Dosimetric risk estimates of radiation-induced malignancies after intensity modulated radiotherapy

Context: The increasing popularity of intensity-modulated radiotherapy (IMRT) stems from its ability to generate a more conformal plan than hitherto possible with conventional planning. As a result, IMRT is in widespread use across diverse indications. However, the inherent nature of IMRT delivery makes it monitor unit inefficient and leads to increased normal tissue integral dose. This in turn may result in an increased risk of radiation-induced second malignancies.
Aim: To calculate the risk of second malignancy post-IMRT.
Settings and Design: Observational study in a tertiary care institute.
Materials and Methods: Eighteen previously untreated patients with head and neck cancers (n = 10) and prostate cancer (n = 8) were selected. In these patients, selected infield organs around the planning target volume were contoured, viz. brain and thyroid in patients with head and neck cancer and bladder, rectum and small intestine in patients with carcinoma prostate. The estimates of radiation-induced malignancies in these organs and the whole of the body were derived using the concept of Organ Equivalent Dose.
Statistical Analysis Used: Descriptive statistics (SPSS version 12).
Results: The modal estimated incidence of radiation-induced malignancies was 129.87, 1.4, 0.10, 3.42, 7.789 and 129.85 per 10,000 person-years for the brain, thyroid, bladder, rectum, small intestine and whole body respectively.
Conclusions: The estimated risk of radiation-induced malignancies in the thyroid and rectum was similar to the available literature, while the risk for bladder carcinomas was lower than that reported. However, the calculated risk of radiation-induced tumors of the brain was more than that reported with conventional radiation therapy. We propose that estimation of the risk of radiation-induced malignancies should be a part of the plan evaluation process and special care should be taken before using this modality in young patients with benign tumors in the head and neck region.

Keywords
Dose-response relationship, intensity-modulated radiotherapy, radiation-induced cancer, risk assessment, therapy-related cancer