Abstract:
Background: Adaptive radiotherapy (ART) is defined as all processes leading to the modification of a treatment plan due to treatment response or change in patient contour secondary to weight loss observed during the course of a treatment. Because of the greater conformity of 3-DCRT and IMRT, these changes can have more severe dosimetric impact especially for patients receiving curative radiotherapy over 6–7 weeks because it is based on a single anatomical snapshot acquired during the planning CT scan. This issue can be overcome by applying ART which requires the availability of image-guidance using on-board imaging such as kilo voltage (kV) or megavoltage (MV) cone beam CT (CBCT).
Material/MethodP: Retrospective analysis was done for 70 patients who were treated with curative intent from March, 2015 to May, 2016 at our institute on a newly installed TRILOGY. On board imaging with kv cone beam CT (KV-CBCT) was done approximately three times during the course of treatment. Image registration with original planning CT, contours propagation and evaluation of cumulative doses were carried out for every CBCT. Total 180 CBCT were available for review.
Results: There were 8 cases of Brain, 20 of Head and Neck, 12 of Thorax, 6 of abdomen and 24 of pelvis. Total 68 (37%) adaptive events were observed out of 180 available CBCT, 40 in H&N, 12 in Thorax and 16 in Pelvis. PTV (mostly PTVLN) reduction was done in all adaptive events of H&N cases, because of weight loss and tumor shrinkage. PTV reduction was done in thorax cases because the tumor shrinkage was leading to the increase in overlapping lung volume. In pelvis, major discrepancy was observed because of organ motion and PTV modulation was carried out in 6 adaptive events. No adaptation was done in cases of Abdomen. Minor DVH changes seen for OAR’s, especially cumulative dose to spinal cord and lung were lower after adaptation. Conclusion: Co registration of CBCT to planning CT for ART is helpful for volume modification, required due to anatomical changes and tumor regression during long course radiotherapy. It provides better conformity for tumor and OAR’s which reduces the toxicity especially in head and neck and pelvic malignancies.
