Absorbed Dose Calculation In Irregular Blocked Radiation Fields: Evaluation of Clarkson’s Sector Integration Method for Radiation Fields Commonly Used in Conventional Radiotherapy

  Irregular/blocked fields are routinely used in radiotherapy. The doses of these fields are usually calculated by means of equivalent square method that is inherently prone to uncertainty. On the other hand, Clarkson’s sector integration method is a dose calculation method which offers far better accuracy in dose calculation of irregular fields. The Scatter Air Ratio (SAR) of an individual sector, in which whole field has been divided, is calculated and averaged over all sectors to find total SAR for the whole field. Percentage depth dose (PDD) and tissue-maximum ratio (TMR) for irregularly shaped beams can be calculated by the SAR values using the standard relationships of these measurement quantities.
Material and The present study was conducted on 40 actual patient treatment fields. The PDD values for depths up to 15 cm were calculated using both Clarkson’s sector integration method and conventional methods, and their results were compared with the measured PDDs for all patients. Maximum deviation for Clarkson’s calculation was under 2.7% for any field size, shape, and depth. However for conventional methods, this value exceeded ±5.5% for some field shapes, specifically at larger depths. Better results of sector integration are more prominent for field shapes with a large field size and a shielded area of regular shape. For the treatment fields with a very large degree of approximation for assessing reduced field size, Clarkson’s method is the most accurate technique for the calculation of absorbed dose.