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Advances in Radiotherapy
& Nuclear Medicine Review of image-guided adaptive radiotherapy

on computed tomography (CT) and other imaging a specific margin to generate a new planning target volume
modalities such as positron emission tomography-CT and (PTV), ensuring adequate radiation dose delivery to the
magnetic resonance imaging (MRI), to optimize treatment tumor. However, this approach can only partially enhance
precision and effectiveness. 6-11 During RT, technicians use tumor coverage while escalating radiation exposure to
auxiliary equipment, such as positioning laser lights, to surrounding normal tissues, thereby constraining the
ensure precise alignment between the irradiation site and feasibility of administering higher therapeutic doses to
the planned target. This approach minimizes the risk of the target lesion. Moreover, the determination of CTV
misdirected radiation or excessive doses to organs at risk expansion distance is typically based on fixed standards
(OARs) resulting from target movement and changes in derived from clinical research summaries, disregarding
patient positioning. patient-specific anatomical variations. Consequently,
in certain cases, the expansion distance may be either
However, considering the prolonged duration of RT, excessive or insufficient. An excessively large expansion
typically spanning approximately 30 – 40 days to complete compromises protection of OARs and restricts the
the entire course, several factors may change during this potential for target dose escalation, whereas an inadequate
period. These include alterations in the patient’s body expansion may result in reduced target dose coverage. 16-21
shape, tumor size and morphology, bladder or organ
filling status, displacement of OARs adjacent to the target To achieve an optimal equilibrium between target
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area, and variations in beam entry distances. 12-15 Such expansion distance and radiation dosage, Yan et al. and
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alterations can cause discrepancies between the patient’s Yan et al. proposed the innovative concept of adaptive
anatomy at the time of each treatment fraction and the RT (ART). ART is a closed-loop radiation treatment
anatomical model used for treatment planning, potentially process where the treatment plan can be modified using
compromising the accuracy of dose delivery. These effects systematic feedback of measurements. ART intends to
are illustrated in Figure 1, which presents examples from improve radiation treatment by systematically monitoring
various RT modalities. treatment variations and incorporating them to re-optimize
the treatment plan early on during the treatment course.
To address this issue, clinicians commonly employ a In this process, field margin and treatment dose can be
strategy of expanding the clinical target volume (CTV) by routinely customized to each patient to achieve a safe dose
escalation. By individually optimizing prescribed dosage
A and expansion distance for each patient, and adapting
initial treatment plans to account for anatomical variations
when necessary, ART aims to increase tumor local control
rates and decrease toxicity of the OARs. 24-29
The ART approach primarily comprises three modes:
(i) Offline ART utilizes offline imaging to adjust treatment
plans between fractions without requiring real-time
capabilities. Although being user-friendly, this mode has
limited capacity to evaluate actual dose delivery due to the
B lack of same-day imaging information; 30-32 (ii) Online ART
employs online imaging to promptly complete target and
OAR delineation, re-optimize treatment plans, and deliver
treatment before each fraction. As the most widely used
mode in ART, it facilitates more effective assessment of
inter-fractional changes; however, further improvements
in optimization algorithms and integration processes are
still needed. 29,31,33-36 . The workflow of online ART is shown
in Figure 2. Artificial intelligence (AI), as the driving force
behind ART, has significantly enhanced the accuracy of
Figure 1. Schematic diagram of anatomical structure variations between automatic delineation for tumor targets and OARs through
localization computed tomography and radiotherapy. (A) Anatomical deep learning. It also enables intelligent processing of
structure on the day of non-ART localization and corresponding image reconstruction–such as synthetic CT generation,
anatomy during treatment; (B) Anatomical structure on the day of ART
radiotherapy localization and corresponding anatomy during treatment. plan optimization, and quality assurance (QA)–thereby
Abbreviations: ART: Adaptive radiotherapy; OAR: Organ at risk. making real-time, dynamic adaptation feasible. Emerging

Volume 3 Issue 3 (2025) 4 doi: 10.36922/ARNM025110012