# Assessment of Online Adaptive MRI-Guided Stereotactic Body Radiotherapy (SBRT) With an Integrated Boost to the Dominant Lesion in Prostate Cancer

## TL;DR

This study shows that using daily MRI-guided adaptive radiotherapy for prostate cancer improves target coverage and protects nearby organs better than non-adaptive methods.

## Contribution

The study introduces daily adaptive planning with integrated boosts to the dominant lesion in prostate cancer SBRT, demonstrating improved dosimetric outcomes.

## Key findings

- Adaptive planning achieved full GTV, CTV, and PTV coverage in all fractions, while non-adaptive fractions had suboptimal coverage in nearly half of cases.
- Adaptive planning significantly reduced anorectum dose constraint violations compared to non-adaptive methods.
- Bladder sparing was observed in adaptive fractions without exceeding dose constraints.

## Abstract

Purpose/objective

Online adaptive radiotherapy (ART) with daily MR imaging can improve dosimetric accuracy by accounting for anatomical changes throughout the course of treatment. Existing daily ART prostate studies have diverse conclusions, but most of these studies do not include simultaneous integrated boosts (SIBs) to the dominant intraprostatic lesion, which has shown an increase in biochemical disease-free survival. The purpose of this study is to evaluate the potential dosimetric benefits of daily ART with SIB to the dominant intraprostatic lesion and other targets, as well as the potential sparing of organs-at-risk (OARs) for prostate cancer, and to compare these results with those reported in other published studies.

Materials/methods

Thirteen patients with prostate cancer treated with MR-guided stereotactic body radiotherapy (SBRT) were included in this study. The prescribed dose to the planning target volume (PTV) was 36.25 Gy in five fractions. The prescription to the intraprostatic boost to the gross tumor volume (GTV) ranged between 40 and 45 Gy. All SBRT fractions employed daily MR-guided setup, and OARs included the bladder, anorectum, bowel, femoral heads, and penile bulb. Daily MRIs and associated contours were used to create adapted treatment plans. For each simulated adapted fraction, the plan was re-optimized based on the contours from the daily setup MRI. Dosimetric metrics for non-adaptive fractions were simulated by re-calculating the dose on each daily setup MRI. PTV and GTV coverage and OAR constraints were used to compare non-adaptive and adaptive approaches.

Results

PTV coverage at the prescription dose level exceeded 90% in 35 out of 64 non-adapted treatment fractions (55%). Conversely, 29 fractions (45%) had less than 90% coverage, with 12 fractions (19%) showing coverage below 85%. The clinical target volume (CTV) coverage was greater than 95% in 52/64 fractions and was below 90% coverage in only 1/64 fractions (1.5%). GTV coverage at the boost dose level ranged from 55% to 100%, with 38 out of 74 fractions (51%) achieving 90% or greater coverage at the prescribed dose. A substantial number of fractions 27/74 (36%) had coverage below 85%, with a handful of fractions exhibiting less than 70% coverage, 5/74 fractions or 7%. All adapted fractions met prescribed coverage for the GTV, CTV, and PTV. Additionally, OAR constraint violations were evaluated and compared for the bladder and anorectum. Very few OAR constraints were exceeded in the non-adaptive setting, and none were exceeded in the adaptive setting. The anorectum dose constraint was exceeded in 22% of the non-adaptive fractions, while all adaptive fractions met the prescribed constraint. All non-adaptive and adaptive fractions met the prescribed bladder dose constraints; however, the adaptive fractions demonstrated some degree of bladder sparing compared to the non-adaptive fractions. Accumulated dose volume histograms were also generated for each patient to evaluate cumulative differences in target coverage and OAR sparing.

Conclusion

Online adaptive MR-guided SBRT for prostate cancer, utilizing daily plan re-optimization, resulted in improved target conformality, enhanced coverage, and increased OAR sparing compared to non-adaptive SBRT. The primary benefit observed was improved GTV coverage. Daily adaptive planning enables satisfactory GTV coverage without the need for added margins, while also providing better OAR sparing.