SU‐E‐T‐167: QA of Dose‐Painting Plans: Risk of Overdosage in the High‐Dose Regions?

Abstract

Purpose: Deliverability of a treatment plan is generally evaluated by the gamma value (γ) and pass rate (PR) describing the fraction of measured points passing a certain γ‐criteria for dose deviation (DD) and distance‐to‐agreement. γ does not distinguish between under/overdosage. When dose‐painting to high doses in head‐and‐neck cancer, excessive dose to small volumes is associated with dose‐limiting toxicity (Madani et al. Radiother Oncol. 2011). Overdosage of high‐dose volumes is therefore a particular concern. Here we focus on the presence and detection of overdosage in high‐dose regions of dose‐painting plans. Methods: 40 dose‐painting head‐and‐neck plans with five prescription levels (52.8, 60, 69.7, 73.1 and 79.7 Gy, all in 34 fractions) were measured with an EPID‐based system. 14 of these plans with the poorest PR were additionally measured with a cylindrical PMMA diode phantom system. PR(γ: 3%/3mm), γ(3%/3mm) and DD were analysed. Correlations between planned dose, DD and γ were quantified by the Spearman rank correlation coefficient (ρ). Results: Mean PR were 98.2% (EPID, range 93.9–100) and 98.4% (PMMA, range 95.5– 99.9). In the PMMA phantom, 262 diodes had a planned dose >2.2 Gy/fractions. Overdosage (between +0.004% and +6.9%) was measured in 248 of these. A high planned dose correlated with high DD (ρ=0.24) and poor γ (ρ=0.19) (p<0.001). The biological effect accentuates the problem; for example, 5% physical overdose at 80 Gy/34 fractions corresponds to +7.3% in biological effect for normal tissue (α/β=3). Conclusion: The dose‐painting plans were considered deliverable based on gamma PR. However, overdosage is common in the high‐dose regions. This cannot be captured with the gamma PR criteria. Unfortunately, the translation of the dose to a phantom diode to a point in the patient is not easily performed. Additional QA may therefore be needed to ensure safe conduct and correct interpretation of the results of prospective dose‐painting trials.