In vivo rectal wall measurements during HDR prostate brachytherapy with MOSkin dosimeters integrated on a trans-rectal US probe: Comparison with planned and reconstructed doses.

BACKGROUND AND PURPOSE: To study if MOSkin detectors coupled to a trans-rectal ultrasound (TRUS) probe may be used for in vivo dosimetry on the rectal wall surface during US-based HDR prostate brachytherapy and to quantify possible discrepancies between planned and delivered doses. MATERIALS AND METHODS: MOSkins are a specific type of MOSFET dosimeter optimized to measure dose in steep dose gradients on interfaces. Two MOSkins were assembled on a TRUS probe used for on-line treatment planning. Measurements of the dose to the rectal wall were performed over 18 treatment sessions and compared to the doses calculated on the pre-treatment plan (DPRE) and reconstructed on post-treatment images (DPOST). RESULTS: Averages of the absolute differences between MOSkin readings and DPRE, MOSkin readings and DPOST and DPRE and DPOST were 6.7 ± 5.1%, 3.6 ± 1.9% and 6.3 ± 4.7%, respectively. Agreement between measurements and DPOST was significantly better than between measurements and DPRE (p=0.002) and DPRE and DPOST (p=0.004). Discrepancy between DPOST and DPRE correlated with the time required for treatment planning. CONCLUSION: MOSkin dosimeters integrated to the TRUS probe proved to be an accurate instrument for measuring the dose delivered to the rectal wall in HDR prostate brachytherapy. The delivered doses may differ significantly from those calculated in the treatment plan.

Postoperative radiotherapy for synovial sarcoma of the head and neck during pregnancy: clinical and technical management and fetal dose estimates.

AIMS AND BACKGROUND: In vivo and phantom dosimetry is reported to estimate the fetal dose and evaluate the effectiveness of a special shielding device to reduce fetal exposure in a woman undergoing postoperative radiation therapy for synovial oral cavity sarcoma at the 30th week of pregnancy. METHODS: In vivo measurements were performed by placing thermoluminescent dosimeters on 3 points for fetal dose estimation: uterine fundus, umbilicus and pubis. A Rando anthropomorphic phantom was used to simulate radiotherapy. We also performed off-axis dose measurements for wedged beams to estimate the dose contribution of this accessory used in the treatment. RESULTS: The special shielding device reduced the fetal dose by 70% on average, despite the presence of wedges, which increased the dose by a factor of about 2.5. Before delivery the patient received 48 Gy, and from the in vivo measurements a fetal dose of 8.5, 1.7 and 0.7 cGy was estimated to the uterine fundus, umbilicus and pubis, respectively. CONCLUSIONS: Pre-treatment simulation in the same irradiation conditions is the only reliable approach to predict the fetal dose. By using a special shielding device, radiotherapy can be optimized while keeping the fetal exposure below the risk of deterministic damage.