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A practical approach to uniform total body photon irradiation
In!. J Radiaion Oncology BKJ/ Phys.. Vol. Printed in the U.S.A. All nghtr mrved.
12. pp. 2033-2039 Copyright
F
03M-30lh/Xh $3.00 + .oo 1986 Peerpamon Journals Ltd
0 Technical Innovations and Notes A PRACTICAL APPROACH TO UNIFORM BODY PHOTON IRRADIATION MARK
J. ENGLER,
TOTAL
PH.D.
Division of Radiation Physics, Department of Radiology, Duke University Medical Center, P.O. Box 3295. Durham. NC 277 10 Photon total body irradiition (TBI) has been applied to treat several systemic malignancies. However, TBI studies have been limited by nonuniform dosimetry. A 16 MV technique was initiated to improve uniformity of dose in a practical manner. For high dose TBI, missing tissue compensators are designed from lateral tissue separations, intra-lung separations, average CT numbers of lung regions, tissue phantom ratios, and off axis ratios. A few days before treatment, CT scans are obtained and TBI is simulated in the treatment room. In the treatment room, back projections of the patient’s lateral silhouette, arm outline, and CT levels are traced on a compensator tray. Lead sheets are scribed through a schematic of the tray, cut, and fixed to their appropriate positions on the tray. Doses are verified with thermoluminescent dosemeters and ion chambers. Most measurements at the temple, chest wall, mid-thighs, and mid-knees have been wlthin 10% of prescribed doses. About 4 hours are required for compensator fabrication and dose verlficrrtion. This approach has been found practical, substantially improving dose unifbrmity relative to prior “OCotechniques applied at this institution. Total body irradiation, Bone marrow transplants,
Radiation pneumonitis,
INTRODUCI’ION
Lung compensators.
body irradiation used for bone marrow transplantation,” eight different points of TBI dose prescription were revealed among nine institutions.” In the 1983 “Symposium on magna-field irradiation: Rationale, technique, result,” six differing ranges of TBI dose heterogeneity were reported without reference to a standard phantom or patient size.‘* These and other reports reveal little standardization of TBI technique among facilities providing it. 1.3-5.7,8,10,11,14,15 Significance of uniformity and standardization is most evident in high dose TBI as used in bone marrow transplants. High dose TBI patients are subject to radiation pneumonitis, however, its quantification remains difficult when doses, dose rates, fmctionations, and adjuvant chemotherapies vary among and within most institutions. To further complicate matters, modem radiotherapy machines and rooms are not customarily designed for TBI.
Geometric limits of collimators and field sizes often force the TBI patient into an uncomfortable position for a treatment lasting at least ten times longer than a treatment of a smaller target. Reporting of TBI depth dose and beam profile data is rarely thorough, and usually inapplicable at locations other than the source of data.5*6*sStandard photon beam calibration phantoms of about 30 X 30 X 30 cm are small relative to TBI fields, and provide less accurate depth doses and dose rates for TBI than for local field irradiation. Depth and scatter dose data of conventional fields are difficult to apply to most TBI fields because the target usually occupies less than a third of the field, with the remainder unblocked. In addition, the beam’s central axis (CAX), widely used as a reference of local field dosimetry, often cannot intercept an anatomic reference point reproducible for different-sized patients. As a result, diverse variables of numerous local targets all coexist in TBI: patient size, shape, and density; beam quality and flatness; source skin distance (SSD); and reproducibility of patient position.’ A 16 MV* TBI technique was initiated at this institution in 198 1, with opposed, diamond-shaped fields (Fig. I), buildup screen for enhancing superficial dose, and missing tissue compensators for the head, neck, and legs.
Presented at the 2nd Annual Meeting of the American Society for Therapeutic Radiology and Oncology, Miami, Florida, 29 September-4 October, 1985; reprint requests to M. J. Engler, Duke University Medical Center, Box 3295, Durham, NC 277 10. Acknowledgements-The author thanks Brenda Stover for help
in preparing the manuscript, Gunilla Bentel and Amy Brown for help with dosimetric procedures, and Drs. Edward Halperin and Leonard Prosnitz for encouraging lung pretection. Accepted for publication 4 June 1986. * SL75/20, Philips Industries, London, Engkmd.
Clinical benefits have been derived from total body irradiation (TBI) in treating certain leukemias, lymphomas, and other diseases.327*B.’ ‘,I5 However, studies of response to TBI are limited by inconsistent and nonuniform doses. In a 1980 report, “The physics and clinical aspects of total
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I. J. Radiation Oncology 0 Biology 0 Physics
Fig.I. TBI compensator tray with diamond-shaped field, outline of patient’s lateral midplane, and other design parameters. At 4 m from the source the light field is 1.2 X 1.2 m, effective field (OAR = 0.90), 1.1 X 1.1 m, and effective diagonal, 1.25 m. IsoOAR relative to OAR at the prescription are shown as dashed lines (OAR = 0.96 to 1.00) and a solid border (OAR = 0.90).
More recently, CT scanning, treatment room simulation, and lung compensation were implemented to further improve dose uniformity for high dose TBI patients. The current TBI protocol is described below.
November 1986, Volume12,Number 11
scans B-D between upper and lower lung borders include substantial lung and are termed “superior,” “central,” and “inferior” lung scans. Central lung, inferior lung, and diaphragm scans are subdivided into anterior and posterior regions, with posterior including arms, and anterior, excluding them. Beam paths of central-anterior lung, inferior-anterior lung, and anteriof diaphragm are sampled at the anterior most edge of thd arms. To determine lung densities and tissue separations, diverse CT subroutines are applied, available on most CT scanners. These subroutines provide intra-lung separations, external tissue separations, mean CT numbers along intra-lung beam paths, and Cartesian coordinates, all annoted on CT scan hard copies. The x axis is taken along the body length, the y axis, as the ordinate of transverse CT scans, and the z axis, as the abscissa of CT scans. The coordinate origin is located at the posterior border of the sagittal midplane at the level of the central lung scan (Figs. 1 and 2). Five beam paths are sampled from lung scans B-D. Lung densities are obtained from areas traced with the CT trackball on screen displays of scans. For example, in the superior lung scan B of Figure 2, two lung areas are traced around sections of the sample beam path, with statistics annoted above the contour. In central and inferior lung scans C and D, areas are traced in anterior and posterior lung. Distances and densities are annoted near as possible to the paths they describe. Because the CT window diameter is limited, in this case to ~48 cm,
SCOUT
METHOD
x
n,
an
80YY
A. CLAVICLE
AND MATERIALS
CT scanning CT scans help determine compensator design (Fig. 2). Simultaneously, CT can document pre-irradiation lung conditions which affect dose reponse. During simulation and therapy, the patient’s hands are folded over the abdomen to lift arms and reduce possible underdose to the lower back. A scout view is obtained to specify table positions at supraclavicular and diaphragmatic lung borders. The table index of the diaphragmatic border is subtracted from the supraclavicular index to give lung length T. The lungs are then divided into four slabs separated by transverse scans x = T/2, T/4, 0, -T/4, -T/2 mm, where x are distances cranial and caudad to the central scan (x = 0). Using the CT laser, these levels are marked on the right chest wall and arm of the patient, and correspond to levels A-E in Figures 1 and 2. Skin markings must be clearly visible in the dim light of the therapy machine’s optical field size indicator, TBI fields having < I/ 16 of the field light intensity at their normal treatment distance of about 1 m from the source. Eight beam paths are sampled from CT scans A-E, one each from A and B, and two each from C-E (dashed lines, Fig. 2). Lung border scans A and E include minimal lung and are termed “clavicle” and “diaphragm.*’ The three
6. SUPERIOR
40MM
LUNG
C. CENTRAL
LUNG
YEU -7n
OYY
“c&*.-776
YEAll-762 YEN--,44 SDEY 90 , SDIV so
474MY
1-n
0.
