Delivery verification in sequential and helical tomotherapy

Abstract

Conformal and conformal avoidance radiation therapy are new therapeutic techniques that are generally characterized by high dose gradients. The success of this kind of treatment relies on quality assurance procedures in order to verify the delivery of the treatment. A delivery verification technique should consider quality assurance procedures for patient positioning and radiation delivery verification. A methodology for radiation delivery verification was developed and tested with our tomotherapy workbench. The procedure was investigated for two cases. The first treatment using a torus-shaped target was optimized for 72 beam directions and sequentially delivered as a single slice to a 33 cm diameter cylinder of homogeneous solid water. For the second treatment, a random pattern of energy fluence was helically delivered for two slices to a 9.0 cm diameter phantom containing inhomogeneities. The presented process provides the energy fluence (or a related quantity) delivered through the multileaf collimator (MLC) using the signal measured at the exit detector during the treatment delivery. As this information is created for every pulse of the accelerator, the energy fluence and state for each MLC leaf were verified on a pulse-by-pulse basis. The pulse-by-pulse results were averaged to obtain projection-by-projection information to allow for a comparison with the planned delivery. The errors between the planned and delivered energy fluences were concentrated between +/-2.0%, with none beyond +/-3.5%. In addition to accurately achieving radiation delivery verification, the process is fast, which could translate to radiation delivery verification in real time. This technique can also be extended to reconstruct the dose actually deposited in the patient or phantom (dose reconstruction).

Tomoterapia helicoidal: IMRT adaptada guiada por imagen

“…Las señales detectadas en el proceso de verificación de la dosis administrada son de gran utilidad para conocer la cantidad de radiación incidente en el paciente 13,14 , y, combinada con la tomoimagen realizada durante el tratamiento proporciona una importante información que tiene que ver con la reconstrucción de la dosis 15 . En una situación ideal, si la anatomía del paciente no cambia, los parámetros de colocación e inmovilización del paciente no varían y la radiación generada en el Acelerador Lineal es correcta, la reconstrucción de la dosis no sería necesaria; sin embargo, en condiciones clínicas, la reconstrucción de la dosis es de gran utilidad para determinar la dosis realmente administrada y compararla con la dosis planificada.…”

A feasible method for clinical delivery verification and dose reconstruction in tomotherapy

“…The theory of this approach to delivery verification and dose reconstruction has been described by Kapatoes et al 1,7 but will be summarized here. For either one accelerator pulse or a full projection, the signal s in any detector element i can be represented as a weighted sum of the energy fluence emitted from each MLC leaf, the weight being d i j :…”

“…Therefore, approximate location of the leaf is known via these signal profiles, and the amount of signal increase for elements in the TAG-P areas can be scaled appropriately for leaves in transition. 1 For the projection-by-projection approach, this information is not available and an approximate TAG-P correction must be applied to the D matrices based on the planned sinogram. However, since the transition in the opening direction tends to nearly compensate for that in the closing direction, and since transition states generally make up a small portion of the leaf states, the resultant errors were expected to be small.…”

“…1,2 This process is referred to as delivery verification ͑DV͒. The transfer matrix is measured by opening each leaf of the multileaf collimator ͑MLC͒ individually, and recording the resulting signal profile in the exit detector for each leaf: The signal profile for each leaf represents a column of the transfer matrix.…”

“…This integration of equipment on a single gantry allows for a host of verification processes. Specifically, patient registration in projection space, 14,15 delivery modification without the need for reoptimization, 16 megavoltage CT imaging ͑MVCT͒, [17][18][19][20] DV, 1,2,7 and DR 4,7-10 will be possible. The transfer matrix approach to DV and DR described previously are verification techniques for general radiotherapy and dynamic intensity modulated radiation therapy as well as tomotherapy: In principle the processes and that described in this manuscript can be used for other delivery strategies ͑e.g., those using dynamic MLCs͒.…”