Focal Boost Prostate SBRT Shows Promise

Stereotactic body radiotherapy to the prostate with focal boost: analysis of the primary endpoint in the DELINEATE trial Cohort E - ScienceDirect

But Higher Urinary Side Effects Than Standard Treatment

UK trial demonstrates safety of targeting visible tumors with extra radiation, though questions remain about necessity

By IPCSG Medical News Team

Men with intermediate and high-risk prostate cancer can safely receive an extra dose of radiation to their visible tumors during ultra-short SBRT treatment, according to new results from the UK's DELINEATE trial. However, the study found that this "focal boost" approach carries a moderate risk of urinary problems that patients and doctors should carefully consider.

The findings, published in the International Journal of Radiation Oncology, Biology, Physics, also raise an important question: With today's excellent cure rates using standard SBRT alone, is the extra radiation even necessary?

What the Study Did

Researchers at the Royal Marsden NHS Foundation Trust treated 49 men between 2018 and 2020 using a specialized radiotherapy approach. Instead of the traditional 20-40 treatments, patients received just 5 treatments over two weeks—what doctors call SBRT (stereotactic body radiotherapy) or ultra-hypofractionation.

The entire prostate received 36.25 Gray (the standard SBRT dose), but tumors visible on MRI scans received an extra boost bringing the dose up to 45 Gray. These boosted areas were typically small, averaging about 1.9 cubic centimeters (roughly the size of a marble). All patients also received hormone therapy, with most getting 6 months of androgen deprivation therapy.

The Good News: Bowel Side Effects Were Minimal

After nearly 4 years of follow-up, the results for bowel toxicity were excellent:

• No patients experienced severe (grade 3 or higher) bowel problems at any point
• Only 6.1% had moderate bowel issues at one year, rising to just 10.2% by three years
• Most patients reported bowel problems were "very small" or "no problem"

These bowel toxicity rates were actually lower than those seen in the PACE-B trial, which used standard SBRT without the focal boost.

The Concern: Urinary Problems Were More Common

Urinary side effects proved more challenging:

• 10.2% of men had moderate urinary problems at one year
• This increased to 20.4% by three years
• More concerning, 12.2% experienced severe urinary toxicity (grade 3) by two to three years

The severe urinary problems included bladder inflammation (cystitis), blood in the urine, and urethral narrowing (stricture). Patients typically experienced a flare of urinary symptoms around 12-18 months after treatment, though this often improved over time.

During treatment itself, about one-quarter of patients (24.5%) had moderate urinary symptoms at week 4, but these largely resolved within a few months.

Disease Control: Excellent Results

The cancer control results were impressive:

• Only one patient out of 49 had disease progression (rising PSA) by 4 years
• No local recurrences were detected
• Two deaths occurred, both unrelated to prostate cancer

Sexual Function: Mixed Picture

About half of men maintained good erectile function at 2 years after treatment. Interestingly, 60% of patients reported improved sexual quality of life compared to before treatment—likely because hormone therapy had ended and testosterone levels recovered.

The Central Question: Is the Extra Dose Needed?

Lead researcher Dr. Alison Tree and colleagues raised a critical issue: recent trials show that standard SBRT without focal boost already achieves excellent results. The landmark PACE-B trial reported 96% cancer control at 5 years using standard-dose SBRT alone in men with favorable-risk disease.

"With almost 100% local control rates with SBRT, our attention now turns to further reducing toxicity," the researchers wrote. They noted that results from PACE-C (testing SBRT in intermediate and high-risk patients similar to those in DELINEATE) are eagerly awaited.

The UK's PIVOTAL-boost trial, which enrolled 2,232 men to directly compare treatment with and without focal boost, should provide definitive answers when results are released.

What This Means for Patients

For men with intermediate-risk disease: Standard SBRT without focal boost may be sufficient based on emerging evidence.

For men with high-risk or locally advanced disease: Focal boost remains a reasonable option, since cancer recurrences almost always occur at the site of the original MRI-visible tumor. However, patients should understand the trade-offs:

• Potential benefit: Possibly better long-term cancer control
• Cost: About 1 in 8 men will experience significant urinary problems by 3 years

Questions to ask your radiation oncologist:

1. What is my specific risk level, and does focal boost make sense for my situation?
2. What are my alternatives (standard SBRT, longer fractionation schedules, different boost approaches)?
3. What is your experience with urinary complications, and how are they managed?
4. Should I wait for PIVOTAL-boost results before deciding?

Future Directions

Researchers are exploring a novel approach called "prostate-sparing" radiotherapy—giving the required high dose only to visible tumors while reducing radiation to the rest of the prostate. This concept, being tested in the DESTINATION trials, could potentially maintain excellent cancer control while reducing side effects.

Bottom Line

Focal boost SBRT is technically feasible and safe for most patients, with minimal bowel problems but a notable risk of urinary side effects. However, whether the extra radiation provides meaningful benefit over standard SBRT remains an open question. Patients with intermediate and high-risk prostate cancer should discuss with their radiation oncologists whether this approach is right for their individual situation, weighing potential benefits against the approximately 10-12% risk of significant urinary problems.

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Sources

1. Murray, J., Satchwell, L., Rajan, Z., Gao, A., McNair, H., Pathmanathan, A., Parker, C., Talbot, J., Dearnaley, D., & Tree, A. (2025). Stereotactic body radiotherapy to the prostate with focal boost: analysis of the primary endpoint in the DELINEATE trial Cohort E. International Journal of Radiation Oncology, Biology, Physics (Journal Pre-proof). https://doi.org/10.1016/j.ijrobp.2025.10.016

2. van As, N., Griffin, C., Tree, A., et al. (2024). Phase 3 Trial of Stereotactic Body Radiotherapy in Localized Prostate Cancer. New England Journal of Medicine, 391(15), 1413-1425.

3. Tree, A.C., Satchwell, L., Alexander, E., et al. (2023). Standard and Hypofractionated Dose Escalation to Intraprostatic Tumor Nodules in Localized Prostate Cancer: 5-Year Efficacy and Toxicity in the DELINEATE Trial. International Journal of Radiation Oncology, Biology, Physics, 115(2), 305-316.

4. Kerkmeijer, L.G.W., Groen, V.H., Pos, F.J., et al. (2021). Focal Boost to the Intraprostatic Tumor in External Beam Radiotherapy for Patients With Localized Prostate Cancer: Results From the FLAME Randomized Phase III Trial. Journal of Clinical Oncology, 39(7), 787-796.

5. Syndikus, I., Cruickshank, C., Staffurth, J., et al. (2020). PIVOTALboost: A phase III randomised controlled trial of prostate and pelvis versus prostate alone radiotherapy with or without prostate boost. Clinical and Translational Radiation Oncology, 25, 22-28. https://www.sciencedirect.com/science/article/pii/S2405630820300690