  During Stereotactic Body Radiotherapy (SBRT) for the treatment of prostate
cancer, an inflatable endorectal balloon (ERB) may be used to reduce motion of
the target and reduce the dose to the posterior rectal wall. This work assessed
the dosimetric impact of manual interventions on ERB position in patients
receiving prostate SBRT and investigated the impact of ERB interventions on
prostate shape. Daily kilovoltage (kV) cone-beam computed tomography (CBCT)
imaging was performed to localize the PTV, and an automated fusion with the
planning images yielded displacements required for PTV re-localization. When
the ERB volume and/or position were judged to yield inaccurate repositioning,
manual adjustment (ERB re-inflation and/or repositioning) was performed. Based
on all 59 CBCT image sets acquired, a deformable registration algorithm was
used to determine the dose received by, displacement of, and deformation of the
prostate, bladder, and anterior rectal wall. This dose tracking methodology was
applied to images taken before and after manual adjustment of the ERB
(intervention), and the delivered dose was compared to that which would have
been delivered in the absence of intervention. Interventions occurred in 24 out
of 35 (69%) of the treated fractions. The direct effect of these interventions
was an increase in the prostate radiation dose that included 95% of the PTV
(D95) and an increase in prostate coverage. Additionally, ERB interventions
reduced prostate deformation in the anterior-posterior (AP) direction, reduced
errors in the sagittal rotation of the prostate, and increased the similarity
in shape of the prostate to the radiotherapy plan. Image-guided interventions
in ERB volume and/or position during prostate SBRT were necessary to ensure the
delivery of the dose distribution as planned.