INFERIOR
474YY
LUNG
-40~
1 E. DIAPHRAGM
-80 MM
477YY
Fig. 2. CT scans annoted with sample beam paths (dashed lines), tissue separations, and statistics of sample lung (shaded areas): coronal scout view and clavicular scan (a); superior (b), central (c), and inferior (d) lung scans; and diaphragmatic scan (e). The x coordinate is printed in the upper right comer of each scan and the maximum tissue separation, in the lower left comer.
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Uniform total body photon irradiation ??M. J. ENGLER Table 1. Form giving effective tissue separations for TBI lung compensation PATIENT: HOSPITAL #:
RT #:_
1 LUNSEPLX(#/lOOO)J’ LAT SEPb S, cm
SITE
EFF LAT SEPd +
MISSING TISSUE, cm
=
S,fi. cm
=
38
=
26
=
39
=
26
=
46
15 x (-0.755) SUPERIOR
(4,3.5)
-
49
I CENTRAL ANTERIOR
((x13)
-
34
11 10.4 x (-0.789) 8
I
17.5 x (-0.776) CENTRAL POSTERIOR
(033)
-
53
14 10.7 X (-0.776)
INFERIOR ANTERIOR
(-4,14)
34
-
8 18.8 X (-0.469)
INFERIOR POSTERIOR a. b. c. d.
(-4,4)
55
-
9
x cm along inferior-superior and y cm along posterior-anterior directions. Total lateral separation parallel to beam rays. L: intra-lung separation parallel to beam rays: #: average CT numbers in regions of interest near beam rays. S,e = S + L#/ 1000, #, negative in sign. SIGNATURE:
S > 48 cm are measured with calipers in the treatment room. Muscle-equivalent, or effective, tissue separations including lung are estimated by s,fl=s-L+L(l =s+L#/1000,
+#/loOO),
(la) (lb)
where S is the uncorrected external separation; L, total intra-lung separation; #, mean CT number of lung traversed; ( 1 + #/ 1000), lung density; L( 1 + #/ 1OOO),muscleequivalent of lung; and L#/ 1000, missing tissue. Intralung separation is obtained by summing paths through both lungs. In eq ( 1a), lung is first subtracted from external separation, and then muscle-equivalent added back in. The abbreviated eq ( 1b) gives total separation combined with missing tissue, # being negative in sign. Values of S, L, and # are taken from CT scans. Each sample ray is located by x,y coordinates, giving a beam’s eye view of its position on the patient’s lateral silhouette, or sagittal midplane. Calculations of S,f are tabulated on a form shown in Table 1. Treatment room simulation After the patient is CT scanned, TBI is simulated in the therapy room. Objectives of the simulation are to es* Donor Chair, Contour Chair Lounge Co., St. Louis, MO.
DATE:-
tablish reproducible treatment position, measure lateral tissue separations, and trace a lateral silhouette on a compensator tray. One lateral field is set up with the following parameters and procedure (Fig. 1): 1. Field size: 1.2 X 1.2 m at the sagittal midplane,
4 m from the source. 2. Gantry angle: 87” -t 1”. 3. Collimator angle: 45” for both lateral fields. 4. The side of the treatment couch is leaned against the wall. Leg posts of the couch nearest the machine are aligned with marks on the floor. 5. A compensator tray is inserted into the blocking tray holder, 68 cm from the source. This tray is a 10 X 10 X $”Lexan sheet, similar to blocking trays, but undrilled. It has tracings of rounded field borders corresponding to OAR = 0.90, and its center is marked to coincide with the shadow of the CAX cross hairs. A point is marked 4 cm below the CAX marking on the tray corresponding to the point of dose prescription. The field light projects shadows of the CAX cross hairs, effective field border, and prescription point from the compensator tray to the treatment couch, patient, and walls. 6. The patient is centered on the treatment couch,* which can be rotated electronically to optimize the patient’s position in the field. The patient’s hands are folded
1. J. Radiation Oncology 0
2036
Biology 0 Physics
over the abdomen, knees are flexed, and feet are taped close to thighs to maintain the feet within the effective field. The feet of a large patient may extend slightly beyond the 0.90 OAR border. 7. With room lights out and field light on, coincidence of beam cross hairs and tray CAX marking is checked. The patient’s lateral silhouette, CT subdivisions, and right arm are then outlined with a marking pen on the compensator tray. The pen tip is held at an angle that allows its shadow to be clearly visible while tracing body contours. 8. External tissue separations S of pelvis, head, neck, knees, and other sites are measured with calipers and recorded on a form “Data for Total Body Irradiation Compensator” shown in Table 2. Separations at levels of the shoulders, diaphragm, diaphragm plus arms, and lungs are obtained from CT scans when possible. The assumed lateral separation of the “tangential” site is discussed below.
