 10 mm collimators, respectively. This was in good agreement with RADPOS corrected field factors of 0.650 ± 0.010, 0.816 ± 0.024 and 0.867 ± 0.010 for the 5, 7.5 and 10 mm collimators, respectively. Both RL and RADPOS total scatter factors agreed within approximately two standard deviations of the GAFCHROMIC film values (average of EBT and EBT2) of 0.640 ± 0.006, 0.806 ± 0.007 and 0.859 ± 0.09. Corrected total scatter factors for all dosimetry systems agreed within one standard deviation for collimator sizes 10-60 mm. Our study suggests that the microMOSFET/RADPOS and optical fibre-coupled RL dosimetry system are well suited for total scatter factor measurements over the entire range of field sizes, provided that appropriate correction factors are applied for the collimator diameters smaller than 10 mm.

Monte Carlo simulated correction factors for output factor measurement with the CyberKnife system-results for new detectors and correction factor dependence on measurement distance and detector orientation.

A previous study of the corrections needed for output factor measurements with the CyberKnife system has been extended to include new diode detectors (IBA SFD and Exradin D1V), an air filled microchamber (Exradin CC01) and a scintillation detector (Exradin W1). The dependence of the corrections on detector orientation (detector long axis parallel versus perpendicular to the beam axis) and source to detector distance (SDD) was evaluated for these new detectors and for those in our previous study. The new diodes are found to over-respond at the smallest (5 mm) field size by 2.5% (D1V) and 3.3% (SFD) at 800 mm SDD, while the CC01 under-responds by 7.4% at the same distance when oriented parallel to the beam. Corrections for all detectors tend to unity as field size increases. The W1 corrections are <0.5% at all field sizes. Microchamber correction factors increase substantially if the detector is oriented perpendicular to the beam (by up to 23% for the PTW 31014). Corrections also vary with SDD, with the largest variations seen for microchambers in the perpendicular orientation (up to 13% change at 650 mm SDD versus 800 mm) and smallest for diodes (~1% change at 650 mm versus 800 mm). The smallest and most stable corrections are found for diodes, liquid filled microchambers and scintillation detectors, therefore these should be preferred for small field output factor measurements. If air filled microchambers are used, then the parallel orientation should be preferred to the perpendicular, and care should be taken to use corrections appropriate to the measurement SDD.

Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system.

Monte Carlo (MC) simulation of dose to water and dose to detector has been used to calculate the correction factors needed for dose calibration and output factor measurements on the CyberKnife system. Reference field ionization chambers simulated were the PTW 30006, Exradin A12, and NE 2571 Farmer chambers, and small volume chambers PTW 31014 and 31010. Correction factors for Farmer chambers were found to be 0.7%-0.9% larger than those determined from TRS-398 due mainly to the dose gradient across the chamber cavity. For one microchamber where comparison was possible, the factor was 0.5% lower than TRS-398 which is consistent with previous MC simulations of flattening filter free Linacs. Output factor detectors simulated were diode models PTW 60008, 60012, 60017, 60018, Sun Nuclear edge detector, air-filled microchambers Exradin A16 and PTW 31014, and liquid-filled microchamber PTW 31018 microLion. Factors were generated for both fixed and iris collimators. The resulting correction factors differ from unity by up to +11% for air-filled microchambers and -6% for diodes at the smallest field size (5 mm), and tend towards unity with increasing field size (correction factor magnitude <1% for all detectors at field sizes >15 mm). Output factor measurements performed using these detectors with fixed and iris collimators on two different CyberKnife systems showed initial differences between detectors of >15% at 5 mm field size. After correction the measurements on each unit agreed within ∼1.5% at the smallest field size. This paper provides a complete set of correction factors needed to apply a new small field dosimetry formalism to both collimator types on the CyberKnife system using a range of commonly used detectors.

Sci-Fri PM: Delivery - 07: Cyberknife relative output factor measurements using fiber-coupled luminescence, MOSFETS and RADPOS dosimetry system.

