P. Mangili et al., In-vivo dosimetry by diode semiconductors in combination with portal filmsduring TBI: reporting a 5-year clinical experience, RADIOTH ONC, 52(3), 1999, pp. 269-276
Citations number
34
Categorie Soggetti
Radiology ,Nuclear Medicine & Imaging","Onconogenesis & Cancer Research
Background and purpose: In-vivo dosimetry is vital to assure an accurate de
livery of total body irradiation (TBI). In-vivo lung dosimetry is strongly
recommended because of the risk of radiation-induced interstitial pneumonia
(IP). Here we report on our 5-year experience with in-vivo dosimetry using
diodes in combination with portal films and assessing the effectiveness of
in-vivo dosimetry in improving the accuracy of the treatment. Moreover, we
wished to investigate in detail the possibility of in-vivo portal dosimetr
y to yield individual information on the lung dose and to evaluate the impa
ct of CT planning on the correspondence between stated and in-vivo measured
doses.
Materials and methods: From March 1994 to March 1999, 229 supine-positioned
patients were treated at our Institute with TBI, using a 6 MV X-rays oppos
ed lateral beam technique. 146 patients received 10 Gy given in three fract
ions, once a day (FTBI), shielding the lungs by the arms; 70 received 12-13
.2 Gy, given in 6-11 fractions, 2-3 fractions per day (HFTBI): in this case
about 2/3 of the lungs were shielded by moulded blocks (mean shielded lung
dose equal to 9 or 9.5 Gy). Thirteen patients received 8 Gy given in a sin
gle fraction (SFTBI, lung dose: 7 Gy). For all HFTBI and FTBI patients, mid
line in-vivo dosimetry was performed at the first fraction by positioning t
wo diodes pairs tone at entrance and one at the exit side) at the waist (um
bilicus) and at the pelvis (ankles). If at least one of the two diodes dose
s (waist-pelvis) was outside +/-5% from the prescribed dose, actions could
be initiated, together with possible checks on the following fractions. Tra
nsit dosimetry by portal films was performed for most patients; for 165 of
them (117 and 48, respectively for FTBI and HFTBI) the midline in-vivo dose
distribution of the chest region was derived and mean lung dose assessed.
As a CT plan was performed for all HFTBI patients, for these patients, the
lung dose measured by portal in-vivo dosimetry was compared with the expect
ed value.
Results: Concerning all diodes data, 528 measurements were available: when
excluding the data of the first fraction(s) of the patients undergoing corr
ections (n = 392), mean and SD were respectively 0.0% and 4.5% (FTBI: -0.3
+/- 4.8%; HFTBI: 0.4 +/- 3.9%). In total 105/ 229 patients had a change aft
er the first fraction and 66/229 were controlled by in-vivo dosimetry for m
ore than one fraction. Since January 1998 a CT plan is performed for FTBI p
atients too: when comparing the diodes data before and after this date, a s
ignificant improvement was found (i.e. rate of deviations larger than 5% re
spectively equal to 30.7% and 13.1%, P = 0.007). When considering only the
patients with a CT plan, the global SD reduced to 3.5%. Concerning transit
dosimetry data, for FTBI, the mean (midline) lung dose was found to vary si
gnificantly from patient to patient (Average 9.13 +/- 0.81 Gy; range 7.4-11
.4 Gy); for the HFTBI patients the mean deviation between measured and expe
cted lung dose was 0.0% (1 SD = 3.8%).
Conclusions: In vivo dosimetry is an effective tool to improve the accuracy
of TBI. The impact of CT planning for FTBI significantly improved the accu
racy of the treatment delivery. Transit dosimetry data revealed a significa
nt inter-patient variation of the mean lung dose among patients undergoing
the same irradiation technique. For patients with partial lung shielding (H
FTBI), an excellent agreement between measured and expected lung dose was v
erified. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.