November 1986, Volume 12, Number
1I
Compensator design Data of the pelvis and twelve other sites used for compensator design are listed under the following column headings on the TBI data form shown in Table 2, left to right: SITE: The total body is divided into a reference (pelvic) and twelve other sites. LAP SEP S: Lateral separations are obtained with calipers and CT scans. S/2 signifies depth of beam penetration at the sagittal midplane z = 0. EFFECTIVE: These separations are taken from Table 1. TPR(S/2): TPR and dose rates (D) were measured in a polystyrene slab phantom placed on the treatment couch at the position of the pelvis about 23 cm below the CAX (Fig. 3) Additional plastic phantom materials 30 X 30 X 30 cm and 25 X 33 X 15 cm were stacked cranial and caudal to the pelvic position to simulate internal scattering from an average sized body. Measurements were made
Table 2. Form indicating geometry and thicknesses of compensator PATIENT HOSPITAL #: DATA FOR TOTAL BODY IRRADIATION
RT #:
COMPENSATOR
16 MV photons, 120 X 120 cm @ 400 cm, l/ 16” Pb buildup screen @ 360 cm Source-compensator distance = 68 cm, Compensator magnification = 5.9 LAT SEP S, cm
TPR(S/Z)
OAR
PELVIS
43
0.63
1.00
1.00
0
HEAD
15
0.87
0.96
0.75
7
NECK
13
0.88
0.97
0.74
7
SHOULDERS
47
0.60
0.97
1.08 (-8%)
0
DIAPHRAGM
33
0.71
0.98
0.91
2
DIA + ARMS
55
0.55
0.97
1.18 (-15%)
0
KNEES
24
0.79
1.oo
0.80
5
[(TPR X OAR) = 0.951
0.66
10
SITE
TANGENTIAL
O-4.0
LUNGS
TRANSb
NUMBER OF l/32” Pb SHEETS
EFFECTIVE”
SUPERIOR
38
0.67
0.97
0.97
0
CENTRAL
ANT
26
0.77
0.99
0.83
4
CENTRAL
POST
39
0.66
0.97
0.98
0
INFERIOR
ANT
26
0.77
1.00
0.82
5
INFERIOR
POST
46
0.61
0.97
1.06 (-6%)
0
a. Lateral separation parallel to beam rays; effective separations include lung density corrections. b. Transmission sought from compensator = TPR,/(TPR X 0AR)i , where OAR is the off axis ratio of site i relative to pelvic or other reference midpoint; values > 1 signify underdose = 100 (1 - l/TRANS)% SIGNATURE:
relative to the reference
midpoint. DATE:
Uniform total body photon irradiation 0
I
I
I
16 MV Photons 120xl20cm
E
0.40:
yE_&gk,?:_::;~
I o’200
10Cm
. . . . . . . _I
I
I
I
I
I
20
30
40
50
60
. 70
Depth,cm Fig. 3. TBI tissue phantom ratios measured with compensator tray and buildup screen in place. Additional plastic was stacked adjacent to the polystyrene to simulate mass of an average-sized patient.
with compensator tray and buildup screen in place to reduce calculations needed for MU prescription. OAR: These are relative to the dose rate with phantom at the pelvic position (Fig. 1, with values of 1.OO,0.97, 0.96, and 0.90 labeled), in contrast to conventional OAR relative to CAX dose rates. OAR are obtained by superposing a tracing paper map of iso-OAR lines onto the compensator tray’s lateral outlines. TRANS: Transmission of each component is given by TRANSi = [TPR(S/2)]J[OAR
X TPR(S/2)]i,
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Compensator assembly A sheet of paper is placed on the compensator tray and lateral outlines are traced with black lighting. Compensator outlines are scribed through the tracing onto a lead sheet, which is then cut according to its scribe markings. Lead sheets are fixed to the tray and to one another with double stick tape (Fig. 5). To reduce the number of layers, one l/ 16” sheet is substituted for every two l/32” sheets required. Largest sheets are cut first, and represent adjoining anatomic regions. Thick compensator edges are beveled by interpolating numbers of lead sheets between adjacent regions. For example, the two sheets for the diaphragm are extended along the anterior edge of the upper arm to interpolate between a value of four at central anterior lung and zero at central posterior lung (Table 2, Fig. 1). Because tissue separations of lower legs may be considerably less than separation of the knees, additional sheets are added at the level of the calves and below. Possible tangential overdose is compensated for only in the chest wall, where adjacent lung complications are possible. Compensator margins of 0.5- 1.Ocm are added to external contours of the tray’s lateral silhouette to allow for slight changes of head and leg position. Compensator edges shadowing internal contours are cut to cast reproducible shadows along patients’ shoulders, arms, and upper legs (Fig. 5).
(2)
where i denotes the ith anatomic site, and ref, the reference, or prescription point, currently the pelvic midpoint (Fig. 4). l/32” Pb SHEETS: TRANS versus number of lead sheets for each site is graphed in Figure 4. Because transmission is an exponential function of lead thickness, the number of lead sheets is obtained from N = (l/p) In [(OAR X TPR)i/TPR,cd,
M. J. ENGLER
Treatments Currently, the high dose TBI regimen consists of 12 Gy delivered in three consecutive days, with two fractions per day, and 2 Gy per fraction. Each fraction is given with two fields, 1 Gy per field. The first, third, and fifth fractions are given between 7 and 10 A.M., and the second, fourth, and sixth between 3 and 6 P.M.
(3)
where ~1is a linear attenuation coefficient with units expressed for convenience as (number of l/32” lead sheets)-‘. From Figure 4, 1/cl is 24 sheets, 1.9 cm thick, corresponding to the transmission 1/e. The negative error in TRANS from lack of scatter is offset by rounding off fractional values of lead thickness ~0.8 to the lower integral value.
0
2
4
6 # of I/$
8
IO
12
14
Pb Sheets
Fig. 4. Desired transmission as a function of compensator thickness.
Fig. 5. TBI lead sheet compensator based upon Figs. l-4, Tables 1 and 2, and eqs (l), (2), and (3).
1. J. Radiation Oncology 0 Biology 0 Physics
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Monitor units of each lateral irradiation are given by M&
= 1OO/(D X TPR),f,
(4)
where Dnr is 0.070 cGy/MU, giving temporal dose rates of about 10 to 25 cGy/min. The therapy machine is limited to 950 MU of continuous irradiation. Thus each lateral field usually requires three sequential irradiations, each with its own manual MU setting, or dialup. A l/ 16” lead buildup screen is leaned directly against the treatment couch at 3.6 m from the source, to avoid underdosing superhcial tissues. A minimum 5 cm air gap is left between the lead screen and the patient. Dose verification Doses are measured on patients during their first TBI fractions to ascertain whether the compensator is functioning properly. Fifteen Calcium Fluoride:Dysprosium (CaF2 :Dy) thermoluminescent dosemeters (TLD)* are applied per verification, including five set aside as calibration standards. The remaining ten are divided into five pairs and placed in vivo: 1. Beneath 1 cm of bolus on the right temple. 2. In the armpit. 3. Beneath 1 cm of bolus on the right chest wall, 1 cm above the anterior edge of the upper right arm. 4. Between thighs, abutting groin. 5. Between knees. These TLD remain in place for both lateral fields of the first fraction, with locations l-3 above receiving both entrance and exit doses Den and D,,. TLD standards are irradiated with 2 Gy at a depth of 3 cm in the standard polystyrene calibration phantom, with SSD of 1 m, and field size of 20 X 20 cm. TBI TLD are read an hour before the second fraction, about 6 hours after irradiation. Dose verification has also been obtained with ionization chambers protected against mechanical damage by thinwalled buildup caps. Midplane dose d’measured during an irradiation of 950 MU is scaled to give a dose representing one fraction of two lateral fields: d(4,5) = 2MUmf d’/950,
+ 4x)/950.
