Dosimetric verification of IMRT and 3D conformal treatment delivery using EPID.

PURPOSE: Electronic portal imaging devices (EPIDs) could potentially be useful for either in-vivo or pre-treatment dosimetric verification of external beam radiation therapy. The accuracy of EPID for dosimetric purposes is highly dependent on the specific method used for the determination of dose-response characteristics. The aim of this study was to develop a simple and time-saving EPID back-projection dosimetry algorithm for 2D dose verification in 3D conformal and intensity-modulated beams. METHODS: The procedure of dose reconstruction includes a first calibration step using ionization chamber measurements to convert the Electronic Portal Image (EPI) pixel values into an absorbed dose in water. Subsequently, several corrections were applied to the Portal Dose Images (PDIs) for the effect of field size, attenuator thickness, scattering radiation, beam hardening and EPID off-axis response. Furthermore, to consider tissue inhomogeneity for accurate dose reconstruction, the patient's water equivalent path length (WEPL) was calculated using a range of digitally reconstructed radiographs (DRRs) obtained at various thicknesses by Plastimatch software. The EPID-derived dose maps accuracy was assessed by comparing with the treatment planning system (TPS) calculated dose in the prostate region of Alderson phantom irradiated with 3D conformal and intensity-modulated beams. RESULTS: The gamma analysis for the dose plane showed agreements of 96.95% and 93.5% for 3D conformal and IMRT fields, respectively, with 3%/3 mm acceptance criteria. CONCLUSION: The presented algorithm can provide accurate absolute 2D dose maps for clinical use in the context of 3DCRT or IMRT Quality Assurance (QA) programs.

Assessment of Thyroid Lobe Dose in Breast Cancer Intraoperative Radiotherapy.

BACKGROUND: Breast cancer is the most common cancer among women. Considering the fact that a high dose is delivered in a single fraction of IORT, the evaluation of the dose at sensitive organs like thyroid is necessary. OBJECTIVE: The current study has aimed to evaluate the received dose to thyroid lobes in the breast IORT technique. MATERIAL AND METHODS: A total of 49 women with breast cancer undergoing IORT were enrolled in this cross-sectional study with census sampling. Immediately after tumor resection, a single dose of 20 Gray at the applicator surface was delivered using 50KV X-ray by an Intrabeam machine. The thyroid dose was detected using thermoluminescent detectors (TLD) 100 at the mid-thyroid line, left and right lobes. RESULTS: The dose at the right and left lobes of the thyroid gland as well as the mid-thyroid line was found to be 40.18±35.44 mGy, 35.50±27.32 mGy, and 40.61±32.47 mGy, respectively. The right lobe received a significantly higher absorbed dose compared to the left lobe when the right breast was under IORT treatment. The same trend was seen with the left lobe and left breast under IORT treatment (P=0.0001 and P=0.018, respectively). The applicator size showed non-significant effects on the absorbed dose at the thyroid gland. Also, the applicator depth had a non-significant inverse effect on thyroid dose. CONCLUSION: According to our findings, the absorbed dose at each thyroid lobe depends on the under-treatment side as well as the applicator size and depth (applicator upper surface distance from the skin).

Evaluation of Breast Cancer Radiation Therapy Techniques in Outfield Organs of Rando Phantom with Thermoluminescence Dosimeter.

BACKGROUND: Given the importance of scattered and low doses in secondary cancer caused by radiation treatment, the point dose of critical organs, which were not subjected to radiation treatment in breast cancer radiotherapy, was measured. OBJECTIVE: The purpose of this study is to evaluate the peripheral dose in two techniques of breast cancer radiotherapy with two energies. MATERIAL AND METHODS: Eight different plans in two techniques (conventional and conformal) and two photon energies (6 and 15 MeV) were applied to Rando Alderson Phantom's DICOM images. Nine organs were contoured in the treatment planning system and specified on the phantom. To measure the photon dose, forty-eight thermoluminescence dosimeters (MTS700) were positioned in special places on the above nine organs and plans were applied to Rando phantom with Elekta presice linac. To obtain approximately the same dose distribution in the clinical organ volume, a wedge was used on planes with an energy of 6 MeV photon. RESULTS: Point doses in critical organs with 8 different plans demonstrated that scattering in low-energy photon is greater than high-energy photon. In contrast, neutron contamination in high-energy photon is not negligible. Using the wedge and shield impose greater scattering and neutron contamination on patients with low-and high-energy photon, respectively. CONCLUSION: Deciding on techniques and energies required for preparing an acceptable treatment plan in terms of scattering and neutron contamination is a key issue that may affect the probability of secondary cancer in a patient.