The dosimetric impact of respiratory breast movement and daily setup error on tangential whole breast irradiation using conventional wedge, field-in-field and irregular surface compensator techniques

摘要

A total of 16 consecutive patients with early-stage breast cancer who received WBI were chosen for this study. Right breasts were treated in 8 patients and left breasts were treated in the other 8. For all patients, the prescribed dose was 50 Gy in 25 fractions. CT image sets were transferred to the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA) for contouring and treatment planning. The patient outline, clinical target volume (CTV), planning target volume (PTV) and whole lung were contoured on each CT slice. The CTV was defined as the entire breast tissue including glandular breast and surrounding soft tissue. The PTV was defined as the CTV plus an expansion of 5 mm in all directions except for the external skin surface. The lung volume was automatically generated using the auto-contouring tool of the treatment planning system. The evaluated CTV (CTV_evl) was defined as the volume of the CTV enclosed by contours drawn 5 mm below the skin surface to eliminate the region of dose build-up [6]. We performed treatment planning using two tangential fields for breast irradiation with three different techniques: CW, FIF and ISC. For field-in-field (FIF), we followed the procedures reported by Kestin et al. [3]. The monitor unit (MU) weights of the open radiation field and those of the sub-fields were adjusted to remove hot spots in each plan. Taking into account dose output stability, we selected sub-fields with more than 6 MU. Irregular surface compensator (ISC), a type of electronic compensator, is a feature installed in Eclipse which enables improved dose homogeneity for irregular surface shapes. The X-ray fluence distribution required to produce an isodose surface perpendicular to the central axis at a specified depth is calculated by Eclipse and then delivered using a dynamic multileaf collimator (MLC). If the dose is not sufficiently homogenous, painting the fluence map can modify fluence distribution to achieve better dose homogeneity. The region enclosed by the 107 % isodose contours is erased by manual fluence painting. The fluence value of the superficial region is also applied in the flash region in order to ease MLC movement in the flash region. A dose–volume histogram (DVH) analysis was performed for all regions of interest. The following dose indices were used to evaluate the plan quality where relative dose means the ratio of the received to the prescribed dose, and relative volume shows the fraction of the whole contoured volume of each region of interest. Based on the CT image set of each patient, an ‘original plan' was created using the usual clinical planning technique of our Institution. Our approach for the evaluation of dose variation was to shift the isocenter of each original plan in every direction and estimate the variation of each dose index with isocenter shift. To analyze movement in the AP direction, the variation of dose indices caused by respiratory motion and setup error were calculated for a single fraction using the method described below. Assuming that daily setup error has a normal distribution for systematic and random error, the dose indices after all fractions () are calculated using , as follows; (3) where is the normal distribution function of the systematic setup error E. M indicates the overall mean error and Σ is the standard deviation (SD) of systematic setup error. The normal distribution function of the random setup error e is defined as where σ is the SD of the random setup error [17]. All patient data were used in the statistical calculation. Student's t-test or Welch's t-test (two-tailed) was applied to compare each dose index between irradiation techniques and to investigate the effects of respiratory motion and setup error (i.e. comparing the dose indices on original and isocenter-shift plans, and ). A P value of ≤0.05 was considered statistically significant. Table 1 summarizes the mean value and 1 SD of each dose index obtained from the original plan. Table 2 shows the dose indices for CTV_evl, lung and body with isocenter shift in the lateral and SI directions. For all techniques, isocenter shifts of 0.5 cm along these axes showed no significant variation from the original plans for most of the indices. However, for the lateral shift in ISC, which indicates a lateral shift away from the midline, V₉₅ significantly decreased from 99.2% to 96.9% and D₉₅ also decreased from 97.3% to 96.0%. The reason is that if a patient shifts away from the midline, the target moves into the flash region (see Fig. 1), where the fluence intensity is small to ensure a smooth movement of MLC, and not sufficient to give the prescribed dose to the target. Table 4 shows the dose indices, taking respiratory motion into account for a single fraction treatment (). Because we confirmed that the dose indices were unaffected by isocenter shifts in the lateral and SI directions (Table 2), both respiratory motion and s