Elements of commissioning step-and-shoot IMRT: Delivery equipment and planning system issues posed by small segment dimensions and small monitor units

The implementation of intensity-modulated radiation therapy (IMRT) in the clinic necessitates commissioning for all systems involved. This paper describes work carried out for the treatment planning system (Helax-TMS version 6.1) and the treatment delivery equipment (Siemens Primus) available at our center. Particular regard was paid to small monitor units (MUs) and small field segments typical of step-and-shoot IMRT plans. The beam profile stability of the Siemens Primus accelerators when delivering small MU was examined with a linear detector array. Dose monitor linearity and intersegment variations were measured with a 0.6-cm3 ionization chamber. Treatment planning system calculated total scatter factors (Scp) and beam profiles for symmetric and asymmetric small fields for 6- and 15MV beams were compared against measurements in water using a 0.125-cm3 ionization chamber and a diamond detector. The 6- and 15MV beams from the Primus accelerators were found to be stable at MUs less than 10. Dose monitor linearity for small exposures under 10 MU was within ± 2% for 6 MV, but found to be not so initially for 15 MV. This could be remedied by an adjustment of a soft spot on the Siemens Primus. The delivery of small MU segments as part of an IMRT sequence was found to be consistent down to segment sizes of 1 MU. The treatment planning system pencil-beam convolution model agreed with measurement within ± 5% for fields collimated down to 3 × 3 cm. The collapsed cone point-kernel model better predicted the output for the smallest field, but displayed some unpredictable shifts in the position of the penumbra. The startup characteristics of Siemens Primus accelerators were found suitable for step-and-shoot IMRT. The diminution in accuracy of Helax-TMS dose calculation models for multileaf collimated fields of less than 2 × 2 cm has led us to avoid these in IMRT treatments at our center.