Dose prescription point in forward intensity-modulated radiotherapy of breast and head/neck cancers

In the forward intensity-modulated radiotherapy (FIMRT) technique of treatment planning, the isocenter cannot fulfill the requirements of the International Commission on Radiation Units and Measurements (ICRU) reference point. This study aimed to propose dose prescription points to be used in breast and head/neck cancer patients treated using the FIMRT technique. Two-hundred patients with head/neck (n = 100) and breast (n = 100) cancers were selected. Treatment plans involved using the FIMRT technique. The suggested reference points (SRPs) for dose prescription were placed at selected locations in the lateral neck site and the tangential site in breast and head/neck cancer patients, respectively. Next, doses at the SRPs (DSRPs) were compared to the planning treatment volume (PTV)-equivalent uniform dose (EUD) and mean dose in the PTV (Dmean) extracted from dose–volume histograms. The differences between DSRPs and Dmean for the PTVs were less than 2% in 78 cases and less than 1% in 28 cases. The DSRPs were comparable with the EUD in both types of PTVs; moreover, paired t test analyses indicated that the SRP doses and PTV EUD were statistically similar (p > 0.05). In 116 cases, the discrepancy between DSRP and EUD was less than 2% and less than 1% in 74 cases. The SRPs satisfy the ICRU requirement that the reference point dose should be clinically relevant and representative of the dose throughout the PTV. A consensus on the definition of the reference point is expected to lead to accurate inter-comparison between results from different treatment centers, facilitating the optimization of treatment procedures. © 2018, Japanese Society of Radiological Technology and Japan Society of Medical Physics.