Particular issues to be considered in small field dosimetry for TrueBeam STx commissioning

This study aims to report the relevant issues concerning small fields in the commissioning of a TrueBeam STx for photon energies of 6MV, 10MV, 6FFF, and 10FFF. Percent depth doses, profiles, and field output factors were measured according to the beam model configuration of the treatment planning system. Multiple detectors were used based on the IAEA TRS-483 protocol as well as EBT3 radiochromic film. Analytical Anisotropic and Acuros XB algorithms, were configured and validated through basic dosimetry comparisons and end-to-end clinical tests.

Assessing the need for adaptive radiotherapy in head and neck cancer patients using an automatic planning tool

Background

Unbiased analysis of the impact of adaptive radiotherapy (ART) is necessary to evaluate dosimetric benefit and optimize clinics' workflows. The aim of the study was to assess the need for adaptive radiotherapy (ART) in head and neck (H&N) cancer patients using an automatic planning tool in a retrospective planning study.

Materials and methods

Thirty H&N patients treated with adaptive radiotherapy were analysed. Patients had a CT scan for treatment planning and a verification CT during treatment according to the clinic's protocol. Considering these images, three plans were retrospectively generated using the iCycle tool to simulate the scenarios with and without adaptation: 1) the optimized plan based on the planning CT; 2) the optimized plan based on the verification CT (ART-plan); 3) the plan obtained by considering treatment plan 1 re-calculated in the verification CT (non-ART plan). The dosimetric endpoints for both target volumes and OAR were compared between scenarios 2 and 3 and the SPIDERplan used to evaluate plan quality.

Results

The most significant impact of ART was found for the PTVs, which demonstrated decreased D98% in the non-ART plan. A general increase in the dose was observed for the OAR but only the spinal cord showed a statistical significance. The SPIDERplan analysis indicated an overall loss of plan quality in the absence of ART.

Conclusion

These results confirm the advantages of ART in H&N patients, especially for the coverage of target volumes. The usage of an automatic planning tool reduces planner-induced bias in the results, guaranteeing that the observed changes derive from the application of ART.

A protocol for absolute dose verification of SBRT/SRS treatment plans using Gafchromic™ EBT-XD films

PURPOSE

To provide a practical protocol for absolute dose verification of stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatment plans, based on our clinical experience. It aims to be a concise summary of the main aspects to be considered when establishing an accurate film dosimetry system.

METHODS

Procedures for film calibration and conversion to dose are described for a dosimetry system composed of Gafchromic™ EBT-XD films and a flatbed document scanner. Factors that affect the film-scanner response are also reviewed and accounted for. The accuracy of the proposed methodology was assessed by taking a set of strips irradiated to known doses and its applicability is illustrated for ten SBRT/SRS treatment plans. The film response was converted to dose using red and triple channel dosimetry. The agreement between the planned and measured dose distributions was evaluated using global gamma analysis with criteria of 3%/2mm 10% threshold (TH), 2%/2mm 10% TH, and 2%/2mm 20% TH.

RESULTS

The differences between the expected and determined doses from the strips analysis were 0.9 ± 0.6% for the red channel and 1.1 ± 0.7% for the triple channel method. Regarding the SBRT/SRS plans verification, the mean gamma passing rates were 99.5 ± 1.0% vs 99.6 ± 1.0% (3%/2mm 10% TH), 96.9 ± 3.5% vs 99.1 ± 1.3% (2%/2mm 10% TH) and 98.4 ± 1.8% vs 98.8 ± 1.5% (2%/2mm 20% TH) for red and triple channel dosimetry, respectively.

CONCLUSIONS

The proposed protocol allows for accurate absolute dose verification of SBRT/SRS treatment plans, applying both single and triple channel methods. It may work as a guide for users that intend to implement a film dosimetry system.

On the complexity of helical tomotherapy treatment plans

PURPOSE

Multiple metrics are proposed to characterize and compare the complexity of helical tomotherapy (HT) plans created for different treatment sites.

