Development of intensity modulated radiation therapy (IMRT) based on radiobiology

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A method of quality assurance in IMRT at our hospital was established. The measured dose distribution in phantoms were consistent with the calculated dose distribution within a difference of 2-4%. We measured the intrafraction and interfraction organ motions and set-up errors for patients with head and neck cancer. Based on the measured systematic and random errors, a planning target volume (PTV) margin of 5 mm was appropriate for our IMRT system.It took 15-20 min for a single fraction of IMRT, which may cause a dose-rate effect or sublethal damage repair during a farction. Cell survival curves of SCCVII tumors and CHO cells irradiated with IMRT and conventional beams were compared. Below the range of 10 Gy, no significant difference in cell survival curves were detected.IMRT using the simultaneous integrated boost (SIB) method was designed for treating malignant gliomas. IMRT delivered 70 Gy/28 fractions/daily 2.5 Gy to the gross tumor volume (GTV) and 56 Gy/28 fractions/daily 2.0 Gy to the surrounding edema defined as the clinical target volume annulus (CTV-a). Although this treatment was feasible both physically and clinically, the local control was not improved.The patterns of local-regional recurrence in patients with head and neck cancer treated by parotid-sparing IMRT were analyzed. The mean doses to the contra- and ipsi-lateral parotid glands were 24.0 Gy and 30.3 Gy, respectively Among the 33 patients, 19 patients (58%) showed only grade-0,1 xerostomia. Thus, parotid-sparing IMRT could reduce the incidence of xerostomia. However, some recurrences were noted at the margin of PTV. PTV delineation in IMRT should be cautious enough.