title: Under conditions of large geometric miss, tumor control probability can be higher for static gantry intensity-modulated radiation therapy compared to volume-modulated arc therapy for prostate cancer.
abstract: The purpose of this work was to compare static gantry intensity-modulated radiation therapy (IMRT) with volume-modulated arc therapy (VMAT) in terms of tumor control probability (TCP) under scenarios involving large geometric misses, i.e., those beyond what are accounted for when margin expansion is determined. Using a planning approach typical for these treatments, a linear-quadratic-based model for TCP was used to compare mean TCP values for a population of patients who experiences a geometric miss (i.e., systematic and random shifts of the clinical target volume within the planning target dose distribution). A Monte Carlo approach was used to account for the different biological sensitivities of a population of patients. Interestingly, for errors consisting of coplanar systematic target volume offsets and three-dimensional random offsets, static gantry IMRT appears to offer an advantage over VMAT in that larger shift errors are tolerated for the same mean TCP. For example, under the conditions simulated, erroneous systematic shifts of 15mm directly between or directly into static gantry IMRT fields result in mean TCP values between 96% and 98%, whereas the same errors on VMAT plans result in mean TCP values between 45% and 74%. Random geometric shifts of the target volume were characterized using normal distributions in each Cartesian dimension. When the standard deviations were doubled from those values assumed in the derivation of the treatment margins, our model showed a 7% drop in mean TCP for the static gantry IMRT plans but a 20% drop in TCP for the VMAT plans. Although adding a margin for error to a clinical target volume is perhaps the best approach to account for expected geometric misses, this work suggests that static gantry IMRT may offer a treatment that is more tolerant to geometric miss errors than VMAT.
authors.affiliation: Medical Physics Department, Tom Baker Cancer Centre, Calgary, Alberta; Jack Ady Cancer Centre, Lethbridge, Alberta. Electronic address: michael.balderson@rmp.uhn.ca.
authors.forename: Michael
authors.initials: M
authors.lastname: Balderson
authors.affiliation: Medical Physics Department, Tom Baker Cancer Centre, Calgary, Alberta; Jack Ady Cancer Centre, Lethbridge, Alberta.
authors.forename: Derek
authors.initials: D
authors.lastname: Brown
authors.affiliation: Medical Physics Department, Tom Baker Cancer Centre, Calgary, Alberta; Jack Ady Cancer Centre, Lethbridge, Alberta.
authors.forename: Patricia
authors.initials: P
authors.lastname: Johnson
authors.affiliation: Medical Physics Department, Tom Baker Cancer Centre, Calgary, Alberta; Jack Ady Cancer Centre, Lethbridge, Alberta.
authors.forename: Charles
authors.initials: C
authors.lastname: Kirkby
mesh_terms: D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D011336:Probability; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D047368:Tumor Burden
keywords: Modeling; Monte Carlo; Prostate cancer; Tumor control probability