ESTRO 37 Abstract book

S1034

ESTRO 37

mastectomy, more patients received breast irradiation during the past three decades. Radiotherapy treatment techniques also advanced from conventional wedged technique to intensity modulated radiotherapy (IMRT). Regardless of the treatment techniques, peripheral dose to the contralateral breast is inevitable. The aim of this study was to compare the contralateral breast dose from the primary breast irradiation using various radiotherapy treatment techniques and types of shielding. Material and Methods Six treatment plans by different treatment techniques, including paired physical wedges (PW-P), a lateral physical wedge only (PW-L), paired enhanced dynamic wedges (EDW-P), a lateral enhanced dynamic wedge only (EDW-L), field-in-field tangential opposing (TO-FiF), and inverse-planned intensity modulated radiotherapy (IMRT- IP), were generated using a female Rando phantom. The phantom was treated by all plans, and 15 metal oxide semiconductor field effect transistor (MOSFET) detectors on the surface and inside the contralateral breast were utilized for measuring the contralateral breast dose for each plan. Measurement was repeated with the application of 0.2, 0.3 and 0.5 cm lead sheets or 0.5 and 1 cm superflab (SF) on the TO-FiF to demonstrate the effect of shielding on the contralateral breast dose. Results The measured contralateral breast doses were: 2.05 Gy for PW-P, 1.44 Gy for PW-L, 1.51 Gy for EDW-P, 1.52 Gy for EDW-L, 1.25 Gy for TO-FiF, and 1.17 Gy for IMRT-IP, corresponding to 2.35% to 4.11% of total dose. For the addition of shielding, the doses were: 1.25 Gy for no shielding, 0.65 Gy for 0.2 cm lead, 0.61 Gy for 0.3 cm lead, 0.49 Gy for 0.5cm lead, 0.76 Gy for 0.5 cm SF, and 0.72 Gy for 1 cm SF. All techniques showed that the surface dose was much higher than the dose at depth, and the dose dropped exponentially from the surface to the internal. Conclusion Contralateral breast dose could be lowered by using TO- FiF or IMRT-IP, as well as by adding either lead or SF shielding. Thus it is recommended that TO-FiF or IMRT-IP with lead or SF shielding is applied for clinical practice to achieve a better treatment outcome. EP-1905 Influence of metallic implant on different dose calculation algorithms in Cyberknife planning system H. Geng 1 , W.W. Lam 1 , C.W. Kong 1 , B. Yang 1 , S.K. Yu 1 , K.Y. Cheung 1 1 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective To evaluate the dosimetric impact of metallic implants using ray-tracing and Monte-Carlo (MC) dose calculation algorithm in Cyberknife M6 treatment planning system (Multiplan, version 5.3, Accuray, US). Material and Methods A cylindrical PMMA phantom (20cm long with 15cm diameter) implanted with four stainless steel inserts (4.5cm long with 0.4cm diameter) was used in this study. The inserts were positioned symmetrically 1.5cm from the central axis of the phantom to simulate the cage used for vertebral column reconstruction. The phantom with metallic inserts was scanned by a CT scanner. To simulate the common clinical practice that contour the artifacts and override their densities, an artifact-free image set was also created by scanning the phantom without metallic inserts and assigning the density of stainless steel to the inserts’ locations on the phantom images. Both CT image sets were imported into Multiplan. A spherical target volume (diameter 1.5cm) was defined at the center of the phantom. A treatment plan was generated on the artifact-free image set. 7.5

Dosimetric parameters are summarized in Table 1 for 2 types of beam plans and in Table 2 for 3 types of photon plans. PTV coverage is best achieved with photon plans. Concerning CI, mean values are in average 0.95 per photons and 0.80 per electrons. For HI indeces mean values are about 1.07 and 1.14 respectively. All dosimetric values for OARs are lower with electrons but all techniques enable low and acceptable values. Major differences can be seen in very low doses. Mean values for V2 in ipsilateral lung are about 2 times higher for photons in left-sided breast, only 2 oblique photon plans give similar values. Mean values for V2 in heart can be as high as 30 times greater with photons in right-sided breast for two perpendicular beams. When comparing 3 different photon techniques the best coverage is reached with 3 oblique beams and the best OARs sparing with 2 oblique beams. Conclusion This work aimed to determine whether photon beams are always suitable for breast boost technique in order to diminish the number of electron beams in clinical use. Regarding our results, it is possible to use photon beams for all boosts. However, it is strongly advisable to have one linac per site with calibrated electron beams for patients with high OARs values especially in the first part of breast treatment. EP-1904 A comparison of contralateral breast dose from different breast radiotherapy techniques K.H. Tse 1 , C.Y.H. Cheng 2 , M.K.A. Chow 3 1 Pamela Youde Nethersole Eastern Hospital, Clinical Oncology, Hong Kong, Hong Kong SAR China 2 St. Teresa's Hospital, Oncology Centre, Hong Kong, Hong Kong SAR China 3 Queen Mary Hospital, Clinical Oncology, Hong Kong, Hong Kong SAR China Purpose or Objective Because of the proof of equivalent efficacy between breast-conserving therapy (BCT) plus radiotherapy and