title: What benefit could be derived from on-line adaptive prostate radiotherapy using rectal diameter as a predictor of motion?
abstract: This study investigated a relationship between rectum diameter and prostate motion during treatment with a view to reducing planning target volume (PTV) margins for an adaptive protocol. One hundred and ninety-four cone-beam computed tomography (CBCT) images of 10 patients were used to relate rectum diameter on CBCT to prostate intrafraction displacement. A threshold rectum diameter was used to model the impact of an adaptive PTV margin on rectum and bladder dose. Potential dose escalation with a 6 mm uniform margin adaptive protocol was compared to a PTV margin of 10 mm expansion of the clinical target volume (CTV) except 6 mm posterior. Of 194 fractions, 104 had a maximum rectal diameter of ≤3.5 cm. The prostate displaced ≤4 mm in 102 of those fractions. Changing from a standard to an adaptive PTV margin reduced the volume of rectum receiving 25, 50, 60, and 70 Gy by around 12, 9, 10, and 16%, respectively and bladder by approximately 21, 27, 29, and 35%, respectively. An average dose escalation of 4.2 Gy may be possible with an adaptive prostate radiotherapy protocol. In conclusion, a relationship between the prostate motion and the diameter of the rectum on CBCT potentially could enable daily adaptive radiotherapy which can be implemented from the first fraction.
authors.affiliation: Radiation Therapy Services, Peter MacCallum Cancer Centre, Bendigo, Victoria, Australia ; Medical Imaging and Radiation Sciences, Monash University, Melbourne, Victoria, Australia.
authors.forename: Richard
authors.initials: R
authors.lastname: Oates
authors.affiliation: Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors.forename: Suki
authors.initials: S
authors.lastname: Gill
authors.affiliation: Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia ; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Farshad
authors.initials: F
authors.lastname: Foroudi
authors.affiliation: Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors.forename: Michael Lim
authors.initials: ML
authors.lastname: Joon
authors.affiliation: Medical Imaging and Radiation Sciences, Monash University, Melbourne, Victoria, Australia.
authors.forename: Michal
authors.initials: M
authors.lastname: Schneider
authors.affiliation: Biostatistics and Clinical Trials, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors.forename: Mathias
authors.initials: M
authors.lastname: Bressel
authors.affiliation: Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia ; Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
keywords: Cancer; image-guided; prostate; radiotherapy; rectum