Introduction: The Challenge of Biochemical Recurrence
For many men undergoing radical prostatectomy (RP) for localized prostate cancer, the procedure is curative. However, a significant proportion of patients—up to 40% within ten years—will experience biochemical recurrence (BCR), defined by a rising prostate-specific antigen (PSA) level. In this clinical scenario, salvage radiotherapy (SRT) to the prostate bed remains the only curative-intent treatment option.
Historically, the optimal dose of radiation in the salvage setting has been a subject of intense debate. Drawing from the experience in definitive radiotherapy (where the prostate gland is still intact), many clinicians hypothesized that dose escalation would lead to better local control and, consequently, improved long-term oncological outcomes. This hypothesis led to the initiation of the SAKK 09/10 trial, a multicenter, randomized phase 3 study designed to compare conventional-dose SRT with dose-intensified SRT. The long-term results of this landmark trial have now been released, providing definitive clarity on a question that has persisted for over a decade.
Study Design and Methodology
The SAKK 09/10 trial, conducted by the Swiss Group for Clinical Cancer Research (SAKK), recruited 350 patients between February 2011 and April 2014. Eligible participants were men who had undergone radical prostatectomy and subsequently developed biochemical progression, defined as two consecutive PSA rises with a final value of at least 0.1 ng/ml or a single value >0.4 ng/ml. Crucially, the trial excluded patients with evidence of macroscopic recurrence or distant metastasis on imaging.
Participants were randomized in a 1:1 ratio to receive either:
1. Conventional-dose SRT:
64 Gy delivered in 32 fractions of 2.0 Gy.
2. Dose-intensified SRT:
70 Gy delivered in 35 fractions of 2.0 Gy.
A key feature of this trial was the absence of concomitant hormonal therapy (androgen deprivation therapy, ADT). This allowed researchers to isolate the therapeutic effect of the radiation dose itself, without the confounding benefits of systemic treatment. The primary endpoint was freedom from biochemical progression (FFBP). Secondary endpoints were comprehensive, including clinical progression-free survival (PFS), time to hormonal treatment, overall survival (OS), and a rigorous assessment of acute and late toxicities, as well as quality of life (QoL).
Patient Population and Baseline Characteristics
The median age of the cohort was 66 years, and the median PSA level at the time of randomization was 0.3 ng/ml. This reflects a population undergoing ‘early’ salvage radiotherapy, which is currently recommended by international guidelines. Most patients had favorable performance status and high-quality surgical outcomes prior to recurrence. The baseline characteristics were well-balanced between the 64 Gy and 70 Gy arms, ensuring that the comparative results were not skewed by external variables.
Key Findings: Oncological Outcomes at 8.6 Years
The long-term data, representing a median follow-up of 8.6 years, provide a robust look at the durability of SRT.
Freedom from Biochemical Progression (FFBP)
In the primary analysis, the dose-intensified arm failed to show superiority. The median FFBP was 8.7 years (95% CI 7.1–NR) for the 64 Gy arm and an identical 8.7 years (95% CI 6.7–NR) for the 70 Gy arm. The hazard ratio (HR) was 1.03 (95% CI 0.75–1.41), with a log-rank p-value of 0.87. These statistics indicate that adding 6 Gy to the treatment plan does nothing to delay or prevent the return of PSA markers.
Clinical Progression and Survival
Beyond PSA levels, the trial tracked actual clinical progression—such as the appearance of metastases on scans or local recurrence. No significant differences were observed in clinical PFS. Similarly, the time to the initiation of hormonal treatment and the overall survival rates were nearly identical between the two groups. For clinicians, this suggests that the biological ceiling for radiotherapy effectiveness in the prostate bed may be reached at the conventional 64 Gy dose, at least when ADT is not utilized.
The Cost of Intensification: Safety and Toxicity
While the oncological benefits were non-existent, the physiological costs of the higher dose were clearly evident. The study utilized the Common Terminology Criteria for Adverse Events (CTCAE) v4.0 to track complications.
Gastrointestinal (GI) Toxicity
The most striking difference between the arms was the rate of late GI toxicity. Patients in the 70 Gy arm experienced significantly higher rates of grade 2 and grade 3 GI symptoms (p = 0.015). Grade 2 toxicities often involve persistent diarrhea, rectal bleeding requiring minor intervention, or significant lifestyle adjustments. Grade 3 toxicities are more severe, potentially requiring surgical intervention or blood transfusions. The long-term data confirmed that the rectum, which is in close proximity to the prostate bed, is highly sensitive to the additional 6 Gy of radiation.
Genitourinary (GU) Toxicity
Interestingly, late genitourinary toxicity (such as urinary frequency, urgency, or hematuria) did not show a statistically significant difference between the two doses in the long-term follow-up, although earlier reports from the trial noted a transient increase in urinary symptoms and a slight worsening in urinary-related quality of life for the dose-intensified group.
Expert Commentary: Contextualizing SAKK 09/10
The results of SAKK 09/10 are a powerful reminder of the ‘therapeutic window’ in oncology—the narrow range where a treatment is effective without being excessively toxic. In the context of salvage radiotherapy, dose escalation to 70 Gy appears to move outside this window, increasing morbidity without enhancing the cure rate.
Several factors may explain these results. First, the biology of early recurrence (at PSA <0.5 ng/ml) might involve microscopic disease that is sufficiently sensitive to 64 Gy. Second, the absence of ADT might have limited the potential for the higher dose to overcome more aggressive, radioresistant clones. However, current evidence from other trials (like GETUG-AFU 16 and RADICALS-RT) suggests that adding ADT to SRT is a more effective way to improve outcomes than simply increasing the radiation dose.
Clinicians should also note that the SAKK 09/10 trial focused on the prostate bed alone. Modern practice often involves elective nodal irradiation for high-risk patients, a strategy not tested here but one that further complicates the dose-toxicity relationship.
Conclusion: Clinical Implications
The long-term follow-up of the SAKK 09/10 trial provides high-level evidence that 64 Gy should remain the standard dose for salvage radiotherapy to the prostate bed in men with biochemical recurrence after radical prostatectomy. Dose intensification to 70 Gy should be avoided as it provides no improvement in biochemical control, clinical progression-free survival, or overall survival, while significantly increasing the risk of long-term gastrointestinal complications.
For the practicing urologist and radiation oncologist, the message is clear: when it comes to salvage radiotherapy dose in the prostate bed, ‘more’ is not ‘better.’ Efforts to improve outcomes in this patient population should focus on better patient selection through advanced imaging (like PSMA-PET), the optimal integration of androgen deprivation therapy, and the potential role of pelvic nodal irradiation, rather than simply escalating the dose to the prostate bed.
Funding and Registration
This trial was supported by the Swiss Group for Clinical Cancer Research (SAKK) and is registered on ClinicalTrials.gov as NCT01272050.
