Document Type : Original Paper
Authors
1
Department of Radiation Oncology, Amrita Institute of Medical Sciences and Research, Amrita School of Medicine, Amrita Vishwa Vidyapeetham, AIMS, Ponekkara P.O., Kochi, Kerala, Pin Code 682041
2
Department of Radiation Oncology, Amrita School of Medicine, Amrita Vishwa Vidyapeetham, AIMS Ponekkara PO, Kochi, Kerala, Pincode 682041
3
Department of Radiation Oncology, Amrita Institute of Medical Sciences and Research, Amrita School of Medicine, Amrita Vishwa Vidyapeetham, AIMS, Ponekkara PO, Kochi, Kerala, Pin code 682041.
4
Department of Medical Physics and Radiation Safety Amrita Institute of Medical Sciences, Amrita University Kochi, Kerala, 682041 India
5
Department of Radiation Oncology Amrita Institute of Medical Sciences, Amrita University Kochi, Kerala, 682041 India
Abstract
Introduction:
Conformal radiation techniques are widely used in prostate cancer treatment as it improves the therapeutic ratio. However, inter and intra-fraction variations remain challenging, which reflects on the planning target volume (PTV) margin. This study aimed to determine the benefit of daily CBCT in reducing the actual PTV margin.
Materials and Methods:
51 patients over 3 years were included in the study. Strict bladder and rectal protocols were followed. PTV was obtained by applying 8mm in left-right (LR) and anterior (A), 5mm in posterior (P) and 8mm in superior-inferior (SI) directions of clinical target volumes (CTV). Pelvic bones of CBCT were matched with the planning CT scan and manually adjusted with soft tissue, assessing the total inter-fraction error (setup + organ motion). Values were registered in left-right (X), superior-inferior (Y), and anterior-posterior (Z) directions.
Results:
The median age was 71 years, 15.6% were intermediate and 84.3% were in the high-risk group. The mean inter-fraction error (Mp) was obtained in all three principal axes (x, y, z). The majority of the patients had shifts ≤ 0.3cm. None recorded a shift >1cm. Thus, random errors were 0.44cm in LR (X), 0.36cm in SI (Y) and 0.40cm in AP (Z) directions. Similarly, systematic errors were 0.05cm in LR (X), 0.04cm in SI (Y) and 0.04cm in AP (Z) directions. The PTV margin was calculated using van Herk’s formula and obtained as 0.68cm.
Conclusion:
IGRT is useful in verifying the targets and organs at risk (OAR) before delivering RT.
Routine use of CBCT images significantly reduces the random and systematic errors and ensures safer dose escalation with reduced PTV margin. However, reducing the PTV margins to less than 7mm may only be safe with proper image guidance.
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