(5W
Settings of other than 950 MU may be substituted for the denomenator of eq (5b): d( I,22 3) = M&f [(d/MU),, + (d/MU)&
* TLD-200, Harshaw Chemical Company, Solon, OH.
where MU,, and MU, are any settings. When verification indicates undesirable heterogeneity, compensator thickness is changed for the remaining fractions to give desired total dose. RESULTS
AND DISCUSSION
Verifications of compensator effectiveness at sample sites of the temple, chest wall, mid-thighs, and mid-knees have indicated doses within +lO% of prescribed pelvic doses. Discrepancies between planned and measured TBI doses may arise from: 1. Compensator design and position errors: tracing the lateral silhouette, taping lead sheets to the compensator tray, and positioning the tray require more precision than positioning blocks and compensators applied at conventional SSD. This is because TBI tray positioning uncertainty is magnified by 6 rather than the usual 1.5 at SSD of 1 m. Furthermore, because of the dim field light at 4 m from the source, shadows of outlines and points are difficult to discern during both simulation and treatment. Often, lead sheets obscure most of the CAX cross hair shadows, making it necessary to align upper extremities of the cross hairs, and to match compensator borders with edges and markings of the patient (Fig. 1). 2. Patient positioning: Differences of shape and internal density are likely to occur between CT planes of the supine CT scanning position and the semi-reclining treatment position. 3. CT lung density measurement: The calculation of S,S from eq (1) assumes that soft tissue electron densities may be inferred from CT densities. However, as the CT beam passes through the patient it is hardened, introducing errors into CT numbers of pixels in its path. Fortunately, even large errors of lung density usually result in beam compensation errors which are negligible. 4. Scattered dose: Neglecting scatter dose when replacing missing tissue with lead compensator may result in a few percent of underdose.
(5a)
where (4, 5) refer to locations listed above. Chamber position is changed during pauses necessary for resetting components of MUwf. Entrance and exit doses 4, and 4, at locations l-3 above are measured by taping the chamber to the site of interest. These doses are then scaled and combined: d( 1,293) = MUd4,
November 1986, Volume 12, Number 11
(5c)
Compensator thickness as a function of relative OAR X TPR Most external tissue separations measured with calipers are maximum lateral separations S,,, of their transverse planes. The determination of lead sheets needed for the compensator consists of successively applying three inverse relationships: TPR as a function of S/2, TRANS as a function of TPR and OAR, and number of lead sheets as a function of TRANS. Compensator thickness is then inversely proportional to S and directly proportional to OAR (eq (3) Figs. 3, 4). Neglecting small variations in OAR relative to TPR, S,,, gives the minimum compensation needed within its transverse plane. Conversely, the
Uniform total body photon irradiation 0 M. J.ENGLER
assumed relative transmission 0.95/TPREfof tangentially irradiated tissues leads to the maximum compensation needed within the transverse plane. Tangentially irradiated tissues
Tangentially irradiated tissues in treatment fields of local targets such as chest wall, larynx, and limbs are normally compensated with standard therapy machine wedges. However, such wedges are inapplicable to TBI because of the highly irregular shape of the tangential volume, including sharp convolutions at the neck, hips, and knees. Uncompensated dose to tangential tissues at depths up to 2 cm represents the maximum possible overdose in the total body. This overdose is about (0.95/TPR,f - 1) 100% where loss of side and back scatter is approximated by the factor 0.95. For example, using the above 16 MV technique with SRfof 36 cm, the tangential overdose would be 38%. However, most of the tangential overdose is considered negligible because it applies to a relatively small, radioresistant body fraction. Other beam compensation techniques
The TBI compensation technique described here is similar to techniques routinely used for compensation of local fields. CT scanning and lung density corrections are gradually becoming integrated in radiotherapeutic treatment planning systems. A more feasible approach to uniform TBI may involve the addition of TBI beams into computer beam libraries, software to determine relative
2039
transmissions and appropriate thicknesses of compensators, and isodose distributions at representative levels of the entire anatomy. Another approach to uniform doses is to measure the distribution of uncompensated transmissions on a large radiograph, and to calculate needed transmissions from film densities. Again, as with computer treatment planning, adaptations must be made for the total body beam.
Combining aspects of several approaches. it is conceivable that compensators may be computer-designed and manufactured automatically from radiographic and caliper measurement input. Despite machine and room design limitations, technology is thus available for delivering total body doses within generally accepted standards of unifo~ity.2*4.12-14
CONCLUSION Because the lungs are at risk from high dose TBI it is recommended that lung doses be well specified and verified. Uniform TBI and adequate follow-up should make it be possible to better understand lung tolerance. Although only few institutions have dedicated efforts towards missing tissue compensation of TBI, possible benelits are deemed worthy of the effort. The dose compensation and verification described above requires about 4 hours per patient. In conclusion, this technique has been found practical, providing substantial improvement to dose uniformity and specification compared to prior parallel opposed 6oCo techniques applied at this institution.
REFERENCES 1. Barrett, A., Depledge, M.H.. Powles, R.L.: Interstitial pneumonitis following bone marrow transplantation after low dose rate total body irradiation. Int. J. Radiat. Oncol. Biol. Phys. 9: 1029-1033,
1983.
2. Boyer, A.L.: Compensating filters for high energy x-rays. Med. Phys. 9: 429-433, 1982. 3. Del Regato, J.A.: Total body irradiation in the treatment of chronic lymphogenous leukemia. Am. J. Roentgenol. 120: 504-520, 1974. 4. Calvin, J.M.. D’Angio, G.J., Walsh, G.: Use of tissue compensators to improve the dose uniformity for total body irradiation. Int. J. Radiat. Oncol. Biol. Phys. 6: 767-771, 1980.
5. Glasgow, G.P.: The dosimetry of fixed, single source hemibody and total body irradiators. Med. Ph,vs. 9: 3 11-323. 1982.
6. Gupta, SK., Cunningham, J.R.: Measurement of tissue-air ratios and scatter functions for large field sizes for cobalt 60 gamma radiation. Br. J. Radiol. 39: 7-11, 1966. 7. Jenkin, R.D.T., Rider, W.D.. Sonley, M.J.: Ewing’s sarcoma, adjuvant total body irradiation, cyclophosphamide and vincristine. Int. J. Radiat. Oncol. Biol. Phys. I: 407-413, 1976.
8. Johnson, R.E.: Total body irradiation (TBI) as primary therapy for advanced lymphosarcoma. Cancer 35: 242-246, 1975.
9. Khan, F.M., Williamson, J.F., Sewchand. W.. Kim, T.H.: Basic data for dosage calculation and compensation. Inf. J. Radial. Oncol. Biol. Phys. 6: 745-75 1, 1980. 10. Kim, T.H., Khan, F.M., Galvin, J.M.: A report of the work party: Comparison of total body irradiation techniques for bone marrow transplantation. Int. J. Radiat. Oncol. Biol. Phys. 6: 779-784,
1980.
1I. King, E.R.: Use of total-body radiation in the treatment of far-advanced malignancies. JAMA 177: 86-89.196 1. 12. Renner, W.D.: Tissue compensators. Treat. Planning. 7: 48, 1982.
13. Renner, W.D., O’Connor, T.P., Burmudez, N.M.: An electronic device for digitizing radiotherapy films for the construction of tissue compensators. Med. Phys. 9: 9 10-916, 1982. 14. Shank, B.: Techniques of magna-field irradiation. Int. J. Radiat. Oncol. Biol. Phys. 9: 1925-1931, 1983. 15. Shank, B.: Hyperfractionated total body irradiation for bone marrow transplantation. Results in seventy leukemia patients with allogenic transplants. Int. J. Radiat. Oncol. Biol. Phys. 9: 1607-1611, 1983.
