What benefit could be derived from on-line adaptive prostate radiotherapy using rectal diameter as a predictor of motion?
Richard Oates1, Suki Gill2, Farshad Foroudi3, Michael Lim Joon2, Michal Schneider4, Mathias Bressel5, Tomas Kron6
1 Radiation Therapy Services, Peter MacCallum Cancer Centre, Bendigo; Medical Imaging and Radiation Sciences, Monash University, Melbourne, Australia
2 Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Australia
3 Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Australia
4 Medical Imaging and Radiation Sciences, Monash University, Melbourne, Australia
5 Biostatistics and Clinical Trials, Peter MacCallum Cancer Centre, Melbourne, Australia
6 Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville; Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia
PO Box 126, Bendigo 3552, Victoria
Source of Support: None, Conflict of Interest: None
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.