Quantitative analysis of errors in fractionated stereotactic radiotherapy

Reproducibility and geometric accuracy of the Fixster system during hypofractionated stereotactic radiotherapy

Background

Hypofractionated radiotherapy has been used for the treatment of AVMs and brain metastases. Hypofractionation necessitates the use of a relocatable stereotactic frame that has to be applied on several occasions. The stereotactic frame needs to have a high degree of reproducibility, and patient positioning is crucial to achieve a high accuracy of the treatment.

Methods

In this study we have, by radiological means, evaluated the reproducibility of the isocenter in consecutive treatment sessions using the Fixster frame. Deviations in the X, Y and Z-axis were measured in 10 patients treated with hypofractionated radiotherapy.

Results

The mean deviation in the X-axis was 0.4 mm (range -2.1 – 2.1, median 0.7 mm) and in the Y-axis -0.3 mm (range -1.4 – 0.7, median -0.2 mm). The mean deviation in the Z-axis was -0.6 (range -1.4 – 1.4, median 0.0 mm).

Conclusion

There is a high degree of reproducibility of the isocenter during successive treatment sessions with HCSRT using the Fixster frame for stereotactic targeting. The high reducibility enables a safe treatment using hypofractionated stereotactic radiotherapy.

[Evaluation of positioning reproducibility using a re-locatable head frame for stereotactic radiotherapy]

In fractionated stereotactic radiotherapy (SRT), the accuracy of patient relocation is very important. The Gill-Thomas-Cosman (GTC) re-locatable stereotactic frame is used for patient immobilization. A depth helmet and measuring probe are used to check the stability of the patient's head position relative to the GTC frame. However, displacement error caused by rotation of the patient's head is not considered in the depth-helmet measurement. Consequently, displacement of the isocenter position cannot be confirmed by the measurement obtained from the depth helmet. In this study, we evaluated the precision of reproducibility by comparing measurement values of the depth helmet with the displacement of anatomical position on a CT image. We analyzed 21 setups of 8 patients immobilized for SRT using the GTC frame, between June 2001 and June 2003. The reproducibility of the GTC frame was checked at each treatment by comparing it with the treatment planning position. The average discrepancy of the GTC frame set-up measured by the depth helmet was 0.6 mm, with a standard deviation of 0.3 mm. The result measured by CT was 0.7 mm, with a standard deviation of 0.4 mm. When the error of each measurement point was within 1.0 mm, the accuracy of relocation of the patient could be considered clinically acceptable. Displacement error not considered in the measurement of the depth helmet could be evaluated by using CT.