ESTRO 2024 - Abstract Book

S3442

Physics - Dose calculation algorithms

ESTRO 2024

Low-dose irradiations in radiation therapy are typically overlooked, as they constitute a minor portion of the therapeutic target dose. However, in specific patient cohorts with extended post-treatment expected survival, such as young Lymphoma patients, it becomes necessary to consider these low doses in treatment planning due to the potential to increase the risk of secondary cancers. Furthermore, compliance with the council directive 2013/59/EURATOM dictates the inclusion of all forms of ionizing radiation from a device, which is not the current standard practice. In response, this study aims to quantify the percentage contribution of cone-beam CT (kV-CBCT), out-of-field (OF), and planning CT doses to the doses absorbed by organs-at-risk (OARs) and planning target volume (PTV) for Lymphoma patients undergoing radiation therapy.

Material/Methods:

The dose contribution from three different low-dose sources was assessed for a cohort of nine Lymphoma patients treated with radiation therapy at the Meixoeiro Hospital (Vigo, Spain). The treatment planning system estimates the dose distribution within and near the open field (TPS – Eclipse®). The kV-CBCT dose maps were generated through Monte Carlo simulations using the Geant4 (GATE) software (v. 9.2) [1] and considering daily CBCT. The modelled kV CBCT was an Elekta’s Synergy XVI. Simulations were performed for each patient on their voxelized planning CT. Doses were considered OF if they extended beyond the plan's 5% isodose. The corresponding OF dose maps were calculated using Periphocal (P3D) software [2] for each patient, using their planning CT, the treatment plan file and parameters of the plan (prescribed dose, monitor units, field size, and location of the isocenter). The dose received due to the acquisition of the planning CT for each patient, acquired with a Philips Big Bore scanner, was computed with VirtualDose [3] using the parameters from CT protocol (mAs, kV, CTDI, pitch, beam collimation, scanned volume). In this case, doses within each organ were considered homogeneous. OARs (see list in table 1) were delineated using TotalSegmentator [4], and physicians manually outlined the PTV. Doses from CBCT and OF sources were calculated by cumulating doses over all treatment fractions for each patient (ranging from 12 – 23 fractions), while a single CT scan was considered. CBCT, OF, and CT doses ranged from 0 mGy to 0.6, 1.6, and 0.06 mGy, respectively. The results indicate that CBCT and OF doses significantly exceed the dose from a single CT scan (Fig. 1), resulting in a proportionate dose increment (Table 1). Notably, OF doses exhibit wider variations compared to CBCT, while CT doses remain more homogeneous between different organs. When considering all modalities, dose increments range from 0.9% for the PTV (only imaging - CBCT, in particular as the contribution of CT is low inside the scanned area – see values in Fig.1) to 50% for the breasts, with the top being showed by the kidneys and liver, with 800 and 315%. These large increments can be attributed to the planner's allocation of very low doses to these organs, as they are far from the PTV (Table 1). Therefore, any additional dose contribution, from imaging or OF, is substantially influential. This underscores the significance of accounting for these doses, particularly in the context of peripheral organs. Fig.2 illustrates an example of a patient's dose-volume histogram (DVH) for PTV and selected OARs. Discrepancies between the estimated and simulated doses are most notable for OARs at lower doses, whereas for the PTV, discrepancies are more pronounced at higher doses. Results: