The effects of prehospital TXA on mortality and neurologic outcomes in patients with traumatic intracranial hemorrhage: a subgroup analysis from the prehospital TXA for TBI trial.

BACKGROUND: In the Prehospital Tranexamic Acid (TXA) for TBI Trial, TXA administered within two hours of injury in the out-of-hospital setting did not reduce mortality in all patients with moderate/severe traumatic brain injury (TBI). We examined the association between TXA dosing arms, neurologic outcome, and mortality in patients with intracranial hemorrhage (ICH) on computed tomography (CT). METHODS: This was a secondary analysis of the Prehospital Tranexamic Acid for TBI Trial (ClinicalTrials.gov [NCT01990768]) that randomized adults with moderate/severe TBI (Glasgow Coma Scale<13) and systolic blood pressure > =90 mmHg within two hours of injury to a 2-gram out-of-hospital TXA bolus followed by an in-hospital saline infusion, a 1-gram out-of-hospital TXA bolus/1-gram in-hospital TXA infusion, or an out-of-hospital saline bolus/in-hospital saline infusion (placebo). This analysis included the subgroup with ICH on initial CT. Primary outcomes included 28-day mortality, 6-month Glasgow Outcome Scale-Extended (GOSE) < = 4, and 6-month Disability Rating Scale (DRS). Outcomes were modeled using linear regression with robust standard errors. RESULTS: The primary trial included 966 patients. Among 541 participants with ICH, 28-day mortality was lower in the 2-gram TXA bolus group (17%) compared to the other two groups (1-gram bolus/1-gram infusion 26%, placebo 27%). The estimated adjusted difference between the 2-gram bolus and placebo groups was -8·5 percentage points (95% CI, -15.9 to -1.0) and between the 2-gram bolus and 1-gram bolus/1-gram infusion groups was -10.2 percentage points (95% CI, -17.6 to -2.9). DRS at 6 months was lower in the 2-gram TXA bolus group than the 1-gram bolus/1-gram infusion (estimated difference -2.1 [95% CI, -4.2 to -0.02]) and placebo groups (-2.2 [95% CI, -4.3, -0.2]). Six-month GOSE did not differ among groups. CONCLUSIONS: A 2-gram out-of-hospital TXA bolus in patients with moderate/severe TBI and ICH resulted in lower 28-day mortality and lower 6-month DRS than placebo and standard TXA dosing. LEVEL OF EVIDENCE: Therapeutic/Care Management, Level II.

An evaluation of an energy independent CT reconstruction algorithm for use in radiotherapy treatment planning.

OBJECTIVE: Radiotherapy treatment planning relies upon density information provided by CT for accurate dose calculations. Hounsfield units (HUs) are converted to electron/physical density via an energy dependant calibration curve. Multiple curves are required to make full use of the available accelerating potentials (kVp). The curves are bi-linear with a discontinuity occurring at soft-tissue densities. The commercial algorithm, DirectDensityTM (Siemens Healthcare GmbH), constructs a single calibration curve covering all available kVp. This enables the optimisation of the CT image quality, e.g. in terms of contrast, or the reduction of the imaging dose, whilst rendering the radiotherapy treatment dose calculation robust to the energy used to acquire the CT image. We report our investigations on the clinical utilisation of the DirectDensityTM algorithm for radiotherapy treatments, by using all accelerating potentials, i.e. from 70 kVp up to 140 kVp, available at our CT treatment simulator, in contrast to previous studies that were limited to accelerating potentials spanning a subset of the available kVp. METHODS: The DirectDensityTM (DD) reconstruction algorithm available on a SOMATOM go.Open Pro CT scanner (Siemens Healthineers) was evaluated using the RayStation v. 9 treatment planning system (RaySearch Laboratories, Stockholm, Sweden) and a CIRS Model 002LFC IMRT Thorax Phantom (SunNuclear, Melbourne, FL), which was imaged at all available kVp with clinical protocols corresponding to various anatomical sites. The DD images were compared to those with the standard reconstruction algorithm acquired only at 120 kVp, as per our routine clinical practice. The effect of increasing kVp on HU is investigated for relevant tissue substitutes. In addition, a dosimetric comparison is performed for a VMAT plan technique with 6 MV X-rays using retrospective patient CT data sets representing four anatomical sites (pelvis, thorax, brain and "head and neck") with five patients for each site. The original dose distributions were calculated on images acquired at 120 kVp using the standard clinical iterative reconstruction (Qr40) and compared with dose distributions recalculated on images reconstructed with the new DD (Sm40) algorithm. RESULTS: The maximum difference for radiotherapy doses calculated using images of the phantom reconstructed with Qr40 (120 kVp) or DD (all available kVp) was 0.73%. The patient plans on the anatomically representative sites studied here showed a mean PTV dose difference of -0.2% (s.d. 0.7) for D99%, -0.4% (s.d. 0.4%) for D50% and -0.3% (s.d. 0.4%) for D2%. Incidentally, we found a previously unreported decrease in HU, mostly notable for bone type inserts (~34 HU (cortical bone)), at 110 kVp for the DD reconstructed images. The effect was not noted for the standard Qr40 reconstructions. CONCLUSION: DD has a minimal dosimetric impact in the dose calculations for radiotherapy treatments and could be implemented with existing clinical workflows. Attention should be paid to the HU values for images acquired at 110 kVp (DD algorithm), which warrants further investigation. ADVANCES IN KNOWLEDGE: This is the first paper where DD was evaluated at all available kVp, leading to the incidental discovery of abnormal HU values at 110 kVp for this algorithm.

Evaluation of the RadCalc collapsed cone dose calculation algorithm against measured data.

This work describes the experimental validation of the RadCalc (Lifeline software Inc, Tyler) collapsed cone dose calculation algorithm against measured data for a range of scenarios. 6 MV photon beam data were measured in a large water tank on a Varian TrueBeam linear accelerator. These were input into the RadCalc software, in conjunction with head geometry and output calibration information, then used to create a collapsed cone beam model. The model performance was assessed by comparison against measurement, using a selection of homogeneous and inhomogeneous geometries not incorporated into the original beam model. Dose calculations generated using the collapsed cone algorithm are generally in good agreement with measurement. However, the primary collimating of the linac is not accounted for in the RadCalc model and hence dose in the corners of large fields is significantly overestimated. Percentage depth doses were within 0.5% beyond a depth of 2 cm. The dose in the build-up region was underestimated by RadCalc Version 7.1.4.1, with (Distance To Agreement) discrepancies of up to 3 mm which were corrected in Version 7.2.2.0. Beam profiles for homogeneous phantom comparisons were within 2% in the central 80% of the field with out of field dose underestimated by no more than 3%. Dose comparisons in heterogeneous geometries were acceptable and generally within 3.5%. The largest observed differences were found at density interfaces and a result of the RadCalc dose resolution of 2 mm against 1 mm measured. Absolute dose comparisons demonstrated that RadCalc agreed with measurement to within 1.2% under homogeneous media irradiation geometries. For static beam IMRT deliveries agreement was within 2% or 2 mm of measured data, and for complex VMAT deliveries within 3% or 2 mm. The implementation of the (model-based) photon collapsed cone algorithm in RadCalc shows generally good agreement with measured data over a range of simple and complex scenarios considered.

Cardiac stereotactic ablative radiotherapy for control of refractory ventricular tachycardia: initial UK multicentre experience.

BACKGROUND: Options for patients with ventricular tachycardia (VT) refractory to antiarrhythmic drugs and/or catheter ablation remain limited. Stereotactic radiotherapy has been described as a novel treatment option. METHODS: Seven patients with recurrent refractory VT, deemed high risk for either first time or redo invasive catheter ablation, were treated across three UK centres with non-invasive cardiac stereotactic ablative radiotherapy (SABR). Prior catheter ablation data and non-invasive mapping were combined with cross-sectional imaging to generate radiotherapy plans with aim to deliver a single 25 Gy treatment. Shared planning and treatment guidelines and prospective peer review were used. RESULTS: Acute suppression of VT was seen in all seven patients. For five patients with at least 6 months follow-up, overall reduction in VT burden was 85%. No high-grade radiotherapy treatment-related side effects were documented. Three deaths (two early, one late) occurred due to heart failure. CONCLUSIONS: Cardiac SABR showed reasonable VT suppression in a high-risk population where conventional treatment had failed.

Comparison of the RayStation photon Monte Carlo dose calculation algorithm against measured data under homogeneous and heterogeneous irradiation geometries.

PURPOSE: This work compares Monte Carlo dose calculations performed using the RayStation treatment planning system against data measured on a Varian Truebeam linear accelerator with 6 MV and 10 MV FFF photon beams. METHODS: The dosimetric performance of the RayStation Monte Carlo calculations was evaluated in a variety of irradiation geometries employing homogeneous and heterogeneous phantoms. Profile and depth dose comparisons against measurement were carried out in relative mode using the gamma index as a quantitative measure of similarity within the central high dose regions. RESULTS: The results demonstrate that the treatment planning system dose calculation engine agrees with measurement to within 2%/1 mm for more than 95% of the data points in the high dose regions for all test cases. A systematic underestimation was observed at the tail of the profile penumbra and out of field, with mean differences generally <0.5 mm or 1% of curve dose maximum respectively. Out of field agreement varied between evaluated beam models. CONCLUSIONS: The RayStation implementation of photon Monte Carlo dose calculations show good agreement with measured data for the range of scenarios considered in this work and is deemed sufficiently accurate for introduction into clinical use.

A systematic review and practical considerations of stereotactic body radiotherapy in the treatment of head and neck cancer.

OBJECTIVES: Stereotactic radiotherapy (SBRT) is gaining popularity although its use in head and neck cancer (HNC) is not well defined. The primary objective was to review the published evidence regarding the use of stereotactic radiotherapy in HNC. METHODS: A literature search was performed by using MEDLINE and EMBASE databases for eligible studies from 2000 to 2019 and 26 relevant studies were identified. RESULTS: Literature demonstrates a heterogeneous use of this technique with regards to patient population, primary or salvage treatment, dose fractionation regimens, outcomes and follow-up protocols. Carotid blow out syndrome is a risk as with other forms of reirradiation but alternative treatment regimens may reduce this risk. CONCLUSION: At present there is a lack of evidence regarding SBRT as a primary treatment option for HNC and definitive answers regarding efficacy and tolerability cannot be provided but there is growing evidence that SBRT reirradiation regimens are safe and effective. In lieu of evidence from large Phase III trials, we define appropriate organ at risk constraints and prescription doses, with accurate plan summation approaches. Prospective randomised trials are warranted to validate improved treatment outcomes and acceptable treatment morbidity. ADVANCES IN KNOWLEDGE: This article provides a comprehensive review of evidence of use of stereotactic radiotherapy in HNC site (either as a primary treatment or as reirradiation). We also provide an evidence-based approach to the implementation and practical consideration of stereotactic radiotherapy in HNC.

Technical Note: Efficient and accurate MRI-only based treatment planning of the prostate using bulk density assignment through atlas-based segmentation.

PURPOSE: This study investigates the dosimetric accuracy as well as the robustness of a bulk density assignment approach to magnetic resonance imaging (MRI)-only based treatment planning of the prostate, with bulk density regions automatically identified using atlas-based segmentation (ABS). METHODS: Twenty prostate radiotherapy patients received planning computed tomography (CT) and MRI scans and were treated with volumetric modulated arc therapy (VMAT). Two bulk densities were set, one for bone and one for soft tissue. The bone contours were created by using ABS followed by manual modification if considered necessary. A range of soft tissue and bone density pairs, between 0.95 and 1.03 g/cm3 with increments of 0.01 for soft tissue, and between 1.15 and 1.65 g/cm3 with increments of 0.05 for bone, were evaluated. Using the density pair giving the lowest dose difference compared to the CT-based dose, dose differences were calculated using both the manually modified bone contours and the bone contours from ABS. Contour overlap measurements between the ABS contours and the manually modified contours were calculated. RESULTS: The dose comparison shows a very good agreement with the CT when using 0.98 g/cm3 for soft tissue and 1.20 g/cm3 for bone, with a dose difference less than 1 % in average dose in all regions of interest. The mean Dice similarity coefficient for bone was 0.94 and the Mean Distance to Agreement was <1 mm in most cases. CONCLUSIONS: Using bulk density assignment on MR images with suitable densities for bone and soft tissue results in clinically acceptable dose differences compared to dose calculated on the CT, for both atlas-based and manual bone contours. This demonstrates that an integrated MRI-only pathway utilizing a bulk density assignment for two tissue types is a feasible and robust approach for patients with prostate cancer treated with VMAT.

A UK wide study of current prostate planning practice.

OBJECTIVES: The objective of this work was to undertake a non-judgemental study of prostate planning practice across the UK by inviting all departments to undertake the same case. METHODS: An invitation to take part in the study was sent to the Heads of all UK radiotherapy departments and posted on the UK Medical Physics mailbase. Individuals interested in participating were able to access a single anonymised CT dataset for download with the prostate gland, seminal vesicles, bladder, rectum, bowel, femoral heads, and penile bulb outlined. A brief patient history was also supplied. Participants were asked to create planning target volumes (PTVs) according to their local clinical protocol and plan to give 60 Gy in 20 fractions to the PTV receiving the highest dose. No guidance was given for acceptable organ at risk doses. Dicom plan and dose information was loaded back into ProKnow for analysis by contributors. RESULTS: There were 102 plan submissions made to the study representing 48 different UK radiotherapy departments. Seventeen distinct methodologies for creating the prescription PTV from the prostate and seminal vesicles were identified with the ethos of the CHHIP trial protocol for margin growing followed in nearly two-thirds of cases. Positive correlations were found when assessing the doses received by the bladder and rectum against the volume of the PTV to which 60 Gy was prescribed. CONCLUSIONS: A national planning study whereby staff from a multitude of radiotherapy departments create plans based solely on a single dataset is feasible. The cohort of data was made available to all participants following the study to enable self-assessment and benchmarking against that of their peers. ADVANCES IN KNOWLEDGE: This is the first UK wide treatment planning study to investigate local clinical prostate planning practice. This has given UK departments the opportunity to evaluate their planning practices against those of their peers.

Evaluation of the RayStation electron Monte Carlo dose calculation algorithm.

The aim of this work was to evaluate the accuracy of the RayStation treatment planning system electron Monte Carlo algorithm against measured data for a range of clinically relevant scenarios. This was done by comparing measured percentage depth dose data (PDD) in water, profiles at oblique incidence and with heterogeneities in the beam path, and output factor data and that generated using the RayStation treatment planning system Monte Carlo VMC++ based calculation algorithm. While electron treatments are widely employed in the radiotherapy setting accurate modelling is challenging (TPS) in the presence of patient being both heterogeneous and nonrectangular. Watertank-based measurements were made on a Varian TrueBeam linear accelerator covering electron beam energies 6 to 18 MeV. These included both normal and oblique incidence, heterogeneous geometries, and irregular shaped cut-outs. The measured geometries were replicated in RayStation and the Monte Carlo dose calculation engine used to generate dosimetric data for comparison against measurement in what were considered clinically relevant settings. Water-based PDDs and profile comparisons showed excellent agreement for all electron beam energies. Profiles measured with oblique beam incidence demonstrated acceptable agreement to the treatment planning system calculations although the correspondence worsened as the angle increased with the planning system overestimating the dose in the shoulder region. Profile measurements under inhomogeneities were generally good. The planning system had a tendency to overestimate dose under the heterogeneity and also demonstrated a broader penumbra than measurement. Of the 170 different output factors calculated in RayStation over the range of electron energies commissioned, 141 were within ± 3% of measured values and 164 within ± 5%. Four of the 6 comparisons beyond 5% were at 18 MeV and all had a cut-out edge within 3 cm of the beam central axis/measurement point. The RayStation implementation of a VMC++ electron Monte Carlo dose calculation algorithm shows good agreement with measured data for a range of scenarios studied and represented sufficient accuracy for clinical use.

Dosimetric evaluation of VMAT for palliative radiotherapy for non-small cell lung carcinoma.