METHODS

A cohort composed of 208 HT plans from head and neck (105), prostate (51) and brain (52) tumor sites was considered. For each plan, 14 complexity metrics were calculated. Those metrics evaluate the percentage of leaves with small opening times or approaching the projection duration, the percentage of closed leaves, the amount of tongue-and-groove effect, and the overall modulation of the planned sinogram. To enable data visualization, an approach based on principal component analysis was followed to reduce the dataset dimensionality. This allowed the calculation of a global plan complexity score. The correlation between plan complexity and pretreatment verification results using the Spearman's rank correlation coefficients was investigated.

RESULTS

According to the global score, the most complex plans were the head and neck tumor cases, followed by the prostate and brain lesions irradiated with stereotactic technique. For almost all individual metrics, head and neck plans confirmed to be the plans with the highest complexity. Nevertheless, prostate cases had the highest percentage of leaves with an opening time approaching the projection duration, whereas the stereotactic brain plans had the highest percentage of closed leaves per projection. Significant correlations between some of the metrics and the pretreatment verification results were identified for the stereotactic brain group.

CONCLUSIONS

The proposed metrics and the global score demonstrated to be useful to characterize and quantify the complexity of HT plans of different treatment sites. The reported differences inter- and intra-group may be valuable to guide the planning process aiming at reducing uncertainties and harmonize planning strategies.

Radiobiological evaluation of forward and inverse IMRT using different fractionations for head and neck tumours

Purpose

To quantify the radiobiological advantages obtained by an Improved Forward Planning technique (IFP) and two IMRT techniques using different fractionation schemes for the irradiation of head and neck tumours. The conventional radiation therapy technique (CONVT) was used here as a benchmark.

Methods

Seven patients with head and neck tumours were selected for this retrospective planning study. The PTV1 included the primary tumour, PTV2 the high risk lymph nodes and PTV3 the low risk lymph nodes. Except for the conventional technique where a maximum dose of 64.8 Gy was prescribed to the PTV1, 70.2 Gy, 59.4 Gy and 50.4 Gy were prescribed respectively to PTV1, PTV2 and PTV3. Except for IMRT2, all techniques were delivered by three sequential phases. The IFP technique used five to seven directions with a total of 15 to 21 beams. The IMRT techniques used five to nine directions and around 80 segments. The first, IMRT1, was prescribed with the conventional fractionation scheme of 1.8 Gy per fraction delivered in 39 fractions by three treatment phases. The second, IMRT2, simultaneously irradiated the PTV2 and PTV3 with 59.4 Gy and 50.4 Gy in 28 fractions, respectively, while the PTV1 was boosted with six subsequent fractions of 1.8 Gy. Tissue response was calculated using the relative seriality model and the Poisson Linear-Quadratic-Time model to simulate repopulation in the primary tumour.

Results

The average probability of total tumour control increased from 38% with CONVT to 80% with IFP, to 85% with IMRT1 and 89% with IMRT2. The shorter treatment time and larger dose per fraction obtained with IMRT2 resulted in an 11% increase in the probability of control in the PTV1 with respect to IFP and 7% relatively to IMRT1 (p < 0.05). The average probability of total patient complications was reduced from 80% with CONVT to 61% with IFP and 31% with IMRT. The corresponding probability of complications in the ipsilateral parotid was 63%, 42% and 20%; in the contralateral parotid it was 50%, 20% and 9%; in the oral cavity it was 2%, 15% and 4% and in the mandible it was 1%, 5% and 3%, respectively.

Conclusions

A significant improvement in treatment outcome was obtained with IMRT compared to conventional radiation therapy. The practical and biological advantages of IMRT2, employing a shorter treatment time, may outweigh the small differences obtained in the organs at risk between the two IMRT techniques. This technique is therefore presently being used in the clinic for selected patients with head and neck tumours. A significant improvement in the quality of the dose distribution was obtained with IFP compared to CONVT. Thus, this beam arrangement is used in the clinical routine as an alternative to IMRT.