12. pp. 2033-2039 Copyright
F
03M-30lh/Xh $3.00 + .oo 1986 Peerpamon Journals Ltd
0 Technical Innovations and Notes A PRACTICAL APPROACH TO UNIFORM BODY PHOTON IRRADIATION MARK
J. ENGLER,
TOTAL
PH.D.
Division of Radiation Physics, Department of Radiology, Duke University Medical Center, P.O. Box 3295. Durham. NC 277 10 Photon total body irradiition (TBI) has been applied to treat several systemic malignancies. However, TBI studies have been limited by nonuniform dosimetry. A 16 MV technique was initiated to improve uniformity of dose in a practical manner. For high dose TBI, missing tissue compensators are designed from lateral tissue separations, intra-lung separations, average CT numbers of lung regions, tissue phantom ratios, and off axis ratios. A few days before treatment, CT scans are obtained and TBI is simulated in the treatment room. In the treatment room, back projections of the patient’s lateral silhouette, arm outline, and CT levels are traced on a compensator tray. Lead sheets are scribed through a schematic of the tray, cut, and fixed to their appropriate positions on the tray. Doses are verified with thermoluminescent dosemeters and ion chambers. Most measurements at the temple, chest wall, mid-thighs, and mid-knees have been wlthin 10% of prescribed doses. About 4 hours are required for compensator fabrication and dose verlficrrtion. This approach has been found practical, substantially improving dose unifbrmity relative to prior “OCotechniques applied at this institution. Total body irradiation, Bone marrow transplants,
Radiation pneumonitis,
INTRODUCI’ION
Lung compensators.
body irradiation used for bone marrow transplantation,” eight different points of TBI dose prescription were revealed among nine institutions.” In the 1983 “Symposium on magna-field irradiation: Rationale, technique, result,” six differing ranges of TBI dose heterogeneity were reported without reference to a standard phantom or patient size.‘* These and other reports reveal little standardization of TBI technique among facilities providing it. 1.3-5.7,8,10,11,14,15 Significance of uniformity and standardization is most evident in high dose TBI as used in bone marrow transplants. High dose TBI patients are subject to radiation pneumonitis, however, its quantification remains difficult when doses, dose rates, fmctionations, and adjuvant chemotherapies vary among and within most institutions. To further complicate matters, modem radiotherapy machines and rooms are not customarily designed for TBI.
Geometric limits of collimators and field sizes often force the TBI patient into an uncomfortable position for a treatment lasting at least ten times longer than a treatment of a smaller target. Reporting of TBI depth dose and beam profile data is rarely thorough, and usually inapplicable at locations other than the source of data.5*6*sStandard photon beam calibration phantoms of about 30 X 30 X 30 cm are small relative to TBI fields, and provide less accurate depth doses and dose rates for TBI than for local field irradiation. Depth and scatter dose data of conventional fields are difficult to apply to most TBI fields because the target usually occupies less than a third of the field, with the remainder unblocked. In addition, the beam’s central axis (CAX), widely used as a reference of local field dosimetry, often cannot intercept an anatomic reference point reproducible for different-sized patients. As a result, diverse variables of numerous local targets all coexist in TBI: patient size, shape, and density; beam quality and flatness; source skin distance (SSD); and reproducibility of patient position.’ A 16 MV* TBI technique was initiated at this institution in 198 1, with opposed, diamond-shaped fields (Fig. I), buildup screen for enhancing superficial dose, and missing tissue compensators for the head, neck, and legs.
Presented at the 2nd Annual Meeting of the American Society for Therapeutic Radiology and Oncology, Miami, Florida, 29 September-4 October, 1985; reprint requests to M. J. Engler, Duke University Medical Center, Box 3295, Durham, NC 277 10. Acknowledgements-The author thanks Brenda Stover for help
in preparing the manuscript, Gunilla Bentel and Amy Brown for help with dosimetric procedures, and Drs. Edward Halperin and Leonard Prosnitz for encouraging lung pretection. Accepted for publication 4 June 1986. * SL75/20, Philips Industries, London, Engkmd.
Clinical benefits have been derived from total body irradiation (TBI) in treating certain leukemias, lymphomas, and other diseases.327*B.’ ‘,I5 However, studies of response to TBI are limited by inconsistent and nonuniform doses. In a 1980 report, “The physics and clinical aspects of total
2033
2034
I. J. Radiation Oncology 0 Biology 0 Physics
Fig.I. TBI compensator tray with diamond-shaped field, outline of patient’s lateral midplane, and other design parameters. At 4 m from the source the light field is 1.2 X 1.2 m, effective field (OAR = 0.90), 1.1 X 1.1 m, and effective diagonal, 1.25 m. IsoOAR relative to OAR at the prescription are shown as dashed lines (OAR = 0.96 to 1.00) and a solid border (OAR = 0.90).
More recently, CT scanning, treatment room simulation, and lung compensation were implemented to further improve dose uniformity for high dose TBI patients. The current TBI protocol is described below.
November 1986, Volume12,Number 11
scans B-D between upper and lower lung borders include substantial lung and are termed “superior,” “central,” and “inferior” lung scans. Central lung, inferior lung, and diaphragm scans are subdivided into anterior and posterior regions, with posterior including arms, and anterior, excluding them. Beam paths of central-anterior lung, inferior-anterior lung, and anteriof diaphragm are sampled at the anterior most edge of thd arms. To determine lung densities and tissue separations, diverse CT subroutines are applied, available on most CT scanners. These subroutines provide intra-lung separations, external tissue separations, mean CT numbers along intra-lung beam paths, and Cartesian coordinates, all annoted on CT scan hard copies. The x axis is taken along the body length, the y axis, as the ordinate of transverse CT scans, and the z axis, as the abscissa of CT scans. The coordinate origin is located at the posterior border of the sagittal midplane at the level of the central lung scan (Figs. 1 and 2). Five beam paths are sampled from lung scans B-D. Lung densities are obtained from areas traced with the CT trackball on screen displays of scans. For example, in the superior lung scan B of Figure 2, two lung areas are traced around sections of the sample beam path, with statistics annoted above the contour. In central and inferior lung scans C and D, areas are traced in anterior and posterior lung. Distances and densities are annoted near as possible to the paths they describe. Because the CT window diameter is limited, in this case to ~48 cm,
SCOUT
METHOD
x
n,
an
80YY
A. CLAVICLE
AND MATERIALS
CT scanning CT scans help determine compensator design (Fig. 2). Simultaneously, CT can document pre-irradiation lung conditions which affect dose reponse. During simulation and therapy, the patient’s hands are folded over the abdomen to lift arms and reduce possible underdose to the lower back. A scout view is obtained to specify table positions at supraclavicular and diaphragmatic lung borders. The table index of the diaphragmatic border is subtracted from the supraclavicular index to give lung length T. The lungs are then divided into four slabs separated by transverse scans x = T/2, T/4, 0, -T/4, -T/2 mm, where x are distances cranial and caudad to the central scan (x = 0). Using the CT laser, these levels are marked on the right chest wall and arm of the patient, and correspond to levels A-E in Figures 1 and 2. Skin markings must be clearly visible in the dim light of the therapy machine’s optical field size indicator, TBI fields having < I/ 16 of the field light intensity at their normal treatment distance of about 1 m from the source. Eight beam paths are sampled from CT scans A-E, one each from A and B, and two each from C-E (dashed lines, Fig. 2). Lung border scans A and E include minimal lung and are termed “clavicle” and “diaphragm.*’ The three
6. SUPERIOR
40MM
LUNG
C. CENTRAL
LUNG
YEU -7n
OYY
“c&*.-776
YEAll-762 YEN--,44 SDEY 90 , SDIV so
474MY
1-n
0.
