Influence of dose calculation algorithms on isotoxic dose-escalation of non-small cell lung cancer radiotherapy

Optimizing collimator margins for isotoxically dose-escalated conformal radiation therapy of non-small cell lung cancer

PURPOSE

Isotoxic dose escalation schedules such as IDEAL-CRT [isotoxic dose escalation and acceleration in lung cancer chemoradiation therapy] (ISRCTN12155469) individualize doses prescribed to lung tumors, generating a fixed modeled risk of radiation pneumonitis. Because the beam penumbra is broadened in lung, the choice of collimator margin is an important element of the optimization of isotoxic conformal radiation therapy for lung cancer.

METHODS AND MATERIALS

Twelve patients with stage I-III non-small cell lung cancer (NSCLC) were replanned retrospectively using a range of collimator margins. For each plan, the prescribed dose was calculated according to the IDEAL-CRT isotoxic prescription method, and the absolute dose (D99) delivered to 99% of the planning target volume (PTV) was determined.

RESULTS

Reducing the multileaf collimator margin from the widely used 7 mm to a value of 2 mm produced gains of 2.1 to 15.6 Gy in absolute PTV D99, with a mean gain ± 1 standard error of the mean of 6.2 ± 1.1 Gy (2-sided P<.001).

CONCLUSIONS

For NSCLC patients treated with conformal radiation therapy and an isotoxic dose prescription, absolute doses in the PTV may be increased by using smaller collimator margins, reductions in relative coverage being offset by increases in prescribed dose.

Will SNPs be useful predictors of normal tissue radiosensitivity in the future?

The ability to predict individual risk of radiation-induced normal tissue complications is a long sought goal in radiobiology. The last decade saw increasing interest in identifying associations between single nucleotide polymorphisms (SNPs) and normal tissue complication risk. Nevertheless, it remains controversial whether SNPs will be useful predictors of normal tissue radiosensitivity. This paper provides a summary of a scientific debate held at the 31st ESTRO conference in which four scientists argued in favor or against the motion that SNPs will be useful predictors of normal tissue radiosensitivity in the future.

(Radio)biological optimization of external-beam radiotherapy

“Biological optimization” (BIOP) means planning treatments using (radio)biological criteria and models, that is, tumour control probability and normal-tissue complication probability. Four different levels of BIOP are identified: Level I is “isotoxic” individualization of prescription dose D_presc at fixed fraction number. D_presc is varied to keep the NTCP of the organ at risk constant. Significant improvements in local control are expected for non-small-cell lung tumours. Level II involves the determination of an individualized isotoxic combination of D_presc and fractionation scheme. This approach is appropriate for “parallel” OARs (lung, parotids). Examples are given using our BioSuite software. Hypofractionated SABR for early-stage NSCLC is effectively Level-II BIOP. Level-III BIOP uses radiobiological functions as part of the inverse planning of IMRT, for example, maximizing TCP whilst not exceeding a given NTCP. This results in non-uniform target doses. The NTCP model parameters (reflecting tissue “architecture”) drive the optimizer to emphasize different regions of the DVH, for example, penalising high doses for quasi-serial OARs such as rectum. Level-IV BIOP adds functional imaging information, for example, hypoxia or clonogen location, to Level III; examples are given of our prostate “dose painting” protocol, BioProp. The limitations of and uncertainties inherent in the radiobiological models are emphasized.

Individualized dose prescription for hypofractionation in advanced non-small-cell lung cancer radiotherapy: an in silico trial

PURPOSE

Local tumor control and outcome remain poor in patients with advanced non-small-cell lung cancer (NSCLC) treated by external beam radiotherapy. We investigated the therapeutic gain of individualized dose prescription with dose escalation based on normal tissue dose constraints for various hypofractionation schemes delivered with intensity-modulated radiation therapy.

METHODS AND MATERIALS

For 38 Stage III NSCLC patients, the dose level of an existing curative treatment plan with standard fractionation (66 Gy) was rescaled based on dose constraints for the lung, spinal cord, esophagus, brachial plexus, and heart. The effect on tumor total dose (TTD) and biologic tumor effective dose in 2-Gy fractions (TED) corrected for overall treatment time (OTT) was compared for isotoxic and maximally tolerable schemes given in 15, 20, and 33 fractions. Rescaling was accomplished by altering the dose per fraction and/or the number of fractions while keeping the relative dose distribution of the original treatment plan.

RESULTS

For 30 of the 38 patients, dose escalation by individualized hypofractionation yielded therapeutic gain. For the maximally tolerable dose scheme in 33 fractions (MTD(33)), individualized dose escalation resulted in a 2.5-21% gain in TTD. In the isotoxic schemes, the number of fractions could be reduced with a marginal increase in TED. For the maximally tolerable dose schemes, the TED could be escalated up to 36.6%, and for all patients beyond the level of the isotoxic and the MTD(33) schemes (range, 3.3-36.6%). Reduction of the OTT contributed to the therapeutic gain of the shortened schemes. For the maximally tolerable schemes, the maximum esophageal dose was the dominant dose-limiting constraint in most patients.

CONCLUSIONS

This modeling study showed that individualized dose prescription for hypofractionation in NSCLC radiotherapy, based on scaling of existing treatment plans up to normal tissue dose constraints, enables dose escalation with therapeutic gain in 79% of the cases.