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1Radiotherapy Department, Centre Antoine Lacassagne, 33 Avenue de Valombrose, 06189 Nice Cedex 2 and 2The Pediatric Unit, Hôpital de l'Archet, 151 Route de St Antoine de Ginestière, BP 3079, 06202 Nice Cedex 3, France
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Abstract |
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Introduction |
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If radiotherapy for a malignant tumour is necessary during pregnancy, it must be based on published evidence and ethical considerations. First, an abortion may be necessary. In discussing the use of ionizing radiation during pregnancy, a physician must explain to the patient all the potential risks for the unborn child. There are reports of radiotherapy for Hodgkin's disease, breast cancer and non-Hodgkin's lymphoma during pregnancy, but few cases of brain irradiation during pregnancy [1–4]. This report describes a new method for reducing fetal dose in a patient treated during pregnancy with cerebral radiotherapy for a solitary brain metastasis from a non-small cell lung cancer. The results are discussed in relation to other published reports.
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Case report |
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Whole brain irradiation was given through right and left lateral 24 cm x 16 cm fields, to 30 Gy in 10 fractions over 2 weeks. At each side of the patient, near the head of the treatment machine, lead screens (2 cm thickness, less than 2 half-value layers (HVLs)) were positioned to reduce leakage from the treatment head. Concrete blocks containing 2.3% FeO2 (density 4.5 g cm-3) were used for shielding under the neck in a supine position with maximal neck extension on 12 cm thickness polystyrene (breadth 47 cm) on an irradiation couch (Figure 1
). No lead shielding was used over the abdomen.
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Calibration of each TLD was performed with the treatment machine at a distance of 1 m and with a 20 x 20 cm2 field at a depth of 0.5 cm, before and after the five measurement sessions (dose to the TLDs 1 Gy).
Dosemeters were placed in the same location during each of the five sessions (Figure 1
). There were five TLDs in the patient's mouth near the radiation field and five in the vaginal vault near the head of the fetus. A marker indicated the level of the vaginal catheter. There were also five TLDs on each side of the abdomen and on the skin in the mid plane of the fetus. After each session, the TLDs were read and regenerated for re-use.
To reduce leakage from the treatment head and/or through collimators for the lead, as well as to reduce scatter from the patient treatment volume for the concrete, several shields were used. Lead screens (2 cm thickness, less than 2 HVLs) were placed at each side of the patient, near the head of the treatment machine, and 12 cm thick concrete blocks (breadth 5 cm) containing FeO2 (density 4.5 g cm-3) were placed under the neck, which was in maximum extension.
Measurements showed that in the phantom the lead screens reduced the dose by 18%, the concrete blocks by 27% and both together by 50%. In the vaginal vault, the TLD measured a dose of 0.09 cGy±6% (average of 15 measurements) and the ionization chamber measured a dose of 0.1 cGy for 3 Gy given at the isocentre in the brain [7] using the phantom. The ionization chamber recorded 0.11 Gy at a point equivalent to the middle of the fetus, 12.9 cGy±8.5% in the mouth, 0.03 cGy±20% in the vaginal vault and 0.075 cGy±9.3% on the abdominal wall.
At the beginning of April 1997, a full-term healthy male baby was delivered by Caesarian section. At last follow-up in June 2000, the child showed normal growth and development, with no malformations. The patient is still alive, with no evidence of recurrence. No complications or sequelae of radiotherapy were observed during or after treatment.
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Discussion |
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Although the patient refused a therapeutic abortion, radiotherapy during pregnancy was possible with appropriate precautions.
Simulation using a phantom with different shielding achieved as low a fetal radiation dose as possible. The principal sources of radiation to the fetus are leakage through the treatment head of the machine, radiation scattered from the collimator and radiation scattered from the volume irradiated [9, 10]. In this case, the dose measurements in the mouth were high (4.3% of ICRU 50 dose [7]) owing to scattered radiation from the brain.
