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DÜZENSİZ NAZOFARENKS ALANLARINA KOBALT-60 İLE RADYOTERAPİ UYGULAMALARINDA KORUMA BLOKLARININ DOZ PARAMETRELERİNE ETKİSİ

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Abstract (2. Language): 
Shielding blocks for protection of vital organs within a radiation field give rise to changes in dose distribution of open field due to decreasing of scattered radiation from the shielded areas. Several methods have been used to calculate depth dose values for irregular fields. However, validity of such methods should be verified before routine use in clinic. Design.- In this study, measured percentage depth doses for selected irregular fields have been compared with percentage depth dose for fields obtained using equivalent squares (negative field) and [4x(A/P)] techniques. Dose rates for blocked and unblocked fields have been measured. Also, dose profiles for open fields at 0.5 cm and blocked fields for three depths–0.5, 5 and 10 cm depth – have been obtained and the effect of blocking on dose profile for open field has been investigated. Results.- When measured percentage DD for the selected fields were compared with percentage DD for equivalent squares technique, the percentage difference was found 0.20 %-2.34 % in the nasopharyngeal fields since the block edge was close to central axis. When measured percentage DDs were compared with percentage DD for [4x (A/P)] technique, the percentage difference was found 0. 20 %-2.34 % for the nasopharygeal fields. But both techniques were convenient for percentage DD except points at block edge. When out-put values for selected blocked fields were compared with those of open fields, if blocked fields were less than 25 %, out-put of open fields could be used for irregular fields. Since blocks were close to center of the fields in cranial fields outputs were found to have 2%-3% difference from outputs of open fields. Conclusion.- The dose under the block for selected field was about 10 % of dose at d max.
Abstract (Original Language): 
Radyoterapide kritik organların korunmas ı amacıyla radyasyon alanı içine konulan bloklar, korunmuş volümden radyasyon saçılmasını azaltarak, açık alanın doz dağılımlarının değişmesine neden olurlar. Düzensiz alanların derin doz yüzde değerlerinin bulunmasına ilişkin çeşitli metodlar geliştirilmiştir. Rutinde kullanı- lan bu metodların ölçümlerle geçerliliğinin incelenmesi gereklidir. Bu çalışmada çeşitli düzensiz alanların ölçülen derin doz yüzde değerleri, kare eşdeğeri (KE-negatif alan ) ve [4x(A/P)] yöntemiyle bulunan alanların derin doz yüzde değerleri ile karşılaştırılmış, bloklu ve a- çık alan doz verimleri ölçülmüştür. Ayrıca üç derinlikte (0.5,5 ve 10 cm derinlikler için) açık ve bloklu alanların doz profilleri çizdirilmiş ve bloklamanın açık alan doz profiline olan etkisi incelenmiştir. Seçilen düzensiz alanlarda yapılan derin doz yüzde değerleri ile yoğun bloklamanın yapıldığı ve ikinci boost alanı olan nazofarenks (faz-III) alanında ise KE yöntemi, [4x(A/P)] ‘ ye göre daha iyi sonuç vermiştir (% 2.34 ve % 0.35). Ancak, her iki hesaplama yöntemi de blok kenarındaki noktalar hariç derin doz yüzde değerleri için uygundur. Her kliniğin kendi hesaplama yöntemini kendi koşullarında belirmesi uygun olacaktır. Seçilen bloklu alanlar ile açık alanların doz verimleri mukayese edildiğinde bloklamanın % 25’den az olduğu vakalarda açık alan (kolimatör) doz veriminin kullanılabileceği görülmüştür. Ancak, bloklamanın ve blokların merkeze yakın olduğu nazofarenks (III) alanında doz verimi, açık alan doz verimine göre % 2 - % 3 arasında farklıdır. Bu çalışmada kullan ılan alanlarda blok altındaki dozlar, tüm alanlar için dmax’ın % 10’u civarındadır.

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