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Advances in Radiotherapy
& Nuclear Medicine 3D-PT-assisted CT-guided I RSI
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have not been previously treated with radiation therapy: ii. Pathological diagnosis was required.
120 – 160 Gy, with an activity range of 0.3 – 0.6 mCi. 29 iii. The necessity of availability of an appropriate puncture
pathway.
3.1.2.2. Evaluation of dosimetric parameters iv. Karnofsky performance scale (KPS) score >70 points.
Dosimetric descriptive parameters include parameters for v. The necessity of tolerability of the RSI procedure.
the target area and OARs, such as D90, the dose received vi. Expected survival time should exceed 3 months.
by 100% of the target area (D100), the volume of the
target area covered by the 100% prescription dose line 3.2.2. Contraindications
(V100), the volume of the target area covered by the 150% i. Severe bleeding with platelet count ≤50×10 /L and
9
prescription dose line (V150), and the volume of the target severe coagulation disorders (prothrombin time
area covered by the 200% prescription dose line (V200). >18 s, prothrombin activity <40%). Anticoagulant
In addition, treatment plan quality is often evaluated using and antiplatelet drugs should be discontinued at least
the conformity index (CI), homogeneity index (HI), and 1 week before seed implantation.
external index (EI). ii. Tumor rupture is contraindicated.
Conformity index is calculated as follows: iii. Severe diabetes is also considered as a contraindication.
iv. Lack of an appropriate puncture pathway renders the
CI = (VT /V ) × (VT/V ) (I) therapy unfeasible if it cannot achieve the planned
ref
ref
ref
where VT is the GTV receiving the prescription dose, target area dose according to the prescription dose
ref
VT is the GTV receiving the prescription dose, and V is design requirements.
ref
the total volume covered by the prescription dose.
3.2.3. Relative contraindications
HI is calculated as follows:
i. Extensive metastasis with an anticipated survival time
HI = ((VT − VT1.5 )/VT ) × 100% (II) of ≤3 months.
ref
ref
ref
where VT1.5 is the GTV receiving 150% of the ii. Severe complications, immunocompromised state,
ref
prescription dose. and renal insufficiency.
iii. Allergy to iodine contrast agents is considered a
EI evaluates the percentage of the prescribed dose that relative contraindication.
falls outside the target area, and is calculated as follows:
3.3. Technical workflow of RSI
EI = (V − VT )/V × 100% (III)
ref
ref
ref
The 3D-PT-assisted CT-guided RSI is primarily employed
3.1.2.3. Limits on OARs for treating recurrent or metastatic head and neck tumors.
In interstitial RSI with I seeds, the impact of radiation It can also be used as salvage treatment in cases where
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doses on normal tissue and their associated side effects tumors have not fully regressed following external beam
remains unclear, highlighting the necessity for further radiation. Adherence to standardized technical procedures
research. At present, it is recommended to refer to the and principles is essential for the successful execution
parameters of high dose rate after loading brachytherapy. of RSI. The standardized process for 3D-PT-assisted
CT-guided RSI for HNC includes eight steps: pre-operative
3.1.2.4. Physical characteristics of I seeds assessment, CT simulation for localization, pre-treatment
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The commonly used clinical radioactive seeds are I with planning design, 3D-PT production, repositioning of
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a half-life of 60 days and emit photons with an energy 3D-PT, insertion of needles, seed implantation, post-
of 27 keV. In recent years, seed chains have gradually treatment evaluation of dosimetric parameters, and
replaced individual seeds internationally due to their ease follow-up (Figure 3, Table 1).
of placement within tissues.
3.4. Classification of RSI for recurrent HNC
3.2. Indications and contraindications for RSI for Given the heterogeneity of head and neck tumors and
recurrent HNC the intricate anatomical structures, it is imperative to
3.2.1. Indications functionally categorize recurrent sites to meet the technical
i. Recurrence of malignant tumors post-surgery or prerequisites for RSI.
post-radiotherapy, with a diameter of ≤5 cm. Local Adhering to a five-region classification scheme is
recurrence post-surgery or post-radiotherapy, essential, which includes the following regions: the skull
or recurrent neck lymph node metastases post- base region, maxillary sinus region, parapharyngeal space
radiotherapy, with a diameter of ≤5 cm. region, the floor of mouth region, and neck lymph node

Volume 2 Issue 4 (2024) 5 doi: 10.36922/arnm.4212