IGRT: Bildgesteuerte Strahlentherapie: Eine Übersicht von der planaren röntgenbasierten Positionierung zum Cone-Beam-CT

Monitoring of tumor oxygenation changes in head-and-neck carcinoma patients breathing a hyperoxic hypercapnic gas mixture with a noninvasive MRI technique

PURPOSE

: To implement and evaluate a noninvasive functional MRI technique for measuring tumor tissue oxygenation changes in head-and-neck carcinoma patients.

PATIENTS AND METHODS

: Tissue oxygenation changes were determined quantitatively in 13 patients with head-and-neck cancer. The MR examinations were performed on a clinical MR scanner at 1.5 T. Different breathing gases (air, 2% CO(2) and 98% O(2), 100% oxygen) were administered to induce oxygenation changes. A multigradient echo sequence was used for quantification of the apparent transverse relaxation time T2*.

RESULTS

: Pixel-by-pixel analysis of the T2* values in tumors showed a shift toward higher values corresponding to oxygenation increase and correlated with a median shift toward positive values in the DeltaT2* fraction under carbogen and oxygen breathing in most but not all patients. A slightly pronounced T2* increase breathing oxygen compared with 2% CO(2)/98% O(2) was found. Furthermore, a statistically significant difference in the heterogeneity of oxygenation changes induced by oxygen or 2% CO(2)/ 98% O(2) breathing was seen.

CONCLUSION

: Measurement of oxygenation changes in head-and-neck tumor patients is feasible by the presented MRI technique. Tumor oxygenation and oxygenation changes were heterogeneous among the investigated patients. To the authors' knowledge, they are the first to describe a statistically significant difference in the heterogeneity of oxygenation changes induced by oxygen or 2% CO(2)/98% O(2) breathing using a noninvasive MRI technique.

Image-guided in vivo dosimetry for quality assurance of IMRT treatment for prostate cancer

PURPOSE

In external beam radiotherapy (EBRT) and especially in intensity-modulated radiotherapy (IMRT), the accuracy of the dose distribution in the patient is of utmost importance. It was investigated whether image guided in vivo dosimetry in the rectum is a reliable method for online dose verification.

METHODS AND MATERIALS

Twenty-one dose measurements were performed with an ionization chamber in the rectum of 7 patients undergoing IMRT for prostate cancer. The position of the probe was determined with cone beam computed tomography (CBCT). The point of measurement was determined relative to the isocenter and relative to an anatomic reference point. The dose deviations relative to the corresponding doses in the treatment plan were calculated. With an offline CT soft-tissue match, patient positioning after ultrasound was verified.

RESULTS

The mean magnitude +/- standard deviation (SD) of patient positioning errors was 3.0 +/- 2.5 mm, 5.1 +/- 4.9 mm, and 4.3 +/- 2.4 mm in the left-right, anteroposterior and craniocaudal direction. The dose deviations in points at corresponding positions relative to the isocenter were -1.4 +/- 4.9% (mean +/- SD). The mean dose deviation at corresponding anatomic positions was 6.5 +/- 21.6%. In the rare event of insufficient patient positioning, dose deviations could be >30% because of the close proximity of the probe and the posterior dose gradient.

CONCLUSIONS

Image-guided dosimetry in the rectum during IMRT of the prostate is a feasible and reliable direct method for dose verification when probe position is effectively controlled.