1、 ANSI/HPS N13.54-2008 American National Standard Fetal Radiation Dose Calculations Approved: January 2008 American National Standards Institute, Inc. ANSI/HPS N13.54-2008 ii Published by Health Physics Society 1313 Dolley Madison Blvd. Suite 402 McLean, VA 22101 Copyright 2008 by the Health Physics
2、Society. All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without prior written permission of the publisher. Printed in the United States of America ANSI/HPS N13.54-2008 iii This standard was consensus-balloted and approv
3、ed by the ANSI-accredited HPS N13 Committee on September 15, 2006. At the time of balloting, the HPS N13 Committee had the following membership: Chairperson Joseph Ring Vice Chairperson Tracy Ikenberry American College of Occupational and Environmental Medicine Bryce Breitenstein American Industrial
4、 Hygiene Assoc. Irene Patrek American Iron and Steel Institute Anthony LaMastra American Mining Congress Scott C. Munson American Nuclear Insurers Bob Oliveira American Nuclear Society Nolan E. Hertel Conference of Radiation Control Program Directors Shawn Seeley Council on Ionizing Radiation Msmts
5、2003. 2.0 Fetal Dose from Radiotherapy 2.1 Introduction As mentioned previously, pregnant women are sometimes exposed to ionizing radiation in the course of medical treatments from radiation therapy. Generalized data and procedures will be presented to assist in the planning and execution of radiati
6、on therapy. Data presented are summarized from the American Association of Medical Physicists in Medicine Task Group Report No. 36 (Stovall et al. 1995) and are general in nature and should not be assumed to adequately characterize all vendors radiation therapy machines. All such data must be interp
7、reted or measured by a qualified medical physicist. Photon Intensity Modulated Radiation Therapy (IMRT) will not be addressed as a separate topic; the procedures described for other photon treatments are applicable for IMRT and should be carefully measured by a medical physicist. Additionally, the p
8、hysician and medical physicist should recognize the increasing role of Computed Tomography (CT) in radiation therapy and carefully review the Diagnostic Radiology section of this standard to effectively estimate fetal dose from such procedures. This section will not address the biological effects of
9、 fetal irradiation. These effects are reviewed in Section 6 in this standard. However, the severity and frequency of adverse effects increase with total dose. Therefore, reduction of fetal dose to a level that as is as low as reasonably achievable is of course advisable to reduce the potential risks
10、 to the fetus. The proper treatment of pregnant women with radiation requires advanced consultation between the radiation oncologist, medical oncologist, obstetrician and medical physicist. Special devices to shield the fetus are often only available at large medical institutions, where only a few s
11、uch patients may be treated annually. As such, technical resources must be allocated to prepare for these patients or they should be referred to those institutions best equipped to manage their treatment. 2.2 Radiation Dose Peripheral to the Planned Treatment Volume Radiation dose outside or periphe
12、ral to a treatment volume is the result of photon leakage from the head of the treatment machine, radiation scattered from the collimators and beam modifiers, and radiation scattered from within the patients treatment volume. Neutrons may measurably add to the peripheral doses for photon energies gr
13、eater than 10 MeV. Collimator scatter accounts for approximately one-third of the peripheral dose and predominates near the edge of the treatment volume, while at greater distances treatment unit head leakage predominates (Green et al. 1983, 1985; Kase et al. 1983). As a general rule of thumb, colli
14、mator scatter plus head leakage is approximately equal to patient internal scatter. However, it should be noted that treatment head leakage might vary by a factor of two, depending upon vendor design (Fraass and van de Geijn 1983). Placing a lead shield between the fetus and the radiation source can
15、 easily reduce the collimator scatter and head leakage component of the total peripheral radiation dose (Fraass and van de Geijn 1983). The use of wedges and other treatment beam modifiers can increase the peripheral dose by a factor of 2 to 5 (Fraass et al. 1985; Sherazi and Kase 1985; McParland 19
16、90). As with head leakage and collimator scatter, these components can be reduced by shielding the critical area. 2.2.1 Photon Dose Outside of the Treatment Volume Figure 2-1 is representative of the radiation dose expected peripheral to the desired treatment volume, as measured without the use of s
17、pecial shielding (Stovall et al. 1995). The AAPM Task Group Report on Fetal Dose provides additional such figures for photon energies ranging from 60Co to 25 MeV (Stovall et al. 1995). Distance from the edge of the treatment volume dominates peripheral radiation dose, decreasing approximately expone
