Basic Study
Copyright ©The Author(s) 2016. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Radiol. Jan 28, 2016; 8(1): 73-81
Published online Jan 28, 2016. doi: 10.4329/wjr.v8.i1.73
Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery
Grigor N Grigorov, James CL Chow
Grigor N Grigorov, Medical Physics Department, Grand River Regional Cancer Center, Kitchener, ON N2G 1G3, Canada
James CL Chow, Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, ON M5G 2M9, Canada
James CL Chow, Department of Radiation Oncology, University of Toronto, Toronto, ON M5G 2M9, Canada
Author contributions: Grigorov GN did the experimental work and collected the data which analysed by Grigorov GN and Chow JCL; The text and figures of the manuscript were prepared by Grigorov GN and edited by Chow JCL.
Institutional review board statement: This is not a patient-related manuscript.
Institutional animal care and use committee statement: No animal is involved in this study.
Conflict-of-interest statement: No potential conflicts of interest relevant to this article were reported.
Data sharing statement: No additional data are available.
Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/
Correspondence to: James CL Chow, PhD, Radiation Medicine Program, Princess Margaret Cancer Center, 610 University Avenue, Toronto, ON M5G 2M9, Canada. james.chow@rmp.uhn.on.ca
Telephone: +1-416-9464501 Fax: +1-416-9466566
Received: April 29, 2015
Peer-review started: April 30, 2015
First decision: October 27, 2015
Revised: November 5, 2015
Accepted: December 3, 2015
Article in press: December 4, 2015
Published online: January 28, 2016
Processing time: 271 Days and 18.6 Hours
Abstract

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT).

METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle3 treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0o with equal separation of 72o, was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations.

RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams.

CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.

Keywords: Multileaf collimator leakage; Overshoot effect; Beam penumbra; Prostate intensity modulated radiation therapy planning

Core tip: This manuscript contains discussion of the effect of inter-leaf leakage in multileaf collimator (MLC)-based step-and-shoot intensity-modulated radiotherapy (IMRT) of cancer, and presents a method to measure and correct for this effect. Based on our experience, and as illustrated with a single IMRT plan, we recommend adjusting the width of treatment field for different segments delivered in the step-and-shoot fashion (to reduce overlap effects), and possibly, to include leakage as optimization parameter in treatment planning. In this study, we introduced a novel empirical model for determining the 2-dimensional distribution of dose resulting from transmission through the MLC.