Novel dosimetry systems based on Al2 O3 :C radioluminescence (RL) and a 4D dosimetry system (RADPOS) from Best Medical Canada were used to measure the relative output factor (ROF) on Cyberknife. Measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm for cones from 5 to 60 mm. ROFs were also measured using a mobileMOSFET system (Best Medical Canada) and EBT1 and EBT2 GAFCHROMIC® (ISP, Ashland) radiochromic films. For cone sizes 12.5-60 mm all detector results were in agreement within the measurement uncertainty. The microMOSFET/RADPOS measurements (published corrections applied) yielded ROFs of 0.650 ± 1.9%, 0.811 ± 0.9% and 0.843 ± 1.7% for the 5, 7.5 and 10 mm cones, respectively, and were in excellent agreement with radiochromic film values (averaged for EBT1 and EBT2) of 0.645 ± 1.4%, 0.806 ± 1.1% and 0.859 ± 1.1%. Monte-Carlo calculated correction factors were applied to the RL readings to correct for excessive scatter due to the relatively high effective atomic number of Al2 O3 (Z=10.2) compared to water for the 5, 7.5 and 10 mm cones. When these corrections are applied to our RL detector measurements, we obtain ROFs of 0.656 ± 0.3% and 0.815 ± 0.3% and 0.865 ± 0.3% for 5, 7.5 and 10 mm cones. Our study shows that the microMOSFET/RADPOS and optical fiber-coupled RL dosimetry system are well suited for Cyberknife cone output factors measurements over the entire range of field sizes, provided that appropriate correction factors are applied for the smallest cone sizes (5, 7.5 and 10 mm).

Calculation of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) for several small detectors and for two linear accelerators using Monte Carlo simulations.

PURPOSE: The scope of this study was to determine a complete set of correction factors for several detectors in static small photon fields for two linear accelerators (linacs) and for several detectors. METHODS: Measurements for Monte Carlo (MC) commissioning were performed for two linacs, Siemens Primus and Elekta Synergy. After having determined the source parameters that best fit the measurements of field specific output factors, profiles, and tissue-phantom ratio, the generalized version of the classical beam quality correction factor for static small fields, k(Q(clin),Q(msr) ) (f(clin),f(msr) ), were determined for several types of detectors by using the egs_chamber Monte Carlo user code which can accurately reproduce the geometry and the material composition of the detector. The influence of many parameters (energy and radial FWHM of the electron beam source, field dimensions, type of accelerator) on the value of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was evaluated. Moreover, a MC analysis of the parameters that influence the change of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) as a function of field dimension was performed. A detailed analysis of uncertainties related to the measurements of the field specific output factor and to the Monte Carlo calculation of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was done. RESULTS: The simulations demonstrated that the correction factor k(Q(clin),Q(msr) ) (f(clin),f(msr) ) can be considered independent from the quality beam factor Q in the range 0.68 ± 0.01 for all the detectors analyzed. The k(Q(clin),Q(msr) ) (f(clin),f(msr) ) of PTW 60012 and EDGE diodes can be assumed dependent only on the field size, for fields down to 0.5 × 0.5 cm². The microLion, and the microchambers, instead, must be used with some caution because they exhibit a slight dependence on the radial FWHM of the electron source, and therefore, a correction factor only dependent on field size can be used for fields ≥ 0.75 × 0.75 and ≥ 1.0 × 1.0 cm², respectively. The analysis of uncertainties gave an estimate of uncertainty for the 0.5 × 0.5 cm² field of about 0.7% (1σ) for k(Q(clin),Q(msr) ) (f(clin),f(msr) ) factor and of about 1.0% (1σ) for the field output factor, Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ), of diodes, microchambers, and microLion. CONCLUSIONS: Stereotactic diodes with the appropriate k(Q(clin),Q(msr) ) (f(clin),f(msr) ) are recommended for determining Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ) of small photon beams.

Quality assurance of volumetric modulated arc therapy: evaluation and comparison of different dosimetric systems.

