Magnetic resonance imaging-guided radiotherapy for intermediate- and high-risk prostate cancer: Trade-off between planning target volume margin and online plan adaption

Abstract

Magnetic resonance-guided radiotherapy with daily plan adaptation for intermediate- and high-risk prostate cancer is time and labor intensive. Fifty adapted plans with 3 mm planning target volume (PTV)-margin were compared with non-adapted plans using 3 or 5 mm margins. Adequate (V95% ≥ 95%) prostate coverage was achieved in 49 fractions with 5 mm PTV without plan adaptation, however, coverage of the seminal vesicles (SV) was insufficient in 15 of 50 fractions. There was no insufficient coverage for prostate and SV using plan adaptation with 3 mm. Hence, daily adaptation is recommended to obtain adequate SV-coverage when using 3 mm PTV.

Keywords: Dosimetry; Interfraction motion; MR-guided radiotherapy; Plan adaptation; Prostate cancer; Seminal vesicles.

Fig. 1

Boxplots for median V95% coverage to prostate and seminal vesicles in compared plans (left). Boxplots for V36.25 Gy to organs at risk in compared plans (right). CTV_PR = clinical target volume of prostate. CTV_SV = clinical target volume of seminal vesicles. * = statiscally significant with p-value < 0.05.

Fig. 2

Locations of implanted fiducial markers in 23 patients need for set-up on a regular linac. Green = clinical target volume. Red = planning target volume.