Prostate Cancer Treatment with Pencil Beam Proton Therapy Using Rectal Spacers sans Endorectal Balloons

Abstract

Purpose: Proton beam radiotherapy (PBT) has been used for the definitive treatment of localized prostate cancer with low rates of high-grade toxicity and excellent patient-reported quality-of-life metrics. Technological advances such as pencil beam scanning (PBS), Monte Carlo dose calculations, and polyethylene glycol gel rectal spacers have optimized prostate proton therapy. Here, we report the early clinical outcomes of patients treated for localized prostate cancer using modern PBS-PBT with hydrogel rectal spacing and fiducial tracking without the use of endorectal balloons.

Materials and methods: This is a single institutional review of consecutive patients treated with histologically confirmed localized prostate cancer. Prior to treatment, all patients underwent placement of fiducials into the prostate and insertion of a hydrogel rectal spacer. Patients were typically given a prescription dose of 7920 cGy at 180 cGy per fraction using a Monte Carlo dose calculation algorithm. Acute and late toxicity were evaluated using the Common Terminology Criteria for Adverse Events (CTCAE), version 5. Biochemical failure was defined using the Phoenix definition.

Results: From July 2018 to April 2020, 33 patients were treated (median age, 75 years). No severe acute toxicities were observed. The most common acute toxicity was urinary frequency. With a median follow-up of 18 months, there were no high-grade genitourinary late toxicities; however, one grade 3 gastrointestinal toxicity was observed. Late erectile dysfunction was common. One treatment failure was observed at 21 months in a patient treated for high-risk prostate cancer.

Conclusion: Early clinical outcomes of patients treated with PBS-PBT using Monte Carlo-based planning, fiducial placement, and rectal spacers sans endorectal balloons demonstrate minimal treatment-related toxicity with good oncologic outcomes. Rectal spacer stabilization without the use of endorectal balloons is feasible for the use of PBS-PBT.

Keywords: endorectal balloon; prostate cancer; proton therapy; radiation therapy; rectal spacer.

Figure 1.

(A) Plan for a patient diagnosed with favorable intermediate risk adenocarcinoma of the prostate. Hydrogel spacer demonstrated by the pink contour creating artificial displacement between anterior aspect of the rectum and posterior aspect of the prostate. (B) Color wash dose distribution of treatment to a total dose of 7920 cGy (RBE) in 44 fractions (initial field including seminal vesicles to a total dose of 4500 cGy [RBE] in 25 fractions) using opposed lateral proton beams. (C) Color wash dose distribution of single lateral plan.

Figure 1.

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Figure 2.

(A) (left) In-room view of single left lateral proton beam field with associated aperture projection. (right) Beam's eye view of aperture projection demonstrating hydrogel spacer displacement, which allows for sparing of rectum and mitigation of radiation dose to this organ at risk. (B) A T2-weighted MRI of the prostate in the coronal (left) and axial (right) planes demonstrating the separation of the prostate (red arrow) and rectum (blue arrow) provided by insertion of a hydrogel spacer (purple arrow).