Evidence suggests that external beam radiation therapy (ERBT) is a well-warranted treatment modality for prostate cancer, and 51.7% to 53.1% of newly diagnosed patients should receive ERBT at least once during their illness .
The clinical goal of the treatment is to permanently stop the growth of the tumor, achieved by sufficient radiation dose application while sparing the surrounding organs at risk (OARs; bladder and rectum) to such an extent that adverse effects of radiation are at least manageable. Geometric uncertainties, such as uncertainties in patient setup and tumor delineation, as well as daily differences in patient anatomy, rectum and bladder changes, and inter- and intrafraction prostate movement increase the CTV-to-PTV margin and make achieving this goal extremely challenging . Decrease in PTV margins, and thus volume receiving the prescribed dose, can be accomplished by mitigation of geometric uncertainties with prostate immobilization. The higher the prostate stabilization, the better the treatment outcomes in terms of safety and efficiency.
Radiation Therapy Immobilization Products and Their Impact on Prostate Movement
With extensive studies of the interfraction movement of prostate contributed to the development of various external immobilization products that reduce patient setup errors (median; mm) and improve ERBT accuracy ,:
- thermoplastic Hipfix - 5.1
- full body alpha cradle - 2.8
- VacLock - 7.1 (anterior–posterior plane), 5.4 (superior–inferior plane)
- BodyFix - 8.2 (AP), 3.5 (SI)
While the use of external radiation therapy immobilization products results in reduced interfraction motion, intrafraction motion still poses a challenge . In a study of 41 prostate cancer patients during the full course of treatment, Kupelian et.al. discovered a prostate displacement of >5 mm in over 75% of all treatment sessions (fractions). >5 mm and >3 mm displacements of 30 s or more were observed in 15% and 41% of fractions.
Intrafraction movement can be reduced with the use of endorectal balloon (ERB). A study of 576 patients without ERB and 567 patients with ERB during daily ERBT revealed that 3D intrafraction prostate displacements (>3 mm, >5 mm) after 600 s of treatment time occurred in 18.1% and 4.6% fractions for non-ERB group and 7.0% and 0.7% fractions for ERB group (5 mm reduction in CTV-to-PTV margin) . Further studies support up to 40% decrease in the 3D symmetrical CTV-to-PTV margin and up to 33% reduction in the asymmetrical internal margin for treatments up to 6 min. After 6 min, symmetrical 3D and asymmetrical internal margins can be reduced between 11% and 57% .