Abstract

Lotte Stubkjaer Fog¹, Christopher Wong¹, Janet Droege¹

¹The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia

Patient specific quality assurance measurements (PSQAMs) facilitate evaluation of differences between measured and calculated doses. In this retrospective work, we investigate correlations in iPlan PSQAM data with plan properties.

Data for 41 patients, comprising 310 fields, calculated in iPlan Dose ™ v 4.5.6  (Brainlab) were used. Calculations were done using the Monte Carlo algorithm. Measurement data were acquired using the PTW 1000 array (for small fields) and the PTW 1500 array (larger fields) detector arrays.

Firstly, the MU/Gy, PTV volume, average jaw opening and gamma pass rate (g) (using dose difference and distance-to-agreement of 2% and 2mm) were extracted. Correlations between field and plan parameters and g were assessed using the significance (S) from regression analyses.

Secondly, in order to investigate array and TPS dependence, measurements were carried out using both arrays, and calculations were done using iplan and  Eclipse v 13.6™ using AAA, for 5 plans comprising 39 fields. Four of these patients were selected for their large PTVs, one small PTV patient was included.  The PTV volumes were 330.57, 248.51, 88.32, 79.29 and 17.96 cm3.

The correlation between g measurement (using the large and small array) and calculation (using iPlan and Eclipse) was determined. 

In the first analysis, the average g was 95.00% (st. dev. 6.50).

For the iPlan plans, g was not found to correlate with MU/Gy (S =0.27), but with PTV volume (S=4.56 x 10^-30) and average jaw opening (S=8.22 x 10^-33), with larger targets and jaw openings resulting in lower g (fig 1).

In the second analysis, no significant difference between the measurement results from the two arrays (S=6.64 x 10^-7) was found. When comparing the measurement (averaged over the two arrays) with calculation, g was 81.82% (st. dev. 12.27) for iPlan and 97.43% (st. dev. 2.51) for Eclipse (table 1).  

Figure 1 Gamma pass rate versus jaw opening for 310 fields.

Table 1. Gamma pass rates for measurements (carried out with a large and a small array) and calculation (using iplan and Eclipse) for 5 patients, 4 of which had large PTVs (PTV volumes: 330.57, 248.51, 88.32, 79.29 and 17.96 cm3)

The agreement between iPlan calculated and measured doses was excellent for most plans, however it was poorer for larger PTVs and field sizes. This trend was observed for the large and the small array. For these plans, Eclipse calculations agreed better with measurements.

TPSs commissioned for small stereotactic targets may be less accurate for larger field sizes, and clinics that use several TPSs may wish to consider PTV size/jaw opening when choosing the appropriate TPS.