18、ntially ANSI/HPS N13.54-2008 3 with increasing distance. The absolute peripheral radiation dose will increase with increasing treatment volume due to increased internal scatter (Stovall et al. 1995). Published data indicate that the change in peripheral dose with depth is small for photon energies g
19、reater than 4 MeV (Fraas and de Geijn 1983; Kase et al. 1983; McParland 1990; Stovall et al. 1995). The exception to this similarity occurs for 60Co, where head leakage dominates the dose (Fraas and de Geijn 1983; Kase et al. 1983; McParland 1990; Stovall et al. 1995). 2.2.2 Photoneutron Dose Outsid
20、e of the Treatment Volume Linear accelerators with photon energies greater than 10 MeV produce neutrons in the accelerating guide, x-ray target, filters, collimators, and the patient (Almond 1979; Ing and Shore 1982; Ing et al. 1982). The neutron dose component, near the treatment volume, is less th
21、an 5% of the total peripheral dose, increasing up to 40% at a distance of 30 cm from the edge of the treatment volume (Stovall et al. 1995). The photoneutron contribution to the total dose increases as the photon energy is increased from 10 to 20 MeV, thereafter remaining approximately constant. Spe
22、cific biological data concerning the risk of neutron exposure to the fetus do not exist. The National Council on Radiation Protection (NCRP 1980) suggests that a negligible biological risk exists from the exposure to incidental neutrons from linear accelerators. The conservative choice is to treat a
23、 pregnant patient with photon energies less than 10 MeV, as long as patient care is not compromised. 2.2.3 Electron Dose Outside of the Treatment Volume Electron therapy is conceptually similar to photon treatments. Due to lower beam currents, head leakage is a smaller fraction of total peripheral d
24、ose; however, other sources of scatter (collimators, blocks, patient) continue to contribute to overall total peripheral dose. The same type of measurements and shields used for photon treatments can be used for electron treatments. The use of the photon shielding for electron treatments will reduce
25、 fetal dose by more than 50%, partly because the electron beams inherent photon contamination has a lower average energy (Antolak et al. 1998). Currently, there is no summary of electron dose values similar to Figure 2-1 available in the literature. 2.2.4 Brachytherapy Implants Fetal dose estimates
26、for brachytherapy implants can be estimated using various computer or manual calculations near the radioactive implant or point dose approximations at greater distances (Nath et al. 1997). Gynecologic brachytherapy procedures typically will not be performed while the mother is pregnant, since shield
27、ing cannot be placed between the radioactive sources and the fetus. Other brachytherapy procedures (i.e., volume implant for sarcomas, eye plaques for ocular melanoma, etc.) may be appropriate if they are sufficiently distant from the fetus. The radiation oncologist, obstetrician, and medical physic
28、ist should carefully review each case. If treatment is determined to be a medical necessity, the medical physicist should estimate the fetal dose by one of the above methods and determine whether appropriate shielding can be provided to the fetus. 2.2.5 Summary Peripheral radiation dose from externa
29、l radiation therapy directly depends on energy, distance from the treatment volume, treatment volume size (i.e., treatment field size), and, to a much lesser extent, depth. Patient internal and collimator scatter dominate peripheral radiation dose within 10 cm of the edge of the treatment volume. Pa
30、tient internal scatter dominates in the distance of 10 to 20 cm from the edge of the treatment volume. Head leakage dominates at a distance of approximately 30 cm, where patient internal scatter and head leakage are approximately equivalent. The use of blocks and wedges may increase dose near the ed
31、ge of the treatment volume by a factor of 2 to 5. It is imperative that the physicist realize that the shielding can only reduce the collimator scatter and head leakage components of peripheral dose. 2.3 Fetal Dose Estimation and Reduction 2.3.1 Dose Estimation The medical physicist is responsible f
32、or estimating the dose to the fetus. Gestational age is of primary importance to the treating physician when estimating fetal dose, as is discussed in the Biological Risk section. Typical anatomical points utilized for fetal dose estimation are the uterine fundus, symphysis pubis, and patient umbili
33、cus (Stovall et al. 1995). ANSI/HPS N13.54-2008 4 The fundus and pubis delineate the limits of the fetal position, and the umbilicus represents a generalized mid-fetal position. Fundus position moves superiorly, with regard to the pubis, as the pregnancy progresses. This position should be carefully
