Comparison of an in-house developed monitor unit double-check program for 3D conformal radiation therapy and treatment planning system verification

Abstract

AimThe treatment planning system (TPS) plays a key role in radiotherapy treatments; it is responsible for the accurate determination of the monitor unit (MU) needed to be delivered to treat a patient with cancer. The main goal of radiotherapy is to sterilise the tumour; however, any imprecise dose delivered could lead to deadly consequences. The TPS has a quality assurance tool, an independent program to double check the MU, evaluate patient plan correctness and search for any potential error.Materials and methodsIn this work, a comparison was carried out between a MU calculated by TPS and an independent in-house-developed monitor unit calculation program (MUCP). The program, written in Cplusplus (C++ Object-Oriented), requires a database of several measured quantities and uses a recently developed physically based method for field equivalence calculation. The ROOT CERN data analysis library has been used to establish fit functions, to extend MUCP use to a variety of photon beams. This study presents a new approach to checking MU correctness calculated by the TPS for a water-like tissue equivalent medium, using our MUCP, as the majority of previous studies on the MU independent checks were based on the Clarkson method. To evaluate each irradiated region, four calculation points corresponding to relative depths under the water phantom were tested for several symmetric, asymmetric, irregular symmetric and asymmetric field cases. A comparison of MU for each radiation fields from readings of the TPS and the MUCP was undertaken.ResultsA satisfactory agreement has been obtained and within the required standards (3%). Additional experimental measurements of dose deposited in a water phantom showed a deviation of <1·6%.FindingsThe MUCP is a useful tool for basic and complex MU verification for 3D conformal radiation therapy plans.

Validation of the geometric equivalent field concept in total scatter factor calculations, for half‐, quarter‐ and off‐isocenter asymmetric square fields

“…confirmed that the differences in MU between manual and TPS calculations for breast radiotherapy increase when these calculations require OAR(r) corrections, and compared 6 MV and Cobalt beams (the OAR correction is neglected for Co‐60) under similar conditions while neglecting influencing factors such as tissue heterogeneities in both calculations. Also, computerized investigations proved that these differences are small at the IC and large away from it under the same conditions 3,24,37 . In addition, Ian et al 25,26 .…”

“…The S cp , S cp , D f %, and 𝚫 S cp were calculated from Equations (3), ( 5), (6), and (7) Also, computerized investigations proved that these differences are small at the IC and large away from it under the same conditions. 3,24,37 In addition, Ian et al 25,26 proved, after correcting the missing tissue in breast, that these differences increase with the decreasing energy of the photon beam. Part of the remaining unresolved difference in MUs can be explained by the fact that the equivalent field in the TPS is defined as the symmetric field in which the PDD of its central axis has the same properties as the PDD in the effective axis of the asymmetric field,and this also applies to the S cp .…”

“…22 According to these formulas, in order to calculate the S cp for asymmetric fields, the S c and S p of their GESF can be used. 3,[23][24][25][26] In the referenced studies this conclusion has been validated for almost all Varian Linacs, such as Clinac 4/100 with 4 MV beam, 600C with 6 MV beam, 2100C with energies 6 and 10 MV, 2100CD with energies 6 and 15 MV, and the Linacs 2100C, 2300C/D, Clinac-DHX (with energies 6 and 18MV), but no relevant data exist for Varian Clinac IX accelerators, or the 23 MV beam energy. Most important, few studies have measured the S cp of half -fields, while fewer studies have measured the S cp of the off -isocenter and quarter-fields.…”

“…Thus, several mathematical formulas were developed to improve the accuracy of calculations for asymmetric fields based on the GESF concept, 12,18–21 the simplest of which is the area‐perimeter ratio 22 . According to these formulas, in order to calculate the S cp for asymmetric fields, the S c and S p of their GESF can be used 3,23–26 …”