A. Méndez Romero

Stereotactic body radiation therapy for colorectal liver metastases

Stereotactic body radiation therapy (SBRT) is a treatment option for colorectal liver metastases. Local control, patient survival and toxicity were assessed in an experience of SBRT for colorectal liver metastases.

Treatment precision of image-guided liver SBRT using implanted fiducial markers depends on marker?tumour distance

The purpose of this study is to assess the accuracy of day-to-day predictions of liver tumour position using implanted gold markers as surrogates and to compare the method with alternative set-up strategies, i.e. no correction, vertebrae and 3D diaphragm-based set-up. Twenty patients undergoing stereotactic body radiation therapy (SBRT) with abdominal compression for primary or metastatic liver cancer were analysed. We determined the day-to-day correlation between gold marker and tumour positions in contrast-enhanced CT scans acquired at treatment preparation and before each treatment session. The influence of marker-tumour distance on the accuracy of prediction was estimated by introducing a method extension of the set-up error paradigm. The distance between gold markers and the centre of the tumour varied between 5 and 96 mm. Marker-guidance was superior to guiding treatment using other surrogates, although both the random and systematic components of the prediction error SD depended on the tumour-marker distance. For a marker-tumour distance of 4 cm, we observed σ = 1.3 mm and Σ = 1.6 mm. The 3D position of the diaphragm dome was the second best predictor. In conclusion, the tumour position can be predicted accurately using implanted markers, but marker-guided set-up accuracy decreases with increasing distance between implanted markers and the tumour.

SU‐E‐T‐630: Adaptive Liver SBRT: Daily Re‐Planning to Compensate for Non‐Rigid Anatomy Changes Improves Dose Distributions

Purpose: In stereotactic body radiotherapy(SBRT) of livertumors, daily image guidance based on contrast enhanced CT‐scans enables precise delivery of high tumordoses with small margins. However non‐rigid patient anatomy changes could still cause OAR dose constraint violations. In this study we evaluated the impact of daily re‐planning based on contrast‐ enhanced CT‐scans to compensate for non‐rigid anatomy changes. Methods: For eight patients with twelve tumors in different liver regions, three planning strategies were simulated. In all strategies, the patient was shifted each fraction to correct for the tumor displacement derived from registration of the daily CT‐scan. In the first strategy, the patient is irradiated with the non‐coplanar treatment plan that was based on the planning CT‐scan. In the second strategy the fluence profiles of the treatment beams were re‐ optimized based on the daily CT scan (beam angles remained the same as for the planning scan). In the last strategy, for each fraction a completely new plan (beam angles and profiles) was generated based on the daily CT‐ scan. Non‐rigid registration and LQ‐corrections were used to obtain cumulative dose distributions. Results: In 50% of the patients, constraint violations occurred for the repeated plan when we look at the dose distributions of the single fractions. For 33% of those patients, severe constraint violations in a single fraction even resulted in constraint violations in the integral dose distribution. In those cases, the beamweight plan and adaptive plan were easily capable of taking those OAR deformations into account and both methods designed good treatment plans without constraint violations. The adaptive plan performed slightly better than the beamweight plan, but the latter requires only some minutes computation time while full re‐optimization would take a few hours. Conclusions: Adaptive re‐optimization of fluence profiles based on daily CT scans can significantly improve dose distributions in liverSBRT.

WE‐D‐351‐07: Daily CT Guidance in Liver Stereotactic Body Radiation Therapy; Impact of a Non‐Rigid Patient Anatomy On OAR Dose

Purpose: To investigate the influence of daily tumor‐based setup corrections, derived from pre‐treatment CT‐scans, on the organs‐at‐risk (OARs) dose distributions in stereotactic body radiation therapy(SBRT) of livertumors.Method and Materials: Fifteen patients diagnosed with liver metastases and treated with SBRT were included in this study. Patients were positioned in a stereotactic body frame and abdominal compression was applied to decrease the respiratory tumor displacement. A total dose of 37.5 Gy at the 65% isodose was delivered in three fractions. Sixty CT data sets, corresponding to the planning and consecutive treatment days, were reviewed. Relevant OARs were delineated on all CT sets. Daily 3D dose distributions were calculated using the daily CT sets, both without and with taking the clinically applied setup correction into account. Dose‐volume histograms and relevant dosimetric parameters for the PTV and all OARs were calculated. Results: Large shape and volume variations were seen for OARs (especially the oesophagus, duodenum and stomach), causing large variations in the dose to these organs. In 27% of the treatment fractions, it was seen that after setup correction the dose to an OAR exceeded the constraints, while the planning was within constraints; in one case, the constraint was exceeded by a factor of two. Setup correction yielded a significant increase in PTV coverage, but for the OARs no significant dosimetric effect was seen. Conclusion: Daily tumor‐based CT‐guidance is effective to increase PTV coverage, but does not significantly affect dose to the OARs. This is due to their non‐rigid motion and volume variations. Both may cause substantial OAR constraint violations, even after set‐up correction. However, for most fractions the dose to the OARs is within constraints. To obtain optimal sparing of OARs, adaptive treatment explicitly accounting for non‐rigid anatomy may be required, especially when the tumordose is escalated.