DOSIMETRIC COMPARISON OF DIFFERENT ARC TREATMENT PLANS IN PATIENTS WITH BILATERAL SURRENAL METASTASES

OBJECTIVE: A retrospective study was designed for dosimetric comparison of different arc treatment plans to investigate the efficacy and safety of ablative stereotactic body radiotherapy (SBRT) in the treatment of bilateral adrenal gland metastases (AGM). MATERIAL AND METHODS: In 14 patients (12 males, 2 females) with AGM of 84 and Karnofsky performance score ≥70, we optimised single-volumetric arc therapy (S-VMAT) and multiple-volumetric arc therapy (M-VMAT) isocentric arc VMAT for SBRT treatments from October 2006 to January 2017. The maximum and mean organ doses in specific volumes of critical organs were examined. The differences between the monitor unit values were statistically analysed. RESULTS: S-VMAT plans had a better dose conformity index (COI) than M-VMAT plans, with a relative improvement of 4% (p=0.02). Similarly, S-VMAT plans had a significantly higher homogeneity index (HI) than M-VMAT plans, with a relative improvement of 13.8% (p=0.001). S-VMAT plans resulted in better UI and GI compared with M-VMAT, with relative improvements of 8.7% and 6.9% respectively (p=0.02). Mean MU per fraction was 2755.21±388.72 in S-VMAT and 4848.79±491.06 in M-VMAT. The S-VMAT schedules reduced MU per fraction used by 76% (p=0.01). CONCLUSIONS: S-VMAT has significantly protected OARs without compromising target coverage compared to the M-VMAT technique. Although the average time to prepare an S-VMAT plan is approximately four times that of an M-VMAT plan, this plan has demonstrated several inherent advantages, including a shorter treatment duration and fewer monitor units (MU). Consequently, it has reduced the quality of M-VMAT plans and caused an increase in the doses to organs at risk (OARs). Furthermore, as it has also caused an increase in the total number of MUs, the treatment duration has been extended and the number of MUs delivered to the patient has increased.