Planning Target Volume Margins For Fractionated Stereotactic Radiotherapy On PerfexionKeywords: radiotherapy, gamma knife, Fractionated radiosurgery, dose planning, MarginInteractive Manuscript
Ask Questions of this Manuscript:
What is the background behind your study?
In the presence of geometric uncertainties in fractionated stereotactic radiotherapy (SRT), use of planning target volume (PTV) margins is a common mitigating strategy to ensure that the gross tumor volume (GTV) received the prescribed radiation dose.
What is the purpose of your study?
The purpose of this study was to calculate PTV margins for frame-based linac and Perfexion (PFX) SRT using the relocatable head frame (RHF) eXtend™.
Describe your patient group.
Patients with large brain metastases are enrolled on the clinical trial.
Describe what you did.
Patients with large brain metastases are currently undergoing hypofractionated (3 fractions) SRT on PFX enrolled on a phase 1 dose-escalation clinical trial. In prior investigation, the performance of the RHF was quantified in stereotactic coordinates using cone-beam CT (CBCT) in fourteen patients undergoing linac-based SRT (median: 30 treatment fractions). Considering only where the check-tool indicated a positional displacement <1mm (action threshold), standard performance metrics – group mean (m), systematic (S) and random (s) uncertainties – were determined in stereotactic coordinates for both positioning and intra-fraction motion using CBCT. These metrics were input to a published margin-determination formula (2.5*S+0.7*s) that assumes many fractions and linac dose distributions. An additional factor of (s/v3) was added to the systematic component of the margin formula when initially designing the PTV for 3 fractions in PFX-SRT. To more accurately account for PFX dose distributions and only 3 treatment fractions, a population-based stochastic modelling approach is being developed to refine the PTV margin for hypofractionated PFX-SRT.
Describe your main findings.
The RHF positioning performance for linac-SRT was mposition={0.3;-0.5;-0.7}mm, Sposition={0.8;0.9;0.6}mm, and sposition={0.3;0.5;0.4}mm in {Right;Superior;Anterior}. One subject outlier was indentified and subsequently removed from analysis, yielding mposition={0.1,-0.2,-0.6}mm, Sposition={0.2;0.8;0.6}mm, and sposition={0.3;0.6;0.4}mm. For intra-fraction motion, mmotion={-0.1;-0.1;0.0}mm, Smotion={0.2;0.2;0.1}mm, and smotion={0.2;0.4;0.2}mm. The standard margin formula indicated an expansion of {1.0;2.6;1.8}mm and {1.6;3.1;2.3}mm for frame-guided linac (30 fractions) and PFX-SRT (3 fractions), respectively. The mposition of 0.2 and 0.6 mm along inferior and posterior directions indicate an anisotropic margin expansion. For two patients treated to date on PFX, mposition=(0.2mm;-0.9mm;-0.8mm), indicating similar trends to the linac cohort.
Describe the main limitation of this study.
This is a retrospective study.
Describe your main conclusion.
To ensure that the GTV receives the prescription dose, PTV margins have been calculated to account for the geometric uncertainties present in PFX-SRT.
Describe the importance of your findings and how they can be used by others.
The margins will be reviewed as more data are collected, RHF refinements are made, and stochastic-based modelling is used.
In the presence of geometric uncertainties in fractionated stereotactic radiotherapy (SRT), use of planning target volume (PTV) margins is a common mitigating strategy to ensure that the gross tumor volume (GTV) received the prescribed radiation dose.
The purpose of this study was to calculate PTV margins for frame-based linac and Perfexion (PFX) SRT using the relocatable head frame (RHF) eXtend™.
Patients with large brain metastases are enrolled on the clinical trial.
Patients with large brain metastases are currently undergoing hypofractionated (3 fractions) SRT on PFX enrolled on a phase 1 dose-escalation clinical trial. In prior investigation, the performance of the RHF was quantified in stereotactic coordinates using cone-beam CT (CBCT) in fourteen patients undergoing linac-based SRT (median: 30 treatment fractions). Considering only where the check-tool indicated a positional displacement <1mm (action threshold), standard performance metrics – group mean (m), systematic (S) and random (s) uncertainties – were determined in stereotactic coordinates for both positioning and intra-fraction motion using CBCT. These metrics were input to a published margin-determination formula (2.5*S+0.7*s) that assumes many fractions and linac dose distributions. An additional factor of (s/v3) was added to the systematic component of the margin formula when initially designing the PTV for 3 fractions in PFX-SRT. To more accurately account for PFX dose distributions and only 3 treatment fractions, a population-based stochastic modelling approach is being developed to refine the PTV margin for hypofractionated PFX-SRT.
The RHF positioning performance for linac-SRT was mposition={0.3;-0.5;-0.7}mm, Sposition={0.8;0.9;0.6}mm, and sposition={0.3;0.5;0.4}mm in {Right;Superior;Anterior}. One subject outlier was indentified and subsequently removed from analysis, yielding mposition={0.1,-0.2,-0.6}mm, Sposition={0.2;0.8;0.6}mm, and sposition={0.3;0.6;0.4}mm. For intra-fraction motion, mmotion={-0.1;-0.1;0.0}mm, Smotion={0.2;0.2;0.1}mm, and smotion={0.2;0.4;0.2}mm. The standard margin formula indicated an expansion of {1.0;2.6;1.8}mm and {1.6;3.1;2.3}mm for frame-guided linac (30 fractions) and PFX-SRT (3 fractions), respectively. The mposition of 0.2 and 0.6 mm along inferior and posterior directions indicate an anisotropic margin expansion. For two patients treated to date on PFX, mposition=(0.2mm;-0.9mm;-0.8mm), indicating similar trends to the linac cohort.
This is a retrospective study.
To ensure that the GTV receives the prescription dose, PTV margins have been calculated to account for the geometric uncertainties present in PFX-SRT.
The margins will be reviewed as more data are collected, RHF refinements are made, and stochastic-based modelling is used.
Project Roles:
