Comparison between manual and automatic segment generation in step-and-shoot IMRT of prostate cancer

Steep dose gradients for simultaneous integrated boost IMRT

Steep dose gradients between two planning target volumes (PTVs) as may be required for simultaneous integrated boosts (SIB) should be an option provided by IMRT algorithms. The aim was to analyse the geometry of the SIB problem and to implement the results in an algorithm for IMRT segment generation denoted two-step intensity modulated radiotherapy (2-Step IMRT). It was hypothesized that a gap between segments directed to the inner and the outer PTV would steepen the dose gradient. The mathematical relationships were derived from the individual dose levels and the geometry (diameters) of the PTVs. The results generated by means of 2-Step IMRT segments were equivalent or better than the segment generation using a commercial IMRT planning system. The dose to both the inner and the outer PTV was clearly more homogeneous and the composite objective value was the lowest. The segment numbers were lower or equal--with better sparing of the surrounding tissue. In summary, it was demonstrated that 2-Step IMRT was able to achieve steep dose gradients for SIB constellations.

Pre-segmented 2-Step IMRT with subsequent direct machine parameter optimisation - a planning study

BACKGROUND

Modern intensity modulated radiotherapy (IMRT) mostly uses iterative optimisation methods. The integration of machine parameters into the optimisation process of step and shoot leaf positions has been shown to be successful. For IMRT segmentation algorithms based on the analysis of the geometrical structure of the planning target volumes (PTV) and the organs at risk (OAR), the potential of such procedures has not yet been fully explored. In this work, 2-Step IMRT was combined with subsequent direct machine parameter optimisation (DMPO-Raysearch Laboratories, Sweden) to investigate this potential.

METHODS

In a planning study DMPO on a commercial planning system was compared with manual primary 2-Step IMRT segment generation followed by DMPO optimisation. 15 clinical cases and the ESTRO Quasimodo phantom were employed. Both the same number of optimisation steps and the same set of objective values were used. The plans were compared with a clinical DMPO reference plan and a traditional IMRT plan based on fluence optimisation and consequent segmentation. The composite objective value (the weighted sum of quadratic deviations of the objective values and the related points in the dose volume histogram) was used as a measure for the plan quality. Additionally, a more extended set of parameters was used for the breast cases to compare the plans.

RESULTS

The plans with segments pre-defined with 2-Step IMRT were slightly superior to DMPO alone in the majority of cases. The composite objective value tended to be even lower for a smaller number of segments. The total number of monitor units was slightly higher than for the DMPO-plans. Traditional IMRT fluence optimisation with subsequent segmentation could not compete.

CONCLUSION

2-Step IMRT segmentation is suitable as starting point for further DMPO optimisation and, in general, results in less complex plans which are equal or superior to plans generated by DMPO alone.

Replacing Pretreatment Verification With In Vivo EPID Dosimetry for Prostate IMRT

PURPOSE

To investigate the feasibility of replacing pretreatment verification with in vivo electronic portal imaging device (EPID) dosimetry for prostate intensity-modulated radiotherapy (IMRT).

METHODS AND MATERIALS

Dose distributions were reconstructed from EPID images, inside a phantom (pretreatment) or the patient (five fractions in vivo) for 75 IMRT prostate plans. Planned and EPID dose values were compared at the isocenter and in two dimensions using the gamma index (3%/3 mm). The number of measured in vivo fractions required to achieve similar levels of agreement with the plan as pretreatment verification was determined. The time required to perform both methods was compared.

RESULTS

Planned and EPID isocenter dose values agreed, on average, within +/-1% (1 SD) of the total plan for both pretreatment and in vivo verification. For two-dimensional field-by-field verification, an alert was raised for 10 pretreatment checks with clear but clinically irrelevant discrepancies. Multiple in vivo fractions were combined by assessing gamma images consisting of median, minimum and low (intermediate) pixel values of one to five fractions. The "low" gamma values of three fractions rendered similar results as pretreatment verification. Additional time for verification was approximately 2.5 h per plan for pretreatment verification, and 15 min +/- 10 min/fraction using in vivo dosimetry.

CONCLUSIONS

In vivo EPID dosimetry is a viable alternative to pretreatment verification for prostate IMRT. For our patients, combining information from three fractions in vivo is the best way to distinguish systematic errors from non-clinically relevant discrepancies, save hours of quality assurance time per patient plan, and enable verification of the actual patient treatment.

