162. Is Intensity modulation useful in radiosurgery? An analysis of clinical applications of IMRS

IntroductionTreatment modalities based on single fraction delivery demand applying techniques that allow sparing critical tissues and appropriate dose distribution in planning target volume. The most attractive seems to be Intensity? Modulated Radiosurgery (IMRS). IMRT is a treatment modality that uses varying energy fluence across the field to assure better conformity and steeper dose gradients than techniques based solely on geometric field shaping. The indications for its use are still discussed, especially in conjunction with stereotactic radiosurgery.ObjectiveThe aim of our study was to analyze treatment plans in order to find the most frequent situations when intensity modulation was used and, thus, to determine the possible indications for IMRS.Material and MethodsThirty-three treatment plans of IMRS of 220 made for radiosurgery between October 2001 and February 2003 were analyzed.ResultsIMRS was most commonly used when target volume was localized in the vicinity of organs at risk (17 cases −51%). In 7 of these cases the need for intensity modulation was caused both by the necessity of sparing organs at risk and geometric complexity of irradiated volume. In 15 cases (45%), including the mentioned above, complex shape of target volume was the reason of intensity modulation application. In 4 cases (12%) – multiple target volumes (2–4) – in brain metastases and bifocal recurrence of malignant glioma. In 3 cases (9%) – dose escalation within the irradiated volume (boost on gross tumor volume inside the PTV). In two cases the use of intensity modulation was the only way to obtain optimal dose distribution in the target volume of simple shape, located away of organs at riskConclusionsIntensity modulation in radiosurgery is a very attractive option when there is a need for irradiation volumes close to organs at risk. It is also useful when “conventional” conformal techniques cannot assure optimal dose distribution in target volumes of complex shapes. Finally, it is a convenient way to boost gross tumor volume inside the PTV when prescribed dose is delivered in single fraction