INFERIOR
474YY
LUNG
-40~
1 E. DIAPHRAGM
-80 MM
477YY
Fig. 2. CT scans annoted with sample beam paths (dashed lines), tissue separations, and statistics of sample lung (shaded areas): coronal scout view and clavicular scan (a); superior (b), central (c), and inferior (d) lung scans; and diaphragmatic scan (e). The x coordinate is printed in the upper right comer of each scan and the maximum tissue separation, in the lower left comer.
1035
Uniform total body photon irradiation ??M. J. ENGLER Table 1. Form giving effective tissue separations for TBI lung compensation PATIENT: HOSPITAL #:
RT #:_
1 LUNSEPLX(#/lOOO)J’ LAT SEPb S, cm
SITE
EFF LAT SEPd +
MISSING TISSUE, cm
=
S,fi. cm
=
38
=
26
=
39
=
26
=
46
15 x (-0.755) SUPERIOR
(4,3.5)
-
49
I CENTRAL ANTERIOR
((x13)
-
34
11 10.4 x (-0.789) 8
I
17.5 x (-0.776) CENTRAL POSTERIOR
(033)
-
53
14 10.7 X (-0.776)
INFERIOR ANTERIOR
(-4,14)
34
-
8 18.8 X (-0.469)
INFERIOR POSTERIOR a. b. c. d.
(-4,4)
55
-
9
x cm along inferior-superior and y cm along posterior-anterior directions. Total lateral separation parallel to beam rays. L: intra-lung separation parallel to beam rays: #: average CT numbers in regions of interest near beam rays. S,e = S + L#/ 1000, #, negative in sign. SIGNATURE:
S > 48 cm are measured with calipers in the treatment room. Muscle-equivalent, or effective, tissue separations including lung are estimated by s,fl=s-L+L(l =s+L#/1000,
+#/loOO),
(la) (lb)
where S is the uncorrected external separation; L, total intra-lung separation; #, mean CT number of lung traversed; ( 1 + #/ 1000), lung density; L( 1 + #/ 1OOO),muscleequivalent of lung; and L#/ 1000, missing tissue. Intralung separation is obtained by summing paths through both lungs. In eq ( 1a), lung is first subtracted from external separation, and then muscle-equivalent added back in. The abbreviated eq ( 1b) gives total separation combined with missing tissue, # being negative in sign. Values of S, L, and # are taken from CT scans. Each sample ray is located by x,y coordinates, giving a beam’s eye view of its position on the patient’s lateral silhouette, or sagittal midplane. Calculations of S,f are tabulated on a form shown in Table 1. Treatment room simulation After the patient is CT scanned, TBI is simulated in the therapy room. Objectives of the simulation are to es* Donor Chair, Contour Chair Lounge Co., St. Louis, MO.
DATE:-
tablish reproducible treatment position, measure lateral tissue separations, and trace a lateral silhouette on a compensator tray. One lateral field is set up with the following parameters and procedure (Fig. 1): 1. Field size: 1.2 X 1.2 m at the sagittal midplane,
4 m from the source. 2. Gantry angle: 87” -t 1”. 3. Collimator angle: 45” for both lateral fields. 4. The side of the treatment couch is leaned against the wall. Leg posts of the couch nearest the machine are aligned with marks on the floor. 5. A compensator tray is inserted into the blocking tray holder, 68 cm from the source. This tray is a 10 X 10 X $”Lexan sheet, similar to blocking trays, but undrilled. It has tracings of rounded field borders corresponding to OAR = 0.90, and its center is marked to coincide with the shadow of the CAX cross hairs. A point is marked 4 cm below the CAX marking on the tray corresponding to the point of dose prescription. The field light projects shadows of the CAX cross hairs, effective field border, and prescription point from the compensator tray to the treatment couch, patient, and walls. 6. The patient is centered on the treatment couch,* which can be rotated electronically to optimize the patient’s position in the field. The patient’s hands are folded
1. J. Radiation Oncology 0
2036
Biology 0 Physics
over the abdomen, knees are flexed, and feet are taped close to thighs to maintain the feet within the effective field. The feet of a large patient may extend slightly beyond the 0.90 OAR border. 7. With room lights out and field light on, coincidence of beam cross hairs and tray CAX marking is checked. The patient’s lateral silhouette, CT subdivisions, and right arm are then outlined with a marking pen on the compensator tray. The pen tip is held at an angle that allows its shadow to be clearly visible while tracing body contours. 8. External tissue separations S of pelvis, head, neck, knees, and other sites are measured with calipers and recorded on a form “Data for Total Body Irradiation Compensator” shown in Table 2. Separations at levels of the shoulders, diaphragm, diaphragm plus arms, and lungs are obtained from CT scans when possible. The assumed lateral separation of the “tangential” site is discussed below.
November 1986, Volume 12, Number
1I
Compensator design Data of the pelvis and twelve other sites used for compensator design are listed under the following column headings on the TBI data form shown in Table 2, left to right: SITE: The total body is divided into a reference (pelvic) and twelve other sites. LAP SEP S: Lateral separations are obtained with calipers and CT scans. S/2 signifies depth of beam penetration at the sagittal midplane z = 0. EFFECTIVE: These separations are taken from Table 1. TPR(S/2): TPR and dose rates (D) were measured in a polystyrene slab phantom placed on the treatment couch at the position of the pelvis about 23 cm below the CAX (Fig. 3) Additional plastic phantom materials 30 X 30 X 30 cm and 25 X 33 X 15 cm were stacked cranial and caudal to the pelvic position to simulate internal scattering from an average sized body. Measurements were made
Table 2. Form indicating geometry and thicknesses of compensator PATIENT HOSPITAL #: DATA FOR TOTAL BODY IRRADIATION
RT #:
COMPENSATOR
16 MV photons, 120 X 120 cm @ 400 cm, l/ 16” Pb buildup screen @ 360 cm Source-compensator distance = 68 cm, Compensator magnification = 5.9 LAT SEP S, cm
TPR(S/Z)
OAR
PELVIS
43
0.63
1.00
1.00
0
HEAD
15
0.87
0.96
0.75
7
NECK
13
0.88
0.97
0.74
7
SHOULDERS
47
0.60
0.97
1.08 (-8%)
0
DIAPHRAGM
33
0.71
0.98
0.91
2
DIA + ARMS
55
0.55
0.97
1.18 (-15%)
0
KNEES
24
0.79
1.oo
0.80
5
[(TPR X OAR) = 0.951
0.66
10
SITE
TANGENTIAL
O-4.0
LUNGS
TRANSb
NUMBER OF l/32” Pb SHEETS
EFFECTIVE”
SUPERIOR
38
0.67
0.97
0.97
0
CENTRAL
ANT
26
0.77
0.99
0.83
4
CENTRAL
POST
39
0.66
0.97
0.98
0
INFERIOR
ANT
26
0.77
1.00
0.82
5
INFERIOR
POST
46
0.61
0.97
1.06 (-6%)
0
a. Lateral separation parallel to beam rays; effective separations include lung density corrections. b. Transmission sought from compensator = TPR,/(TPR X 0AR)i , where OAR is the off axis ratio of site i relative to pelvic or other reference midpoint; values > 1 signify underdose = 100 (1 - l/TRANS)% SIGNATURE:
relative to the reference
midpoint. DATE:
Uniform total body photon irradiation 0
I
I
I
16 MV Photons 120xl20cm
E
0.40:
yE_&gk,?:_::;~
I o’200
10Cm
. . . . . . . _I
I
I
I
I
I
20
30
40
50
60
. 70
Depth,cm Fig. 3. TBI tissue phantom ratios measured with compensator tray and buildup screen in place. Additional plastic was stacked adjacent to the polystyrene to simulate mass of an average-sized patient.
