ICRU reference dose in an era of intensity-modulated radiation therapy clinical trials: correlation with planning target volume mean dose and suitability for intensity-modulated radiation therapy dose prescription.

Background And Purpose: IMRT clinical trials lack dose prescription and specification standards similar to ICRU standards for two- and three-dimensional external beam planning. In this study, we analyzed dose distributions for patients whose treatment plans incorporated IMRT, and compared the dose determined at the ICRU reference point to the PTV doses determined from dose-volume histograms. Additionally, we evaluated if ICRU reference type single-point dose prescriptions are suitable for IMRT dose prescriptions.

Materials And Methods: For this study, IMRT plans of 117 patients treated at our institution were randomly selected and analyzed. The treatment plans were clinically applied to the following disease sites: abdominal (11), anal (10), brain (11), gynecological (15), head and neck (25), lung (15), male pelvis (10) and prostate (20). The ICRU reference point was located in each treatment plan following ICRU Report 50 guidelines. The reference point was placed in the central part of the PTV and at or near the isocenter. In each case, the dose was calculated and recorded to this point. For each patient--volume and dose (PTV, PTV mean, median and modal) information was extracted from the planned dose-volume histogram.

Results: The ICRU reference dose vs PTV mean dose relationship in IMRT exhibited a weak positive association (Pearson correlation coefficient 0.63). In approximately 65% of the cases studied, dose at the ICRU reference point was greater than the corresponding PTV mean dose. The dose difference between ICRU reference and PTV mean doses was 2% in approximately 79% of the cases studied (average 1.21% (+/-1.55), range -4% to +4%). Paired t-test analyses showed that the ICRU reference doses and PTV median doses were statistically similar (p=0.42). The magnitude of PTV did not influence the difference between ICRU reference and PTV mean doses.

Conclusions: The general relationship between ICRU reference and PTV mean doses in IMRT is similar to that in 3D CRT distributions. Point doses in IMRT are influenced by the degree of intensity modulation as well as calculation grid size utilized. Although the ICRU reference point type prescriptions conceptually may be extended for IMRT dose prescriptions and used as a representative of tumor dose, new universally acceptable dose prescription and specification standards for IMRT based on RTOG IMRT prescription model incorporating dose-volume specification would likely lead to greater consistency among treatment centers.