Bladder Tumor-Focused Adaptive Radiation Therapy: Clinical Outcomes of a Phase I Dose Escalation Study

We determine the maximum tolerated tumor-focused dose (MTD) for the radical treatment of muscle invasive bladder cancer enabled by image guided adaptive radiation therapy and long-term clinical outcomes.

Fifty-nine patients with T2 to T4aN0M0 unifocal urothelial muscle invasive bladder cancer suitable for daily radical radiation therapy were recruited prospectively to an ethics-approved protocol (NCT01124682). The uninvolved bladder (PTV_bladder) was planned to 52 Gy in 32 fractions. The bladder tumor (PTV_tumor) was planned to an assigned dose level of 68, 70, 72, or 74 Gy. If organ at risk dose constraints were violated, then PTV_tumor was planned to 64 Gy. Dose level allocation was determined by concurrent toxicity assessment of all previous patients recruited. Acute toxicity was evaluated using Common Terminology Criteria for Adverse Events v3.0; late toxicity was evaluated using Radiation Therapy Oncology Group criteria. The MTD was predefined as the highest dose level with an estimated probability of ≤ 15% ≥ G3 late toxicity and an observed rate of <50% acute G3 and <10% acute G4 toxicity.

Twenty-six patients were assigned to 68 Gy, of whom 6 were planned to 64 Gy; 29 patients were assigned to 70 Gy of whom 1 was planned to 68 Gy, 2 patients were assigned and planned to 72 Gy; no patients were assigned to 74 Gy. Three patients did not complete the treatment as planned, of whom only 1 patient stopped treatment because dose-limiting toxicity occurred. The MTD was 70 Gy. Acute genito-urinary and gastro-intestinal G3 acute toxicity was seen in 19% and 7% of patients, respectively. No acute G4 genito-urinary or gastro-intestinal toxicity was seen. Late toxicity (any) G3 and G4 was seen in 14% and 2% of patients, respectively. The 5-year overall survival was 58% (95% CI, 44%-71%). The bladder preservation rate was 89% (95% CI, 88%-96%) with 6 patients not retaining native bladder function.

Bladder tumor-focused dose escalation to 70 Gy using image guided adaptive radiation therapy is feasible with acceptable toxicity. This dose level has been evaluated in a phase II randomized control trial (RAIDER NCT02447549).