External beam radiotherapy has been widely accepted as an efficient treatment modality for localized prostate cancer, with dose escalated treatment variations even further reducing the incidence of biochemical failure . However, with the clear dose-response relationship of ERBT, regarding both desired, as well as adverse clinical outcomes, late rectal toxicity remains a primary dose-limiting factor of radiotherapy treatment for prostate cancer .
Immobilization Techniques in Radiotherapy – Mitigation of Dosimetric and Anatomic Risk Factors Associated with Late Rectal Toxicity with Endorectal Balloon (ERB)
In addition to the delivered dose and the volume of OARs within specific isodoses, late rectal toxicity is also linked to anatomic factors (prostate shape and position, bladder filling, rectum anatomy) and clinical factors (inflammatory bowel disease, hemorrhoids, and similar) ,. The use of ERB reduces these factors from multiple aspects:
- Interfraction prostate immobilization – with reproducible placement, ERB reduces interfraction setup uncertainties
- Intrafraction prostate immobilization – ERB reduces the effect of rectum and bladder filling on prostate displacements, in turn limiting prostate motion during each treatment fraction and allowing lower CTV-to-PTV margins
- Rectal wall distension – by distancing the anterior rectal wall from the prostate, ERB increases overall rectum sparing while also reducing the volume of the rectum within high isodoses
Treatment and Outcome Implication - A Comparison of Rectal DVH with and without Prostate Immobilization
Cho et al. studied DVHs and rectal dose statistics, such as mean rectal dose and VnGy, for 3D-CRT and IMRT treatment plans. For both groups of patients, the use of ERB significantly reduced the mean rectal dose from 51.73 to 41.98 Gy for the 3D-CRT group and from 41.1 to 33.8 Gy for IMRT group. In 3D-CRT group, V45Gy, V50Gy, V60Gy and V65Gy were also reduced with the use of ERB, from 69,96% to 44,05%, 59,63% to 36, 13%, 41,33% to 19, 53% and 29,68% to 14, 89% respectively . In another study, DVH analysis revealed that the use of ERB during 15 and 40 fractions of an escalated 3D-CRT treatment plan reduces the portion of rectal volume receiving over 70 Gy from 25% (ERB not used) to 7.5% and 3.6% respectively .
In terms of clinical implications, 3D-CRT with ERB can produce dosimetric results comparable to non-ERB IMRT treatment modality, according to the comparison of DVHs for respective groups. In the IMRT group, the main clinical implication was a significantly reduced incidence of Grade I rectal complications while Grade II complications were not observed ,.