Moderate hypofractionation has become an important approach in prostate cancer radiotherapy, but its use becomes more complex when pelvic nodal irradiation and dose escalation to gross disease are added.
The phase 2 MOB-RT study evaluated whether moderately hypofractionated radiotherapy to the prostate and pelvic nodes, with optional boosts to intraprostatic disease and involved nodes, could be delivered safely in patients with high-risk or node-positive prostate cancer.
Published in Radiotherapy and Oncology, the study focused on acute safety and patient-reported quality of life outcomes.
Why This Study Matters
Patients with high-risk or node-positive prostate cancer often require treatment beyond the prostate alone.
Pelvic elective nodal irradiation may improve regional disease control, while dose escalation to visible disease could potentially improve local control. However, treating larger pelvic volumes and adding boosts can increase concerns about gastrointestinal and genitourinary toxicity.
MOB-RT addresses this practical clinical question: can prostate, pelvic nodes, and selected gross disease be treated with a moderately hypofractionated schedule while maintaining acceptable acute toxicity and quality of life?
Study Design
MOB-RT was a prospective, single-center, single-arm phase 2 study.
Patients with high-risk or node-positive prostate cancer received external beam radiotherapy with androgen deprivation therapy. The radiotherapy schedule included: 60 Gy in 20 fractions to the prostate. 48 Gy elective nodal irradiation to the pelvis. A simultaneous integrated boost up to 68 Gy to MRI-defined intraprostatic gross disease, when indicated.
A simultaneous integrated boost up to 55 Gy to radiographically involved lymph nodes, when present. The primary endpoint was acute gastrointestinal side effects at 3 months after radiotherapy, assessed using CTCAE version 5.0.
The study tested whether acute grade 2 or higher gastrointestinal toxicity would remain below a predefined threshold of 35%. Secondary endpoints included acute genitourinary toxicity and patient-reported quality of life using EPIC-26 and IPSS.
Patient Population
A total of 100 patients were enrolled, and 97 completed treatment. The median age was 73 years. Nearly half of patients had grade group 4 to 5 disease. The median PSA was 22.2 ng/mL.
Twenty-one percent of patients had clinically node-positive disease. Intraprostatic boost was delivered in 24% of patients, and nodal boost was delivered in 19%.
Key Safety Results
The study met its primary safety objective. Acute grade 2 or higher gastrointestinal side effects occurred in 15.5% of patients, below the predefined threshold of 35%. The one-sided 95% upper confidence bound was 22.8%. Acute grade 2 or higher genitourinary side effects occurred in 26.8% of patients.
Higher rectal and bowel D1cc doses and use of intraprostatic boost were associated with increased acute gastrointestinal side effects on univariable analysis.
Nodal boost was not significantly associated with increased toxicity.
Quality of Life Outcomes
Patient-reported outcomes were reassuring. There was no statistically significant decline in mean gastrointestinal or genitourinary quality of life at 3 months after treatment. This is important because acute toxicity rates alone may not fully reflect how patients experience treatment.
The lack of significant short-term deterioration in GI or GU quality of life supports the tolerability of this approach in appropriately selected patients.
Clinical Interpretation
The MOB-RT study suggests that moderate hypofractionated radiotherapy to the prostate and pelvic nodes can be delivered with acceptable acute toxicity in patients with high-risk or node-positive prostate cancer.
The addition of selective boosts to MRI-defined intraprostatic disease and radiographically involved nodes also appeared feasible. These findings are clinically relevant because they support a shorter radiotherapy schedule while still addressing the prostate, elective pelvic nodes, and visible disease.
At the same time, the association between rectal and bowel dose parameters, intraprostatic boost, and gastrointestinal toxicity highlights the need for careful treatment planning and dose constraints.
Limitations
This was a phase 2, single-arm study without a randomized comparator. The reported outcomes focus on acute safety and quality of life at 3 months, so longer follow-up is needed to assess late toxicity, biochemical control, metastasis-free survival, and overall oncologic outcomes. The use of boosts was selective and based on clinical indication and feasibility, so the study does not definitively establish which patients benefit most from dose escalation.
Clinical Takeaway
MOB-RT supports the acute safety of moderate hypofractionated prostate and pelvic nodal radiotherapy in high-risk or node-positive prostate cancer. In this phase 2 study, treatment with 60 Gy in 20 fractions to the prostate, 48 Gy to pelvic nodes, and selective boosts to gross prostate or nodal disease resulted in lower-than-expected acute gastrointestinal toxicity and no significant short-term decline in mean GI or GU quality of life. Longer follow-up will be important to define late toxicity and cancer control outcomes, but these early results support the feasibility of this approach in carefully planned treatment.
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