Radiotherapy for localized prostate cancer has steadily moved toward more precise treatment delivery. While dose escalation can improve cancer control, increasing the dose to the entire prostate gland may also raise the risk of urinary and gastrointestinal toxicity.
A new ESTRO clinical practice consensus addresses how radiation oncologists can selectively intensify treatment to the most clinically significant tumour areas within the prostate. The recommendations focus on focal boosting of intraprostatic tumours, outlining which patients may benefit, how target lesions should be defined, and how treatment can be delivered safely in routine practice.
Why Focus the Boost on the Tumour?
Local recurrences after prostate radiotherapy often occur at the site of the original dominant tumour. This has led to increasing interest in focal boosting: delivering a higher dose directly to visible intraprostatic lesions while maintaining standard treatment to the whole gland.
The aim is not simply to give more radiation. It is to direct additional dose where the cancer burden is greatest while preserving surrounding structures, including the urethra, bladder, and rectum.
Whole-gland dose escalation has improved biochemical progression-free survival in several studies. However, it has also been associated with higher toxicity. Focal boosting offers a more selective approach, with treatment planning designed to prioritize normal tissue protection over the intended boost dose whenever necessary.
FLAME Trial Provides the Strongest Evidence
The phase III FLAME trial remains the central randomized study supporting focal boost radiotherapy in prostate cancer.
The trial included 571 men with intermediate- and high-risk disease. All participants received 77 Gy in 35 fractions to the prostate. Patients assigned to the focal boost arm received an isotolerated boost of up to 95 Gy to the gross tumour volume.
At five years, biochemical progression-free survival was 92% in the focal boost group, compared with 85% in the standard radiotherapy group. The benefit remained evident after longer follow-up. At 10 years, biochemical progression-free survival was 86% with focal boosting, compared with 71% without focal boosting.
The focal boost approach also reduced intraprostatic recurrence and was associated with fewer regional lymph node and distant recurrences. Importantly, the trial did not show a significant increase in acute or late side effects when dose constraints for organs at risk were respected.
These findings established the principle that higher radiation doses can be delivered selectively to prostate tumours without necessarily compromising quality of life.
Which Patients Are Most Likely to Benefit?
Patient selection is central to focal boost treatment. ESTRO experts agreed that the strongest indication is in men with unfavourable intermediate-risk, high-risk, and very-high-risk localized prostate cancer.
Patients with ISUP grade group 1 or 2 disease generally have excellent outcomes with standard treatment approaches and are not expected to derive a clear benefit from intraprostatic boosting. In contrast, patients with more aggressive local disease may be appropriate candidates, particularly those with:
- ISUP grade group 4 or 5 disease;
- T3a, T3b, or T4 N0M0 disease;
- PSA levels above 20 ng/mL;
Unfavourable intermediate-risk disease, including ISUP grade group 3 tumours with bulky disease, cribriform morphology, or intraductal carcinoma.
A clearly visible intraprostatic lesion is essential. Patients without a defined gross tumour volume on imaging are not suitable for focal boosting, although whole-gland brachytherapy boost may remain an option in selected cases.
The consensus also notes that extensive tumour involvement may limit the value of a “focal” approach. When tumour volume occupies more than half of the prostate, treatment may effectively become whole-gland dose escalation rather than selective boosting.
Imaging Defines the Target
Focal boost radiotherapy depends on accurate imaging. Multiparametric MRI remains the cornerstone of tumour identification and contouring.
T2-weighted MRI and apparent diffusion coefficient imaging were considered mandatory for gross tumour volume delineation. Diffusion-weighted imaging was strongly supported as an additional tool. Lesions suitable for focal boosting are commonly PI-RADS 4 or 5 abnormalities, reflecting a high or very high likelihood of clinically significant prostate cancer.
Pre-biopsy MRI is preferred whenever possible. If unavailable, MRI should ideally be performed before hormone therapy begins, as androgen deprivation therapy can reduce tumour visibility and complicate target definition.
PSMA-PET/CT may provide further information, particularly when MRI findings are uncertain or when there is concern that MRI may underestimate the full tumour extent. However, the consensus did not establish PSMA-PET/CT as mandatory for all patients. Questions remain regarding uptake thresholds, lesion significance, spatial resolution, and differences between PET-identified and MRI-visible disease.
Histological confirmation of a visible lesion is preferred whenever feasible, although the expert panel did not consider it mandatory when isotolerated focal boosting is used.
