[en] Full text: Fractionated stereotactic radiotherapy uses relocatable stereotactic head fixation systems along with positioning and imaging techniques originally developed by Leksell and others for stereotactic radiosurgery. The aim is to achieve acceptable spatial positioning accuracy approaching that for stereotactic radiosurgery without the invasive fixation ring, and therefore to make possible fractionated treatments which have a known radiobiological advantage. At Royal Adelaide Hospital we have carried out a series of quality assurance measurements using a fibre glass head mould attached to a ring which can be reproducibly fixed to the treatment couch. Initially the target is set to the isocentre (from known co-ordinates for treatment, or the measured position of a 1mm lead shot for QA) by using positioning micrometers on a box frame attached to the ring. The laser beam positions are then marked on the mould so that the position can be accurately regained for later fractions. The QA was carried out with the head of an anthropomorphic phantom. Skin was simulated by PVC foam wrap whilst hair was simulated by bubblewrap. Two sets of experiments were performed. The first set evaluated the accuracy of the mask repositioning on the head. This was done through a series of measurements of lead shot co-ordinates after the phantom head was repeatedly repositioned inside the mask. The second set evaluated the total accuracy of targeting. The reproducibility of the head positioning inside the mould was checked many times with a simulator X-ray and with a 6 MeV linac by determining the positions of four 1mm lead shots. Also the position accuracy of a known shot relative to the treatment collimator was measured. Reproducibility of head positioning inside the mould is ±1.5 mm while collimator accuracy relative to a known shot is ±1 mm. Copyright (2001) Australasian College of Physical Scientists and Engineers in Medicine

[en] Full text: Patient positioning errors and organ motion during the course of a radiotherapy treatment will introduce uncertainty in the amount of radiation dose delivered to both the tumour and contiguous normal tissue. For prostate treatments the desired dose distribution is carefully planned to avoid excess dose to the rectum and bladder, while depositing toxic doses to the tumour. We investigate the effect of variable fractional doses to the rectum, on the probability of normal tissue (rectum) complication. The simulation uses published data for random patient movements, organ motion, and tissue sensitivities with the Lyman/Kutcher normal tissue complication probability (NTCP) model

[en] Inter-fraction dose fluctuations, which appear as a result of setup errors, organ motion and treatment machine output variations, may influence the radiobiological effect of the treatment even when the total delivered physical dose remains constant. The effect of these inter-fraction dose fluctuations on the biological effective dose (BED) has been investigated. Analytical expressions for the BED accounting for the dose fluctuations have been derived. The concept of biological effective constant dose (BECD) has been introduced. The equivalent constant dose (ECD), representing the constant physical dose that provides the same cell survival fraction as the fluctuating dose, has also been introduced. The dose fluctuations with Gaussian as well as exponential probability density functions were investigated. The values of BECD and ECD calculated analytically were compared with those derived from Monte Carlo modelling. The agreement between Monte Carlo modelled and analytical values was excellent (within 1%) for a range of dose standard deviations (0-100% of the dose) and the number of fractions (2 to 37) used in the comparison. The ECDs have also been calculated for conventional radiotherapy fields. The analytical expression for the BECD shows that BECD increases linearly with the variance of the dose. The effect is relatively small, and in the flat regions of the field it results in less than 1% increase of ECD. In the penumbra region of the 6 MV single radiotherapy beam the ECD exceeded the physical dose by up to 35%, when the standard deviation of combined patient setup/organ motion uncertainty was 5 mm. Equivalently, the ECD field was ∼2 mm wider than the physical dose field. The difference between ECD and the physical dose is greater for normal tissues than for tumours

[en] Dose profiles produced by wedge filters in the non-wedged direction can exhibit a 7% or greater dose reduction at the outer ends of the field compared with open field profiles. However, many planning systems use open field profiles to model wedged dose distributions. In the present work, wedges have been modified to reproduce open field profile shapes. This modification involved removing varying thicknesses of the wedge using a simple milling machine. The wedge thickness was calculated using the assumption that dose is proportional to primary collision kerma. The discrepancies in dose between wedged field and open field profile shapes of up to 7% were reduced to less than 3% with the modifications, even for varying depths and off-axis distances. The necessary measurements are simple to perform, and hence this technique could be applied to improve wedged field dose distributions in other radiotherapy departments. (author)

