Interplay effects in highly modulated stereotactic body radiation therapy lung cases treated with volumetric modulated arc therapy

Abstract

Interplay effects in highly modulated stereotactic body radiation therapy lung cases treated with volumetric modulated arc therapy.

Purpose: To evaluate the influence of tumor motion on dose delivery in highly modulated stereotactic body radiotherapy (SBRT) of lung cancer using volumetric modulated arc therapy (VMAT).

Methods: 4D-CT imaging data of the quasar respiratory phantom were acquired, using a GE Lightspeed 16-slice CT scanner, while the phantom reproduced patient specific respiratory traces. Flattening filter-free (FFF) dual-arc VMAT treatment plans were created on the acquired images in Pinnacle³ treatment planning system. Each plan was generated with varying levels of complexity characterized by the modulation complexity score. Static and dynamic measurements were delivered to GafChromic EBT3 film inside the respiratory phantom using an Elekta Versa HD linear accelerator. The treatment prescription was 10 Gy per fraction for 5 fractions. Comparisons of the planned and delivered dose distribution were performed using Radiological Imaging Technology (RIT) software.

Results: For the motion amplitudes and periods studied, the interplay effect is insignificant to the GTV coverage. The mean dose deviations between the planned and delivered dose distribution never went below -2.00% and a minimum dose difference of -5.05% was observed for a single fraction. However for amplitude of 2 cm, the dose error could be as large as 20.00% near the edges of the PTV at increased levels of complexity. Additionally, the modulation complexity score showed an ability to provide information related to dose delivery. A correlation value (R) of 0.65 was observed between the complexity score and the gamma passing rate for GTV coverage.

Conclusions: As expected, respiratory motion effects are most evident for large amplitude respirations, complex fields, and small field margins. However, under all tested conditions target coverage was maintained.

Keywords: 4D-CT; MLC; SBRT; VMAT; interplay.

Fig. 1

Quasar Phantom (a), Cedary lung tumor insert (b), CT scan of cedar lung tumor insert (c). The insert is composed of cedar wood and contains an offset, 3 cm diameter, plastic sphere to be used as lung and tumor surrogates respectively. The CT number and density of cedar ranges 290–400 and 0.25–0.32 g/cm³, which compares favorably with lung (140–300 and 0.15–0.33 g/cm³ respectively). The CT number and density of the plastic sphere was 950 and 0.98 g/cm³.

Fig. 2

MCS vs MU. The relationship between the number of MU and the MCS is shown. The graph shows that there was a strong linear correlation (R = 0.92) between MCS and the number of MU.

Fig. 3

Longitudinal profiles. Longitudinal profile measurements between static, dynamic, and calculated dose distributions for several cases are shown. The plots also display the isocenter and the extent of the GTV, ITV, and PTV at the 95% prescription level (950 cGy). Figures 3a and 3b show longitudinal profile measurements for a simplified case (MCS = 0.70) at amplitudes of 2 cm and 1 cm respectively. Figures 3c and 3d show longitudinal profile measurements for the most complex case (MCS = 0.40) at amplitudes of 2 cm and 1 cm respectively.

Fig. 4

Lateral profiles. Lateral profile measurements among static, dynamic, and calculated dose distributions for several cases are shown. Figures 4a and 4b show lateral profile measurements for a simplified case (MCS = 0.70) at amplitudes of 2 cm and 1 cm respectively. Figures 4c and 4d show lateral profile measurements for the most complex case (MCS = 0.40) at amplitudes of 2 cm and 1 cm respectively.

Fig. 5

Profile Widths. The width of the 100% (1000 cGy) and 95% (950 cGy) prescription dose along the longitudinal axis for patient trace 1 are shown. In each measurement it can be seen that the width of static dose distribution is wider than the planned dose distribution. For the 2 cm deliveries, it can be seen that the dynamic dose distributions have the shortest width and that the 95% width fails to meet the required 6 cm to cover the entire PTV as prescribed.

Fig. 6

Gamma Analysis. The results show that as plan complexity increases (ie decreasing modulation complexity score) the gamma analysis passing rate decreases. Additionally, the results show that the dynamic deliveries generally have lower gamma passing rates compared to its corresponding static delivery and commonly the 2 cm dynamic deliveries have the lowest gamma passing rate.