Risk of second cancer from scattered radiation of intensity-modulated radiotherapies with lung cancer
© Kim et al.; licensee BioMed Central Ltd. 2013
Received: 24 December 2012
Accepted: 24 February 2013
Published: 4 March 2013
To compare the risk of secondary cancer from scattered and leakage doses following intensity-modulated radiotherapy (IMRT), volumetric arc therapy (VMAT) and tomotherapy (TOMO) in patients with lung cancer.
IMRT, VMAT and TOMO were planned for five lung cancer patients. Organ equivalent doses (OEDs) are estimated from the measured corresponding secondary doses during irradiation at various points 20 to 80 cm from the iso-center by using radio-photoluminescence glass dosimeter (RPLGD).
The secondary dose per Gy from IMRT, VMAT and TOMO for lung cancer, measured 20 to 80 cm from the iso-center, are 0.02~2.03, 0.03~1.35 and 0.04~0.46 cGy, respectively. The mean values of relative OED of secondary dose of VMAT and TOMO, which is normalized by IMRT, ranged between 88.63% and 41.59% revealing 88.63% and 41.59% for thyroid, 82.33% and 41.85% for pancreas, 77.97% and 49.41% for bowel, 73.42% and 72.55% for rectum, 74.16% and 81.51% for prostate. The secondary dose and OED from TOMO became similar to those from IMRT and VMAT as the distance from the field edge increased.
OED based estimation suggests that the secondary cancer risk from TOMO is less than or comparable to the risks from conventional IMRT and VMAT.
For earlier stages of lung cancer, the surgical resection has played the main role in its treatment. However there are some opportunities for radiation therapy when the tumor is located in the superior sulcus or is close to the critical normal organ, such as the esophagus and spinal cord, or for patients with positive lymph nodes. Furthermore, when the patient is in an in-operable situation according to their lung function, cardiac function, bleeding tendency or other reasons including the patient’s refusal for surgery, radiation therapy will be a beneficial option.
The radiation therapy technique has developed significantly over the last few decades. We have moved from simple 2 dimensional treatment to 3 dimensional conventional radiotherapy using the treatment fields to an increasingly conformal radiotherapy technique based on 3 dimensional computed tomography (CT) information such as three dimensional conformal therapy (3DCRT) and intensity modulated radiotherapy (IMRT) [1–7]. Recently, volumetric modulated arc therapy (VMAT) and helical tomotherapy (TOMO) have been introduced which can deliver rotational intensity-modulated therapy with more degrees of freedom of gantry speed, multileaf collimator (MLC) leaf motion and dose rates to maximize the target conformity and sparing the normal tissue dose [8–13]. In this study, we estimate the secondary cancer risk of normal organs which are out-of-field for IMRT, VMAT and TOMO with lung cancer patients.
In general, when tumors in cancer patients during the radiation treatment are exposed to high doses which are prescribed for a definitive or palliative goal, the surrounding normal tissues are exposed to intermediate doses which is due to the primary radiation in the beam path. Therefore, the main goal of the treatment planning is finding the right option to satisfy two conflicting priorities such as reducing the exposed dose into the surrounding normal organ and focusing the prescription dose into a target volume. However, out-of-field exposure is another interesting item to be concern. The rest of the body is also exposed to low doses during the radiation treatment which is due primarily to out-of-field radiation resulting from scattering and leakage. Therefore, it will be interesting to measure and estimate the exposed dose for normal organs in out-of-field regions. Furthermore, the evaluation of secondary cancer risk from the out-of-field dose would be interesting, too.
To date, there have been many measurements and calculations of secondary scattered dose and secondary cancer risk [14–22]. In 2003, Hall E. and Wuu C. S. reported the radiation induced second cancers. They are concerned that the secondary cancer risk may be increased by moving from 3DCRT to IMRT which use more fields and monitor units to increase the exposed normal tissue volume by low dose and total body exposure due to leakage radiation. They reported that IMRT induces almost double the incidence of second malignancies compared with 3DCRT . Kim S. et al. presented the secondary radiation doses of intensity-modulated radiotherapy and proton therapy in patients with lung and liver cancer . They measured the secondary scattered dose of IMRT at 20–50 cm from iso-center, ranging from 5.8 and 1.0 mGy per Gy. However, they did not present the calculation results of secondary risk from their measurement .