PURPOSE: To compare and evaluate different dosimetric techniques and devices for the QA of VMAT plans created by two treatment planning systems (TPSs). METHODS: A total of 50 VMAT plans were optimized for treatment of anatomical sites of various complexities by two TPSs which use rather different approaches to VMAT optimization. Dosimetric plan verifications were performed both as part of commissioning and as patient specific QA of clinical treatments. Absolute point doses were measured for all plans by a micro ion chamber inserted in a dedicated water-filled cylindrical phantom. Delivered dose distributions were verified by four techniques based on different detectors: radiographic and gafchromic films, two systems based on 2D diode arrays and an ion chamber array. Gamma index analysis with various tolerance levels (3%, 3 mm and 3%, 2 mm) was used to analyze differences between calculated and delivered doses. Sensitivity to possible delivery errors was also evaluated for three of the considered devices introducing +/-3 mm shifts along the three directions and a 3 degrees gantry offset. RESULTS: Ion chamber measured point doses were within 3% of calculated ones for 48 out of 50 values. For delivered dose distribution, the average fraction of passed gamma values using 3% and 3 mm criteria was above 95% for both TPSs and all detectors except gafchromic film which yielded on average of 91.4%. For 49 out of 50 plans, a pass-rate above 94% was obtained by at least one of the four techniques. Shrinking the tolerance to 3% and 2 mm, the average pass-rate by all detectors (except film) was still above 95% for one of the two TPSs, but lower for the other one. The detector sensitivity to 3 mm shifts and to gantry angle offset was strongly plan and partially detector dependent: the obtained pass-rate reduction ranged from 2% to 30%. CONCLUSIONS: The presented results for VMAT plans QA assess the reliability of the delivered doses for both TPSs. The slightly lower pass-rate obtained for one of the considered TPS can be attributed to a higher level of complexity of the optimized plans. The results by different dosimetric techniques are coherent, apart from a few measurements by gafchromic films. The detector sensitivity to delivery errors, being strongly plan dependent, is not easy to evaluate.

Use of motion tracking in stereotactic body radiotherapy: Evaluation of uncertainty in off-target dose distribution and optimization strategies.

Spatial accuracy in extracranial radiosurgery is affected by organ motion. Motion tracking systems may be able to avoid PTV enlargement while preserving treatment times, however special attention is needed when fiducial markers are used to identify the target can move with respect to organs at risk (OARs). Ten patients treated by means of the Synchrony system were taken into account. Sparing of irradiated volume and of complication probability were estimated by calculating treatment plans with a motion tracking system (Cyberknife Synchrony, Sunnyvale, CA, USA) and a PTV-enlargement strategy for ten patients. Six patients were also evaluated for possible inaccuracy of estimation of dose to OARs due to relative movement between PTV and OAR during respiration. Dose volume histograms (DVH) and Equivalent Uniform Dose (EUD) were calculated for the organs at risk. In the cases for which the target moved closer to the OAR (three cases of six), a small but significant increase was detected in the DVH and EUD of the OAR. In three other cases no significant variation was detected. Mean reduction in PTV volume was 38% for liver cases, 44% for lung cases and 8.5% for pancreas cases. NTCP for liver reduced from 23.1 to 14.5% on average, for lung it reduced from 2.5 to 0.1% on average. Significant uncertainty may arise from the use of a motion-tracking device in determination of dose to organs at risk due to the relative motion between PTV and OAR. However, it is possible to limit this uncertainty. The breathing phase in which the OAR is closer to the PTV should be selected for planning. A full understanding of the dose distribution would only be possible by means of a complete 4D-CT representation.

Preliminary study on the use of nonrigid registration for thoraco-abdominal radiosurgery.

The inclusion of organ deformation and movement in radiosurgery treatment planning is of increasing importance as research and clinical applications begin to take into consideration the effects of physiological processes, like breathing, on the shape and position of lesions. In this scenario, the challenge is to localize the target in toto (not only by means of marker sampling) and to calculate the dose distribution as the sum of all the contributions from the positions assumed by the target during the respiratory cycle. The aim of this work is to investigate the use of nonrigid registration for target tracking and dynamic treatment planning, i.e., treatment planning based not on one single CT scan but on multiple CT scans representative of the respiration. Twenty patients were CT scanned at end-inhale and end-exhale. An expert radiation oncologist identified the PTV in both examinations. The two CT data sets per patient were nonrigidly registered using a free-form deformation algorithm based on B-splines. The optimized objective function consisted of a weighted sum of a similarity criterion (Mutual Information) and a regularization factor which constrains the transformation to be locally rigid. Once the transformation was obtained and the registration validated, its parameters were applied to the target only. Finally, the deformed target was compared to the PTV delineated by the radiation oncologist in the other study. The results of this procedure show an agreement between the center of mass as well as volume of the target identified automatically by deformable registration and manually by the radiation oncologist. Moreover, obtained displacements were in agreement with body structure constraints and considerations usually accepted in radiation therapy practice. No significant influence of initial target volume on displacements was found. In conclusion, the proposed method seems to offer the possibility of using nonrigid registrations in radiosurgery treatment planning, even if more cases need to be investigated in order to give a statistical consistency to parameter setup and proposed considerations.

