Session Item

In adjuvant partial breast irradiation (PBI), the geometric accuracy of the treatment is critical to avoid geographical miss. This is even more important if the number of fractions is reduced. The tumor bed is often not clearly visible on pretreatment imaging, so fiducials can be used for daily image-guidance. There are various types of fiducials available, and it is currently unknown what  the impact is of a given fiducial type on the PTV margin. The purpose of this study is to select the optimal fiducial for patient positioning in PBI regarding the required PTV margin.

Fourteen patients from the CK-APBI trial (NL6802) were included and ten patients analyzed, excluding the first 4 participants assuming a learning curve. All patients had ≥3 tantalum surgical clips placed during lumpectomy, 3 interstitial gold markers placed outside the lumpectomy area, and a gold marker taped on the areola skin. Breath-hold planning CT and 5 daily CT scans were available for all patients. The daily CT scans were acquired with an in-room CT-on-rails integrated with a CyberKnife system. The surgical clips, interstitial markers, and skin markers were identified in all CT scans. In MIM (version 6.9.3), the tumor bed in the planning CT was registered to the tumor bed in each daily CT. Subsequently, the displacement of the center of mass (CoM) per set of fiducials was calculated. This gives the residual error for the use of fiducials for daily image guidance. We calculated the group mean residual error M, the standard deviation of the systematic error Σ, the standard deviation of the random error σ, and the required margin according to van Herk (2000) for a 5-fraction and 15-fraction schedule.

The displacements of the CoM of the different fiducial types with respect to the tumor bed are shown in Table 1. The group mean error was not significantly different from zero. The skin marker showed the largest systematic and random errors, followed by the interstitial markers. Figure 1 shows the margins required to correct for fiducial displacement for a 5-fraction and 15-fraction schedule. The margins required were ≤ 2 mm in all directions for the surgical clips. Interstitial markers required a larger and anisotropic margin, of up to 5 mm in the left-right direction. Due to the relatively large random error, fractionation has the biggest impact on the margin for a skin marker. The average PTV volume (5 fractions) would be 113 cc for the surgical clips, 134 cc for the interstitial markers, and 169 cc for the skin marker, which is a 50% larger volume.

Based on the data taken from in-room acquired daily CT scans, surgical clips most accurately represent the position of the tumor bed. A larger margin is required if interstitial fiducials are used and a single skin marker is insufficient to accurately localize the tumor bed for daily image guidance.