Evaluation of online dose-guided adaptive fractionation for intact prostate radiotherapy

Dose-guided adaptive fractionation (DGAF) was proposed as a simpler alternative of adaptive radiation therapy, using organ variation simulation for proof of concept. However, its dosimetric benefit using actual daily anatomy changes in prostate cancer patients remains unclear.

The purpose of this study was to retrospectively evaluate the dosimetric benefit and potential risk of DGAF for prostate cancer patients treated with moderately hypofractionated radiotherapy and stereotactic body radiotherapy using daily cone-beam CT.

Patients assessed in this study were treated with moderately hypofractionated radiotherapy (MHRT, n = 11, 244 total fractions) and stereotactic body radiotherapy (SBRT, n = 11, 55 total fractions). The optimizer algorithm computed the desired fractional dose based on favorable or unfavorable anatomical conditions derived from daily imaging, quantified by the sparing factor threshold (μ) and spread (σ). Three methods of estimating subsequent fractionation schemes were explored. The assessment relative to patient-specific probabilities observed post-treatment includes the cohort-based probability (CBP) approach, the individual-based probability (IBP) approach, and the hybrid approach that begins as CBP and transitions to the IBP after a certain number of fractions. The benefits/risks of DGAF were assessed by calculating the equivalent dose in 2 Gy fractions (EQD2) for both the CTV and the rectum in comparison to regular fractionation.

DGAF demonstrated potential dosimetric benefits for intact prostate patients. The average and standard deviation of the sparing factor was 0.51 ± 0.11 for MHRT patients and 0.42 ± 0.04 for SBRT patients treated with endorectal-balloon. The hybrid approach for estimating the patients' threshold μ and spread σ outperformed other approaches assessed for both cohorts. The highest dosimetric benefit was retrospectively seen in MHRT patients where a hybrid approach were used, showing equal or decreased D15% EQD2.

DGAF demonstrated considerable rectum sparing potential, and the benefits increased with longer fractionation and advanced approaches toward subsequent fraction estimation.