A dosimetric phantom study of dose accuracy and build-up effects using IMRT and RapidArc in stereotactic irradiation of lung tumours

Abstract

Background and purpose: Stereotactic lung radiotherapy (SLRT) has emerged as a curative treatment for medically inoperable patients with early-stage non-small cell lung cancer (NSCLC) and the use of intensity-modulated radiotherapy (IMRT) and volumetric modulated arc treatments (VMAT) have been proposed as the best practical approaches for the delivery of SLRT. However, a large number of narrow field shapes are needed in the dose delivery of intensity-modulated techniques and the probability of underdosing the tumour periphery increases as the effective field size is decreased. The purpose of this study was to evaluate small lung tumour doses irradiated by intensity-modulated techniques to understand the risk for dose calculation errors in precision radiotherapy such as SLRT.

Materials and methods: The study was executed with two heterogeneous phantoms with targets of Ø1.5 and Ø4.0 cm. Dose distributions in the simulated tumours delivered by small sliding window apertures (SWAs), IMRT and RapidArc treatment plans were measured with radiochromic film. Calculation algorithms of pencil beam convolution (PBC) and anisotropic analytic algorithm (AAA) were used to calculate the corresponding dose distributions.

Results: Peripheral doses of the tumours were decreased as SWA decreased, which was not modelled by the calculation algorithms. The smallest SWA studied was 2 mm, which reduced the 90% isodose line width by 4.2 mm with the Ø4.0 cm tumour as compared to open field irradiation. PBC was not able to predict the dose accurately as the gamma evaluation failed to meet the criteria of ±3%/±1 mm on average in 61% of the defined volume with the smaller tumour. With AAA the corresponding value was 16%. The dosimetric inaccuracy of AAA was within ±3% with the optimized treatment plans of IMRT and RapidArc. The exception was the clinical RapidArc plan with dose overestimation of 4%.

Conclusions: Overall, the peripheral doses of the simulated lung tumours were decreased by decreasing the SWA. To achieve adequate surface dose coverage to small lung tumours with a difference less than 1 mm in the isodose line radius between the open and modulated field, a larger than 6 mm SWA should be used in the dose delivery of SLRT.

Figure 1

Measurement set-up. The outer PMMA cylinder was attached to a stereotactic frame (A) and the cork cylinders (B) were inserted inside. A frontal view of the Ø4.0 cm tumour surrounded by cork with film inside (C).

Figure 2

Measured central doses of the tumours (Ø1.5 cm and Ø4.0 cm) with the calculated MUs of PBC and AAA. The dose prescription was 1.0 Gy to the centre of the mimicked tumour. The error bars represent the 95% confidence intervals of the measurements.

Figure 3

Film measurements of the RA1 (A) and RA2 (D) treatment plans of the Ø4.0 cm tumour. The corresponding dose distributions of AAA calculations (B and E) and the related absolute differences (C and F) (calculated - measured). The border of the tumour is visualized with a dashed line.

Figure 4

Measured dose distributions of 2 mm (A) and 15 mm (B) SWAs were subtracted from the corresponding open field measurement of 2.5 × 2.5 cm²(C and D). Films were perpendicular to the beam axis in the middle of the Ø1.5 cm tumour.

Figure 5

Difference in measured surface doses represented by the difference in isodose line widths of the treatment plans of 3D-CRT and various SWAs (SWA - 3D-CRT). The mimicked lung tumours of Ø1.5 cm and Ø4.0 cm were irradiated with the nine field treatment plans. The widths of normalized isodose lines (50%, 80%, 85%, 90% and 95%) were measured from the irradiated films to X- and Y-direction.

Figure 6

Measured dose distribution of the IMRT treatment plan of Ø4.0 cm tumour (A) was subtracted from the calculated dose distributions of PBC (B) and AAA (C) (D and E), respectively. The border line of the tumour is visualized with a dashed line.