Evaluation of Patient-Specific Quality Assurance (PSQA) using Octavius 4D in IMRT Planning for Nasopharyngeal Cancer Cases

Rizki Budi Rahayu, Rustika Alifiani, Aditya Prayugo Hariyanto, Levina Almira, Endarko Endarko

Abstract

Patient-Specific Quality Assurance (PSQA) is an important procedure for verifying the accuracy of dose planning before administering radiation therapy to patients. This study investigated the effects of progressively stricter OAR constraints on fluence complexity and Patient-Specific Quality Assurance (PSQA) performance in nasopharyngeal cancer IMRT as a preliminary case-based investigation. Three IMRT plan variations were created for two patients (tumor volumes: 56.16 cm³ and 75.35 cm³) using TPS Monaco: Plan0 (no OAR constraints), Plan1 (moderate constraints), and Plan2 (strict constraints) for the parotid glands, brainstem, and spinal cord. PSQA was performed using the PTW Octavius 4D phantom with gamma index criteria of 3%/3 mm, 3%/2 mm, and 2%/2 mm, in accordance with the AAPM TG-218. All plans achieved GPR values above the 95% tolerance limit for the 3%/2 mm criterion (95.5-99.8%) and above 90% for the 2%/2 mm criterion, confirming clinically acceptable deliverability across all optimization levels. Notably, the GPR in the patient with a larger tumor volume (ID2, 75.35 cm³) did not decrease with stricter constraints. The Plan2 GPR (99.5%) exceeded the Plan1 GPR (98.5%), whereas the patient with a smaller tumor volume (ID1, 56.16 cm³), whose GPR declined consistently from Plan0 to Plan2. This divergence reveals that the relationship between OAR constraint stringency and plan deliverability is patient-specific and governed by the geometric interaction between the tumor volume and surrounding OARs; larger targets afford the TPS greater dosimetric flexibility, thereby reducing the need for narrow MLC segments even under strict constraints. These findings suggest that Octavius 4D-based PSQA serves not only as a routine delivery verification tool but also as a patient-specific plan selection tool, and that the 3%/2 mm criterion is recommended as the primary clinical evaluation parameter for nasopharyngeal IMRT.

Keywords

Gamma Index; IMRT; Nasopharyngeal Cancer; Octavius 4D; PSQA

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References

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DOI: https://doi.org/10.13057/ijap.v16i1.111531

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