Comparing measured and treatment planning system calculated dose outside the irradiated volume

The introduction of advanced techniques and technology in radiotherapy has greatly improved our ability to deliver highly conformal tumor doses while minimizing the dose to the adjacent organs at risk. Throughout clinical routine, there is a temptation for the medical physics specialist to try to estimate the dose delivered to points far away from the treatment site by means of the treatment planning system. This study goal was to analyse how much agreement there is between measured and calculated doses given by the treatment planning system, identifying the dosimetric and geometric accuracy limit that can be considered meaningful provided by the treatment planning system. In order to do this, we made use of a simple solid water slabs setup and a farmer type ionization chamber with 0,6 cm³ of sensitive volume, which where sequentially irradiated by fixed MU and size fields for each couch longitudinal displacement done where the solid water setup were placed upon. Results shown that the percentage deviation between measured and treatment planning system calculated doses is below 10% for distances as far as 7 centimeters from the irradiated field edge and the treatment planning system based estimative analysis have to be cautiously done between 7 and 12 centimeters from the field's edge. Even for distances greater than half a meter, absolute absorbed dose have to be considered. The use of treatment planning system to predict doses outside the treated volume is possible if it is used as an auxiliary tool, not forgetting that treatment planning system can systematically calculate doses bellow the ionization chamber measured ones by the irradiation of phantoms, being they anthropomorphic or not, even when it don't have heterogeneities like found in a real human body, they are essential to a patient safe treatment. (author)