Potential for toxicity reduction using intensity modulated radiation therapy (IMRT) for retroperitoneal sarcoma

Abstract

Purpose/Objective: Radiation therapy for retroperitoneal sarcoma remains challenging because of proximity to surrounding organs at risk(OAR). We report the use of IMRT in the treatment of retroperitoneal sarcomas to minimize dose to OAR while concurrently optimizing tumor dose coverage. Materials/Methods: From January 2000 to October 2002, 10 patients with retroperitoneal sarcoma and 1 with inguinal sarcoma were treated with radiation at Emory Clinic. Average patient age was 58 years. 9 patients were treated preoperatively and 2 postoperatively. Prescription dose varied from 36 Gy at 1.2 Gy BID for a re-irradiation case to 59.4 Gy at 1.5 Gy QD for a postoperative case. The majority of patients received a preoperative dose of 50.4 at 1.8 Gy qd. CT simulation was used in each patient; 3 patients were treated with three-dimensional conformal radiotherapy (3D-CRT) and 8 received MLC-based IMRT. IMRT treatment fields ranged from 8–11 and average volume treated was 3498 cc. For both techniques, the field encompassed gross tumor plus a 5 cm margin in the superior/inferior dimension and a 2 cm margin in the anterior/posterior and medial/lateral dimension; when a boost was used a field reduction with a 2 cm margin was employed. Optimal 3D-CRT plans were generated and compared with IMRT with respect to tumor coverage and OAR dose toxicity. Dose volume histograms (DVH) were compared for both the 3D-CRT and IMRT plans. Results: With IMRT, average dose to 50% of small bowel was 27 Gy with less than 9% of the bowel volume exceeding 50 Gy. Mean dose to both 50% of the left kidney and the entire left kidney was 23 Gy. 50% of the right kidney received 14.6 Gy and mean dose to the entire right kidney was 14.6 Gy. 50% of the liver received 20 Gy and the entire liver received a mean dose of 23 Gy. With 3-D CRT, average dose to 50% of small bowel was 36 Gy and 24% of the bowel volume exceeded 50 Gy. 50% of the left kidney received a mean dose of 24 Gy and mean dose to the entire left kidney was 28 Gy. 50% of the right kidney received 18 Gy and mean dose to the entire right kidney was 15 Gy. 50% of the liver received 22 Gy and the entire liver received a mean dose of 28 Gy. For the same dose constraints assigned to liver, small bowel, kidney, and PTV, IMRT resulted in improved coverage of the PTV and reduced dose to critical organs at risk. Mean dose to small bowel was decreased from 36 Gy with 3D-CRT to 27 Gy using IMRT and tumor coverage as measured by the V95 was increased from 95.3% with conventional to 98.6% using IMRT. Both the maximum and minimum doses delivered to the PTV were significantly increased by 6% and by 22 % respectively (p=.011, p=.055) resulting in better dose distribution within the tumor volume. The volume of small bowel receiving > 30 Gy was significantly decreased from 63.5% to 43.1% with IMRT compared with conventional treatment (p=.043). The volume of left kidney receiving a dose greater than 25 Gy decreased from 49% to 37% with the use of IMRT. All patients were evaluated for toxicity using the RTOG toxicity scale. The most common symptoms were nausea and vomiting and less frequently diarrhea. 7 patients developed grade 2 nausea, 3 developed grade 2 diarrhea and 1 patient with primary groin involvement experienced grade 2 skin toxicity. One patient, who had extensive liver involvement and received conventional treatment developed Grade 3 liver toxicity six months after his radiation and had to be hospitalized for management of ascites. Other than this patient, there have been no other delayed toxicities related to radiation. No GU or wound toxicities were observed and no treatment breaks were necessary. At a median follow up time of 52 weeks, there were no local recurrences and only one patient developed disease progression with distant metastasis occurring in the liver. Conclusions: IMRT for retroperitoneal sarcoma allows for enhanced tumor coverage and better sparing of dose to critical normal structures such as small bowel, liver and kidney. Escalation of dose has been shown to have a positive impact on local control for retroperitoneal sarcoma and IMRT may be an effective method to achieve this goal.