Dosimetric impacts of cone-beam computed tomography (CBCT)-based anatomic changes in intensity-modulated radiotherapy for cervical cancer

BACKGROUND: To evaluate the effects of dose to tumors and organs at risk (OARs) on inter-fractional anatomic changes. METHODS: We evaluated nine patients with cervical cancer treated with intensity-modulated radiotherapy (IMRT) (45 Gy in 25 fractions) using kV cone-beam computed tomography (CBCT) image guidance once or twice a week before treatment. For each patient, the original plan on the computed tomography (CT) image was copied to merged images, and then the fractional doses were calculated. Subsequently, deformable accumulated doses were obtained by summing the fractional absolute doses into a single dose in MIM Maestro software. The volume changes in the target and OARs were compared between the original CT and merged CBCT images, and the differences in the fractional and accumulated doses were also evaluated. RESULTS: Sixty-nine merged CBCT images were obtained and analyzed in this study. For the target areas, the volume changes in the clinical target volume (CTV) and planning target volume (PTV) reached −18.05% and −24.11% at most, respectively. The fractional D(2%) of the CTV and PTV was generally higher than the original plans, and the accumulated deviations were 2.27%±0.82% (P<0.01) and 2.42%±1.28% (P<0.01), respectively. The fractional D(98%) of the PTV was underdosed up to 18.28% for 78% of patients, and the accumulated deviations were −2.06% to −17.29% (P<0.05). For the OARs, the bladder volume changes were the most dramatic, reducing up to 93.60%. The fractional Dmean and D(2cc) of the bladder were generally higher than the original plans, and there were significant differences in their accumulated values (P<0.05). There was no obvious trend of rectal volume change with −69.65% to 74.20%. The rectum D(mean) and D(2cc) of the accumulated were not significantly different from the planned dose (P>0.05). CONCLUSIONS: For patients with cervical cancer, the changes in bladder and rectal volume were greater than in the target volume. Although the volume changes in the bladder and rectum had no significant effect on D(98%) of the CTV and PTV, they had a significant effect on their own D(2cc) and the D(2%) of the CTV and PTV. More attention should be paid to the volume changes in the bladder and rectum in clinical work.