In this study, we compared the secondary cancer risk by out-of-field radiation for three treatment modalities using the concept of organ equivalent dose (OED) for radiation-induced cancer.
Methods and materials
Patient data and treatment planning
PTV volume (cm3)
Calibration of the radio-photoluminescence glass dosimeter
A radio-photoluminescence glass dosimeter (RPLGD) is newly introduced, as a substitution of the themoluminescence dosimeter (TLD) or other, which was commonly used for in-vivo measurement [23–27]. In this study, we used commercially available RPLGD (GD-302M, Asahi Techno Glass Co., JAPAN). An RPLGD measured the absorbed dose by counting the orange light (500 ~ 700 nm) from the dosimeter, when 365 nm of mono-energetic light was exposed on the irradiated dosimeter. RPLGD has relatively good reproducibility at about 1% and low energy dependency, at higher than 200 keV energy [23–27]. In addition, RPLGD has relatively small incident beam angular dependency, and low toxicity inside the human body, compared with a TLD or optically stimulated luminescence dosimeter (OSLD) [28–30]. A geometrical shape of RPLGD is a rod with 0.15 cm of the diameter and 0.85 cm length.
For estimating the dose response of RPLGD, 10 × 10 cm  open field photon beam was exposed into RPLGD at a 10 cm depth, and 100 cm of Source Surface Distance (SSD). The reproducibility of RPLGD is estimated by calculating the standard deviation of the dose measurements, which the photon beam exposed 3 times into the same detector. Also, the deviations of each RPLGD detector are measured to characterize each RPLGD.
Measurement of secondary dose during IMRT, VMAT and Tomotherapy treatment
Secondary photon doses were measured using an RPLGD on the surface of couch table; thus, the secondary dose, measured at various distances from the iso-center, was the maximum possible dose at that distance and decreased with depth in the body. Therefore, the actual doses at certain body depths at each distance from the iso-center will be smaller than the measured doses.
Cancer risk estimation attributable to secondary doses
where β is the initial slope, f(D) is a function of dose, and g(s, e, a) is a modifying function of population dependent variables such as gender (s), age at exposure (e), and age attained (a), respectively. In Eq. (1), the fit parameters are gender (s) averaged (+ for female, - for male) and mean age at exposure (e) of 37 years old and attained age of 46 years old. The function of dose f(D), which is the dose-dependent portion of Eq. (1), is OED, when averaged over the whole body of volume. It should be noted that OED, a dose–response-weighted dose variable, is proportional to cancer risk in a defined population (same gender, same age of exposure, and same attained age).
where V is the whole body volume, V i is a volume and D i is the dose elements, respectively. In these models, parameters such as α and σ are used to determine the dose–response curve for specific organs. We compared the radiation-induced secondary cancer risk resulting from IMRT, VMAT and TOMO for NSCLC patients, based on analysis of OEDs.
Results and discussion
Treatment planning information
# of fields (or arcs)
At each points, the secondary scattered dose measurements as percentage of prescription dose
Modality \ Distance
Relative percentage OED which is normalized by OED of IMRT
Modality \ Site
We compared secondary scattered doses and OED which is related with radiation induced secondary malignancy risk. We found that the secondary dose depended on the distance from the iso-center and their modalities. The secondary dose and OED from TOMO is less than or compatible to the secondary dose from conventional IMRT and VMAT. The secondary dose and OED from TOMO became similar to IMRT and VMAT as the distance from the field edge increased.
Written informed consent was obtained from the patient for publication of this report and any accompanying images.
This work was supported by general Researcher Program (2012003174) and Radiation Safety Program (2011–31115) through the National Research Foundation.
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