Statistical motion modelling for robust evaluation of clinically delivered accumulated dose distributions after curative radiotherapy of locally advanced prostate cancer

Publication date: Available online 22 June 2018
Source:Radiotherapy and Oncology
Author(s): Liv B. Hysing, Christian Ekanger, Ándras Zolnay, Svein Inge Helle, Mana Rasi, Ben J.M. Heijmen, Marcin Sikora, Matthias Söhn, Ludvig Paul Muren, Sara Thörnqvist
PurposePlanned doses are used as surrogate for the actually delivered dose in radiotherapy. We have estimated the delivered dose in a dose-escalation trial of locally advanced prostate cancer by statistical dose-accumulation and by DVH-summation, and compared to planned dose.Materials and methodPrescribed dose-escalation to the prostate was 67.5 Gy/25fr., corresponding to 81GyEQD2 assuming α/β = 1.5. The 21 patients had three targets (i.e. CTV67.5 + 2 mm, CTV60 + 5 mm, CTV50 + 10 mm) irradiated by a simultaneous-integrated-boost technique. Analysis was based on 213 CT scans and 5-years of follow-up. For statistical dose-accumulation, we modelled 10000 possible treatment courses based on planned dose and deformation-vector-fields from contour-based registration. For DVH-summation we recalculated dose on repeat-CTs and estimated median D98%/EUD. Groups with/without disease recurrence were compared.ResultsDiscrepancies between planned and accumulated dose were mostly seen for CTV67.5, where under-dosage was found at different locations in the prostate in 12/21 patients. Delivered dose-escalation (D98%) was on average 73.9GyEQD2 (range: 68.3–78.7GyEQD2). No significant difference in accumulated-D98% was found in patients with (n = 8) and without (n = 13) recurrence (p > 0.05). Average D98%/EUD with statistical dose-accumulation vs DVH-summation was significantly different in CTV60, CTV50, rectum and bladder but not in CTV67.5.ConclusionThe planned dose escalation was not received by more than half-of-the patients. Robustness of the prostate target (CTV67.5) should therefore be better prioritized in these patients given the low toxicity profile. Estimates of delivered dose were less conservative for dose-accumulation due to interaction of random organ motion with the dose matrix.



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