Clinical Practice Evolvement for Post-Operative Prostate Cancer Radiotherapy-Part 2: Feasibility of Margin Reduction for Fractionated Radiation Treatment with Advanced Image Guidance

Simple Summary While the definition of the prostate bed clinical target volume (CTV) has been proposed in consensus guidelines, various PTV expansions have been proposed. Depending on the institution, isotropic or non-isotropic expansions on the prostate bed CTV may be considered. This is a preliminary study evaluating PTV margin reduction accounting for inter-fractional uncertainties using high quality iCBCT image guidance. This analysis revealed that 2 mm and 4 mm margins may be considered for margin expansion with and without an endorectal balloon. Purpose: Planning target volume (PTV) expansion for post-prostatectomy radiotherapy is typically >= 5 mm. Recent clinical trials have proved the feasibility of a reduced margin of 2-3 mm for treatments on MRI-linac. We aim to study the minimum PTV margin needed using iterative cone-beam CT (iCBCT) as image guidance on conventional linacs. Materials/Methods: Fourteen patients who received post-prostatectomy irradiation (8 with an endorectal balloon and 6 without a balloon) were included in this study. Treatment was delivered with volumetric modulated radiation therapy (VMAT). Fractional dose delivery was evaluated in 165 treatment fractions. The bladder, rectal wall, femoral heads, and prostate bed clinical tumor volume (CTV) were contoured and verified on daily iCBCT. PTV margins (0 mm, 2 mm, and 4 mm) were evaluated on daily iCBCT. CTV coverage and OAR dose parameters were assessed with each PTV margin. Results: CTV D100% was underdosed with a 0 mm margin in 32% of fractions in comparison with 2 mm (6%) and 4 mm (6%) PTV margin (p <= 0.001). CTV D95% > 95% was met in 93-94% fractions for all PTV expansions. CTV D95% > 95% was achieved in more patients with an endorectal balloon than those without: 0 mm-90/91 (99%) vs. 63/74 (85%); 2 mm-90/91 (99%) vs. 65/75 (87%); 4 mm-90/90 (100%) vs. 63/73 (86%). There was no difference in absolute median change in CTV D95% (0.32%) for 0-, 2-, and 4 mm margins. The maximum dose remained under 108% for 100% (0 mm), 97% (2 mm), and 98% (4 mm) of images. Rectal wall maximum dose remained under 108% for 100% (0 mm), 100% (2 mm), and 98% (4 mm) of images. Conclusions: With high-quality iCBCT image guidance, PTV margin accounting for inter-fractional uncertainties can be safely reduced for post-prostatectomy radiotherapy. For fractionated radiotherapy, an isotropic expansion of 2 mm and 4 mm may be considered for margin expansion with and without the endorectal balloon. Future application for margin reduction needs to be further evaluated and considered with the advent of shorter post-prostatectomy radiation courses.

Automated summary

This study aimed to evaluate the feasibility of reducing the PTV margin using high-quality iCBCT image guidance. The analysis revealed that both 2 mm and 4 mm margins are feasible for patients requiring radiotherapy. For patients requiring an endorectal balloon, a 2 mm or 4 mm margin expansion is also feasible.