Session Item

To investigate two different methods for predicting minimum non-isotropic and asymmetric (NI-AS) treatment margins required for taking into account prostate intrafraction motion occurring during moderate hypofractionated treatments.

Prostate intrafraction 3D translations were recorded using Clarity transperineal ultrasound probe (TP-US) (Elekta AB, Stockholm) in 46 prostate cancer patients (876 sessions) treated by moderate hypofractionated radiotherapy at our institution. All patients underwent volumetric modulated arc therapy. The prescribed dose was 60 Gy in 20 fractions to the Clinical-Target-Volume (CTV) prostate. The treatment goal was that 100% of the prescribed dose must cover 99% of the CTV-prostate. For 18 patients (346 sessions) randomly selected from the cohort, treatment plans were recomputed increasing CTV-to-PTV margins from 0 to 6mm with an auto-planning optimization algorithm. Then, the voxel shifting method (VSM) was used to move the CTV-prostate structure every five seconds of treatment, according to the movements retrieved by the TP-US, and to calculate time-dependent margins. The obtained results were compared to those obtained using van Herk’s margin formula.

Mean intra-fraction prostate displacements observed were -0.02±0.52mm, 0.27±0.78mm and -0.43±1.06mm in left-right (LR), supero-infero (SI) and antero-posterior (AP) directions, respectively. On average, the largest displacements were observed in inferior and posterior directions. The CTV dosimetric coverage increased with increasing CTV-to-PTV margins and decreased with time. Using van Herk’s formula, after 7min of treatment, a margin of 0.4 and 0.5mm was needed in LR, 0.7 and 1.5mm in SI, 1.1 and 3.2mm in AP directions, respectively, for considering prostate intrafraction motion. On the other hand, margins of 0mm in LR direction, 2mm in superior direction, 3mm in inferior and anterior directions, and 5mm in posterior direction were obtained with the VSM. When applying the intrafraction shifts on the PTVs generated with the VSM margins, the impact of motion on the CTV coverage was the same as using 5mm homogeneous margin. Finally, after 7min of treatment, intrafraction motion triggered an increased in the average dose (D_mean) of 2 and 1.5Gy to the rectum and bladder walls, respectively, with respect to the initial treatment plan, using a 5mm CTV-to-PTV margin. Conversely, a reduction in rectum and bladder walls D_mean of 2Gy was reported using the margins obtained with the VSM.

Prostate movements impact dose distribution and target coverage. NI-AS margins would be required to optimally take into account intrafraction motion and spare organs-at-risk.