OBJECTIVE:: To compare the dosimetric consequences of volumetric modulated arc therapy (VMAT) for high-dose palliative thoracic radiotherapy through comparison with conventionally used isocentric parallel opposed pair (POP) of fields. METHODS:: 20 consecutive patients with non-small cell lung cancer who received 36 Gy in 12 fractions using a POP technique were re-planned using a single VMAT arc. Salient dosimetric parameters were compared between the plans using a paired t-test. RESULTS:: VMAT demonstrated dosimetric superiority; all PTV dose parameters were significantly improved and importantly the volume of normal lung receiving a high dose was also significantly reduced (mean volume of normal lung receiving 36 Gy was 12.9% in POP vs 1.8% in VMAT, p < 0.005). CONCLUSION:: The standard POP technique does not take into account tissue densities which results in higher doses to the normal tissue outside the target volume and reduced conformity to the PTV. ADVANCES IN KNOWLEDGE:: With the help of modern VMAT techniques, it is possible to effectively achieve highly conformal dose delivery which may provide an opportunity to escalate the dose to the tumour in this group of patients.

The accuracy of treatment planning system dose modelling in the presence of brass mesh bolus.

AIM: This work assesses the dosimetric accuracy of three commercial treatment planning system (TPS) photon dose calculation algorithms in the presence of brass mesh used as a bolus. BACKGROUND: Bolus material is used in radiotherapy to provide dose build-up where superficial tissues require irradiation. They are generally water equivalent but high density materials can also be used. MATERIALS AND METHODS: Dose calculations were performed on Monaco and Masterplan TPS (Elekta AB, Sweden) using phantoms defined by the three DICOM CT image sets of water equivalent blocks (no bolus, 1 layer and 2 layers of brass mesh) exported from the CT scanner. The effect of the mesh on monitor units, build-up dose, phantom exit dose and beam penumbra were compared to measured data. RESULTS: Dose calculations for 6 and 15 MV photon beams on plain water equivalent phantoms were seen to agree well with measurement validating the basic planning system algorithms and models. Dose in the build-up region, phantom exit dose and beam penumbra were poorly modelled in the presence of the brass mesh. The beam attenuation created by the bolus material was overestimated by all three calculation algorithms, at both photon energies, e.g. 1.6% for one layer and up to 3.1% for two layers at 6 MV. The poor modelling of the physical situation in the build-up region is in part a consequence of the high HU artefact caused by the mesh in the CT image. CONCLUSIONS: CT imaging is not recommended with the brass mesh bolus in situ due to the poor accuracy of the subsequent TPS modelling.

Dosimetric characteristics of brass mesh as bolus under megavoltage photon irradiation.

OBJECTIVE: This article presents a set of dosimetric measurements describing the properties of brass mesh (Whiting and Davis, Attleboro Falls, MA) under megavoltage photon irradiation conditions, with particular relevance to its use in breast radiotherapy. METHODS: The effectiveness of brass mesh as a bolus material was investigated using 6-, 15- and 6-MV flattening filter-free photon beams. The effect on dose build-up at the entrance surface, build-down at the beam-exit surface, dose with surface entrance obliquity, beam profiles, penumbra and percentage depth doses were investigated. RESULTS: One layer of the brass mesh produces a build-up effect equivalent to 1.1 mm of water at 6 MV and 1.9 mm at 15 MV. The brass generates a backscattered component of dose, if the photon beam exits through it. Percentage depth-dose curves are largely unaffected by the mesh and are shown to be equivalent to plain-field data. Beam penumbra and profiles are unchanged by the brass except within the first millimetre (mm) of phantom, where a periodic pattern of dose enhancement is seen. CONCLUSION: The data presented demonstrate that one layer of brass mesh provides a similar dose build-up effect equivalent to only a few millimetres of water. However, backscatter from the high atomic number (Z) mesh, at the beam exit, contributes appreciably to the overall dose surface enhancement. This dosimetric consequence cannot be neglected and indeed should be considered and accounted for, when determining the bolus effect of the brass mesh in the case of tangential breast irradiation. Advance in knowledge: This article provides dosimetric data necessary for the introduction of brass mesh bolus into the clinical setting for external-beam breast radiotherapy.

Empirical determination of collimator scatter data for use in Radcalc commercial monitor unit calculation software: Implication for prostate volumetric modulated-arc therapy calculations.