The vaginal dose was very low owing to the different shields, the distance (70 cm) and the depth (16 cm). Stovall et al [11] gave a value of 0.1% at a distance of 55 cm and a depth of 10 cm. With a cobalt unit, a lead screen is useful but, if possible, the thickness should be increased. In the technique described here, concrete blocks were used with neck extension. However, some of the scattered radiation crosses to the opposite side as only one-quarter to one-half of the irradiated volume is behind the shield. This method enables the use of one large shield to avoid as much as possible of the scattered radiation.
The fetus is nearer to the beam than the vaginal vault. To estimate the difference between these two points, the simulation was done with the same dose as used for the treatment. The measurements at the vaginal vault are a good approximation of the dose to the brain of a fetus with a cephalic presentation. Phantom measurements were higher than those on the patient's abdomen because the simulation of the pregnant woman is difficult. Mazonakis et al [12] used an anthropomorphic phantom to estimate fetal dose at different gestational ages, field sizes and distances from field isocentre, demonstrating a useful method for the physical measurements before treatment. The discrepancy in this study between the simulation and in vivo measurements shows that in vivo dosimetry is necessary. The comparison between TLD and ionization chamber measurements at the vaginal vault shows a difference of 10%, but the characteristics of the TLD calibrator (0.5 cm depth in the beam, 1 Gy) are different from this measurement (out of beam 0.001 Gy). Nevertheless, for this low dose, TLDs are useful for measuring the dose to the fetus.
Radiotherapy during pregnancy has been most commonly reported in association with Hodgkin's disease and breast cancer. There are only a few reports of head and neck or brain irradiation during pregnancy. Wong et al [4] treated a pregnant patient for head and neck cancer using 6 MV photons with lead shielding over the abdomen. The estimated fetal dose was 5.7 cGy for a dose of 62.5 Gy to the nasopharynx and 39.2 Gy to the neck. Scrimger and Urtasum [2] estimated the fetal dose to be 10.0 cGy in a pregnant patient treated with 40 Gy to a brain tumour. Sneed et al [3] estimated that the cumulative total fetal doses were 6.3 cGy and 3.0 cGy for two patients who received 68 Gy and 78 Gy, respectively, to the whole brain using 6 MV photons. Lagrange et al [1] described three patients who had radiotherapy during pregnancy, including one patient with a head and neck tumour treated with 70 Gy to the tumour site, the estimated fetal dose being 4.0 cGy. In our case, using a new positioning method, the fetal dose is 0.30 cGy for 30 Gy delivered to the brain.
Using a new method of shielding, a lower fetal radiation dose was achieved than previously reported in the literature. However, complications may occur many years after radiotherapy and long-term follow-up will be necessary. The effect of ionizing radiation depends on the stage of pregnancy [13]. The main effects of radiation exposure during the fetal period are malformations and changes in growth or other functional changes, mental retardation, induction of malignant disease including leukaemia, and death. Death of the embryo or fetus, anatomical malformations or mental retardation may follow a dose–response curve with a threshold dose of at least 5 cGy [14]. Induction of malignancies and leukaemia as a consequence of radiation exposure in the pre-natal period follows a stochastic curve. Doll and Wakeford [15] concluded that radiation doses of the order of 1 cGy received by the fetus in utero result in an increased risk of childhood cancer. The excess absolute risk at this level of exposure is approximately 6% per Gy, although the exact value of this risk coefficient remains uncertain. However, no congenital abnormalities were reported in 16 babies delivered after the mothers had received supradiaphragmatic radiation while shielding the uterus with five HVLs of lead, and all offspring are physically and mentally normal, with none developing malignancy [16].
In conclusion, radiotherapy with appropriate techniques to reduce the dose to the fetus to a minimum is possible during pregnancy. Lack of information about the threshold dose and the need for careful follow-up should encourage the creation of an international or national registry of these cases.
Received for publication July 20, 2000. Accepted for publication April 17, 2001.
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