Influence of the simultaneous calibration of multiple ring dosimeters on the individual absorbed dose.

Ring dosimeters for personal dosimetry are calibrated in accredited laboratories following ISO 4037-3 guidelines. The simultaneous irradiation of multiple dosimeters would save time, but has to be carefully studied, since the scattering conditions could change and influence the absorbed dose in nearby dosimeters. Monte Carlo simulations using PENELOPE-2014 were performed to explore the need to increase the uncertainty ofHp0.07in the simultaneous irradiation of three and five DXT-RAD 707H-2 (Thermo Scientific) ring dosimeters with beam qualities: N-30, N-80 and N-300. Results show that the absorbed dose in each dosimeter is compatible with each of the others and with the reference simulation (a single dosimeter), with a coverage probability of 95% (k= 2). Comparison with experimental data yielded consistent results with the same coverage probability. Therefore, five ring dosimeters can be simultaneously irradiated with beam qualities ranging, at least, between N-30 and N-300 with a negligible impact on the uncertainty ofHp0.07.

Correlation between eye lens doses and over apron doses in interventional procedures.

Measurements of eye lens dose using over apron dosimeters with a geometric correction factor is an international accepted practice. However, further knowledge regarding geometric correction factors in different contexts is required. The authors studied the correlation between eye lens dose and over apron dosimetry for different medical specialties in eleven hospitals, using a standardized protocol, two independent over apron dosimeters (worn at chest and at neck levels) and a dedicated calibration procedure. The results show good correlation between subjects working on the same medical specialty for 5 specialties: Interventional Radiology, Vascular Surgery, Vascular Radiology, Hemodynamics and Neuroradiology. The geometric correction factors resulting from this study could be used to estimate eye lens dose using over apron dosimeters, which are more comfortable than eye lens dosimeters, as reported by the study subjects, as long as the increased uncertainty of the over apron dosimetry compared to the dedicated eye lens dosimetry is acceptable.

Calibration of a thermoluminescent dosimeter worn over lead aprons in fluoroscopy guided procedures.

Fluoroscopy guided interventional procedures provide remarkable benefits to patients. However, medical staff working near the scattered radiation field may be exposed to high cumulative equivalent doses, thus requiring shielding devices such as lead aprons and thyroid collars. In this situation, it remains an acceptable practice to derive equivalent doses to the eye lenses or other unprotected soft tissues with a dosimeter placed above these protective devices. Nevertheless, the radiation backscattered by the lead shield differs from that generated during dosimeter calibration with a water phantom. In this study, a passive personal thermoluminescent dosimeter (TLD) was modelled by means of the Monte Carlo (MC) code Penelope. The results obtained were validated against measurements performed in reference conditions in a secondary standard dosimetry laboratory. Next, the MC model was used to evaluate the backscatter correction factor needed for the case where the dosimeter is worn over a lead shield to estimate the personal equivalent dose H p (0.07) to unprotected soft tissues. For this purpose, the TLD was irradiated over a water slab phantom with a photon beam representative of the result of a fluoroscopy beam scattered by a patient. Incident beam angles of 0° and 60°, and lead thicknesses between the TLD and phantom of 0.25 and 0.5 mm Pb were considered. A backscatter correction factor of 1.23 (independent of lead thickness) was calculated comparing the results with those faced in reference conditions (i.e., without lead shield and with an angular incidence of 0°). The corrected dose algorithm was validated in laboratory conditions with dosimeters irradiated over a thyroid collar and angular incidences of 0°, 40° and 60°, as well as with dosimeters worn by interventional radiologists and cardiologists. The corrected dose algorithm provides a better approach to estimate the equivalent dose to unprotected soft tissues such as eye lenses. Dosimeters that are not shielded from backscatter radiation might underestimate personal equivalent doses when worn over a lead apron and, therefore, should be specifically characterized for this purpose.

IMRT: preliminary results in a series of advanced head-and-neck cancer patients

PURPOSE: To determine retrospectively 2-3 year local and regional control (LRC), free-of-disease survival (FDS) and overall survival (OS), as well as summarized toxicities in a group of 31 advanced head-and-neck cancer patients, treated at our institution between 2004 and 2011 with definitive IMRT low-dose concomitant boost, the majority of them with concurrent chemotherapy based on cisplatin. The results are also shown in the sub-group of nasopharyngeal cancer patients (NPC: 15 cases). PATIENTS AND METHODS: Radiological basal and contrasted CT series, MR-CT or PET/CT fused images in the setup position with immobilization mask were registered in simulation therapy patients. Planed doses were: 70 Gy in primary tumor and positive nodes >1 cm; 63 Gy in high-risk areas of microscopic diseases +10 mm safety margin; and 56 Gy in low risk of diseases regional lymph nodes. Treatment was delivered using a Varian 2100 Clinac with sliding windows IMRT. Spinal cord doses were limited to a strict maximum of 45 Gy, and optimization aimed for mean doses in parotid glands below 26 Gy, especially in the contralateral parotid gland. Online DRR-portal X-ray comparison images were taken every day with a deviation module tolerance ≤3 mm. RESULTS: The mean follow-up since IMRT was 34 months (interval: 8-89; median 31 months). Median follow-up in living patients was 22 months. The 2-year rate for global LRC was 64 %, for FDS 61 % and OS 77 %. For the NPC group after 2 years, LRC was 73 %, FDS 73 % and OS 93 %. The 3-year rates were similar. Seven patients died as a consequence of local and/or regional progression (mean time 10 months). Relapses were observed in eight patients (26 %), but only seven could be confirmed by biopsy (22.6 %; mean time to relapse: 8.6 months). Global acute mucositis was 61 % and chronic mucositis was shown in six cases which developed xerostomia (19 %) in the first control after IMRT, but 1 year later it was reduced to only four patients, two Grade 2 and two Grade 1. CONCLUSIONS: No excessive, unwarranted toxicities were observed using concomitant low doses boost in IMRT. High rates of compliance to concurrent chemotherapy were achieved. Late xerostomia associated with this regime decreased 1 year after conclusion of treatment. The implementation of IMRT requires advances in imaging for better tumor delineation; otherwise the physician loses the advantage of dose modulation or faces a risk of geographical miss (AU)