The impact of margin reduction on radiation dose distribution of ultra-hypofractionated prostate radiotherapy utilizing a 1.5-T MR-Linac

Abstract

Background: We examined the effects of reducing the planning target volume (PTV) margin in MR-guided radiotherapy (MRgRT) on the distribution of radiation dose to target volumes and organs-at-risk (OARs). Thus, we compared MR-Linac (MRL) plans with and without reduced margin and intensity-modulated radiotherapy (IMRT) plan with conventional linac for low-risk prostate cancer patients receiving 36.25 Gy in five fractions of ultra-hypofractionated radiation therapy.

Materials and methods: Twenty low-risk prostate cancer patients treated with 1.5 T MR-Linac were evaluated. The same planning CT images were used for four plans: the MRL-R plan with reduced margin planning target volume (PTV-R) and the MRL-N plan with normal margin PTV (PTV-N), which is also used for IMRT plan. In four plans, PTV doses, organs-at-risk (OARs) doses, the homogeneity index (HI), and monitor units were compared.

Results: All plans met the criteria for PTV coverage and OARs dose constraints. The maximum and mean PTV doses were significantly higher in the MRL-R and MRL-N plans compared to the IMRT plan. The HI was lowest in the IMRT plan (0.040 ± 0.013) and highest in the MRL-N plan (0.055 ± 0.012; p < 0.001). There was no significant difference in the PTV dosimetric parameters between the MRL-R and the MRL-N plans. The high doses in the rectum was significantly lower in the MRL-R compared to other plans. The bladder V36.25 Gy was significantly lower in the MRL-R plan (2.43 ± 1.87 Gy) compared to MRL-N (4.50 ± 2.42 Gy; p < 0.001), and IMRT plans (4.76 ± 2.77 Gy; p < 0.001). There was no significant difference in the low-dose volumes of the body, maximum femur doses, or monitor units across each plan.

Conclusions: Ultra-hypofractionated MR-guided RT with 1.5 T MRL is dosimetrically feasible for patients with prostate cancer. The improved soft tissue contrast and the online adaptive plan for 1.5 T MR-Linac allows for PTV margin reduction resulted in a significant dose reduction in OARs.

Keywords: MR-linac; dosimetry; prostate cancer; radiotherapy; stereotactic body radiotherapy.

FIGURE 1

Prostate (green line), PTV with reduced margin (yellow line) and PTV with standard margins demonstrated in (a) axial, (b) sagittal, (c) coronal CT slices, and (d) beams‐eye‐view reconstruction.

FIGURE 2

Representative axial CT slices showing 95% of prescribed dose distributions (green area) for the (a) MR‐Linac plan with reduced margin (MRL‐R), (b) MR‐Linac plan with normal margin (MRL‐N), and (c) intensity‐modulated radiotherapy plan and 50% of prescribed dose distributions (blue area) for the (d–f) same consecutive plans (red area represents 95% isodose area).

FIGURE 3

Box and whisker plot demonstrating rectum and bladder doses according to the dose volume parameters across each plan.

FIGURE 4

Box and whisker plot demonstrating rectum, bladder and femur doses received by 1 cc volume and mean doses measured in four different plans.