A new tool for dose conformity evaluation of radiosurgery treatment plans

Purpose: A novel method for dose conformity evaluation of treatment plans p roduced by the stereotactic radiosurgery treatment planning system is postu lated. Methods and Materials: By consolidating the information contained in the in tegral dose-volume histogram and the treatment volume ratio, a plot of trea tment volume ratio versus percentage dose may be considered as a useful too l for plan evaluation. To validate the suggested argument, two simple exper iments simulating the conformal and nonconformal cases mere conducted on th e geometric phantom that is commercially available from Radionics, An actua l patient treatment plan is also included to explore the effectiveness of t he proposed parameters. It is an attempt to establish the baseline of a con formal plan, Results: A plot showed the ability to give the user an idea whether the siz e of the collimator was adequate to cover the delineated Lesion when the us er-defined criteria had been in place. Two parameters, namely take-off dose (TOD) and take-off volume (TOV) were defined. The former was defined as th e maximum dose level found on the surface of the target volume. The TOD is also the maximum possible dose to be received by the adjacent normal tissue . The latter was defined as the percentage of the target volume that receiv ed the TOD, Another parameter, irradiated percentage volume (IPV), was defi ned here as the percentage of the target volume receiving at least the pres cribed dose. When the prescribed dose is also the TOD, the IPV becomes the TOV, They were proved to be effective in evaluating the dose conformity, An other term known as equivalent fall-off distance (EFOD) was defined as the equivalent radial distance calculated between two isodose lines. In fact, t he dose fall-off rate can also act as a measuring index for plan comparison , because a fast dose fall-off rate is often a requirement for radiosurgery in order to minimize the risk of radiation damage to the surrounding struc tures. The two phantom studies showed consistent results with the theoretic al predictions. The ability of the plot was further explored in the patient treatment plan studies. It was demonstrated that the plot had a remarkable ability to check whether the hot spot is in the vicinity of the lesion. A baseline of a conformal plan was also established; for example, a plan is s aid to be conformal if its IPV has attained a value of not less than 95% an d its associated TVR is not greater than 2, Conclusion: The proposed method has demonstrated the effectiveness in dose conformity evaluation. It supplements the integral dose-volume histogram to provide a complete information of a treatment plan in terms of dose unifor mity and conformity. (C) 1999 Elsevier Science Inc.