with compensator tray and buildup screen in place to reduce calculations needed for MU prescription. OAR: These are relative to the dose rate with phantom at the pelvic position (Fig. 1, with values of 1.OO,0.97, 0.96, and 0.90 labeled), in contrast to conventional OAR relative to CAX dose rates. OAR are obtained by superposing a tracing paper map of iso-OAR lines onto the compensator tray’s lateral outlines. TRANS: Transmission of each component is given by TRANSi = [TPR(S/2)]J[OAR
X TPR(S/2)]i,
2037
Compensator assembly A sheet of paper is placed on the compensator tray and lateral outlines are traced with black lighting. Compensator outlines are scribed through the tracing onto a lead sheet, which is then cut according to its scribe markings. Lead sheets are fixed to the tray and to one another with double stick tape (Fig. 5). To reduce the number of layers, one l/ 16” sheet is substituted for every two l/32” sheets required. Largest sheets are cut first, and represent adjoining anatomic regions. Thick compensator edges are beveled by interpolating numbers of lead sheets between adjacent regions. For example, the two sheets for the diaphragm are extended along the anterior edge of the upper arm to interpolate between a value of four at central anterior lung and zero at central posterior lung (Table 2, Fig. 1). Because tissue separations of lower legs may be considerably less than separation of the knees, additional sheets are added at the level of the calves and below. Possible tangential overdose is compensated for only in the chest wall, where adjacent lung complications are possible. Compensator margins of 0.5- 1.Ocm are added to external contours of the tray’s lateral silhouette to allow for slight changes of head and leg position. Compensator edges shadowing internal contours are cut to cast reproducible shadows along patients’ shoulders, arms, and upper legs (Fig. 5).
(2)
where i denotes the ith anatomic site, and ref, the reference, or prescription point, currently the pelvic midpoint (Fig. 4). l/32” Pb SHEETS: TRANS versus number of lead sheets for each site is graphed in Figure 4. Because transmission is an exponential function of lead thickness, the number of lead sheets is obtained from N = (l/p) In [(OAR X TPR)i/TPR,cd,
M. J. ENGLER
Treatments Currently, the high dose TBI regimen consists of 12 Gy delivered in three consecutive days, with two fractions per day, and 2 Gy per fraction. Each fraction is given with two fields, 1 Gy per field. The first, third, and fifth fractions are given between 7 and 10 A.M., and the second, fourth, and sixth between 3 and 6 P.M.
(3)
where ~1is a linear attenuation coefficient with units expressed for convenience as (number of l/32” lead sheets)-‘. From Figure 4, 1/cl is 24 sheets, 1.9 cm thick, corresponding to the transmission 1/e. The negative error in TRANS from lack of scatter is offset by rounding off fractional values of lead thickness ~0.8 to the lower integral value.
0
2
4
6 # of I/$
8
IO
12
14
Pb Sheets
Fig. 4. Desired transmission as a function of compensator thickness.
Fig. 5. TBI lead sheet compensator based upon Figs. l-4, Tables 1 and 2, and eqs (l), (2), and (3).
1. J. Radiation Oncology 0 Biology 0 Physics
2038
Monitor units of each lateral irradiation are given by M&
= 1OO/(D X TPR),f,
(4)
where Dnr is 0.070 cGy/MU, giving temporal dose rates of about 10 to 25 cGy/min. The therapy machine is limited to 950 MU of continuous irradiation. Thus each lateral field usually requires three sequential irradiations, each with its own manual MU setting, or dialup. A l/ 16” lead buildup screen is leaned directly against the treatment couch at 3.6 m from the source, to avoid underdosing superhcial tissues. A minimum 5 cm air gap is left between the lead screen and the patient. Dose verification Doses are measured on patients during their first TBI fractions to ascertain whether the compensator is functioning properly. Fifteen Calcium Fluoride:Dysprosium (CaF2 :Dy) thermoluminescent dosemeters (TLD)* are applied per verification, including five set aside as calibration standards. The remaining ten are divided into five pairs and placed in vivo: 1. Beneath 1 cm of bolus on the right temple. 2. In the armpit. 3. Beneath 1 cm of bolus on the right chest wall, 1 cm above the anterior edge of the upper right arm. 4. Between thighs, abutting groin. 5. Between knees. These TLD remain in place for both lateral fields of the first fraction, with locations l-3 above receiving both entrance and exit doses Den and D,,. TLD standards are irradiated with 2 Gy at a depth of 3 cm in the standard polystyrene calibration phantom, with SSD of 1 m, and field size of 20 X 20 cm. TBI TLD are read an hour before the second fraction, about 6 hours after irradiation. Dose verification has also been obtained with ionization chambers protected against mechanical damage by thinwalled buildup caps. Midplane dose d’measured during an irradiation of 950 MU is scaled to give a dose representing one fraction of two lateral fields: d(4,5) = 2MUmf d’/950,
+ 4x)/950.
