Room: Exhibit Hall | Forum 6
Purpose: To investigate the dosimetric feasibility and clinical efficacy of proton therapy of prostate cancer with simultaneous integrated boost (SIB) to the MRI-defined dominant intraprostatic lesions.
Methods: For 6 patients with high-risk prostate cancer who had received conventional radiotherapy, dominant intraprostatic lesions were delineated using diagnostic multiparametric MRI, based on hypointense appearance on T1/T2-weighted and apparent diffusion coefficient (ADC) maps. After transferring the contours to the planning CT via image registration, new pencil beam scanning (PBS) proton therapy plans were created using conventional (70.4 Gy(RBE) to prostate and 51.2 Gy(RBE) to seminal vesicles) and SIB (83.2 Gy(RBE) to dominant lesions, 70.4 Gy(RBE) to prostate and 51.2 Gy(RBE) to seminal vesicles) dose prescriptions, in 32 fractions. The spot size was 3 mm median sigma at isocenter for 90â€“230MeV range in air. Tumor control probability (TCP) and normal tissue complication probability (NTCP) were compared between the two scenarios.
Results: In all cases boosting the intraprostatic lesions to 83.2 Gy was achievable without significantly increasing the dose to bladder and rectum. The average lesion 2Gy-equivalent D98 and Dmean increased by 29.3 and 34.2% for SIB compared to conventional plans, respectively. For prostate volume, this increase was 13.7 and 18.5%. For organs at risk the average increase in mean dose was within 1 Gy. The average increase in bladder/rectum D2 was 1.22 and 1.07 Gy, and remained below the clinical tolerance for all cases. The TCP of the lesion increased by 59-67% for the SIB plans depending on TCP model parameters used. The bladder and rectum NTCP increase remained below 1%.
Conclusion: Proton therapy with MRI-guided SIB to intraprostatic lesions of 83.2 Gy and 2Gy-equivalent dose of ~100 Gy(EQD2) is feasible and yields significant increase in the lesion tumor control probability without increasing the dose to the OARs.