Purpose: To determine the value of preoperative adaptive radiotherapy (ART) for soft tissue sarcoma patients (STS) by modeling the dosimetric consequences of tumour volume changes (TVC) using different external beam radiotherapy techniques.
Methods and materials: A subset of 22 STS patients from a recent trial (NCT00188175) underwent a repeat CT scan (CT2) prompted by TVC>1cm during IMRT; 14 tumours grew, 8 shrank. Conformal and conventional plans were modelled in addition to IMRT replicating original criteria from the initial planning dataset (CT1):95% PTV encompassed by 97% prescribed dose. CT1 RT parameters for all plans were applied to CT2 for dosimetric assessment of TVC. Co-registration of CT1 and CT2 permitted comparison of original and new contours.
Results: Mean TVC was 45% for growing and 33% for the shrinking cohort with TVC prompting CT2 at a mean of 13 fractions. For growers, the lack of target coverage on CT2 was statistically significant but was adequate for shrinkers.
Conclusion: GTV expansion of >1cm during RT may result in target underdosage independent of RT technique. ART applied offline for TV increases >1cm is a practical adaptive strategy to ensure tumour coverage during RT. TV shrinkage may allow for normal tissue sparing, which should be investigated prospectively.
Keywords: Adaptive; Preoperative; Radiotherapy; Sarcoma.
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