Analysis of inter-fraction setup errors and organ motion by daily kilovoltage cone beam computed tomography in intensity modulated radiotherapy of prostate cancer

Marcella Palombarini, Stefano Mengoli, Paola Fantazzini, Cecilia Cadioli, Claudio Degli Esposti, Giovanni Piero Frezza, Marcella Palombarini, Stefano Mengoli, Paola Fantazzini, Cecilia Cadioli, Claudio Degli Esposti, Giovanni Piero Frezza

Abstract

Background: Intensity-modulated radiotherapy (IMRT) enables a better conformality to the target while sparing the surrounding normal tissues and potentially allows to increase the dose to the target, if this is precisely and accurately determined. The goal of this work is to determine inter-fraction setup errors and prostate motion in IMRT for localized prostate cancer, guided by daily kilovoltage cone beam computed tomography (kVCBCT).

Methods: Systematic and random components of the shifts were retrospectively evaluated by comparing two matching modalities (automatic bone and manual soft-tissue) between each of the 641 daily kVCBCTs (18 patients) and the planning kVCT. A simulated Adaptive Radiation Therapy (ART) protocol using the average of the first 5 kVCBCTs was tested by non-parametric bootstrapping procedure.

Results: Shifts were < 1 mm in left-right (LR) and in supero-inferior (SI) direction. In antero-posterior (AP) direction systematic prostate motion (2.7 ± 0.7 mm) gave the major contribution to the variability of results; the averages of the absolute total shifts were significantly larger in anterior (6.3 ± 0.2 mm) than in posterior (3.9 mm ± 0.2 mm) direction. The ART protocol would reduce margins in LR, SI and anterior but not in posterior direction.

Conclusions: The online soft-tissue correction based on daily kVCBCT during IMRT of prostate cancer is fast and efficient. The large random movements of prostate respect to bony anatomy, especially in the AP direction, where anisotropic margins are needed, suggest that daily kVCBCT is at the present time preferable for high dose and high gradients IMRT prostate treatments.

Figures

Figure 1
Figure 1
Histograms of the shifts along × (LR), Y (SI), Z (AP) for the two matching modalities and corresponding scatterplots. Distributions of the shifts (in mm) along left-right (X) (a), supero-inferior (Y) (b), and antero-posterior (Z) (c) axes for the two matching modalities (B-match and T-match), along with the scatterplots of the corresponding shifts. Bell curves are computed using mean and standard deviation of the underlining variables. Dashed lines represent the bisectors. The scatterplots shown in Figure 1a (XT vs XB), Figure 1b (YT vs YB), and Figure 1c (ZT vs ZB), have correlation coefficients of 0.94, 0.74 and 0.78, respectively, all highly statistically significant (p-value < 0.001). Note that ZT tends to be larger than ZB, especially for larger values of ZB (Figure 1c).
Figure 2
Figure 2
Percentage of the total fractions outside the margins, for margins from 1 to 8 mm. The data (all the fractions, all the patients) are computed by using the absolute values of the shifts given by the T-match. Dotted-dashed lines show the results if no correction is made on patient position. Solid lines show the results of a simulated off-line protocol based on the first 5 CBCTs. The area in grey represent the 5% and 95% confidence levels. Dashed lines show the results if a daily correction is made on the basis of daily CBCT. For the antero-posterior (Z) axis the percentages are divided into two cases, Z positive and Z negative, because there is no symmetry in the margins we apply for the positive and negative directions. The vertical lines are positioned at the margins currently used.

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