PURPOSE: The aim of this work was to determine, by measurement and independent monitor unit (MU) check, the optimum method for determining collimator scatter for an Elekta Synergy linac with an Agility multileaf collimator (MLC) within Radcalc, a commercial MU calculation software package. METHODS: The collimator scatter factors were measured for 13 field shapes defined by an Elekta Agility MLC on a Synergy linac with 6MV photons. The value of the collimator scatter associated with each field was also calculated according to the equation Sc=Sc(mlc)+Sc(corr)(Sc(open)-Sc(mlc)) with Sc(corr) varied between 0 and 1, where Sc(open) is the value of collimator scatter calculated from the rectangular collimator-defined field and Sc(mlc) the value using only the MLC-defined field shape by applying sector integration. From this the optimum value of the correction was determined as that which gives the minimum difference between measured and calculated Sc. Single (simple fluence modulation) and dual-arc (complex fluence modulation) treatment plans were generated on the Monaco system for prostate volumetric modulated-arc therapy (VMAT) delivery. The planned MUs were verified by absolute dose measurement in phantom and by an independent MU calculation. The MU calculations were repeated with values of Sc(corr) between 0 and 1. The values of the correction yielding the minimum MU difference between treatment planning system (TPS) and check MU were established. RESULTS: The empirically derived value of Sc(corr) giving the best fit to the measured collimator scatter factors was 0.49. This figure however was not found to be optimal for either the single- or dual-arc prostate VMAT plans, which required 0.80 and 0.34, respectively, to minimize the differences between the TPS and independent-check MU. Point dose measurement of the VMAT plans demonstrated that the TPS MUs were appropriate for the delivered dose. CONCLUSIONS: Although the value of Sc(corr) may be obtained by direct comparison of calculation with measurement, the efficacy of the value determined for VMAT-MU calculations are very much dependent on the complexity of the MLC delivery.

A comparison of phantom scatter from flattened and flattening filter free high-energy photon beams.

Flattening filter free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this work was to characterize the collimator scatter (Sc) and total scatter (Scp) from 3 FFF beams of differing quality indices and use the resulting mathematical fits to generate phantom scatter (Sp) data. The similarities and differences between Sp of flattened and FFF beams are described. Sc and Scp data were measured for 3 flattened and 3 FFF high-energy photon beams (Varian 6 and 10MV and Elekta 6MV). These data were fitted to logarithmic power law functions with 4 numerical coefficients. The agreement between our experimentally determined flattened beam Sp and published data was within ± 1.2% for all 3 beams investigated and all field sizes from 4 × 4 to 40 × 40cm(2). For the FFF beams, Sp was only within 1% of the same flattened beam published data for field sizes between 6 × 6 and 14 × 14cm(2). Outside this range, the differences were much greater, reaching - 3.2%, - 4.5%, and - 4.3% for the fields of 40 × 40cm(2) for the Varian 6-MV, Varian 10-MV, and Elekta 6-MV FFF beams, respectively. The FFF beam Sp increased more slowly with increasing field size than that of the published and measured flattened beam of a similar reference field size quality index, i.e., there is less Phantom Scatter than that found with flattened beams for a given field size. This difference can be explained when the fluence profiles of the flattened and FFF beams are considered. The FFF beam has greatly reduced fluence off axis, especially as field size increases, compared with the flattened beam profile; hence, less scatter is generated in the phantom reaching the central axis.

A comparison of small-field tissue phantom ratio data generation methods for an Elekta Agility 6 MV photon beam.

Tissue-phantom ratios (TPRs) are a common dosimetric quantity used to describe the change in dose with depth in tissue. These can be challenging and time consuming to measure. The conversion of percentage depth dose (PDD) data using standard formulae is widely employed as an alternative method in generating TPR. However, the applicability of these formulae for small fields has been questioned in the literature. Functional representation has also been proposed for small-field TPR production. This article compares measured TPR data for small 6 MV photon fields against that generated by conversion of PDD using standard formulae to assess the efficacy of the conversion data. By functionally fitting the measured TPR data for square fields greater than 4cm in length, the TPR curves for smaller fields are generated and compared with measurements. TPRs and PDDs were measured in a water tank for a range of square field sizes. The PDDs were converted to TPRs using standard formulae. TPRs for fields of 4 × 4cm(2) and larger were used to create functional fits. The parameterization coefficients were used to construct extrapolated TPR curves for 1 × 1 cm(2), 2 × 2-cm(2), and 3 × 3-cm(2) fields. The TPR data generated using standard formulae were in excellent agreement with direct TPR measurements. The TPR data for 1 × 1-cm(2), 2 × 2-cm(2), and 3 × 3-cm(2) fields created by extrapolation of the larger field functional fits gave inaccurate initial results. The corresponding mean differences for the 3 fields were 4.0%, 2.0%, and 0.9%. Generation of TPR data using a standard PDD-conversion methodology has been shown to give good agreement with our directly measured data for small fields. However, extrapolation of TPR data using the functional fit to fields of 4 × 4cm(2) or larger resulted in generation of TPR curves that did not compare well with the measured data.

