Intrafractional organs movement in three-dimensional image-guided adaptive pulsed-dose-rate cervical cancer brachytherapy: assessment and dosimetric impact

Purpose: To prospectively evaluate the intrafractional movements of organs at risk (OARs) and their dosimetric impact during the delivery of pulsed-dose-rate brachytherapy in cervical cancer.

Patients and methods: An MRI on Day 1 was used for treatment planning in 19 patients. CT scans were acquired at Days 1, 2, and 3 with delineation of the OARs. The MRI plan was transferred to each CT. The intersection volume between the 10 Gy isodose and the OARs were monitored, reflecting movement. Lower dose evaluated in the maximally exposed 0.1 cm(3) of an organ and lower dose evaluated in the maximally exposed 2 cm(3) of an organ (D(2cm3)) were evaluated on each CT and compared. Results were averaged considering that each CT reflected one-third of the treatment course to evaluate the delivered dose.

Results: No major movements of the sigmoid and bladder were observed, whereas the rectum got significantly closer to the implant at Day 2. The consequence was an increase of 6% ± 5.3 (3.7 Gy, α/β = 3 Gy) of the delivered D(2cm3) from the planned dose, in contrast to 0.2% ± 6.1 for the bladder and 1.1% ± 6.4 for the sigmoid. The increase of the D(2cm3) of the rectum was reported in 17 patients, ranging from 0.4 to 9.4 Gy, leading to a 10.5% overcoming of the dose constraint (75 Gy). Similar tendencies were reported for lower dose evaluated in the maximally exposed 0.1 cm(3) of an organ.

Conclusions: A significant systematic variation was observed for the rectum (+3.7 Gy). As significant random variations were observed, caution should be exercised when the planned D(2cm3) is close to the dose constraints.