[en] RapidArc radiotherapy technology from Varian Medical Systems is one of the most complex delivery systems currently available, and achieves an entire intensity-modulated radiation therapy (IMRT) treatment in a single gantry rotation about the patient. Three dynamic parameters can be continuously varied to create IMRT dose distributions-the speed of rotation, beam shaping aperture and delivery dose rate. Modeling of RapidArc technology was incorporated within the existing Vancouver Island Monte Carlo (VIMC) system (Zavgorodni et al 2007 Radiother. Oncol. 84 S49, 2008 Proc. 16th Int. Conf. on Medical Physics). This process was named VIMC-Arc and has become an efficient framework for the verification of RapidArc treatment plans. VIMC-Arc is a fully automated system that constructs the Monte Carlo (MC) beam and patient models from a standard RapidArc DICOM dataset, simulates radiation transport, collects the resulting dose and converts the dose into DICOM format for import back into the treatment planning system (TPS). VIMC-Arc accommodates multiple arc IMRT deliveries and models gantry rotation as a series of segments with dynamic MLC motion within each segment. Several verification RapidArc plans were generated by the Eclipse TPS on a water-equivalent cylindrical phantom and re-calculated using VIMC-Arc. This includes one 'typical' RapidArc plan, one plan for dual arc treatment and one plan with 'avoidance' sectors. One RapidArc plan was also calculated on a DICOM patient CT dataset. Statistical uncertainty of MC simulations was kept within 1%. VIMC-Arc produced dose distributions that matched very closely to those calculated by the anisotropic analytical algorithm (AAA) that is used in Eclipse. All plans also demonstrated better than 1% agreement of the dose at the isocenter. This demonstrates the capabilities of our new MC system to model all dosimetric features required for RapidArc dose calculations. (note)

[en] The Monte Carlo (MC) method provides the most accurate to-date dose calculations in heterogeneous media and complex geometries, and this spawns increasing interest in incorporating MC calculations in the treatment planning quality assurance process. This process involves MC dose calculations for the treatment plans produced clinically. Commonly used in radiotherapy, MC codes are BEAMnrc and DOSXYZnrc, which transport particles in a coordinate system (c.s.) that has been established historically and does not correspond to the c.s. of treatment planning systems (TPSs). Relative rotations of these c.s. are not straightforward, especially for non-coplanar treatments. Transformation equations are therefore required to re-calculate a treatment plan using BEAM/DOSXYZnrc codes. This paper presents such transformations for beam angles defined in a DICOM-compliant treatment planning coordinate system. Verification of the derived transformations with two three-field plans simulated on a phantom using TPS as well as MC codes has been performed demonstrating exact geometrical agreement of the MC treatment fields' placement. (note)

[en] Full text: Stereotactic radiosurgery (SRS) is a high technology treatment technique in which many assumptions have been made to speed the dose calculation. Included in these (in our Leibinger system) are tissue homogeneity, normal incidence on the body contour, and a depth-independent dose profile. In this work the validity of these assumptions is investigated and deviations quantified. Monte Carlo (EGS4) techniques have been employed to quantify the effect of air gaps on absorbed dose for SRS beams. TMRs were calculated at the isocentre for semi-infinite layers of water-air-water. Air gap layers of 5, 10, and 20 mm thick were positioned at 26 mm deep, after d_max for our 6 MV photon beam spectrum. Also MC simulations were performed to calculate dose profiles at the isocentre at 6 cm depth in water, where a 5 mm air gap was positioned after 54 mm depth in water. To quantify the change in profile characteristics with depth, profiles for 7 and 23 mm SRS collimators were measured at the isocentre at 2, 6 and 15 cm depths in solid water, using Kodak X-Omat V films. Monte Carlo calculated TMRs showed reductions of 45%, 65% and 75% for a 7 mm collimator and 19%, 30% and 45% for a 23 mm collimator immediately beyond 5, 10, and 20 mm thick air gaps respectively; beyond this secondary buildup region dose is increased. Also MC calculation of dose profiles after a 5 mm air gap showed about 40% and 200% increase in the 90%-20% penumbra width for 7 and 23 mm collimators, respectively. The presence of an air cavity will cause an under-dose to a target located after the cavity and an over-dose to the normal tissue adjacent to the target volume. Using film dosimetry, it was shown that 90%-50% and 90%-10% penumbra widths increased by 22% and 32% for 23 mm collimator, when depth changed from 2 to 15 cm. Consequently, in treating lesions at different depths, the volume inside the 90% isodose will be under- or over-estimated if the lesion site is shallower or deeper than average (6 cm). Assumptions of normal incidence of the beam on the body surface and a finite number of the beam entry points for each arc in calculating average TMR was found to be insignificant. It has been shown that, although the assumption of a homogeneous medium for the brain is valid in most cases, in some conditions this could cause significant errors (such as in using small number of static fields). Also, it was shown that the constancy of the dose profile with depth is not a good assumption, especially when the target volume is close to a critical organ. This may lead to incorrect choice of collimator size or prescription dose and will reduce the therapeutic ratio