CT-3D rotational angiography automatic registration: a sensitivity analysis.

Preprocessing, binning and dataset subsampling are investigated with regard to simultaneous maximisation of the speed, accuracy and robustness of CT-3D rotational angiography (3DRA) registration. Clinical diagnosis and treatment can both take advantage of this integration, because 3DRA allows the shape of vessel structures to be evaluated three-dimensionally with respect to standard 2D projective angiography. The method for optimising preprocessing, binning and subsampling consisted of independent variation of the corresponding parameters to maximise robustness and speed while maintaining subvoxel accuracy; the latter was computed as the sum of the mean squared errors initially present in the registrations with the errors relative to both binning and subsampling. The results suggest the choice of 256 bins, steps between 14 mm (coarse optimisation) and 2.5 mm (fine optimisation) and bone segmentation by threshold, for binning, subsampling and preprocessing, respectively. The application of this parameter set-up to 50 CT-3DRA registrations resulted in a saving, on average, of 40% of the time with respect to the method previously used, while registration error was maintained within 2 mm (1.97 mm, 90% confidence interval) and robustness was increased, so that no manual initial realignment was needed in 48 registrations. Validation by the registration of images acquired for a head phantom showed subvoxel residual errors. In conclusion, the proposed procedure can be considered a satisfactory strategy to optimise CT-3DRA registration.

Interstitial radiosurgery with the photon radiosurgery system in the minimally-invasive treatment of selected deep-seated brain tumors.

The purpose of this study was to evaluate the results of interstitial radiosurgery (IR) with Photon Radiosurgery System (PRS) in 18 patients (P) with deep-seated brain primary or secondary tumors. Follow-up varied from 2 to 53 months (mean, 13.6 mo). Seven P with glioblastomas died due to tumor progression. Five P with metastases died for systemic disease while local control was achieved in all. Six P with low-grade astrocytomas were well and imaging showed tumor control. We conclude that PRS IR is effective in the treatment of metastases while it provides lower benefit in malignant gliomas. It could play a major role in low-grade astrocytomas.

Dose verification of an IMRT treatment planning system with the BEAM EGS4-based Monte Carlo code.

Intensity modulated radiation therapy (IMRT) has been increasingly used in radiotherapy departments during the last several years. A major advantage of IMRT in comparison to traditional three-dimensional conformal radiotherapy is the higher capability in providing dose distributions that conform very tightly to the target even for very complex shapes such as, for instance, concave regions. This results in a significant sparing of adjacent normal tissues. Different types of algorithms are employed in the IMRT dose calculation, from the simple pencil beam method, such as the finite-size pencil beam algorithm, to the more sophisticated algorithms, such as the kernel-based convolution/superposition ones. With the latter ones, electronic disequilibrium and inhomogeneities are better dealt with in comparison to the correction-based models like pencil beam. Nevertheless, even these types of algorithms may have some approximations that can potentially affect the dose results, especially considering that in an IMRT plan small segments or beamlets may be present for which electronic disequilibrium and inhomogeneities effects are of paramount importance. The goal of this work was to determine the accuracy in monitor units (MU) and dose distribution calculation of the algorithm implemented in the commercial treatment planning system PINNACLE3 (P3), for two IMRT plans with 6 MV photon beams. This system is based on a convolution/superposition with the Collapsed Cone approximation algorithm. The "BEAM" Monte Carlo (MC) code was employed as a benchmark in comparing the MU calculation and the dose distribution of P3. The model used to calculate the MU, with the separation of collimator scatter from the phantom scatter, valid for broad beams, was verified for narrow and irregular segments. The attention was focused on the way P3 calculates output factors (OF). A difference of 8% compared to MC was found for a particularly narrow segment analyzed. A dependence of the results on field size was found. For the complete plan, the agreement of dose distribution and MU calculation with MC results (affected by a dose uncertainty less than 0.5%) is very good: the dose difference at isocenter is 2.1% (1 standard deviation) for a "Prostate" site and 2.9% (1 standard deviation) for the "Head and Neck" site.

