Dose de tolérance des tissus sains : la thyroïde

Organs at risk radiation dose constraints

Dose constraints are essential for performing dosimetry, especially for intensity modulation and for radiotherapy under stereotaxic conditions. We present the update of the recommendations of the French society of oncological radiotherapy for the use of these doses in classical current practice but also for reirradiation.

[Thyroid function after hypofractionated adjuvant radiotherapy for localized breast cancer]

PURPOSE

We aimed to evaluate the impact on the thyroid function of hypofractionated adjuvant radiotherapy of localized breast cancer.

MATERIAL AND METHOD

This was a prospective study, including 50 patients with breast cancer treated by hypofractionated adjuvant radiation, only half of whom received radiotherapy on the supraclavicular region. The analysis focused on clinical, dosimetric and biological data collected through periodic dosing of thyroid hormones.

RESULTS

The incidence of hypothyroidism was 4% of the patients. The multivariate correlation analysis showed a significant association between initial thyroid volume, volume receiving 50Gy, 40Gy and 30Gy and the incidence of hypothyroidism, no statistical link was found between the bilateral breast, chemotherapy, hormone therapy and the type of surgery and the incidence of radiation-induced hypothyroidism.

CONCLUSION

Although hypofractionation theoretically exposes the thyroid gland to late radiotherapy complications, radiation-induced dysthyroidism remains a complication underestimated by clinicians, and in the absence of prevention and treatment guidelines, it is necessary to delineate systematically the thyroid gland, to try to minimize as much as possible the doses received by this organ, and to monitor the thyroid function by periodic serum assays.

Pattern of radiation-induced thyroid gland changes in nasopharyngeal carcinoma patients in 48 months after radiotherapy

OBJECTIVES

Radiation-induced hypothyroidism is the most common thyroid disorder after radiotherapy in nasopharyngeal carcinoma (NPC) patients. This study evaluated the pattern of radiation-induced thyroid gland changes in 48 months after radiotherapy in NPC patients and the association of hypothyroidism incidence with thyroid dose.

METHODS

Fifty-six NPC patients treated by intensity modulated radiotherapy in 2013 were recruited. All patients received baseline thyroid hormones (fT3, fT4 and TSH) tests and CT scan before radiotherapy. Repeated measures of the thyroid hormones and gland volume were performed at 3, 6, 12, 18, 24, 30, 36 and 48 months after treatment. Trend lines of the thyroid volume and hormone level changes against time were plotted. The incidence of hypothyroidism patients and its relationship with the dose were also evaluated.

RESULTS

The mean thyroid volume followed a decreasing trend after radiotherapy, reaching a minimum (-39.8%) at 30 months and slightly increased afterward. The fT4 level followed a similar pattern with its mean value dropped by 21.5% at 30 months and became steady after 36 months. TSH level showed gradual rise from just after radiotherapy, reaching a peak at 24 months and became relatively steady after 36 months. The incidence of hypothyroidism increased to a maximum at 24 months (28.6%) and dropped afterwards. Thyroid Dmean and D50 were significantly correlated with hypothyroidism incidence in 12 to 30 months (ρ > 0.40, p < 0.05).

CONCLUSION

The patterns of radiation induced thyroid volume shrinkage and fT4 level reduction were similar, with both of them showed decreasing trend from 0 to 30 months. The thyroid volume and function reached a relatively steady state after 36 months. The incidence of hypothyroidism increased up to 24 months and its frequency was associated with the thyroid dose.

Cimetidine administration decreases radiogenic damage on the thyroid gland in mice

PURPOSE

To study if Cimetidine administration could ameliorate the thyroid damage in external radiation.

MATERIALS AND METHODS

Forty healthy male adult mice were used in the present study. The animals were randomized into four groups. Untreated mice (Group 1) that received 1 mg/kg saline intraperitoneally (IP). Group 2 received a single 10 Gy gamma radiation dose with 1 mg/kg saline IP and group 3 were treated with Cimetidine IP. Group 4 was irradiated 1 hour after treatment with Cimetidine. The serum were assayed for the contents of triiodothyronine (T3), tetraiodothyronine (T4), thyroid stimulating hormone (TSH), Free T4 (FT4) and Cortisol using a radioimmunological technique 7 days following radiation. The thyroid tissue was processed and stained with hematoxylin and eosin (H&E), and periodic acid-Schiff (PAS) for histological examination. Data were statistically analyzed using Tukey's post-hoc test and were considered significant at p < 0.05.

RESULTS

External radiation resulted in weight loss and reduction of serum thyroid hormone levels. However, Cimetidine administration prevented marked changes. Histological study showed that Cimetidine injection to irradiated mice minimized the thyroid damage.

CONCLUSIONS

These findings suggested that Cimetidine administration 1 hour before radiation exposure was potent in ameliorating the thyroid damages.

Genetic alterations in thyroid tumors from patients irradiated in childhood for tinea capitis treatment

OBJECTIVE

Exposure to ionizing radiation at young age is the strongest risk factor for the occurrence of papillary thyroid carcinoma (PTC). RET/PTC rearrangements are the most frequent genetic alterations associated with radiation-induced PTC, whereas BRAF and RAS mutations and PAX8-PPARG rearrangement have been associated with sporadic PTC. We decided to search for such genetic alterations in PTCs of patients subjected in childhood to scalp irradiation.

DESIGN

We studied 67 thyroid tumors from 49 individuals irradiated in childhood for tinea capitis scalp epilation: 36 malignant (12 cases of conventional PTC (cPTC), two cPTC metastases, 20 cases of follicular variant PTC (FVPTC), one oncocytic variant of PTC and one follicular carcinoma) and 31 follicular thyroid adenomas.

METHODS

The lesions were screened for the BRAF(V600E) and NRAS mutations and for RET/PTC and PAX8-PPARG rearrangements.

RESULTS

BRAF(V600E) mutation was detected in seven of 14 (50%) cPTC and two of 20 FVPTC (10%) (P=0.019). NRAS mutation was present in one case of FVPTC (5%). RET/PTC1 rearrangement was found, by RT-PCR, in one of 17 cases (5.9%) and by fluorescence in situ hybridization in two of six cases (33%). PAX8-PPARG rearrangement was not detected in any carcinoma. None of the follicular adenomas presented any of the aforementioned genetic alterations.

CONCLUSIONS

The prevalence of BRAF(V600E) mutation in our series is the highest reported in series of PTCs arising in radiation-exposed individuals. The prevalence of RET/PTC1 rearrangement fits with the values recently described in a similar setting.

[Organs at risk and target volumes: definition for conformal radiation therapy in breast cancer]

Adjuvant radiotherapy is a standard component of breast cancer treatment. The addition of radiotherapy after breast conserving surgery has been shown to reduce local recurrence rate and improve long-term survival. Accurate delineation of target volumes and organs at risk is crucial to the quality of treatment planning and delivered accomplished with innovate technologies in radiation therapy. This allows the radiation beam to be shaped specifically to each individual patient's anatomy. Target volumes include the mammary gland and surgical bed in case of breast conserving surgery, the chest wall in case of mastectomy, and if indicated, regional lymph nodes (axillary, supra- and infraclavicular and internal mammary). Organs at risk include lungs, thyroid, brachial plexus, heart, spinal cord and oesophagus. The aim of this article is to encourage the use of conformal treatment and delineation of target volumes and organs at risk and to describe specifically the definition of these volumes.