Session Item

In this study, the dosimetric effects of intrafractional motion during the treatment of breast cancer patients, as evaluated by surface guided radiation therapy (SGRT), is analysed. On a side, the possibility of assessing the delivered dose patient- and fraction wise was explored; on the other side, suitable SGRT monitoring threshold values were determined with a treatment plan robustness analysis on the basis of dose coverage constraints.    

22 patients were treated on two C-arm linacs equipped with an SGRT system. After positioning using either SGRT only or additional CBCT scans, intrafractional motion during treatment was recorded using the surface scanner and was analysed statistically. Dose calculations were carried out with the original treatment planning system for different 6-DOF shifts, mimicking rigid patient motion. The dosimetric effect of the observed intrafractional motion was calculated and evaluated with respect to target coverage and OAR dose. The effect of fixed, pre-set shifts (1 mm / 1 ° steps) was analysed systematically and maximum shift values were determined using dosimetric thresholds (minimum relative dose coverage of 95 %) in order to obtain suitable SGRT monitoring threshold values for application during treatment.

The analysis of the effect of pre-set shifts partly showed large deviations from the originally planned PTV dose coverage (V_95% of prescribed dose). A pronounced asymmetry for vertical translations was observed (see figure 1). The maximum shift values (based on mean ± standard deviation) are marked by red arrows in figure 1; for example, for vertical translations we obtained maximum values of -5 mm (positioned too high) and +2 mm (positioned to low), respectively. For DIBH patients, the SGRT based intrafraction motion showed bell-shaped relative frequency distributions with standard deviations of approximately 1 mm and 1 °, respectively, centered around zero. Only for vertical translations, the distribution was shifted slightly towards positive values. The dosimetric effects of intrafractional motion of the observed shifts were comparable to partial volume effects stemming from resampling of the dose distribution, which is necessary for dose summation of different scenarios; details thereof are currently under investigation.

Figure 1 Relative PTV dose coverage for 6-DOF shifts and shift threshold values

Dosimetric effects of intrafractional motion during treatment of breast cancer patients have been evaluated by calculating the dose distribution of the clinical treatment plans under simulated 6‑DOF shifts. Shift threshold values suitable for SGRT based monitoring of intrafractional motion were identified. The results underline the importance of position control during treatment and robust treatment planning.