**5. IMRT plan verification**

In external beam radiation therapy, the energy deposition is three-dimensional in nature. As such particles not only interact with the tumor site but also deposit some of their energy into the adjacent area. Consequently, neighboring normal tissues also receive some amount of radiation dose in this process. Therefore, normal tissue dose tolerance becomes a limiting factor to the success of the treatment. Therefore, a scheduled quality assurance program should be established to verify the plans generated on Treatment Planning System (TPS).

The dose distribution given by Intensity Modulated Radiation Therapy (IMRT) is highly conformal, compared to conventional radiotherapies. But due to the presence of large numbers of fields and irregular shape and size of the treatment segments, the accuracy of Intensity Modulated Radiation Therapy (IMRT) delivery needs to be verified via measurement of dose. Based on the recommendations of the International Atomic Energy Agency (IAEA) published in technical reports series number 277 and 398, there are several techniques to attain accuracy in dosimetry.

Different dosimetry techniques are available to compare the planned dose with delivered dose using an ionization chamber and commercially available phantom, such as slab phantom that measures the point dose at a particular desired reference depth. For reference dosimetry, radiographic film or radiochromic film is placed at a particular depth in slab phantom, and a planned dose is delivered on it. The film quality assurance dosimetry system, for instance, OmniPro IMRT correlates the resultant density of film with the planned dose at each point.

Luminescence dosimetry is also performed using an optically stimulated luminescence (OSL) system and thermoluminescent dosimeters (TLD). It can be also used for in vivo dosimetry in which OSL or TLD are placed on patient's body at reference points for measurement. The electronic portal imaging device is also utilized for reference dosimetry. In addition, many detector-based phantoms are available, for reference dosimetery, such as Accua Check, Delta 4 phantom.

To evaluate an institution's ability to deliver the planned dose to patients, an indigenous heterogeneous phantom has been designed.