A dosimetric calibration method for a double-focused multileaf collimator

A method based on penumbra measurements and not on light field, is proposed for the calibration of the Siemens 82-leaf Optifocus multileaf collimator (MLC). The measurements were performed using MLCSoft version 1.2.0.4. (PTW-Freiburg, Germany), which is a software tool based on the LA48 measuring system (a linear array of 47 liquid-ionization chambers). The new leaf positions for each of the leaf banks are calculated from the MLCSoft results using an external spreadsheet. Using the proposed method the MLC calibration is directly performed in terms of dosimetry settings with improved precision. The nominal resulting accuracy of 0.5 mm is highly imposed by the limitations of the moving mechanism of the leaves and not by the method itself. To test the results of the MLC calibration, in-water measurements of off-axis dose profiles have been measured in order to verify different field sizes.

Phylogenetic status and matrilineal structure of the biting midge, Culicoides imicola, in Portugal, Rhodes and Israel

The biting midge Culicoides imicola Kieffer (Diptera: Ceratopogonidae) is the most important Old World vector of African horse sickness (AHS) and bluetongue (BT). Recent increases of BT incidence in the Mediterranean basin are attributed to its increased abundance and distribution. The phylogenetic status and genetic structure of C. imicola in this region are unknown, despite the importance of these aspects for BT epidemiology in the North American BT vector. In this study, analyses of partial mitochondrial cytochrome oxidase subunit I gene (COI) sequences were used to infer phylogenetic relationships among 50 C. imicola from Portugal, Rhodes, Israel, and South Africa and four other species of the Imicola Complex from southern Africa, and to estimate levels of matrilineal subdivision in C. imicola between Portugal and Israel. Eleven haplotypes were detected in C. imicola, and these formed one well-supported clade in maximum likelihood and Bayesian trees implying that the C. imicola samples comprise one phylogenetic species. Molecular variance was distributed mainly between Portugal and Israel, with no haplotypes shared between these countries, suggesting that female-mediated gene flow at this scale has been either limited or non-existent. Our results provide phylogenetic evidence that C. imicola in the study areas are potentially competent AHS and BT vectors. The geographical structure of the C. imicola COI haplotypes was concordant with that of BT virus serotypes in recent BT outbreaks in the Mediterranean basin, suggesting that population subdivision in its vector can impose spatial constraints on BT virus transmission.

Spatial distribution of Culicoides species in Portugal in relation to the transmission of African horse sickness and bluetongue viruses

Surveillance of Culicoides (Diptera: Ceratopogonidae) biting midge vectors was carried out at 87 sites within a 50 x 50 km grid distributed across Portugal, using light trap collections at the time of peak midge abundance. Culicoides imicola (Kieffer) made up 66% of the 55 937 Culicoides in these summer collections. It was highly abundant in the central eastern portion of Portugal, between 37 degrees 5' N and 41 degrees 5' N, and in a band across to the Lisbon peninsula (at around 38 degrees 5' N). Of all the complexes, its distribution was most consistent with that of previous outbreaks of Culicoides-borne disease, suggesting that it may remain the major vector in Portugal. Its distribution was also broadly consistent with that predicted by a recent climate-driven model validating the use of remote sensing datasets for modelling of Culicoides distribution. Adult C. imicola were found to have overwintered at 12 of 20 sites re-surveyed in winter but it did so in very low numbers. Culicoides obsoletus (Meigen) and Culicoides pulicaris (Linnaeus) complex midges were widespread despite their low summer abundance. The observed coincidence of high abundances of C. imicola and high abundances of C. pulicaris in summer lead us to suggest that C. imicola could bring African horse sickness virus or bluetongue virus into contact with C. pulicaris and the latter complex, together with C. obsoletus, could then transmit these viruses across much wider areas of Europe. The fact that adult C. pulicaris are present in high abundances in winter may provide a mechanism by which these viruses can overwinter in these areas.