Dosimetric evaluation of incorporating patient geometric variations into adaptive plan optimization through probabilistic treatment planning in head-and-neck cancers

Publication date: Available online 5 April 2018
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Qiang Liu, Jian Liang, Dingyi Zhou, Daniel J. Krauss, Peter Y. Chen, Di Yan
Purpose4D adaptive treatment planning (4D ART) is an alternative to the conventional margin-based treatment planning approach. In 4D ART, interfraction patient geometric variations, gathered from CT or cone-beam CT (CBCT) images acquired during the patient treatment course, are directly incorporated into the adaptive plan optimization through a probabilistic treatment planning method. The goal of this planning study was to evaluate the dosimetric differences between 4D ART and conventional margin-based adaptive planning strategies for head-and-neck (HN) cancers. In addition, we examined whether the dose differences achieved with 4D ART would translate into clinically relevant toxicity reductions according to existing normal tissue complication probability (NTCP) models.Methods and MaterialsFor 18 HN cancer patients, treatment plans were retrospectively generated for four different treatment strategies, including a solely image-guided radiotherapy (IGRT) strategy (IGRT-only), two conventional adaptive planning strategies using 3- and 0-mm planning target volume (PTV) margins (3-mm ART and 0-mm ART), respectively, and the 4D ART strategy. In the IGRT-only strategy, a conventional 3-mm PTV margin treatment plan was applied during the entire treatment course. In the two conventional adaptive strategies, two new treatment plans were generated during the treatment course using diagnostic planning CTs acquired after the 10th and 22nd fractions. The 4D ART followed the same adaptive schedule except that the 4D adaptive plan was generated using 5 CBCT images that were acquired over the 5 most recent treatment fractions. For each strategy, the actual delivered dose for the entire treatment course was constructed by calculating the daily doses on 35 CBCTs, deforming back to the pre-treatment planning CT and accumulating over all 35 fractions. The target coverage was evaluated using V100% (the percentage target volume receiving ≥100% prescription dose) and D99 (the minimum dose to 99% target volume). It was considered adequate if V100% was ≥95% and dose deficit in D99 was ≤2 Gy (with respect to the prescription dose). For each strategy, the dose received by organs-at-risk (OARs) was also evaluated and corresponding NTCP values were subsequently calculated using three NTCP models.ResultsAdequate target coverage was achieved for the primary clinical target volume (CTV1) and elective nodal CTV (CTV2) with 3-mm PTV margin regardless of adaptation. 3-mm ART reduced OAR mean doses (Dmean) by 1-2 Gy over IGRT-only. 0-mm ART further reduced OAR dose by another 2-3 Gy at the expense of target coverage: 3 and 1 patients had V100% <95%, 6 and 5 patients had >2 Gy dose deficit in D99, for CTV1 and CTV2, respectively. 4D ART improved target coverage while attaining similar OAR sparing as 0-mm ART: the number of patients that had V100% <95% and >2 Gy D99 deficit dropped to 0 and 0 for CTV1, 0 and 2 for CTV2, respectively. NTCP calculations suggested that 4D ART could benefit a substantial portion of patients as compare to IGRT-only (e.g., 17 and 12 patients had ≥5% and ≥10% NTCP reductions for parotid toxicity, 18 and 3 patients had ≥5% and ≥10% NTCP reductions for swallowing toxicity, respectively).ConclusionsCompared to margin-based adaptive planning strategies, 4D ART provides a better balance between target coverage and OAR sparing. NTCP estimation predicts theoretical clinical benefits that warrant further clinical validation.

Teaser

Dosimetric benefit of incorporating patient geometric variations into offline adaptive plan optimization through probabilistic treatment planning was investigated in head-and-neck cancer patients. The study results demonstrate that this novel adaptive planning approach (4D ART) can achieve improved organ-at-risk sparing while maintaining adequate target coverage.


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