34、 monitored during patient treatment and taken into account when determining the total expected fetal radiation dose. Fetal orientation changes frequently, and no single point can adequately describe the location of the fetal brain or other organs. Dose estimates often require measurements, either in
35、 water, a solid phantom, or an anthropomorphic phantom utilizing ionization chambers, diodes, or thermoluminescent dosimeters (TLD). The physicist should first estimate/measure the dose to the fetus without special shielding. Additional dose measurement should then be repeated using special shields
36、as appropriate. Treatment modification and shielding devices are the primary methods to reduce fetal dose. 2.3.2 Modification of Treatment Techniques Treatment modification is usually a combination of many factors, such as (1) changing treatment angles, (2) reducing the planned treatment volume (i.e
37、., field size), (3) choosing a different energy, or (4) use of the lower collimators to define the field edge nearest the fetus. 2.3.3 Use of Special Shields 2.3.3.1 General Considerations The medical physicist must review all treatment parameters to minimize the risk of injury to the patient or per
38、sonnel when using any special shielding. Shielding design must allow for treatment with anterior, posterior, and lateral orientations, above the diaphragm and the extremities. Two types of shielding arrangements will be described. Additional details may be found in the scientific literature (Stovall
39、 et al. 1995). 2.3.3.1.2 Bridge over Patient A basic shield design consists of a bridge over the patients abdomen that supports five half-value layers of lead, allowing the patient to lie in either a supine or prone position. The superior edge of the shield is as near as possible to the inferior edg
40、e of the treatment volume. This position easily attenuates much of the dose contribution due to head leakage and collimator scatter. For posterior treatments, the patient may lie prone on a false tabletop, with the shielding bridge placed over her back, or the lead may be supported on a shelf below
41、the treatment couch. 2.3.3.1.3 Mobile Shields Treatment versatility can be increased by coupling the basic bridge over patient design with a frame that supports the lead such that it does not rest upon the treatment couch (Stovall et al. 1995). A vertical adjustment motor is added to allow for treat
42、ment at a source-to-skin distance of 80 to 125 cm, and appropriate wheels are attached to allow for easy movement by the treatment staff. The addition of side shielding allows for lateral treatments. The weight of such a unit may approach 200 kg; therefore, the physicist must ensure that appropriate
43、 safety procedures are in place. For posterior treatments, a shield is designed to fit between the existing treatment couch and head of the treatment machine (Stovall et al. 1995). Although attached to the treatment couch, the combination of shield and patient weight typically will not exceed vendor
44、s couch design limits. 2.4 Example of Estimation and Reduction of Fetal Dose Table 2-1 demonstrates the expected fetal dose for a pregnant patient being treated for Hodgkins disease. The fetal dose was measured in an anthropomorphic phantom using diodes and shows approximately a 50% decrease compare
45、d to the unshielded values. Additional examples may be found in the scientific literature (Yudelev et al. 1982 ; Nair et al. 1983 ; Sneed et al. 1995 ; Cygler et al. 1997 ; Antypas et al. 1998 ; Mazonakis et al. 1999 ; Podgorsak et al. 1999 ; Greskovich et al. 2000 ; Prado et al. 2000 ; Islam et al.
46、 2001 ; Magne et al. 2001 ; Ioffe et al. 2002 ; Nuyttens et al. 2002).ANSI/HPS N13.54-2008 5 Table 2-1. Hodgkins disease Machine: Varian Cl 2100C, 6 MV, 100 cm SSD Field configuration: Anterior and posterior mantles Prescribed dose: 40 Gy midline Therapy to be given over 6 weeks Gestational stage at
47、 beginning of therapy: 34 weeks Shielding: 5 HVL (6.7 cm) lead Uterine Fundus Mid-Fetus Pubis 15.5 28.5 41.5 0.42 0.14 0.06 Distance from nearest field edge (cm) Dose to unshielded fetus (Gy, 10 cm depth) Dose to shielded fetus (Gy, 10 cm depth) 0.17 0.04 0.02 Machine: Varian Cl 2100C, 6 MV, 100 cm
48、SSD 2.5. Recommendations Specific requirements vary for individual patients treatments. However, the medical physicist in conjunction with the radiation oncologist should carefully review all aspects of the pregnant patients treatment. The following are minimal considerations prior to and during the
49、 patients treatment (Stovall et al. 1995): 1. Plan the patients treatment normally (as if not pregnant). Modify the treatment plan as appropriate. Possible modifications include changing the treatment volume (field size and angle), selecting a different radiation energy, etc. 2. Estimate fetal dose without shielding. Use peripheral dose data measured on the specific treatment unit to be utilized. Typical points of interest are the uterine fundus, the symphysis pubis, and a midpoint (umbilicus). 3. If dose estimates in item 2 above are not acceptable, design