Delivering the Boost: Precision Is Essential
Modern intensity-modulated radiotherapy and volumetric modulated arc therapy are central to focal boost delivery. These techniques allow high-dose radiation to be shaped around the tumour while maintaining steep dose gradients near the rectum, bladder, and urethra.
The consensus recommends simultaneous integrated boost techniques over sequential boosts whenever possible. A simultaneous integrated boost allows the prostate and tumour lesion to be treated during the same course, improving treatment efficiency and dose conformity.
For conventional fractionation, the recommended whole-prostate dose is 77 Gy in 35 fractions, with an isotolerated focal boost of up to 95 Gy where organ-at-risk constraints permit.
For moderately hypofractionated radiotherapy, 60 Gy in 20 fractions is considered standard for the whole prostate. Most experts recommended a focal boost of at least 67 Gy, although no full consensus was reached on the optimal dose.
Ultra-hypofractionated radiotherapy in five fractions is increasingly being studied. Trials including hypo-FLAME have reported promising biochemical control and acceptable toxicity. However, the consensus notes that stereotactic body radiotherapy with focal boosting is not yet considered standard of care by most experts. Treatment delivered in fewer than five fractions should currently be restricted to clinical trials.
Protecting the Urethra, Bladder, and Rectum
The defining safety principle of focal boosting is clear: protection of normal tissue must take priority over boost target coverage.
The urethra deserves particular attention. Analyses from the FLAME trial found that higher urethral and bladder doses were associated with more genitourinary toxicity. For the conventional 35-fraction FLAME schedule, the consensus suggests aiming for a urethral D0.1 cc of 80 Gy or less when possible.
Rectal dose constraints are also critical. Studies from FLAME have linked higher rectal dose exposure with late gastrointestinal symptoms. Treatment teams are encouraged to contour relevant organs carefully and avoid hotspots in the urethra, bladder, rectum, and associated substructures.
When a tumour lies close to the urethra, rectum, bladder neck, or seminal vesicles, clinicians may need to compromise the boost dose to maintain safety. In selected cases, focal brachytherapy boost may provide an alternative route to achieve adequate tumour coverage.
Daily Image Guidance Cannot Be Optional
The prostate can move between and during treatment sessions. This makes consistent preparation and daily image guidance essential when delivering a focal boost.
Patients should follow standardized bladder- and rectum-filling protocols throughout simulation and treatment. The goal is an empty rectum and a comfortably filled bladder, helping reduce prostate displacement and minimizing variation in treatment anatomy.
Daily interfraction motion correction is considered mandatory. Alignment should focus on the prostate and the region of the gross tumour volume rather than relying solely on bony anatomy.
Fiducial markers can support accurate localization on X-ray-based platforms, although they are not mandatory when high-quality volumetric imaging is available. On MRI-guided systems, fiducials are generally not required because the prostate and adjacent organs can be directly visualized.
Real-time intrafraction motion monitoring may further improve accuracy, particularly when high doses are delivered in fewer fractions. The consensus considers intrafraction motion management mandatory for treatment courses involving fewer than five fractions.
Barriers Still Limit Wider Adoption
Despite growing evidence, focal boosting is not yet routinely available in every radiotherapy department.
The main barriers include access to high-quality MRI, confidence in identifying and contouring intraprostatic tumours, MRI-to-CT registration challenges, workflow demands, and concerns about toxicity. PSMA-PET/CT integration also remains complex, particularly because small lesions detected on PET may not always correspond to MRI findings.
Training, multidisciplinary imaging review, and consistent planning procedures will be important as more centers introduce focal boost treatment.
The ESTRO consensus provides a structured pathway for departments seeking to incorporate focal boosting into prostate cancer radiotherapy. Its central message is practical: dose escalation can improve biochemical control, but the benefit depends on careful patient selection, accurate imaging, strict organ-at-risk protection, and reliable image guidance throughout treatment.
Title:”How to focal boost in prostate cancer radiotherapy: ESTRO clinical practice consensus recommendations”
Authors: T. Willigenburg, C. Draulans, F.J. Pos, T.M. Seibert, I. Syndikus, A. Henry, A. Mitchell, M. Buijs, B.A. Jereczek-Fossa, S.K.B. Spohn, T. Pommer, B.R. Pieters, A.C. Tree, J.R.N. van der Voort van Zyp
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