KEY POINTS
- The illustrated patient had unfavourable intermediate-risk prostate cancer with biopsy-confirmed PI-RADS 5 and PI-RADS 4 lesions. Treatment delivered 70 Gy in 28 fractions to the prostate and proximal seminal vesicles, with simultaneous integrated boosts to 77 Gy and concurrent androgen-deprivation therapy.
- The authors recommend routinely boosting PI-RADS 3–5 lesions with concordant positive biopsy findings. Pre-biopsy multiparametric MRI generally provides the clearest tumour definition, while a planning MRI close to simulation is particularly valuable after androgen-deprivation therapy, fiducial placement, rectal-spacer insertion, or a long biopsy-to-treatment interval.
- MRI remains the primary modality for gross-tumour delineation, with PSMA PET used as complementary information. In the illustrated case, the dominant lesion had an SUVmax of 35.4, but PET uptake was more diffuse than the MRI abnormality and was therefore not used to define its full extent.
- The boost planning target volume is generally identical to the MRI-defined gross tumour volume, without an additional margin. When the lesion abuts the urethra, rectum, bladder neck, or external sphincter, normal-tissue protection takes precedence; optional 2-mm planning-risk volumes can support isotoxic optimization.
- A common principle is to prescribe at least 110% of the whole-gland dose to the visible tumour. Example regimens include 66–74 Gy in 20 fractions after 60–62 Gy to the gland, 75.6 to ≥80 Gy in 28 fractions after 70 Gy, and approximately 42–50 Gy in five fractions after 35–40 Gy.
- The intended boost dose should generally be achieved by GTV D95% or D98%, with the minimum GTV dose remaining above the prostate prescription and hotspots confined within the tumour. Hotspots may reach 5–15% above the intended boost, but whole-gland de-escalation should not be attempted outside a clinical trial.
- Isotoxic planning can materially reduce the delivered boost. In hypo-FLAME, the nominal target was 50 Gy in five fractions, but the median achieved dose was approximately 44 Gy because organ-at-risk limits took priority; robust daily image guidance and intrafraction motion management are therefore essential.
CLINICAL TAKEAWAY
Focal boosting is a practical option across conventional, moderately hypofractionated, and stereotactic prostate radiotherapy when the lesion is clearly defined on MRI. The central rule is isotoxicity: urethral, rectal, bladder-neck, and sphincter protection should not be sacrificed to force complete boost coverage.