Point dose verification for intensity modulated radiosurgery using Clarkson's method

In clinical radiation physics chart checking, the dose calculation results generated by computer treatment planning software are usually verified by an independent computerized monitor unit calculation routine, or by "hand calculation" using percent depth dose (PDD), tissue phantom ratio (TPR), scatter factors, and the machine calibration factors. For intensity-modulated radiosurgery (IMRS) or intensity-modulated radiation therapy (IMRT), the "hand calculation" becomes not feasible due to the sophisticated multileaf collimator (MLC) segments created for intensity-modulated dose delivery. Therefore, an independent computerized dose calculation routine is needed for fast and reliable dose verification. In this work, a point dose calculation routine for IMRS/IMRT plan verification is developed by directly applying Clarkson's method. The method includes preparing data table by measuring TPRs for circular fields with diameters ranging 6 to 98 mm, extrapolating TPR for the zero field size (TPR0) from measured data and generating scatter phantom ratio (SPR) for each individual circular field. The segmented MLC sequences created by IMRS/IMRT inverse planning are converted into irregular fields for Clarkson's calculation. This method has been tested using 29 IMRS/IMRT cases. The results indicate that it is reliable, fast, and accurate. The average time to calculate one field is about 2 s with a 300 Mhz CPU.