Background and purpose: The purpose of this study was to investigate the effect of CT-based delineation and planning on the irradiated boost volume. For this specific purpose we used the data as derived from 2 prospective phase III randomised trials. Patients and methods: Data from 1331 patients (≤50 years) were analyzed with a reported boost volume from a simulation-based treatment plan (EORTC boost vs no boost trial, n = 922), and a CT-scan-based treatment plan (Young Boost Trial, n = 409) group. Tumour diameter, irradiation technique (photons vs electrons), lumpectomy size, and age were used as covariates. Results: Median V95%in the conventional simulation-based treatment plans was 99 cc (range 9-628) for photons and was 98 cc (13-651) for electrons, whereas in the CT-planned patients, these figures were 178 cc (37-2699) and 150 cc (43-1272), respectively. Multivariable analysis showed an association of the irradiated boost volume with tumour size (p < 0.0067), lumpectomy size (p < 0.0002), and boost technique (p < 0.0004). The use of a CT-scan for volume delineation and treatment planning remained significant (p < 0.0001). Conclusions: The use of a CT-scan for delineation and treatment planning led to a significant increase of the irradiated boost volume by a factor of 1.5-1.8, compared to conventional simulator-based plans.

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Radiotherapy & Oncology
Erasmus MC: University Medical Center Rotterdam