Quasi-IMAT technique and secondary cancer risk in prostate cancer.

Background And Purpose: Estimates of secondary cancer risk after radiotherapy are relevant for treatment-planning comparison. Recently, the authors investigated the potential of a step-and-shoot intensity-modulated arc therapy (quasi-IMAT [qIMAT]) to improve the intensity-modulated radiotherapy (IMRT) plan quality. Here, the effect of the primary dose distribution, photon scatter and neutron dose, and the risk of secondary malignancies after qIMAT technique were analyzed and compared to IMRT.

Methods: qIMAT plans with 36 beam directions and IMRT plans with six beam directions were created for 15-MV photons. Both plans were calculated for each of five prostate cancer patients. The obtained differential dose-volume histograms, photon scatter and neutron dose were used to determine the organ-equivalent dose (OED), which is proportional to the secondary cancer risk. Because of the uncertainty of the applicability of biological models to the OED concept both the linear-exponential and the plateau model for the dose-response relationship were applied.

Results: Both models gave similar results. The OED in scanned CT volume was lower for the qIMAT technique, but higher in the volume not scanned, compared to IMRT. Using a maximum of 36 segments, the increase of risk resulting from qIMAT was < 1% compared to IMRT for both models. By setting the number of segments to 72, an increase of 8% in secondary cancer risk resulted from qIMAT using the linear-exponential model, compared to IMRT (plateau model: 7%). The primary dose is responsible for 88% of the total OED in IMRT and for 86% in qIMAT.

Conclusion: Although qIMAT uses a large number of fields and therefore the volume of normal tissue that receives low-dose radiation is larger than for IMRT, the total OED (by considering primary and secondary contributions of radiation) does not increase the risk of developing a secondary cancer compared to a conventional IMRT plan.