Dosimetric Comparison of Helical TomoTherapy Treatment and Step-and-Shoot Intensity-Modulated Radiotherapy Treatment on Retroperitoneal Sarcoma
Purpose/Objective: To evaluate the feasibility of Helical TomoTherapy for retroperitoneal sarcoma, and to compare the dose uniformity and normal organ sparing between plans of Helical TomoTherapy and step-and-shoot intensity-modulated radiotherapy (IMRT) plans by the NOMOS CORVUS (V5.0) system.
Materials/Methods: Retroperitoneal soft tissue sarcoma constitutes a difficult management problem. Surgical resection is often difficult or impossible, current chemotherapy is not very effective. External radiation therapy requires treatment using large fields for complex shaped volumes. Prescribed dose is limited by tolerance of adjacent organs at risk, including kidneys, spinal cord, liver and small bowel. Seven patients with large retroperitoneal sarcoma recently treated by either Linac based step-and-shoot IMRT (five patients) or Helical Tomotherapy (two patients) were analyzed. Tumor maximum dimension ranged from 8 to 20cm with treatment volume (PTV) varied from 354 cc to 5337 cc. Treatment plans were generated for each case on both Tomotherapy and CORVUS with identical anatomic contouring and prescription. Five (3 patients) or seven (4 patients) coplanar beams were used in the step-and-shoot IMRT planning. The two treatment plans were compared for tumor coverage, dose uniformity and normal organ sparing.
Results: Clinically relevant DVH points were selected for dosimetric comparison, specifically, V30 and V45 for the small bowel, V30 for the liver, V15 and V18 for the kidneys. Maximum spinal cord dose was limited to 45Gy in any plans. In general, helical tomotherapy plans resulted in better tumor volume coverage, dose uniformity and normal organ sparing compared to step-and-shoot IMRT. The average percent of CTV received less than prescription dose was 1.4% for helical tomotherapy compared to 3.8% for IMRT. The average uniformity index was 1.09 (range: 1.05 to 1.16) for helical tomotherapy and 1.25 (range: 1.20 to 1.32) for IMRT. Statistical analysis showed that Tomotherapy plans significantly reduced the dose to small bowel and kidney compared to the step-and-shoot IMRT plans. In Tomotherapy plans, the average ipsilateral kidney receiving greater than 15Gy was reduced to 22% from 56% and the small bowel volume enclosed by the 30Gy isodose surface showed an average reduction of 159.5cc compared to IMRT plans. The average helical Tomotherapy treatment time, excluding the patient setup time, was 12 minutes, slightly less than step-and-shoot IMRT treatment where any single gantry angle had to be divided into multiple fields due to large treatment volumes.
Conclusions: Both step-and-shoot IMRT and helical tomotherapy can be used to deliver full doses to tumor volumes for retroperitoneal sarcoma while significantly sparing organs at risk to reduce acute and late toxicities. Compared to step-and-shoot IMRT plans, helical tomotherapy significantly reduced the dose to small bowel and ipsilateral kidney. Dose escalation to tumor is going to be studied with helical tomotherapy while keeping the dose in the adjacent organs at risk at tolerable level.