A simple method for bremsstrahlung spectra reconstruction from transmission measurements.

A new method for evaluation of bremsstrahlung spectra from transmission measurements has been developed. In this method some very well known facts relating to thick target bremsstrahlung spectra are a priori included in the calculation procedure. Some characteristics of the method are preliminarily illustrated on a 6 MV therapy linear accelerator.

Employ of a new device for intra-operative radiotherapy of intracranial tumours.

BACKGROUND: This paper presents the operating experience acquired during 18 months of use of the Photon Radiosurgery System manufactured by Photo-electron Corp. as an intra-operative radiation therapy device. The device is based on a miniature x-ray source that delivers low energy x-rays from the tip of a 3.2 mm thick needle-like probe. The interstitial stereotactic employ of the source has already been reported, while there is no evidence of the open-field intra-operative use in the literature. METHOD: Open field intra-operative radiation therapy (IORT) is possible inserting the probe into spherical applicators with diameters ranging from 1.5 to 5 cm. The applicators are made to fit the surgical cavity in order to provide uniform distance between the x-ray source and the tumour bed. Delivery of the prescribed dose takes typically 10 to 45 minutes. Radiation characteristics of the source were measured by means of ionization chambers and radiochromic films positioned in a water phantom. Operating procedures aimed at quality assurance and radiation safety were developed. IORT was administered to 14 patients affected by malignant intracranial tumours. Doses from 10 to 15 Gy at 5 mm depth from the tumour bed were delivered after tumour removal. FINDINGS: This preliminary experience does not afford any clinical evidence of IORT efficacy for intracranial lesions: it permits one however to state the feasibility and safety of the procedure. INTERPRETATION: This system could favour a rapid and significant increase of the experience of intra-operative irradiation in the treatment of CNS tumours. A role in the treatment of extracranial neoplasms can be also foreseen but needs to be more extensively investigated.

Photon dose calculation of a three-dimensional treatment planning system compared to the Monte Carlo code BEAM.

The purpose of this work is to compare the photon dose calculation of a commercially available three-dimensional (3D) treatment planning system based on the collapsed cone convolution technique against BEAM, a Monte Carlo code that allows detailed simulation of a radiotherapy accelerator. The first part of the work is devoted to the commissioning of BEAM for a 6 MV photon beam and to the optimization of the linac description to fit the experimental data. This step also involves a comparison with radiochromic film data on an inhomogeneous phantom built to simulate electronic nonequilibrium conditions. Commissioning the selected photon beams required a careful description of the treatment head and the fine tuning of physical parameters such as electron beam energy and radius. The second part shows the dose comparison for real patient's CT data sets: A mediastinal treatment and a breast treatment were simulated. Doses in terms of absolute values per monitor unit were calculated based on the BEAM simulation of the CT data sets. For comparisons of real-patient cases, differences between the treatment planning system and BEAM ranged from 0 to 2.6% and were within +/-2 standard deviations for the dose calculated at the prescription point. Dose-volume histogram analysis indicated that there is no consistent difference between the Monte Carlo and the convolution calculations. On the basis of the results presented in this study, we can conclude that the CCC algorithm is capable of giving results absolutely comparable to those of a Monte Carlo calculation, as far as common 3D radiotherapy planning is concerned.

2D and 3D dose distribution determination in proton beam radiotherapy with GafChromic film detectors.

This paper presents the results obtained using radiochromic (MD-55 GafChromic) film for the 2D and 3D dosimetric reconstruction of the dose delivered by a proton beam under the real conditions of a programme of radiotherapy treatment for ocular tumours. Standard microdensitometric measurements were used to determine the variation in film optical density (O.D.) vs dose. Calibration curves were obtained by least-square fitting of the experimental OD values using a second order polynomial. This allows conversion of O.D. to dose. With this procedure it was possible to determine the distribution of the dose delivered by the proton beam in a phantom composed of layers of GafChromic film, with high surface spatial resolution and, through sections, the complete mapping of the dose delivered to a volume subjected to irradiation, as in a course of radiotherapy treatment.