Development of external dosimetry audits for advanced technology in radiotherapy dose delivery: An IAEA coordinated research project

The influence of basic plan parameters on calculated small field output factors - A multicenter study

PURPOSE

The influence of basic plan parameters such as slice thickness, grid resolution, algorithm type and field size on calculated small field output factors (OFs) was evaluated in a multicentric study.

METHODS AND MATERIALS

Three computational homogeneous water phantoms with slice thicknesses (ST) 1, 2 and 3 mm were shared among twenty-one centers to calculate OFs for 1x1, 2x2 and 3x3 cm² field sizes (FSs) (normalized to 10x10 cm² FS), with their own treatment planning system (TPS) and the energy clinically used for stereotactic body radiation therapy delivery. OFs were calculated for each combination of grid resolution (GR) (1, 2 and 3 mm) and ST and finally compared with the OFs measured for the TPS commissioning. A multivariate analysis was performed to test the effect of basic plan parameters on calculated OFs.

RESULTS

A total of 509 data points were collected. Calculated OFs are slightly higher than measured ones. The multivariate analysis showed that Center, GR, algorithm type, and FS are predictive variables of the difference between calculated and measured OFs (p < 0.001). As FS decreases, the spread in the difference between calculated and measured OFs became larger when increasing the GR. Monte Carlo and Analytical Anisotropic Algorithms, presented a dependence on GR (p < 0.01), while Collapsed Cone Convolution and Acuros did not. The effect of the ST was found to be negligible.

CONCLUSIONS

Modern TPSs slightly overestimate the calculated small field OFs compared with measured ones. Grid resolution, algorithm, center number and field size influence the calculation of small field OFs.

The role of SSDL in quality assurance in radiotherapy

This paper describes the role of the Polish Secondary Standard Dosimetry Laboratory (SSDL) in quality assurance in radiotherapy by means of providing calibration of ionisation chambers, TLD postal dosimetry audits and end-to-end audits for radiation therapy. A historical review of the methods and results are presented. The influence of the SSDL in Warsaw on radiation protection of patients in Poland is discussed. The International Atomic Energy Agency together with World Health Organisation (IAEA/WHO), through its network of SSDLs around the world, propagates newly developed methods for calibration and auditing. Suitable high quality equipment was provided by the IAEA, as well as special materials and technical support to the SSDL in Warsaw. The activity of the SSDL and the services provided for Polish radiotherapy centres have resulted in a reduction of discrepancies between planned doses and doses delivered to patients. The newly tested IAEA methods of end-to-end on-site dosimetry audits allow for monitoring and improving the quality of IMRT in Poland. The traceability of standards used for the calibration of therapy level dosimeters from Polish radiotherapy centres is assured by the IAEA dosimetry laboratory. The consistency of methods performed in the Polish SSDL with the ISO:17025 norm is supervised by the Polish Centre for Accreditation - a member of International Laboratory Accreditation Cooperation (ILAC), for calibration and testing. Due to the rapid technological development of radiotherapy, special attention has to be paid to newly developed methods for dosimetry auditing and institutions which provide services for assuring radiation safety of patients.

A multi-centre analytical study of small field output factor calculations in radiotherapy

An audit methodology was developed and applied for output factor (OF) calculations in radiotherapy. The auditees were asked to calculate OFs for field sizes from 10 × 10 cm² to 2 × 2 cm². Sixty five beams were audited; missing reference OFs were interpolated. The calculated OFs were in 73% of cases higher than the reference data. The smaller the field size, the higher the overestimations which were observed in the higher fraction of cases. Treatment planning systems generally overestimated OFs for small fields. The reference dataset helped radiotherapy centres to identify discrepancies which were higher than typical.