Determination of monitor unit check tolerances based on a comparison with measurement and treatment planning system data.

This work describes the experimental validation of treatment planning system monitor unit (MU) calculations against measurement for a range of scenarios. This, together with a comparison of treatment planning system MUs and an independent MU check method, allows the derivation of confidence intervals for the check process. Data were collected for open and 60° motorized wedge fields using an Elekta Synergy linac at 6 and 8MV using homogeneous and heterogeneous phantoms. Masterplan (Version 4.0) pencil-beam and collapsed cone algorithms were used for the primary MU calculations with full inhomogeneity correction. Results show that both algorithms agree with measurement to acceptable tolerance levels in the majority of the cases studied. The confidence interval for the pencil-beam algorithm MU against an independent check was determined as + 1.6% to -3.4%. This is modified to + 2.3% to -2.5% when data collected with low-density heterogeneities are removed as this algorithm is not used clinically for these cases. The corresponding interval for the collapsed cone algorithm was + 1.2% to -4.3%, indicating that an offset tolerance for the independent check is appropriate. Analysis of clinical conformal treatment plan data generated using the pencil-beam algorithm (1393 beams) returned 93% of beams within the independent check tolerance. Similarly, using the collapsed cone algorithm as the primary MU calculation, 77% (of 1434 beams) were within the confidence interval.

Experimental verification of convolution/superposition photon dose calculations for radiotherapy treatment planning.

This work describes an experimental verification of the two-photon dose calculation engines available on the Helax-TMS (version 6.1) commercial radiotherapy treatment planning system. The performance of the pencil beam convolution and the collapsed cone superposition algorithms was examined for 4, 6, 15 MV beams, under a range of clinically relevant irradiation geometries. Comparisons against measurements were carried out in terms of absolute dose, thus assessment of the accuracy of monitor unit (MU) calculations was also carried out. Results show that both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The collapsed cone algorithm slightly overestimates the penumbra width and this is mainly due to discretization effects of the fluence matrix. The accuracy of this algorithm strongly depends on the resolution of the patient density matrix. It is recommended that the density matrix voxel size used for dose calculations is less than 5 x 5 x 5 mm3. The dose in media irradiated under missing tissue geometry, or in the presence of low or high-density heterogeneities, is modelled best with the collapsed cone algorithm. This is of particular clinical interest in treatment planning of the breast and of the thorax. For these treatment sites, a retrospective study of treatment plans indicated in certain cases significant overestimation of the dose to the planning target volume when using the pencil beam convolution model.

Evaluation of the dosimetric consequences of adding a single asymmetric or MLC shaped field to a tangential breast radiotherapy technique.

Fifteen consecutive patients had standard treatment plans generated using our departmental protocol and two further plans produced using either an asymmetric, or MLC shaped additional field, from each tangential direction. The mean percentage of the PTV receiving over 107% of the isocentre dose was 19.8% for the standard planned patients (95% confidence interval 12.3-27.4%). This was reduced to 6.0% for the asymmetric field technique (95% confidence interval 4.1-8.0%) and 5.3% for the MLC technique (95% confidence interval 2.8-7.7%). These high dose volume reductions were therefore significant at the 95% confidence level. It was also concluded that both alternative planning techniques offer the greatest potential when the standard plan indicated that more than 20% of the PTV would receive greater than 107% of the prescribed dose. Under these circumstances the segmented field techniques led to a reduction of at least 15 percentage points in this figure. It is this group of patients who stand to benefit most from application of these simple additional field techniques.