Significance of different conformity indices for evaluation of radiosurgery treatment plans for vestibular schwannomas

VMAT and step-and-shoot IMRT in head and neck cancer: a comparative plan analysis

PURPOSE

Rotational IMRT is a new technique, whose value still has to be assessed. We evaluated its adequacy for the treatment of head and neck (H&N) cancer compared to the well-established step-and-shoot IMRT.

MATERIALS AND METHODS

A total of 15 patients, who were treated with either IMRT (13 patients) or VMAT (2 patients) in the H&N region, were chosen. For each patient, a treatment plan with the respective other technique was calculated. To compare the resulting dose distributions, the dose-volume histograms (DVHs) were evaluated. To quantify the differences, a new quality index (QI) was introduced, as a measure of the planning target volume (PTV) coverage and homogeneity. A conformity function (CF) was defined to estimate normal tissue sparing.

RESULTS

The QI for VMAT amounts to 36.3, whereas for IMRT the mean value is 66.5, indicating better PTV coverage as well as less overdosage for the rotational technique. While the sparing of organs at risk (OAR) was similar for both techniques, the CF shows a significantly better sparing of healthy tissue for all doses with VMAT treatment.

CONCLUSIONS

VMAT results in dose distributions for H&N patients that are at least comparable with treatments performed with step-and-shoot IMRT. Two new tools to quantify the quality of dose distributions are presented and have proven to be useful.

The relation between various conformity indices and the influence of the target coverage difference in prescription isodose surface on these values in intracranial stereotactic radiosurgery

OBJECTIVE

The purpose of this study was to describe the relation between various frequently used conformity indices (CIs) and to examine the influence of the target coverage (TC) difference in prescription isodose surface (IDS) on these CI values in dynamic conformal arc (DCA) plans.

METHOD

73 plans for simple-shaped brain metastases that were previously characterised for dose distribution with regard to the effect of the target volume (TV) and the depth from the skin surface were reviewed. Three different-definition CI values for each TV were calculated at the 80% IDS, and at D99, D95, D90 and D85, considering the interplanner variability in the TC values for the prescription IDS.

RESULTS

The CI used as the Radiation Therapy Oncology Group criterion showed nearly perfect values at D90. The CI defined in the BrainSCAN (BrainLAB AG, Feldkirchen, Germany) treatment planning system (CI(BS)) denoted lower (superior) values as the TC of the reference IDS decreased. Nakamura's CI (NCI) had lower variability but demonstrated lower (superior) values at D95. NCI showed the most stringent (higher) values at an 80% IDS, but the differences between the plans were less distinct with NCI.

CONCLUSION

The TC difference in IDS chosen for dose prescription or evaluation significantly led to CI value variability in a definition-dependent manner, even when NCI was applied. Definition of the reference IDS at a specific TC value according to clinical situation would reduce the CI value variability to a minimum and would make the CI(BS) sufficient for the objective metric with a perfect value of 1.