The step‐and‐shoot IMRT overshooting phenomenon: a novel method to mitigate patient overdosage

Abstract

The goal of this work is to evaluate the dosimetric impact of an overshooting phenomenon in step‐and‐shoot IMRT delivery, and to demonstrate a novel method to mitigate the issue. Five pelvis IMRT patients treated on Varian 2100C EX linacs with larger than +4.5% phantom ion chamber point‐dose difference relative to planned dose were investigated. For each patient plan, 5 fractions were delivered. DynaLog files were recorded and centi‐MU pulses from dose integrator board for every control point (CP) were counted using a commercial pulse counter. The counter recorded CP MU agrees with DynaLog records, both showing an ~0.6 MU overshoot of the first segment of every beam. The 3D patient dose was recalculated from the counter records and compared to the planned dose, showing that the overshoot resulted in on average 2.05% of PTV D95 error, and 2.49%, 2.61% and 2.45% of D1cc error for rectum, bladder, and bowel, respectively. The initial plans were then modified by inserting a specially designed MLC segment to the start of every beam. The modified plans were also delivered five times. The dose from the modified delivery was calculated using counter recorded CP MU. The corresponding Dx parameters were all within 0.31% from the original plan. IMRT QA results also show a 2.2% improvement in ion chamber point‐dose agreement. The results demonstrate that the proposed plan modification method effectively eliminates the overdosage from the overshooting phenomenon.PACS number(s): 87.55.Qr, 87.55.km

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“…For block correction, the new first segment and the new last segment were set out of field. However, setting the two new segments as closed will yield better results, as per the method proposed by Zhen et al (2016). Block correction is limited by the fact that the transmission of MLC in the field would be increased when the new two segments are irradiated.…”

“…These two principles make editing the MLC.log file difficult for all three corrections, especially with respect to the treatment fields. A program can be used to edit the MLC.log file, and the treatment fields can be tested with the corrections during the dose agreement, as performed in Zhen et al (2016).…”

“…Therefore, in the MLC log file, they changed the dose by +60% and −60% for the first and last segments, respectively, thereby compensating for the overshoot effect. Zhen et al [8] added two closed segments, one before the first segment and another after the last segment. Thus, when the overshoot effect is constrained to take effect at the two closed segments, the doses at the other segments are constrained to be in accordance with the treatment plan.…”