ESTRO 2024 - Abstract Book

S2313

Clinical - Urology

ESTRO 2024

365

Digital Poster

Ultrafractionated RT for prostate cancer: Dose escalation using MR-Linac

Aysenur Elmali 1 , Gungor Arslan 1 , Cagdas Yavas 1 , Esma Efe 1 , Guler Yavas 1 , Cem Onal 2,1

1 Baskent University, Faculty of Medicine, Radiation Oncology, Ankara, Turkey. 2 Baskent University, Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Radiation Oncology, Adana, Turkey

Purpose/Objective:

Several randomized and non-randomized studies found that patients with localized prostate cancer had lower rates of biochemical failure, post-treatment positive biopsies, and distant metastases. Literature data suggest that 36.25 Gy is an appropriate dose to avoid urinary side effects, but it may not be sufficient to ensure proper local control, particularly for prostate cancer with a high-to-intermediate risk. In addition, increasing the total dose may improve local control at the expense of increased urinary and rectal toxicity. The purpose of this study is to assess the dosimetric feasibility of increasing intraprostatic lesion (IPL) dose to 40 Gy with 1.25 Gy increments delivered in 5 fractions in patients receiving 36.25 Gy to the entire prostate in terms of target volume coverage and organs at risk (OARs) doses.

Material/Methods:

Eighteen low- and intermediate-risk prostate cancer patients treated with ultra-hypofractionated RT using a 1.5T MR-Linac were retrospectively evaluated. Four treatment plans, including a control plan with no SIB and three SIB plans with escalating doses (37.5 Gy, 38.75 Gy, and 40 Gy), were generated with same planning CT images for each patient. Statistical analysis compared differences in PTV doses, OAR doses, and PTV-SIB homogeneity index (HI), gradient index (GI) and conformity index (CI). All plans were computed utilizing the 1.5 T-MR-Linac (Unity® MR Linac System, Elekta AB, Stockholm, Sweden) with 7 MV flattening filter-free (FFF) photons.

Results:

The mean PTV and PTV-SIB volumes were 81.0±26.1 cm³ and 12.5±7.3 cm³, respectively. All treatment plans met coverage criteria for prostate PTV and IPL PTV-SIB. Figure 1 shows target dose distributions of 4 different plans. The 40 Gy SIB plan exhibited higher maximum doses in both the prostate and IPL compared to other plans. Dose homogeneity in the prostate worsened with increasing IPL doses. The PTV HI was significantly higher in the 40 Gy SIB plan (0.135±0.007) compared to the lower dose plans (SIB 38.75 Gy plan (0.099±0.007; p= 0.001), SIB 37.5 Gy (0.067±0.008; p < 0.001), and no SIB plan (0.049±0.010; p < 0.001)). However, there were no significant differences in the HI, GI, and CI for the IPL among the three SIB PTVs. OAR doses for the rectum and bladder were similar across all plans, except for the urethra, which was significantly higher D5 in SIB 40 Gy plan compared to no SIB plan (37.7±1.1 Gy vs. 37.0±0.7 Gy; p=0.009) and SIB 37.5 Gy plan (36.9±0.8 Gy; p=0.008). Monitor units did not differ significantly among the four plans.