Is a 3-mm intrafractional margin sufficient for daily image-guided intensity-modulated radiation therapy of prostate cancer?

Abstract

Purpose: To determine whether a 3-mm isotropic target margin adequately covers the prostate and seminal vesicles (SVs) during administration of an intensity-modulated radiation therapy (IMRT) treatment fraction, assuming that daily image-guided setup is performed just before each fraction.

Materials and methods: In-room computed tomographic (CT) scans were acquired immediately before and after a daily treatment fraction in 46 patients with prostate cancer. An eight-field IMRT plan was designed using the pre-fraction CT with a 3-mm margin and subsequently recalculated on the post-fraction CT. For convenience of comparison, dose plans were scaled to full course of treatment (75.6 Gy). Dose coverage was assessed on the post-treatment CT image set.

Results: During one treatment fraction (21.4+/-5.5 min), there were reductions in the volumes of the prostate and SVs receiving the prescribed dose (median reduction 0.1% and 1.0%, respectively, p<0.001) and in the minimum dose to 0.1 cm(3) of their volumes (median reduction 0.5 and 1.5 Gy, p<0.001). Of the 46 patients, three patients' prostates and eight patients' SVs did not maintain dose coverage above 70 Gy. Rectal filling correlated with decreased percentage-volume of SV receiving 75.6, 70, and 60 Gy (p<0.02).

Conclusions: The 3-mm intrafractional margin was adequate for prostate dose coverage. However, a significant subset of patients lost SV dose coverage. The rectal volume change significantly affected SV dose coverage. For advanced-stage prostate cancers, we recommend to use larger margins or improve organ immobilization (such as with a rectal balloon) to ensure SV coverage.

Fig. 1

The percentage volume dosimetric difference in (a) the prostate and (b) the seminal vesicles before and after a daily radiation treatment fraction at three distinct dose levels (V_75.6, V₇₀, and V₆₀.) Each short horizontal bar represents one patient case and the vertical bar represents the range of differences at each dose level.

Fig. 2

Histograms of the minimal dose differences to 0.1 cm³ of (a) the prostate and (b) the seminal vesicles (SV) before and after administration of a daily radiation treatment fraction.

Fig. 3

(a) The displacement vector field of a patient with extreme bladder filling during a radiation treatment fraction. The anatomy shown in (a) is before the treatment fraction. The red arrows indicate the direction that the anatomy will shift during the treatment fraction. The dose distributions are calculated in (b) before and (c) immediately after treatment for the same patient. (d) shows the dose distribution of (c) mapped to the anatomy of (b) by the deformable image registration. The structure in orange is the seminal vesicle with a blue PTV of 5mm expansion. The isodose lines are: yellow 45 Gy; orange 60 Gy; red: 75.6 Gy.

Fig. 4

(a) The displacement vector field of a patient with rectal gas migration during a treatment fraction. The dose distributions are calculated in (b) before and (c) immediately after treatment for the same patient. (d) shows the dose distribution of (c) mapped to the anatomy of (b) by the deformable image registration.

Fig. 5

(a) The displacement vector field of a patient with possible pelvic and gluteal muscular tension during a treatment fraction. The red arrows indicate the difference in thickness of the clenched pelvic muscles between the “before” and “after” images and the resulting shift of the target organs and bladder relative to the mid pubic symphysis. (b) The planned treatment fraction as calculated before treatment for that patient. (c) The planned treatment fraction calculated directly after treatment for this patient (d) The dose distribution after treatment was mapped to the before fraction anatomy by the deformable image registration method.