Influence of Dosimetric Considerations in Evaluating Second Cancer Risks in Prostate Cancer

Influence of dosimetric considerations in evaluating second cancer risks in prostate cancer. Material and methods: Fifteen patients in this study suffering from early stage of prostate cancer, each patient underwent three plans: 1) Three-dimensional conformal radiation therapy (3DCRT), 2) Rotation therapy (Arc therapy), and 3) intensity-modulated radiation therapy (IMRT) plan. Estimate secondary metastasis risk models: Excess Relative Risk (ERR) and Excess absolute risk (EAR) based on age of exposure by taking dosimetry data from Dose Volume Histograms (DVHs) to calculate risk models. Result: The second cancer risk models (ERR and EAR) for organs at risk OARs decrease with increasing age of exposure for 3D-CRT, ARC and IMRT and there is no significant difference for ERR and EAR model for developing second cancer risk in 3D-CRT, ARC and IMRT.

region (RECRAB), prostate cancer is the second most common cancer in men after lung cancers with standardized incidence to the world population.
Ten-year's US data from the SEER program show relative survival of 91.7% [2]. Prostate cancer has reported either an increased risk of secondary malignancies or no relation to estimate the secondary cancers risk after radiotherapy and negligible risk of secondary malignancies after radiotherapy from other previous review where it is important for both patients and physicians to consider the risk [3].
Radiotherapy has an important role in the treatment of prostate cancer. Three-dimensional conformal radiation therapy (3D-CRT), intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques are all applied for this purpose [4].
The importance of dose escalation for tumor control in the management of localized prostate cancer after External beam radiotherapy (EBRT) as a definitive treatment has been shown in numerous trials where by increasing the radiation dose then the developing complications caused by injury to organs at risk (bladder, prostatic urethra and rectum) also increase [5].
Rotation Therapy technique or ARC therapy is the treatment delivery whereas gantry rotates around the patient, it looks like an infinite extension of the multiple-field technique. The most useful of this technique is when applied to small, symmetric, deep-seated tumors and limited to the treatment of centrally located lesions (i.e., an equal amount of tissue in all directions around the lesion). To reduce dose to critical normal structures one or more sectors of a 360-degree rotation are skipped then the high-dose region is shifted away from the skipped region. ARC therapy dose distributions are not very sensitive to the energy of the photon beam [6].
Intensity-modulated radiotherapy (IMRT) has shown to significantly reduce acute toxicity rates compared with what has been observed with 3D-CRT [7].
Intensity modulated radiation therapy allows conformity to the tumor while saving the adjacent normal structures. IMRT generates high gradient doses on the target, with rapid drop in the latter to the level of normal structures, result of IMRT technique requires extremely high precision in treatment but with very minor positioning errors can affect the target cover and increase the dose to organs at risk (OARs] [8].
Is It expect that 220,800 new cases of prostate cancer and around 27,540 deaths from estimation at 2015. Prostate cancer is the second most common cancer and the second leading cause of cancer death for men in the United States. External beam radiation therapy is commonly used to treat prostate cancer. Studies have shown the benefits of 76 Gy or higher conventionally fractionated treatments, although there is a substantial risk of gastrointestinal toxicity, particularly stemming from the rectum dose. In these cases, radiation doses better conforming to the prostate are necessary to reduce possible rectal complications [9].

Material and Methods
Fifteen patient in this study suffering from early stage of prostate cancer, Selec- tion process based on low staging T1-T2a and PSA < 10 ng/mL and GS 6, Age range from 51 -69 and ECOG Performance Score for patients was 0 -1.

Acquisition and Simulation
All patients underwent a computed tomography (CT) scan with 2 mm slice thickness. After the patients scanning the CT images has transferred to focal contouring system to delineate Target volume and organs at risk on the CT images on each axial slice then CT slices were transferred to treatment planning system.

3D-CRT, ARC and IMRT Technique
For the three techniques we used different beam arrangement, energy and number of beams depending on the recommendation for each technique to give better coverage and spare for organs at risk. 3D-CRT, ARC and IMRT data are displayed in Table 1.

Treatment Planning Evaluation
The new treatment planning systems have many tools for qualitative and quantitative evaluation of the treatment plans. The visual slice-by slice review of the treatment plans using isodose lines distribution can be used as a qualitative evaluation for the treatment plans. The qualitative evaluation is important to know the location of the hot and cold areas in the treatment plans. The quantitative evaluation included the maximum, minimum, mean doses, the dose volume histograms (DVHs) and several indices. Dose Volume Histogram (DVH) was generated 2) Cancer risk models for Radiation-Induced Cancer, The radiation risk model developed by ICRP Publication 103 for use in its recommendations. Excess relative risk (ERR) and Excess absolute risk (EAR) models were developed for cancer incidence and mortality incidence as a function of age at exposure.
where βs is referred to β male or β female which means sex specific estimation of ERR per Sv. D = mean organ dose (Sv), e = age at exposure (years) and a = attained age (years), The coefficients β male, β female, γ , η , are given in tables 4.2 and 4.3 of ICRP Publication 103 in terms of cancer incidence [10].
It's a dosimetric study so there is no follow up taken to check difference between plans.

Results and Discussion
Estimation of second cancer risk calculation based on equivalent dose calculation from mean absorbed dose for fifteen patients with early stage prostate cancer with three different plans (3DCRT, Arc and IMRT) for rectum, bladder, left and right femoral head.

Absorbed Dose (Gy)
The mean dose in Gy and the SD for 3D-CRT for rectum, bladder, left and right femoral head for 3D-CRT were (38.08 ± 4.6), (44

R. A. Elgendy et al.
Murray et al. [17] investigated that SPC risk from modern EBRT by comparing: 3D-CRT, IMRT, volumetric arc therapy (VMAT), and stereotactic ablative radiotherapy (SABR), the excess absolute risks have been determined for out-of-field organs, organ equivalent dose calculations and a linear model was employed for the estimation of the excess absolute risk. 3D-CRT Compared to both IMRT and VMAT showed an increased risk in several out-of-field organs, by up to 26% and 55%, respectively. The absolute risks were low for all irradiation methods, and so were the absolute differences among technique [18].

Conclusions
The results showed mean doses of the rectum, bladder with IMRT lower than 3DCRT plans and ARC therapy plans. But for femoral heads mean dose with IMRT is lower than rotational but not in 3DCRT technique.
The second cancer risk models (ERR and EAR) for OARs decrease with increasing age of exposure for 3D-CRT, ARC and IMRT and there is no significant difference for ERR and EAR model for developing second cancer risk in 3D-CRT, ARC and IMRT.