A Robust and Affordable Table Indexing Approach for Multi-isocenter Dosimetrically Matched Fields

Abstract

Purpose Radiotherapy treatment planning of extended volume typically necessitates the utilization of multiple field isocenters and abutting dosimetrically matched fields in order to enable coverage beyond the field size limits. A common example includes total lymphoid irradiation (TLI) treatments, which are conventionally planned using dosimetric matching of the mantle, para-aortic/spleen, and pelvic fields. Due to the large irradiated volume and system limitations, such as field size and couch extension, a combination of couch shifts and sliding of patients are necessary to be correctly executed for accurate delivery of the plan. However, shifting of patients presents a substantial safety issue and has been shown to be prone to errors ranging from minor deviations to geometrical misses warranting a medical event. To address this complex setup and mitigate the safety issues relating to delivery, a practical technique for couch indexing of TLI treatments has been developed and evaluated through a retrospective analysis of couch position. Methods The indexing technique is based on the modification of the commonly available slide board to enable indexing of the patient position. Modifications include notching to enable coupling with indexing bars, and the addition of a headrest used to fixate the head of the patient relative to the slide board. For the clinical setup, a Varian Exact Couch^TM (Varian Medical Systems, Inc, Palo Alto, CA) was utilized. Two groups of patients were treated: 20 patients with table indexing and 10 patients without. The standard deviations (SDs) of the couch positions in longitudinal, lateral, and vertical directions through the entire treatment cycle for each patient were calculated and differences in both groups were analyzed with Student's t-test. Results The longitudinal direction showed the largest improvement. In the non-indexed group, the positioning SD ranged from 2.0 to 7.9 cm. With the indexing device, the positioning SD was reduced to a range of 0.4 to 1.3 cm (p < 0.05 with 95% confidence level). The lateral positioning was slightly improved (p < 0.05 with 95% confidence level), while no improvement was observed in the vertical direction. Conclusions The conventional matched field TLI treatment is error-prone to geometrical setup error. The feasibility of full indexing TLI treatments was validated and shown to result in a significant reduction of positioning and shifting errors.

Keywords: dosimetric match fields; safety; shift error; table indexing; tli.

Figure 1. The field matching for total lymphoid radiation (TLI) treatment.

(A) Anterior-posterior view of the field with the multileaf collimator (MLC) outlined in orange. In order to achieve the dosimetric match, the absolute couch shift is required. The first couch shift is the distance between the first isocenter and second isocenter. The second couch shift is the distance between the second isocenter and third isocenter. (B) Lateral view of three TLI fields with the diverging border (red and purple). (C) MLC arrangement for each field.

Figure 2. Steps for assembly of the indexing device and subsequent treatment of tall patients

Figure 3. The standard deviation of the couch positions across the entire treatment without and with the implementation of the indexing technique

The standard deviation (SD, cm) without (squares, n = 10) and with (diamonds, n = 20) the implementation of the indexing technique for mantle, spleen, and pelvic fields. The SD in longitudinal (A-C) and lateral (D-F) directions reduces significantly. The SD in the vertical (G-I) direction remains similar. P-values of Student’s t-test are 10^-22, 10^-9, and 0.16 for longitudinal, lateral, and vertical directions, respectively.

Figure 4. Individual cases (patients) representing the worst cases for indexed and non-indexed treatments

(A-C) Patients are too tall to treat the pelvic field. In order to treat the pelvic field, the patient will need to be moved superiorly. (D-F) For patients of regular height, no sliding is required. Indexed couch positions are presented with white squares and non-indexed couch positions are presented with black squares.

Figure 5. The limitation of distance between the first isocenter and the third isocenter

The longitudinal couch shift limitation is dependent on the field setup and the patient’s anatomy. If the shift from the first isocenter to the third isocenter is more than 51 cm, it is very possible to reach the couch longitudinal extension limit.