Session Item

Ultra-hypofractionated (³6Gy/fraction) radiotherapy for localized prostate cancer has been delivered at our institution with either online CBCT-guided 3 degrees of freedom (3DOF) translational correction or online MR-guided adaptation. Herein, we compared the daily dosimetry and acute toxicity between these two systems.

Fifty patients recruited to a Phase 2 trial evaluating the efficacy of External Beam Radiation Therapy (EBRT) plus High Dose Rate Brachytherapy intraprostatic boost (1500cGy/1) were included in this analysis. Hydrogel spacer was inserted at the discretion of physicians. An EBRT reference plan was generated to deliver 3000cGy/5 to the CTV (Prostate ± Seminal Vesicles) with a margin of 5mm using VMAT for 25 patients treated with CBCT-3DOF. The remainder were planned with IMRT and MR-guided adapt-to-shape (AtS). Daily EBRT delivered dose to CTV, bladder and rectum was computed using the daily CBCT or MR images. Deviation of >10% from reference dose was considered clinically significant. Acute toxicity was prospectively recorded using Common Terminology Criteria for Adverse Events (ver 4.0).

Delivered dose per fraction to 95% of CTV was acceptable for all 250 fractions, with a median of 660cGy (Range: 649–680cGy). Hydrogel spacer was present in 17 patients in the CBCT-3DOF and 15 patients in the MRL cohort. VMAT with CBCT-3DOF was associated with lower daily rectal D50% but higher D1cm³ compared to IMRT using MR-AtS, especially in the presence of hydrogel spacer (Table 1). The use of MR-AtS resulted in more fractions delivering a significantly lower rectal D20% and D1cm³ than the reference dose, when compared to CBCT-3DOF (56 vs 26 for D20%, 51 vs 2 for D1cm³; Figure 1). In contrary, difference between reference and delivered dose to 5cm³ of bladder was significant in <15 fractions by either systems. Median follow up was 26 months for CBCT-3DOF and 25 months for MR-AtS. The cumulative incidence of G2+ urinary/gastrointestinal toxicity by 6 months was 8%/8% for the CBCT-3DOF cohort and 20%/0% for the MR-AtS.

Significant deviation from the reference plan dose for rectum was observed with both systems due to large range of anatomical changes exhibited between planning and treatment sessions. MR-guided adaptation was able to deliver a lower than reference plan dose to the rectum in the high dose region more frequently than CBCT-guided 3DOF, and these patients experienced a lower incidence of G2+ GI acute toxicity. Follow up is ongoing and will assess the clinical efficacy of these two treatment delivery systems.