IMRT plan validation

The lecture encompassed the following topics: Utility of radiographic and radiochromic film dosimetry; Diode and chamber arrays; 3D gel dosimetry; 4D dosimetry; Experimental design for dosimetry; In vivo measurements. and Portal dosimetry. In conclusion, the following pitfalls, potential errors and possible actions are pointed to: (i) Lacking algorithm in the TPS for tongue-and-groove effect. Action: Design and verify a new plan in which the tongue-and-groove effect plays a minor role. Discuss the issue with the TPS manufacturer. (ii) Systematic deviations between TPS calculations and ionisation chamber measurements at the isocentre for plans with many small segments due to uncertainties in the output factor calculation. Action: Rescale the number of MUs. Discuss the issue with the TPS manufacturer. (iii) Large regions with gamma values larger than one during repeated film measurements, while ionisation chamber measurements are correct. Action: Check if the film batch is not expired and if so repeat the measurement with a new batch. (iv) Missing significant errors, e.g., resulting from MLC displacements, due to the limited resolution of the measuring device. Action: Move the device in different directions and repeat the measurement. (v) Missing errors at other parts of the PTV or in OARs by performing only one ionisation chamber measurement or an independent MU calculation at a point. Action: Perform also measurements in a plane for representative clinical cases. (vi) Wrong parameter in the TPS for the definition of leaf position. Action: Understand and verify the definition of leaf position in your TPS. (P.A.)