Session Item

In December 2020 a new Image Guided Adaptive RT protocol was introduced at our department for patients with prostate and nodal radiotherapy. This protocol consists of a daily online grey value registration on the prostate to correct for translational shifts of the prostate and an offline adaptive strategy to correct for systematic residual Prostate Rotations (R_PROS) and residual Lymph Nodes Translations (T_LN) and Rotations (R_LN). If required, a synthetic CT (sCT) is created for replanning, using compactly supported radial basis functions to deform the planning CT (pCT) based on the average of the rigid registrations of the prostate as well the bony anatomy of the first 4 fractions (Fx) as explained in figure 1. Using this strategy, the CTV to PTV margins could be reduced from 10 to 5 mm for the prostate and from 10 to 8 mm for the lymph nodes. The aim of this study was to evaluate the first clinical results of this IGART protocol and the reduced margins. 

The first 47 patients treated on prostate and LN with 35x2 Gy using the new IGART protocol were evaluated. The number of patients requiring replanning on sCT, timing of replanning and the required number of interventions according to our decision support protocol for anatomical changes, was scored. The coverage of the prostate and LN by the PTV was assessed by a retrospective review of 1610 CBCTs. For both patients groups, with replanning (ART) and no replanning (No ART), the systematic residual R_PROS, T_LN and R_LN were calculated for the original plan and for the adaptive plan. These residuals were statistically compared with a homogeneity of variances test. For the no ART group this was calculated for Fx 1-6 and 7-35 to simulate plan adaptation according to protocol.

In 19/47 (40.4%) patients replanning on sCT was required. In 12 patients the adaptive plan was started on Fx 7 according to protocol. In 2 patients the ART procedure was delayed to Fx 9 and 10; in 5 patients it was repeated at Fx 10 (2), 14, 18 and 30 due to significant changes in anatomy later in the treatment course. The prostate or LN were outside of the PTV in only 0.9% and 1.1% of the 1610 CBCTs reviewed, demonstrating an excellent coverage. In 1.7% of the Fx an intervention (extra hydration/toilet/re set up) was performed to improve PTV coverage due to differences in bladder preparation, rectal fill or set up. Table 1 shows that the systematic residual R_PROS and T_LN/R_LN are reduced after plan adaptation for the ART group, which is mainly significant for the LN in the AP axis and around the LR-axis (p<0.05).

The new IGART protocol for prostate and LN radiotherapy patients was implemented successfully with an acceptable workload and has led to a reduction of the residual R_PROS and T_LN/R_LN. This is however dependent of the chosen intervention thresholds. With this protocol, a significant reduction of the margin could be implemented, while still maintaining excellent coverage of the target and a low number of interventions.