Hydrogel Spacer Reduces Rectal Dose during Proton Therapy for Prostate Cancer: A Dosimetric Analysis

Praveen Polamraju, BS; Alexander F. Bagley, MD, PhD; Tyler Williamson, BS, CMD; X. Ronald Zhu, PhD; Steven J. Frank, MD

Patients receiving radiation therapy for prostate cancer are at risk of developing treatment-related rectal toxicity, particularly as hypofractionated and stereotactic ablative approaches have become more prominent. Toxicity can manifest as rectal bleeding or bowel urgency, and the risk correlates with dosimetric parameters such as overall dose and the volume of rectum receiving at least 70 Gy (rectal V70). The increase in fractional doses raises concerns regarding greater rectal toxicity, but longer-term results are needed to clarify this issue. Approaches to minimize rectal radiation doses and thereby reduce treatment-related morbidity have become increasingly important in the management of prostate cancer.

Proton therapy has been used as a strategy to minimize radiation dose to adjacent structures including the rectum. Prior reports indicate that rectal volumes receiving 10 to 80 Gy are significantly lower with proton therapy (eg, V70 = 7.9%) than with intensity-modulated (photon) radiation therapy (IMRT) (eg, V70 = 14%), although others have questioned whether proton therapy alone is sufficient to reduce the rectal volume receiving high radiation doses. The 2 primary proton modalities, passive scattering proton therapy (PSPT) and intensity-modulated proton therapy (IMPT), have been compared for their relative ability to spare the rectum. An emerging approach aimed at further rectal sparing involves the use of biodegradable hydrogel spacers that physically displace the prostate from the rectal wall during treatment. In one randomized trial, use of such a spacer led to a relative reduction in mean rectal V70 of 74%. These studies suggest that significant dosimetric benefit requires at least 7- to 15-mm separation between the prostate and rectal wall.

The purpose of this study is to determine the effect of a biodegradable, injectable hydrogel spacer on rectal dose in treatment plans for PSPT and IMPT for prostate cancer. We analyzed a variety of clinically relevant dosimetric parameters for both modalities in the presence and absence of these spacers, and we correlated the extent of displacement between the prostate and rectal wall (with the spacer in place) with rectal V70 to determine the optimal amount of displacement in terms of reducing rectal dose in both modalities.

(A) : Dose-volume histogram for prostate, rectum, and seminal vesicles for representative patient ; (B) : isodose lines for PSPT without spacer (C) : PSPT with spacer (D) : IMPT without spacer (E) : IMPT with spacer . Normal tissues depicted with colorwash include prostate (blue), seminal vesicles (orange), and rectum (green). Abbreviations: IMPT, intensity-modulated proton therapy; PSPT, passive scattering proton therapy.

Conclusion: Use of biodegradable hydrogel spacers for prostate cancer treatment provides a significant reduction of radiation dose to the rectum with proton therapy. Significant reductions in rectal dose occurred in both PSPT and IMPT plans, with the greatest reduction for IMPT-with-spacer relative to PSPT alone. Prospective studies are ongoing to assess the clinical impact of reducing rectal dose with hydrogel spacers.

International Journal of Particle Therapy: Spring 2019, Vol. 5, No. 4, pp. 23-31.https://www.theijpt.org/doi/pdf/10.14338/IJPT-18-00041.1

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IMPT versus VMAT for Pelvic Nodal Irradiation in Prostate Cancer: A Dosimetric Comparison

Thomas J. Whitaker, David M. Routman, Heather Schultz, William S. Harmsen, Kimberly S. Corbin, William W. Wong, and Richard Choo International Journal of Particle Therapy

In the setting of prostate and elective, pelvic nodal radiotherapy for prostate cancer, Intensity Modulated Proton Therapy (IMPT) can significantly reduce the dose to Organs At Risk (OAR) , in comparison to Volumetric-Modulated Arc Therapy (VMAT), and provide adequate target coverage.

The Clinical Target Volumes (CTV) coverage was adequate for both plans with 99% of CTVs receiving ≥ 100% of the prescription doses.

Mean doses to the bladder, rectum, large bowel, and small bowel were lower with IMPT versus VMAT. Mean femoral head dose was greater with IMPT.

The percentage of volumes of rectum receiving ≤ 47.5 Gy, large bowel receiving ≤ 27.5 Gy, small bowel receiving ≤ 30 Gy, and bladder receiving ≤ 37.5 Gy was less with IMPT versus VMAT, largely because of reduction in the low-dose “bath” associated with VMAT.

https://www.theijpt.org/doi/abs/10.14338/IJPT-18-00048.1

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