Dosimetric and Clinical Evaluation of Stereotactic MR-Guided Online Adaptive Radiation Therapy (SMART) for Treatment of Adrenal Metastases

Abstract

Stereotactic MR-guided Adaptive Radiotherapy (SMART) is an appealing treatment option for irradiation of intra-abdominal tumors. Daily adaptation with SMART has the advantage of dose escalation while protecting organs at risk (OARs). We aimed to document dosimetric parameters during SMART and analyze local control for adrenal metastases.Fifteen patients treated between September 2018 and October 2020 for adrenal metastases were included in the study. The institutional constraints were used to evaluate the doses to OARs and target volumes. For each fraction, an adaptive plan was generated following registration and delineation of the daily disposition. A comparison was done between predicted plans (original plan on the daily anatomy) and adapted plans (new plan after re-optimization accounting the new contours) in terms of target volumes coverage and OARs doses. Patients were treated every other day.The median age was 63 years (range 36-74 years) and 73.3% of patients were male. The median delivered total dose was 50 Gy (range 35-50 Gy); with a median fraction number of 5 (range 3-5 fractions) and the median fraction dose was 10 Gy (range 7-15 Gy), corresponding to a median BED10 of 100 Gy (range 60-113 Gy). Nine patients presented with a left-sided lesion, five with right-sided and one patient with both left and right-sided lesions. All of the initial plans met target and OARs constraints. Among 70 fractions 56 fractions (80%) were re-optimized. The dosimetric findings for PTV and OARs on predicted and re-optimized fractions were compared. PTV V100 (90.9% vs 94.3%) and PTV D98 (40.8 Gy vs 44.5 Gy) were better with re-optimization and the differences were statistically significant (P < 0.05). The OARs doses for 0.5 cc, 1 cc, 5 cc and 10 cc were compared. The right and left sided lesions were analyzed separately. For the left sided metastases there was OARs dose violation for stomach in one patient and for bowel in one patient. For the right sided metastases there was OARs dose violation for duodenum in two patients and for bowel in two patients. After re-optimization all dosimetric parameters met the institutional constraints. Local control rate was 93.8% with a median follow up time of 11.7 months (range 4.5-26.4 months). One patient with progressive disease underwent adrenalectomy. All treatments were well tolerated. The most common radiation induced acute toxicities were grade 1-2 fatigue and nausea. There were no grade 3 or higher acute or late toxicities experienced during the treatment and follow-up course.The use of re-optimization with SMART improved the PTV coverage and OARs doses for treatment of adrenal metastases. SMART has some advantages when compared to other radiotherapy techniques due to its better visualization of soft tissue, real-time tumor tracking without fiducial markers and potentially reduced toxicity to OARs. These results need to be confirmed with more patients and longer follow-up.