Incidence of late rectal bleeding in high-dose conformal radiotherapy of prostate cancer using equivalent uniform dose-based and dose-volume-based normal tissue complication probability models

PURPOSE

Accurate modeling of rectal complications based on dose-volume histogram (DVH) data are necessary to allow safe dose escalation in radiotherapy of prostate cancer. We applied different equivalent uniform dose (EUD)-based and dose-volume-based normal tissue complication probability (NTCP) models to rectal wall DVHs and follow-up data for 319 prostate cancer patients to identify the dosimetric factors most predictive for Grade > or = 2 rectal bleeding.

METHODS AND MATERIALS

Data for 319 patients treated at the William Beaumont Hospital with three-dimensional conformal radiotherapy (3D-CRT) under an adaptive radiotherapy protocol were used for this study. The following models were considered: (1) Lyman model and (2) logit-formula with DVH reduced to generalized EUD, (3) serial reconstruction unit (RU) model, (4) Poisson-EUD model, and (5) mean dose- and (6) cutoff dose-logistic regression model. The parameters and their confidence intervals were determined using maximum likelihood estimation.

RESULTS

Of the patients, 51 (16.0%) showed Grade 2 or higher bleeding. As assessed qualitatively and quantitatively, the Lyman- and Logit-EUD, serial RU, and Poisson-EUD model fitted the data very well. Rectal wall mean dose did not correlate to Grade 2 or higher bleeding. For the cutoff dose model, the volume receiving > 73.7 Gy showed most significant correlation to bleeding. However, this model fitted the data more poorly than the EUD-based models.

CONCLUSIONS

Our study clearly confirms a volume effect for late rectal bleeding. This can be described very well by the EUD-like models, of which the serial RU- and Poisson-EUD model can describe the data with only two parameters. Dose-volume-based cutoff-dose models performed worse.

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.

Treatments of exceptionally large prostate cancer patients with low-energy intensity-modulated photons

An inverse planning technique using 6‐MV intensity‐modulated photon beams was developed for treating large‐size patients with prostate cancer. Comparisons of treatment plans using 6‐MV and 18‐MV intensity‐modulated beams were carried out for a cohort of 10 patient cases. For these cases, we analyzed the dependence of plan quality on the beam energies. We found that 6‐MV beams resulted in plans equivalent to those for 18‐MV beams both for targets and for critical structures such as the rectum and bladder. The differences between the plans in the integral dose and the mean dose to the normal tissue surrounding the target were found to be small, in contrast to those for 3D conformal plans. Our findings showed that the low entrance dose of the high‐energy photon beams is mostly compensated by the high exit dose for even exceptionally large patients. In conclusion, 6‐MV intensity‐modulated beams are a feasible choice for treating large‐size patients with prostate cancer, provided that proper inverse planning techniques are adopted.

PACS number: 87.50.Gi, 87.53.Tf

The sensitivity of dose distributions for organ motion and set-up uncertainties in prostate IMRT

BACKGROUND AND PURPOSE

To determine the effect of organ motion and set-up uncertainties on IMRT dose distributions for prostate.

METHODS

For five patients, IMRT techniques were designed to irradiate the CTV (prostate plus seminal vesicles). Technique I delivered 78 Gy to PTV1 (CTV+10 mm margin). Technique II delivered 68 Gy to PTV1, and a 10 Gy boost to PTV2 (CTV+an anisotropic margin of 0 to 5 mm). Technique III delivered 68 Gy to PTV1 and simultaneously 78 Gy to PTV2. Uncertainties were simulated using population statistics of organ motion and set-up accuracy. The average TCP (TCPpop) of the CTV and average NTCP (NTCPpop) of the rectal wall were calculated.

RESULTS

The planning TCP was a good predictor for TCPpop for Techniques I and II. Technique III was sensitive for geometrical uncertainties, reducing TCPpop by 0.8 to 2.4% compared to planning. NTCPpop was reduced for Technique III by a factor 2.6 compared to Technique I. For all plans, the planning NTCP was strongly correlated with NTCPpop.

CONCLUSIONS

Dose distributions created with Techniques I and II are insensitive for geometrical uncertainties, while Technique III resulted in a reduction of TCPpop. This reduction can be compensated by a small dose escalation, while still resulting in an NTCPpop of the rectal wall that is lower or comparable to Technique I.

Additional dose constraints for analytical beam weighting optimization in IMRT

This work presents an improvement to an algorithm for analytical beam weighting optimization where a flexible objective function, which considers 'importance factors' for each anatomical region and 'allowed deviations' from the prescribed dose, is defined. This upgrading allows forcing the mean value of the dose distribution to be the desired value, by using Lagrange multipliers. A real case is presented to show the effect of this change.