ESTRO 36 Abstract Book

S839 ESTRO 36 _______________________________________________________________________________________________

4 University of North Carolina, Radiation Oncology, Chapel Hill, USA 5 Baylor Scott & White Healthcare Temple Clinic, Radiation Oncology, Temple- TX, USA 6 University of Freiburg & German Cancer Consortium DKTK- Partner Site, Radiation Oncology-, Freiburg, Germany Purpose or Objective To use radiobiological metrics to estimate the effectiveness of IMRT and Conformal Radiation Therapy (CRT) modalities in treating prostate tumors. Additionally, to estimate the risk of secondary malignancies in bladder and rectum due to radiotherapy from these treatment modalities. Material and Methods For ten prostate cancer patients, IMRT and CRT plans were developed. For the IMRT plans, two beam energies (6 and 18 MV) and two treatment protocols were used (RTOG 0415 and FCCC). For the evaluation of the deterministic effects of these plans, the tumor control probabilities (TCP) and normal tissues complication probabilities (NTCP) were calculated using the LQ-Poisson and Relative Seriality models. Additionally, the complication-free tumor control probability and the biologically effective uniform dose were calculated for each plan. The risks for secondary malignancies were calculated for bladder and rectum for the different treatment modalities using the LQ model proposed by UNSCEAR. Results The deterministic response probabilities of bladder were lower than those of rectum in all the plans. For bladder, the highest value was for the IMRT FCCC-18X (0.03%) and the lowest for the CRT-18X modality (0.0%). For rectum, the highest value was for the IMRT RTOG-6X (3.52%) and the lowest for the IMRT FCCC-18X modality (0.41%). The average risk for secondary malignancy was lower for bladder (0.37%) compared to rectum (0.81%) based on all the treatment plans of the ten prostate cancer patients. The highest average risk for secondary malignancy for bladder and rectum was for the CRT-6X modality (0.46% and 1.12%, respectively) and the lowest was for the IMRT RTOG-18X modality (0.33% and 0.56%, respectively).

Conclusion Using a minimum dose concept for dose painting with only an overall maximum constraint gives more robust plans than a voxel-by-voxel exact dose prescription, while maintaining maximum dose constraints. Highest adherence to dose painting degree of heterogeneity could be obtained with the minimum-map approach. EP-1558 Dosimetric evaluation of incidental radiation of internal mammary chain in breast cancer with 3D RT A.C. Ciafre 1 , J. Castilla 1 , C. Domingo 1 , E. Jorda 1 , D. Dualde 1 , E. Ferrer 1 1 Hospital Clinico Universitario, Oncologia Radioterapica, Valencia, Spain Purpose or Objective The internal mammary chain represents a less common yet important rout of lymphatic drainage of breast cancer, constituting a matter of debate in the current clinical practice. Poortmans P. 2015 suggests an improvement of disease-free survival, distant disease-free survival and a reduction of breast cancer mortality when they included the internal mammary and medial supraclavicular lymph- node irradiation. This study intends to determinate the incidental coverage and dose distribution of the internal mammary chain in breast cancer treatment, using tangential and opposite fields with 3D External Radiotherapy. Material and Methods We randomly reviewed 47 female patients treated at one institution between January and December of 2013 with an average age of 62 years. Thirty seven patients (78%) had invasive ductal carcinoma (IDC), and 10 patients (21.2%) other histology types; Her2: Negative (82%), hormonal receptors were positive in all of them, 30 affecting the left and 17 the right breast. The majority of tumours were <2cm (n:41/47). Lumpectomy + Sentinel node biopsy (SLN) was performed in 36 patients, one of them with posterior axillary lymph node dissection due to positivity of SNL and the remaining patients were candidates for axillary lymph node dissection. External radiotherapy was administrated exclusively on the breast with tangential and opposite fields with total dose: 50Gy in 5 weeks with 5 fractions per week (200cGy per fraction). Once the treatment was completed, we contour the internal mammary chain according to the Breast Atlas for radiation therapy planning consensus definitions of the RTOG. Results With an average volume of the internal mammary chain of 4.13 cm3, the median minimal dose and maximal dose delivered was 278cGy and 4008cGy respectively, this last one corresponding to 8% of Total dose prescribed for the mammary gland. The median V95 was 297.04cGy. Conclusion The radiation of the internal mammary chain on patients of this study have showed minimal incidental doses, therefore we concluded that the contouring, volume delimitation and dose prescription has been appropriated by not affecting unwanted areas. In the other hand unintended radiation of internal mammary chain turns out as insufficient to treat subclinical disease. EP-1559 Optimizing the risks for deterministic effects and secondary malignancies in bladder and rectum G. Komisopoulos 1 , C. Buckey 2 , S. Stathakis 3 , M. Mavroeidi 4 , G.P. Swanson 5 , D. Baltas 6 , N. Papanikolaou 3 , P. Mavroidis 4 1 University Hospital of Larissa and General Hospital of Larissa- Greece, Medical Physics, Larissa, Greece 2 Mayo Clinic- AZ, Radiation Oncology, Scottsdale, USA 3 University of Texas Health Sciences Center at San Antonio, Radiation Oncology, San Antonio- TX, USA

Fig. 1 shows the dose-responses of bladder and rectum for a range of uniform doses for the different radiation modalities. Additionally, the average response of each modality is plotted accompanied by error bars indicating the standard deviations of response and dose within the patient group. In Fig. 2, the average distributions of the risk for secondary malignancy in bladder (upper) and rectum (lower) as a function of dose are presented for the different modalities and protocols. The vertical lines indicate the mean doses that are delivered to the bladder and rectum by the examined treatment plans. The thick solid line corresponds to uniform irradiation of the organ. Conclusion IMRT plans produced using the RTOG 0415 criteria had equivalent dosimetric results with the CRT plans. General