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Updated: Jul 6, 2025
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Published on: February 6, 2019
Optimal planning target margin for prostate radiotherapy based on interfractional and intrafractional variability
Jina Kim1, Jiwon Sung1, Seo Jin Lee1
1Department of Radiation Oncology, Gangnam Severance Hospital, Yonsei University College of Medicine, Seoul, Republic of Korea.
Introduction:
We analyzed daily pre-treatment- (PRE) and real-time motion monitoring- (MM) MRI scans of patients receiving definitive prostate radiotherapy (RT) with 1.5 T MRI guidance to assess interfractional and intrafractional variability of the prostate and suggest optimal planning target volume (PTV) margin.
Materials And Methods:
Rigid registration between PRE-MRI and planning CT images based on the pelvic bone and prostate anatomy were performed. Interfractional setup margin (SM) and interobserver variability (IO) were assessed by comparing the centroid values of prostate contours delineated on PRE-MRIs. MM-MRIs were used for internal margin (IM) assessment, and PTV margin was calculated using the van Herk formula.
Results:
We delineated 400 prostate contours on PRE-MRI images. SM was 0.57 ± 0.42, 2.45 ± 1.98, and 2.28 ± 2.08 mm in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively, after bone localization and 0.76 ± 0.57, 1.89 ± 1.60, and 2.02 ± 1.79 mm in the LR, AP, and SI directions, respectively, after prostate localization. IO was 1.06 ± 0.58, 2.32 ± 1.08, and 3.30 ± 1.85 mm in the LR, AP, and SI directions, respectively, after bone localization and 1.11 ± 0.55, 2.13 ± 1.07, and 3.53 ± 1.65 mm in the LR, AP, and SI directions, respectively, after prostate localization. Average IM was 2.12 ± 0.86, 2.24 ± 1.07, and 2.84 ± 0.88 mm in the LR, AP, and SI directions, respectively. Calculated PTV margin was 2.21, 5.16, and 5.40 mm in the LR, AP, and SI directions, respectively.
Conclusions:
Movements in the SI direction were the largest source of variability in definitive prostate RT, and interobserver variability was a non-negligible source of margin. The optimal PTV margin should also consider the internal margin.
Insights
This study on prostate radiotherapy found that superior-inferior movements cause the most variability. Incorporating internal margin is crucial for determining the optimal planning target volume (PTV) margin.
Area of Science:
- Radiotherapy
- Medical Imaging
- Oncology
Background:
- Definitive prostate radiotherapy (RT) requires precise targeting to minimize dose to surrounding tissues.
- Assessing interfractional and intrafractional motion variability is critical for optimizing treatment margins.
- 1.5 T MRI guidance offers advanced visualization for prostate RT.
Purpose of the Study:
- To analyze prostate motion variability using daily MRI scans during definitive prostate radiotherapy.
- To determine optimal planning target volume (PTV) margins by considering interfractional and intrafractional uncertainties.
- To evaluate the contribution of interobserver variability to the overall margin requirements.
Main Methods:
- Daily pre-treatment (PRE) and real-time motion monitoring (MM) MRI scans were analyzed.
- Rigid registration was performed using pelvic bone and prostate anatomy.
- Interfractional setup margin (SM), interobserver variability (IO), and internal margin (IM) were assessed.
- PTV margin was calculated using the van Herk formula.
Main Results:
- Superior-inferior (SI) direction showed the largest interfractional variability.
- Interobserver variability was a significant factor, particularly in the SI direction.
- Calculated PTV margins were 2.21 mm (LR), 5.16 mm (AP), and 5.40 mm (SI).
Conclusions:
- SI movements represent the primary source of variability in prostate RT.
- Interobserver variability significantly impacts margin requirements and must be accounted for.
- Optimal PTV margins necessitate the inclusion of internal margin assessments for accurate planning.