(5W
Settings of other than 950 MU may be substituted for the denomenator of eq (5b): d( I,22 3) = M&f [(d/MU),, + (d/MU)&
* TLD-200, Harshaw Chemical Company, Solon, OH.
where MU,, and MU, are any settings. When verification indicates undesirable heterogeneity, compensator thickness is changed for the remaining fractions to give desired total dose. RESULTS
AND DISCUSSION
Verifications of compensator effectiveness at sample sites of the temple, chest wall, mid-thighs, and mid-knees have indicated doses within +lO% of prescribed pelvic doses. Discrepancies between planned and measured TBI doses may arise from: 1. Compensator design and position errors: tracing the lateral silhouette, taping lead sheets to the compensator tray, and positioning the tray require more precision than positioning blocks and compensators applied at conventional SSD. This is because TBI tray positioning uncertainty is magnified by 6 rather than the usual 1.5 at SSD of 1 m. Furthermore, because of the dim field light at 4 m from the source, shadows of outlines and points are difficult to discern during both simulation and treatment. Often, lead sheets obscure most of the CAX cross hair shadows, making it necessary to align upper extremities of the cross hairs, and to match compensator borders with edges and markings of the patient (Fig. 1). 2. Patient positioning: Differences of shape and internal density are likely to occur between CT planes of the supine CT scanning position and the semi-reclining treatment position. 3. CT lung density measurement: The calculation of S,S from eq (1) assumes that soft tissue electron densities may be inferred from CT densities. However, as the CT beam passes through the patient it is hardened, introducing errors into CT numbers of pixels in its path. Fortunately, even large errors of lung density usually result in beam compensation errors which are negligible. 4. Scattered dose: Neglecting scatter dose when replacing missing tissue with lead compensator may result in a few percent of underdose.
(5a)
where (4, 5) refer to locations listed above. Chamber position is changed during pauses necessary for resetting components of MUwf. Entrance and exit doses 4, and 4, at locations l-3 above are measured by taping the chamber to the site of interest. These doses are then scaled and combined: d( 1,293) = MUd4,
November 1986, Volume 12, Number 11
(5c)
Compensator thickness as a function of relative OAR X TPR Most external tissue separations measured with calipers are maximum lateral separations S,,, of their transverse planes. The determination of lead sheets needed for the compensator consists of successively applying three inverse relationships: TPR as a function of S/2, TRANS as a function of TPR and OAR, and number of lead sheets as a function of TRANS. Compensator thickness is then inversely proportional to S and directly proportional to OAR (eq (3) Figs. 3, 4). Neglecting small variations in OAR relative to TPR, S,,, gives the minimum compensation needed within its transverse plane. Conversely, the
Uniform total body photon irradiation 0 M. J.ENGLER
assumed relative transmission 0.95/TPREfof tangentially irradiated tissues leads to the maximum compensation needed within the transverse plane. Tangentially irradiated tissues
Tangentially irradiated tissues in treatment fields of local targets such as chest wall, larynx, and limbs are normally compensated with standard therapy machine wedges. However, such wedges are inapplicable to TBI because of the highly irregular shape of the tangential volume, including sharp convolutions at the neck, hips, and knees. Uncompensated dose to tangential tissues at depths up to 2 cm represents the maximum possible overdose in the total body. This overdose is about (0.95/TPR,f - 1) 100% where loss of side and back scatter is approximated by the factor 0.95. For example, using the above 16 MV technique with SRfof 36 cm, the tangential overdose would be 38%. However, most of the tangential overdose is considered negligible because it applies to a relatively small, radioresistant body fraction. Other beam compensation techniques
The TBI compensation technique described here is similar to techniques routinely used for compensation of local fields. CT scanning and lung density corrections are gradually becoming integrated in radiotherapeutic treatment planning systems. A more feasible approach to uniform TBI may involve the addition of TBI beams into computer beam libraries, software to determine relative
2039
transmissions and appropriate thicknesses of compensators, and isodose distributions at representative levels of the entire anatomy. Another approach to uniform doses is to measure the distribution of uncompensated transmissions on a large radiograph, and to calculate needed transmissions from film densities. Again, as with computer treatment planning, adaptations must be made for the total body beam.
Combining aspects of several approaches. it is conceivable that compensators may be computer-designed and manufactured automatically from radiographic and caliper measurement input. Despite machine and room design limitations, technology is thus available for delivering total body doses within generally accepted standards of unifo~ity.2*4.12-14
CONCLUSION Because the lungs are at risk from high dose TBI it is recommended that lung doses be well specified and verified. Uniform TBI and adequate follow-up should make it be possible to better understand lung tolerance. Although only few institutions have dedicated efforts towards missing tissue compensation of TBI, possible benelits are deemed worthy of the effort. The dose compensation and verification described above requires about 4 hours per patient. In conclusion, this technique has been found practical, providing substantial improvement to dose uniformity and specification compared to prior parallel opposed 6oCo techniques applied at this institution.
REFERENCES 1. Barrett, A., Depledge, M.H.. Powles, R.L.: Interstitial pneumonitis following bone marrow transplantation after low dose rate total body irradiation. Int. J. Radiat. Oncol. Biol. Phys. 9: 1029-1033,
1983.
2. Boyer, A.L.: Compensating filters for high energy x-rays. Med. Phys. 9: 429-433, 1982. 3. Del Regato, J.A.: Total body irradiation in the treatment of chronic lymphogenous leukemia. Am. J. Roentgenol. 120: 504-520, 1974. 4. Calvin, J.M.. D’Angio, G.J., Walsh, G.: Use of tissue compensators to improve the dose uniformity for total body irradiation. Int. J. Radiat. Oncol. Biol. Phys. 6: 767-771, 1980.
5. Glasgow, G.P.: The dosimetry of fixed, single source hemibody and total body irradiators. Med. Ph,vs. 9: 3 11-323. 1982.
6. Gupta, SK., Cunningham, J.R.: Measurement of tissue-air ratios and scatter functions for large field sizes for cobalt 60 gamma radiation. Br. J. Radiol. 39: 7-11, 1966. 7. Jenkin, R.D.T., Rider, W.D.. Sonley, M.J.: Ewing’s sarcoma, adjuvant total body irradiation, cyclophosphamide and vincristine. Int. J. Radiat. Oncol. Biol. Phys. I: 407-413, 1976.
8. Johnson, R.E.: Total body irradiation (TBI) as primary therapy for advanced lymphosarcoma. Cancer 35: 242-246, 1975.
9. Khan, F.M., Williamson, J.F., Sewchand. W.. Kim, T.H.: Basic data for dosage calculation and compensation. Inf. J. Radial. Oncol. Biol. Phys. 6: 745-75 1, 1980. 10. Kim, T.H., Khan, F.M., Galvin, J.M.: A report of the work party: Comparison of total body irradiation techniques for bone marrow transplantation. Int. J. Radiat. Oncol. Biol. Phys. 6: 779-784,
1980.
1I. King, E.R.: Use of total-body radiation in the treatment of far-advanced malignancies. JAMA 177: 86-89.196 1. 12. Renner, W.D.: Tissue compensators. Treat. Planning. 7: 48, 1982.
13. Renner, W.D., O’Connor, T.P., Burmudez, N.M.: An electronic device for digitizing radiotherapy films for the construction of tissue compensators. Med. Phys. 9: 9 10-916, 1982. 14. Shank, B.: Techniques of magna-field irradiation. Int. J. Radiat. Oncol. Biol. Phys. 9: 1925-1931, 1983. 15. Shank, B.: Hyperfractionated total body irradiation for bone marrow transplantation. Results in seventy leukemia patients with allogenic transplants. Int. J. Radiat. Oncol. Biol. Phys. 9: 1607-1611, 1983.