[en] Purpose: The dosimetric accuracy of the recently released Acuros XB advanced dose calculation algorithm (Varian Medical Systems, Palo Alto, CA) is investigated for single radiation fields incident on homogeneous and heterogeneous geometries, and a comparison is made to the analytical anisotropic algorithm (AAA). Methods: Ion chamber measurements for the 6 and 18 MV beams within a range of field sizes (from 4.0x4.0 to 30.0x30.0 cm²) are used to validate Acuros XB dose calculations within a unit density phantom. The dosimetric accuracy of Acuros XB in the presence of lung, low-density lung, air, and bone is determined using BEAMnrc/DOSXYZnrc calculations as a benchmark. Calculations using the AAA are included for reference to a current superposition/convolution standard. Results: Basic open field tests in a homogeneous phantom reveal an Acuros XB agreement with measurement to within ±1.9% in the inner field region for all field sizes and energies. Calculations on a heterogeneous interface phantom were found to agree with Monte Carlo calculations to within ±2.0%(σ_MC=0.8%) in lung (ρ=0.24 g cm^-3) and within ±2.9%(σ_MC=0.8%) in low-density lung (ρ=0.1 g cm^-3). In comparison, differences of up to 10.2% and 17.5% in lung and low-density lung were observed in the equivalent AAA calculations. Acuros XB dose calculations performed on a phantom containing an air cavity (ρ=0.001 g cm^-3) were found to be within the range of ±1.5% to ±4.5% of the BEAMnrc/DOSXYZnrc calculated benchmark (σ_MC=0.8%) in the tissue above and below the air cavity. A comparison of Acuros XB dose calculations performed on a lung CT dataset with a BEAMnrc/DOSXYZnrc benchmark shows agreement within ±2%/2mm and indicates that the remaining differences are primarily a result of differences in physical material assignments within a CT dataset. Conclusions: By considering the fundamental particle interactions in matter based on theoretical interaction cross sections, the Acuros XB algorithm is capable of modeling radiotherapy dose deposition with accuracy only previously achievable with Monte Carlo techniques.

[en] Full text: The aim of this research was to construct a wedge for a radiotherapy beam which reproduced the dose distribution planned by a treatment planning computer in the non-wedged axis. Dose profile curves in the non-wedged axis for a wedged field irradiation show a decrease in dose (sagging) with off-axis distance. This sagging is mainly due to the softening of the energy spectrum off-axis (Chui C-S and LoSasso T, Med. Phys. 21:1685-90, 1994). Other contributing factors include the increase in scattered photon fluence and the increase in the pathlength of off-axis photons due to the angulation of the beam off axis. Because of this sagging, there is an underdosing of up to 7% in the non-wedged direction. This discrepancy increases with off-axis distance. Ideally, the non-wedged profile shape should match that of an open field profile, as the planning computer assumes this shape in the dose calculation. Theory based on the dose distribution beneath the old wedge was used to calculate the required thickness of the new wedge at each off-axis point. Two assumptions were made about the particle transport: (i) that dose = collision kerma (which is a valid assumption at the measurement point) and (ii) that the photon spectrum at a point beneath the wedge is unchanged if the wedge thickness at that point is changed slightly. This second assumption is valid if the change in wedge thickness is small, and hence assumes that the change in wedge thickness will only affect the photon number beneath the wedge. Using the wedge thicknesses derived from the theoretical calculations, a new 60deg wedge was milled, and the dose distribution beneath this wedge was measured, and compared to open field profiles in the non-wedged axis direction. Dose profile results using the new wedge in the non-wedged axis direction differed from the open field profile by less than 1%, an improvement of over 6% on the old wedge. Making new wedges as described above will give a dose distribution which does not exhibit sagging in the non-wedged direction, and is in better agreement with the dose distribution calculated by the treatment planning computer

[en] RapidArc(TM), recently released by Varian Medical Systems, is a novel extension of IMRT in which an optimized 3D dose distribution may be delivered in a single gantry rotation of 360 deg. or less. The purpose of this study was to investigate the accuracy of the analytical anisotropic algorithm (AAA), the sole algorithm for photon dose calculations of RapidArc(TM) treatment plans. The clinical site chosen was oropharynx and the associated nodes involved. The VIMC-Arc system, which utilizes BEAMnrc and DOSXYZnrc for particle transport through the linac head and patient CT phantom, was used as a benchmarking tool. As part of this study, the dose for a single static aperture, typical for RapidArc(TM) delivery, was calculated by the AAA, MC and compared with the film. This film measurement confirmed MC modeling of the beam aperture in water. It also demonstrated that the AAA dosimetric error can be as high as 12% near isolated leaf edges and up to 5% at the leaf end. The composite effect of these errors in a full RapidArc(TM) calculation in water involving a C-shaped target and the associated organ at risk produced a 1.5% overprediction of the mean target dose. In our cohort of six patients, the AAA was found, on average, to overestimate the PTV60 coverage at the 95% level in the presence of air cavities by 1.0% (SD = 1.1%). Removing the air cavities from the target volumes reduced these differences by about a factor of 2. The dose to critical structures was also overestimated by the AAA. The mean dose to the spinal cord was higher by 1.8% (SD = 0.8%), while the effective maximum dose (D_2%) was only 0.2% higher (SD = 0.6%). The mean dose to the parotid glands was overestimated by ∼9%. This study has shown that the accuracy of the AAA for RapidArc(TM) dose calculations, performed at a resolution of 2.5 mm or better, is adequate for clinical use.