PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): Normal tissue toxicity, especially rectal toxicity, is the limiting factor for prostate dose escalation. Several studies reported the effectiveness of rectal dose sparing by using hydrogel spacer during conformal prostate radiation therapy. This study evaluated the dosimetric benefits of using both hydrogel spacer and endorectal balloon (ERB) for patients with escalated prostate dose during image-guided proton therapy.
Materials/Methods: Twenty-nine patients with prostate cancer (stage: T1c-T2c N0M0; Gleason score ≤8; PSA≤20ng/ml) treated with parallel opposed passive scattering proton beams at our institution were retrospectively reviewed. Eleven patients received hypofractionated 70 GyRBE in 28 fractions and eighteen patients received conventional fractionated 79.2 GyRBE in 44 fractions, with intermediate-risk patients receiving 45 GyRBE to the prostate and proximal seminal vesicles and 34.2 GyRBE to the prostate only. PTV was derived from CTV with 5 mm uniform margin. A hydrogel spacer was injected in the recto-prostatic space. The ERB with 50 cc filling prevented prostate mobilization and allowed rectal gas to pass through the balloon’s internal lumen during setup and delivery of daily treatment while allowing reduced beam aperture margins. Dosimetric evaluation was performed for bladder, rectum, CTV and PTV in the treatment planning system.
Results: Mean V100 for CTV and PTV were 100% and 97.0%±1.8%, respectively. Compared with RTOG constraints, reduction in bladder V65&V64 was 75.8% while reduction in rectum V65&V64, V70&V69, and V75&V74 were 82.6%, 85.2%, and 88.0%, respectively (Table 1).
Conclusion: To our knowledge, this is the first ever report on the simultaneous use of both ERB and hydrogel spacer in prostate irradiation with passive scattering proton beams. Using ERB and hydrogel spacer for patients treated with escalated prostate dose can substantially reduce rectum and bladder doses while maintaining sufficient target coverage. Either ERB or hydrogel spacer serves a different purpose and is applied independently in prostate cancer treatment using IMRT and proton therapy. The hydrogel spacer provides extended separation between the prostate and the anterior rectal wall and has level 1 evidence showing a reduction in rectal dose and treatment related toxicity in IMRT. Further studies are warranted to evaluate both acute and late toxicities in this clinical scenario.
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