An Evaluation of Beam Cath®in the Verification Process for Prostate Cancer Radiotherapy

Comparison of a simple dose-guided intervention technique for prostate radiotherapy with existing anatomical image guidance methods

OBJECTIVES

A simple dose-guided intervention technique for prostate radiotherapy using an isodose overlay method combined with soft-tissue-based corrective couch shifts has been proposed previously. This planning study assesses the potential clinical impact of such a correction strategy.

METHODS

10 patients, each with 8-11 on-treatment CT studies (n=97), were assessed using this technique and compared with no intervention, bony anatomy intervention and soft-tissue intervention methods. Each assessment technique used a 4-mm action level for intervention. Outcomes were evaluated using measures of sensitivity, specificity and dosimetric effect, and compared across intervention techniques. Dosimetric effect was defined as the change in dosimetric coverage by the 95% isodose from the no intervention case of an evaluation construct called the verification target volume.

RESULTS

Bony anatomy, soft tissue and dosimetric overlay-based interventions demonstrated sensitivity of 0.56, 0.73 and 1.00 and specificity of 0.64, 0.20 and 0.66, respectively. A detrimental dosimetric effect was shown in 7% of interventions for each technique, with benefit in 30%, 35% and 55% for bony anatomy, soft tissue and dosimetric overlay techniques, respectively.

CONCLUSION

Used in conjunction with soft-tissue-based corrective couch shifts, the dosimetric overlay technique allows effective filtering out of dosimetrically unnecessary interventions, making it more likely that any intervention made will result in improved target volume coverage.

Accuracy of image-guided radiotherapy of prostate cancer based on the BeamCath® urethral catheter technique

BACKGROUND AND PURPOSE

To examine the accuracy of the BeamCath urethral catheter technique for prostate localization during radiotherapy.

MATERIALS AND METHODS

Sixty-four patients were CT scanned twice with the BeamCath catheter, and once without the catheter. The catheter contains radiopaque fiducial markers for prostate visualization on setup images. It is held in place by a balloon inflated with air in the bladder. The repeated CT scans were co-registered and the relative shifts of the BeamCath isocenter fiducial, the prostate center-of-mass, and external skin markers were evaluated. The displacement of the BeamCath isocenter fiducial relative to its position at the planning CT scan was also determined on setup films for 53 consecutive patients (222 setup films).

RESULTS

The standard deviation (SD) of the prostate movement relative to the BeamCath isocenter fiducial was 0.9 mm in the left-right (LR), 2.8 mm in the cranial-caudal (CC), and 1.6 mm in the anteroposterior (AP) directions, respectively. When the balloon radius differed more than 1mm between the CT scans (14 of 64 cases) the SD in the CC direction increased to 4.5 mm. The SD of the prostate movement relative to the pelvic bone was 0.6 mm (LR), 2.4 mm (CC), and 2.8 mm (AP), while the prostate movement relative to external skin markers was considerably larger. Removal of the catheter resulted in a mean cranial prostate movement of 1.5mm relative to the pelvic bone. Caudal catheter displacements of 7-30 mm were observed in 5% of the setup films. In these cases, recatherization was necessary to obtain reliable prostate localization.

CONCLUSIONS

The BeamCath catheter technique markedly improved prostate localization in all directions when compared with skin markers. In the AP direction, the BeamCath technique was also superior to the use of bony structures. However, in the CC direction the catheter position was very vulnerable to changes in the balloon volume.