Cone-beam CT-based inter-fraction localization errors for tumors in the pelvic region

To evaluate inter-fraction tumor localization errors (TE) in the RapidArc® treatment of pelvic cancers based on CBCT. Appropriate CTV-to PTV margins in a non-IGRT scenario have been proposed.

Data of 928 patients with prostate, gynecological, and rectum/anal canal cancers were retrospectively analyzed to determine systematic and random localization errors. Two protocols were used: daily online IGRT (d-IGRT) and weekly IGRT. The latter consisted in acquiring a CBCT for the first 3 fractions and subsequently once a week. TE for patients who underwent d-IGRT protocol were calculated using either all CBCTs or the first 3.

The systematic (and random) TE in the AP, LL, and SI direction were: for prostate bed 2.7(3.2), 2.3(2.8) and 1.9(2.2) mm; for prostate 4.2(3.1), 2.9(2.8) and 2.3(2.2) mm; for gynecological 3.0(3.6), 2.4(2.7) and 2.3(2.5) mm; for rectum 2.8(2.8), 2.4(2.8) and 2.3(2.5) mm; for anal canal 3.1(3.3), 2.1(2.5) and 2.2(2.7) mm. CTV-to-PTV margins determined from all CBCTs were 14 mm in the AP, 10 mm in the LL and 9-9.5 mm in the SI directions for the prostate and the gynecological groups and 9.5-10.5 mm in AP, 9 mm in LL and 8-10 mm in the SI direction for the prostate bed and the rectum/anal canal groups. If assessed on the basis of the first 3 CBCTs, the calculated CTV-to-PTV margins were slightly larger.

without IGRT, large CTV-to-PTV margins up to 15 mm are required to account for inter-fraction tumor localization errors. Daily IGRT should be used for all hypo-fractionated treatments to reduce margins and avoid increased toxicity to critical organs.

Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB). 2018 Feb [Epub]

Cristina Garibaldi, Cristiana Fodor, Giulia Riva, Damaris Patricia Rojas, Samantha Dicuonzo, Elisa Pace, Giuseppe Fanetti, Paolo De Marco, Veronica Dell'acqua, Giulia Marvaso, Maria Cristina Leonardi, Roberta Lazzari, Federica Cattani, Marta Cremonesi, Roberto Orecchia, Barbara Alicja Jereczek-Fossa

Radiation Research Unit, European Institute of Oncology, Milano, Italy. Electronic address: ., Division of Radiation Oncology, European Institute of Oncology, Milano, Italy., Division of Radiation Oncology, European Institute of Oncology, Milano, Italy; Department of Oncology and Hemato-Oncology, University of Milan, Milano, Italy., Medical Physic Unit, European Institute of Oncology, Milano, Italy., Radiation Research Unit, European Institute of Oncology, Milano, Italy., Scientific Director, European Institute of Oncology, Milano, Italy.

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