Journal of the National Cancer Institute Advance Access originally published online on August 26, 2008
JNCI Journal of the National Cancer Institute 2008 100(17):1264-1265; doi:10.1093/jnci/djn238
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© The Author 2008. Published by Oxford University Press.
CORRESPONDENCE |
Re: Intensity-Modulated Radiation Therapy Dose Prescription, Recording, and Delivery: Patterns of Variability Among Institutions and Treatment Planning Systems
Affiliations of authors: Department of Radiotherapy and Oncology (CRC, JAL) and Cancer Center (CYC), China Medical University Hospital, Taichung City, Taiwan
Correspondence to: Ji-An Liang, MD, Department of Radiotherapy and Oncology, China Medical University Hospital, 2nd Yuh-Der Road, Taichung City, Taiwan (email: a0080{at}ms2.hinet.net).
We read the excellent pattern-of-care study about dose prescription, recording, and delivery of intensity-modulated radiation therapy (IMRT) by Das et al. (1). We feel that this is an important topic, but some issues remained to be clarified.
First, the main result of this study was the variation in the planned dose that was delivered to the patient. It seems that IMRT possibly provides lower minimal doses to the target when compared to conventional criteria. However, Das et al. do not clarify whether their results were based on the dose to the planned target volume (PTV) or the dose to the clinical target volume (CTV). This information is important for interpreting their result because a dose gradient exists in the peripheral portion of the target such that the dose to the CTV is usually higher than of the dose to the PTV.
Second, radiotherapy dosage was generally prescribed to the PTV, as recommended by the recent International Commission on Radiation Units and Measurements (ICRU) report (2) or Radiation Therapy Oncology Group (RTOG) protocols. For example, one of the dose specifications in RTOG0615 was that PTV70 (CTV70 + margin) will receive 70 Gy in 33 fractions (3). Although the PTV was introduced to ensure delivery of the minimal dose to the CTV, the actual minimal dose to the CTV during a course of radiotherapy is usually higher than the minimal dose to the PTV. To illustrate this point, we performed a simple simulation using WinBUGS software [MRC Biostatistics Unit, Cambridge, UK (4)]. The statistical program is provided as an Appendix. We assumed a one-dimensional dose profile, as shown in Figure 1, A. The CTV (diameter = 30 mm) to PTV (diameter = 40 mm) margin was 5 mm, which is typically used in head and neck radiotherapy (3). This margin is usually estimated on the basis of systematic and random errors during radiotherapy (5). On the basis of a review (6), we assumed a value of 2 mm for SDs of both errors in our simulation. We also assumed a radiotherapy course that consisted of 35 fractions. Although the minimal dose to the PTV was only 80% of the prescribed dose, we found that the median actual minimal dose to the CTV was 90% (95% confidence interval = 82% to 98%) of the prescribed dose after 5000 rounds of Monte Carlo simulation. The cumulative probability distribution of the actual minimal dose to the CTV after 35 fractions of radiotherapy is illustrated in Figure 1, B. Therefore, although the minimal dose to the PTV in radiotherapy treatment planning may be low compared with conventional criteria, the actual minimal dose to the CTV after fractionated radiotherapy delivery remains to be investigated in the IMRT era.
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Appendix: WinBUGS program for estimation of the actual minimal dose to the CTV after 35 fractions of radiotherapy.
The annotations were preceded by the symbol "#."
model {
#N means number of fractions
for (i in 1:N) {
#actual dose at the margin of CTV in each fraction
dose_margin[i]<-0.9-0.02*d[i];
#the actual dose at the margin of CTV is affected by the random and systematic error
d[i]
dnorm(mean,0.25)}
meandnorm(0,0.25);
# the actual delivered dose at the margin of CTV after N fractions
min<-sum(dose_margin[1:N])/(N);
}
Data
list(N=35)
REFERENCES
1. Das IJ, Cheng CW, Chopra KL, Mitra RK, Srivastava SP, Glatstein E. Intensity-modulated radiation therapy dose prescription, recording, and delivery: patterns of variability among institutions and treatment planning systems. J Natl Cancer Inst. (2008) 100(5):300–307.
2. ICRU 62. Prescribing, recording, and reporting photon beam therapy (supplement to ICRU Report 50). (1999) Bethesda, MD: International commission on Radiation Units and Measurements. ICRU Report 62.
3. RTOG 0615 Protocol. http://www.rtog.org/members/protocols/0615/0615.pdfAccessed April 7, 2008.
4. The BUGS Project. http://www.mrc-bsu.cam.ac.uk/bugsAccessed April 7, 2008.
5. van Herk M. Errors and margins in radiotherapy. Semin Radiat Oncol. (2004) 14(1):52–64.[CrossRef][Web of Science][Medline]
6. Hurkmans CW, Remeijer P, Lebesque JV, Mijnheer BJ. Set-up verification using portal imaging; review of current clinical practice. Radiother Oncol. (2001) 58(2):105–120.[CrossRef][Web of Science][Medline]
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J Natl Cancer Inst 2008 100: 1265-1266.
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