[en] Background and purpose: This paper compares the quality control checks performed in departments in the United Kingdom with those recommended by the Institute of Physics and Engineering in Medicine (IPEM) in Report 81. Method: The data were gathered by the quality assurance team for the START trial, during visits to individual departments. Data are compared with the frequencies suggested in IPEM report 81. Where applicable, data are also analyzed with respect to machine manufacturer. Results: All departments with linear accelerators check the output at least weekly for both photons and electrons, however 11% of departments do not perform a constancy check of the output on a daily basis. The majority of departments check flatness at gantry zero on a monthly basis. However 27% of departments never check at non-zero gantry angles. In 51% of departments no fluoroscopy checks are performed on simulators on a daily basis. Conclusion: The majority of departments are following the frequencies suggested in IPEM report 81 although there are a number of discrepancies particularly for simulators. Accelerator type needs to be considered when designing quality assurance checks. In many departments more time than is currently allocated is needed on equipment, particularly simulators, to complete all of the checks suggested in Report 81

[en] Purpose: To examine the ability of computer planning systems to calculate the dose to the breast correctly in three dimensions. Both the absolute dose at the center of the breast and the accuracy of the isodose distributions were investigated. Methods and Materials: Measurements were performed in a water-filled breast phantom using an ionization chamber. Thirty-six sets of data obtained during the Standardization of Breast Radiotherapy breast fractionation trial quality assurance program were included in the analysis. The planning systems were grouped according to the algorithms used on the basis of the definitions given in International Commission on Radiation Units and Measurements Report No. 24. Results: Thirty-two of the 36 planning systems overestimated the dose to the center of the breast, with a mean measured/calculated dose ratio of 0.979 (SD 0.013). The relative dose within 2 cm of the lung was also overestimated. Conclusion: Only one algorithm (collapsed cone) investigated in this study was able to calculate the dose at the center of the breast correctly in tangential breast radiotherapy. With modern algorithms, it is important to include a correction for the lower density of the lung, because the dose close to the interface between breast and lung tissue will also be lower than anticipated

[en] This paper describes dosimetry measurements performed prior to departments entering patients into the START Trial, a breast fractionation trial. Absolute and relative doses were measured in semi-anatomical breast and chest wall phantoms, as part of a quality assurance programme visit. Doses were measured using an ionization chamber and the resulting distributions were compared with those calculated by the department. The mean ratio of measured to calculated dose at the START reference point was found to be 0.981 for the breast phantom and 0.978 for the chest wall phantom. This average measured dose was significantly less than the prescribed dose (p<0.001). Differences were found between 2D and 3D planning systems and for departments using cobalt 60 beams. A number of departments had deviations of greater than 4%, which was the tolerance applied for this trial. It is essential for dose measurements of this type to be performed for randomized clinical trials involving radiotherapy, particularly where dose fractionation regimes are being compared. (author)

[en] Background and purpose: Linear quadratic models predict that hypofractionation increases the biological effect of physical dose inhomogeneity. The clinical significance of this effect was tested retrospectively in a trial of adjuvant breast hypofractionation. Methods: The UK FAST trial randomised 915 women after breast conservation surgery between standard fractionation and two dose levels of a 5-fraction regimen delivering 5.7 or 6.0 Gy fractions in 5 weeks, using 3D dosimetry. Logistic regression tested for association between the absolute volumes receiving different isodose level >100% of prescribed dose (hotspots) and the risk of change in 2-year photographic breast appearance. The strength of this association was compared between control and hypofractionated groups. Results: Three hundred and ninety datasets from 11 participating centres were available for analysis. At 2 years post-randomisation, 81 (20.8%) had mild change and 24 (6.2%) had marked change in photographic breast appearance. After adjusting for breast size and surgical deficit, there was no statistically significant association between the risk of 2-year change in breast appearance and dose inhomogeneity in either the control or hypofractionated schedules, according to the various definitions of hotspots analysed. The magnitude of the effect of dosimetry on 2-year change in breast appearance did not vary significantly between control and hypofractionated schedules for any of the dosimetry parameters (p > 0.05 for all heterogeneity tests). Conclusion: Dose inhomogeneity had no greater impact on the risk of 2-year change in photographic breast appearance after hypofractionated breast radiotherapy than after standard fractionation.

[en] Purpose: Early breast cancer radiotherapy aims for local disease control and reduced recurrence. Treatment is directed to breast or chest wall alone using tangential fields, or includes regional lymph nodes with a separate anterior field. The complex geometry of this region necessitates matching adjacent radiation fields in three-dimensions. Potential exists for overdosage or underdosage and cosmetic results may be compromised if fields are not accurately aligned. Methods and materials: A study of dosimetry across the match line region using different techniques, as reported in the multicentre START Trial Quality Assurance programme, was undertaken. A custom-made anthropomorphic phantom assessed dose distribution in three-dimensions using film dosimetry. Results: Methods with varying degrees of complexity were employed for field matching. Techniques combined half beam blocking and machine rotations to achieve geometric alignment. Asymmetric beam matching allowed use of a single isocentre technique. Where field matching was not undertaken a gap between tangential and nodal fields was employed. Results demonstrated differences between techniques and variations for similar techniques in different centres. Geometric alignment techniques produced more homogenous dose distributions in the match region than gap techniques or those techniques not correcting for field divergence. Conclusions: Field matching techniques during the START trial varied between centres. Film dosimetry used in conjunction with a breast-shaped phantom provided relative dose information. The study highlighted difficulties in matching treatment fields to achieve homogenous dose distribution through the region of the match plane and the degree of inhomogeneity as a consequence of a gap between treatment fields.

[en] Background: Rotational IMRT (VMAT and Tomotherapy) has now been implemented in many radiotherapy centres. An audit to verify treatment planning system modelling and treatment delivery has been undertaken to ensure accurate clinical implementation. Material and methods: 34 institutions with 43 treatment delivery systems took part in the audit. A virtual phantom planning exercise (3DTPS test) and a clinical trial planning exercise were planned and independently measured in each institution using a phantom and array combination. Point dose differences and global gamma index (γ) were calculated in regions corresponding to PTVs and OARs. Results: Point dose differences gave a mean (±sd) of 0.1 ± 2.6% and 0.2 ± 2.0% for the 3DTPS test and clinical trial plans, respectively. 34/43 planning and delivery combinations achieved all measured planes with >95% pixels passing γ < 1 at 3%/3 mm and rose to 42/43 for clinical trial plans. A statistically significant difference in γ pass rates (p < 0.01) was seen between planning systems where rotational IMRT modelling had been designed for the manufacturer’s own treatment delivery system and those designed independently of rotational IMRT delivery. Conclusions: A dosimetry audit of rotational radiotherapy has shown that TPS modelling and delivery for rotational IMRT can achieve high accuracy of plan delivery