Repeated CT examinations in limited volume irradiation of brain tumors: quantitative analysis of individualized (CT-based) treatment plans

A longitudinal magnetic resonance elastography study of murine brain tumors following radiation therapy

An accurate and noninvasive method for assessing treatment response following radiotherapy is needed for both treatment monitoring and planning. Measurement of solid tumor volume alone is not sufficient for reliable early detection of therapeutic response, since changes in physiological and/or biomechanical properties can precede tumor volume change following therapy. In this study, we use magnetic resonance elastography to evaluate the treatment effect after radiotherapy in a murine brain tumor model. Shear modulus was calculated and compared between the delineated tumor region of interest (ROI) and its contralateral, mirrored counterpart. We also compared the shear modulus from both the irradiated and non-irradiated tumor and mirror ROIs longitudinally, sampling four time points spanning 9-19 d post tumor implant. Results showed that the tumor ROI had a lower shear modulus than that of the mirror ROI, independent of radiation. The shear modulus of the tumor ROI decreased over time for both the treated and untreated groups. By contrast, the shear modulus of the mirror ROI appeared to be relatively constant for the treated group, while an increasing trend was observed for the untreated group. The results provide insights into the tumor properties after radiation treatment and demonstrate the potential of using the mechanical properties of the tumor as a biomarker. In future studies, more closely spaced time points will be employed for detailed analysis of the radiation effect.

Does the standardized helmet technique lead to adequate coverage of the cribriform plate? An analysis of current practice with respect to the ICRU 50 report

PURPOSE

To investigate whether the standardized helmet technique adequately covers the cribriform plate.

METHODS AND MATERIALS

For 11 patients with acute leukemia or primary intracerebral neoplasms undergoing irradiation with the standardized helmet technique, three-dimensional isodose distributions were evaluated with special respect to the dose to the cribriform plate and the ocular lenses.

RESULTS

The average dose received by 95% of the cribriform plate with the standardized helmet technique was 85% of the prescribed dose. To enclose the cribriform plate by the 95% isodose (according to the ICRU 50 report) with a 10-mm safety margin allowing for deviations during treatment planning and delivery, the eye block had to be moved in the ventrocaudal direction with an average vector length of 13.6 mm. Consequently, the mean dose received by 5% of the lenses rose from 18% to 91% of the prescribed total dose.

CONCLUSION

Sufficient lens shielding is usually not compatible with safe irradiation of the frontobasis by the standardized helmet technique.

Geometric accuracy of field alignment in fractionated stereotactic conformal radiotherapy of brain tumors

PURPOSE

To assess the accuracy of field alignment in patients undergoing three-dimensional (3D) conformal radiotherapy of brain tumors, and to evaluate the impact on the definition of planning target volume and control procedures.

METHODS AND MATERIALS

Geometric accuracy was analyzed in 20 patients undergoing fractionated stereotactic conformal radiotherapy for brain tumors. Rigid head fixation was achieved by using cast material. Transfer of stereotactic coordinates was performed by an external positioning device. The accuracy during treatment planning was quantitatively assessed by using repeated computed tomography (CT) examinations in treatment position (reproducibility of isocenter). Linear discrepancies were measured between treatment plan and CT examination. In addition, for each patient, a series of 20 verifications were taken in orthogonal projections. Linear discrepancies were measured between first and all subsequent verifications (accuracy during treatment delivery).

RESULTS

For the total group of patients, the distribution of deviations during treatment setup showed mean values between -0.3-1.2 mm, with standard deviations (SD) of 1.3-2.0 mm. During treatment delivery, the distribution of deviations revealed mean values between 0.7-0.8 mm, with SDs of 0.5-0.6 mm, respectively. For all patients, deviations for the transition to the treatment machine were similar to deviations during subsequent treatment delivery, with 95% of all absolute deviations between less than 2.8 and 4.6 mm.

CONCLUSION

Random fluctuations of field displacements during treatment planning and delivery prevail. Therefore, our quantitative data should be considered when prescribing the safety margins of the planning target volume. Repeated CT examination are useful to detect operator errors and large random or systematic deviations before start of treatment. Control procedures during treatment delivery appear to be of limited importance. In addition, our findings should help to determine "cut-off points" for corrective actions in stereotactic conformal radiotherapy of brain tumors.

Accuracy of field alignment in abdominal radiation therapy

PURPOSE

To assess the accuracy of field alignment in a homogeneous group of patients undergoing radiotherapy of the abdomen (adjuvant treatment of the paraaortic region in Stage I testicular seminoma). To evaluate the predictive value of the first verification on field placement errors during subsequent treatment delivery.

METHODS AND MATERIALS

In 45 patients, linear and rotational discrepancies were measured between simulation and first check and between 10 consecutive verification films.

RESULTS

For the total group of patients, the distribution of all deviations showed mean values between 2.3 mm and -2.7 mm with standard deviations of 3.9 mm to 4.7 mm for linear discrepancies, and -0.5 degree to 0.3 degree with standard deviations of 1.2 degrees to 2.1 degrees for rotational discrepancies, respectively. For all patients, deviations for the transition from simulator to the treatment machine were similar to deviations during subsequent treatment delivery, with 95% of all absolute deviations < 10.0 mm and 4 degrees, respectively. When performing correlation analysis between deviations at first check and during treatment delivery, a correlation for lateral displacements and a borderline correlation for caudal displacements could be found. There was no correlation for cranial and rotational displacements.

CONCLUSIONS

Although a trend of deviations for subsequent treatment delivery may be shown at first check, our analysis indicates that the first verification cannot reliably predict inaccuracies during treatment delivery. Random fluctuations of field displacements of up to 1.0 cm prevail. They must be considered when prescribing the safety margins of the planned target volume and determining cutoff points for corrective actions in abdominal radiation therapy.

Accuracy of field alignment in radiotherapy of head and neck cancer utilizing individualized face mask immobilization: a retrospective analysis of clinical practice

Deviations between simulation and first check films were quantitatively assessed for 95 unselected head and neck cancer patients. All measured deviations--calculated on the basis of a total of 190 simulation and 380 verification films--were normally distributed, with mean values of 0-3 mm and standard deviations of 3-5 mm. Of the absolute deviations, 50% and 95% were within 3 mm and 9 mm, respectively. These results should be considered in clinical practice when prescribing safety margins and adequate cut off doses for sparing critical organs in head and neck cancer.