First experiences with a noninvasive patient set-up system for radiation therapy of the prostate

Purpose: Highly conformal radiotherapy techniques require precise patient p ositioning. We report our first experience with a new cast system for fixat ion of the pelvis during stereotactically guided intensity modulated radiot herapy (IMRT) of the prostate with respect to positioning accuracy of the p rostate. Material and Methods: The immobilization device consists of a custom-made w rap-around body cast that extends from the abdomen to the thighs and a sepa rate head mask, both made from Scotchcast(R), and attaches to a frame for e xtracranial stereotaxy. Sixteen CT-studies (greater than or equal to 25 sli ces, thickness: 3 mm) of 2 patients who were immobilized for IMRT of prosta te tumors were evaluated with respect to set-up accuracy of bony structures and the prostate itself. CT-studies were performed immediately before or a fter a treatment fraction. Deviations of bony landmarks and anatomical land marks inside the planning target volume were measured in all 3 dimensions. Results: Mean patient movements of 0.15 +/- 0.3 mm (latero-lateral), 0.9 +/ - 1 mm (anterior-posterior), 1 +/- 1 mm (transversal vectorial error) and < 3 mm slice thickness (craniocaudal) were recorded using bony landmarks and 0.9 +/- 0.9 mm (latero-lateral), 1.8 +/- 1.5 mm (anterior-postserior), 2.2 +/- 1.5 mm (transversal vectorial error) and < 3 mm (craniocaudal) using t he confines of, or landmarks within the prostate. Standard deviations of ab solute positioning, error as an often used metric for positioning accuracy ranged between 0.3 and 1.7 mm in the transversal plane. The worst case tran sversal vectorial deviation for the prostate was 4.4 mm. Figure 3 summarize s the set-up, accuracy of bony landmarks and the prostate. Conclusion: The presented combination of a body cast and head mask system i n a rigid stereotactic body frame ensures reliable noninvasive patient fixa tion for fractionated extracranial stereotactic radiotherapy. It provides p recise and reliable positioning of the prostate and meets the requirements for highly conformal radiotherapy such as IMRT. No further improvement of r epositioning can be achieved with external immobilization devices since the positioning error of the target relative to the skeleton exceeds the accur acy of the positioning of the skeleton itself.