Purpose: Evaluation of secondary malignancy risks after radiotherapy for organs peripheral to the treatment fields can be performed using a comprehensive Monte Carlo (MC) computational chain to accurately calculate the typical low doses imparted to these areas. This work presents a comprehensive TOPAS extension to accomplish such evaluations, including dose calculation and cancer risk models.
Methods: Partial-body CT image stacks were extended using the ICRP computational human phantoms of matching height and weight, adopting the Anatomically Predictive Extension (APE) method . Hybrid CT-APE phantoms from prostate, liver and head and neck for patients treated with proton pencil beam radiotherapy (PBS) were generated. The MC TOPAS toolkit was used to (1) convert the plan information in DICOM format into MC components (i.e., geometry and source); (2) perform absorbed dose calculations to normal tissues far from the treated tumor area; and (3) estimate excess absolute risk (EAR), excess relative risk (ERR), and lifetime attributable risk (LAR) for secondary malignancy. The formalism proposed by Paganetti et al.  was implemented as a TOPAS extension and accounts for the inhomogeneity of the organ dose distribution. Our extension offers flexibility for users to change several parameters for the secondary risk estimation (e.g., carcinoma/sarcoma induction), as well as revise the models if desired.
Results: We present the implementation of the new TOPAS extension and demonstrate its use based on the example of LAR₇₀ for carcinoma in prostate, liver and head and neck cancer patients treated with PBS. Simulations involving different treatment modalities and cancer patients are currently being performed.
Conclusion: This extension will allow the radiation therapy community to assess the risk of secondary malignancies over large cohorts of patients by running pre-set MC calculations.1) Kuzmin GA, et al (doi: 10.1667/RR14999.1).2) Paganetti H, et al (doi: 10.1016/j.radonc.2020.05.035).
TH- External Beam- Particle/high LET therapy: Proton therapy – out of field dosimetry/risk analysis