Background and purpose: In radiotherapy (RT) of bladder cancer, dose escalation without increased adverse effects could be achieved with a concomitant bladder tumour boost. In this study we quantified (1) the fraction of patients suitable for this approach, and (2) the potential of intensity-modulated RT (IMRT) to achieve this boost while also sparing normal tissues.
Materials and methods: The fraction of patients suitable for this boost approach was quantified using both a series of 30 radical therapy candidates, and a series of 15 consecutive RT patients. IMRT plans with 3, 5, 7 and 9 equi-spaced beams were set up for the patients in the RT series found suitable for a boost. Two sets of targets were defined, with (i) wide and (ii) narrow margins around both the tumour (prescribed 120% dose) and the non-involved bladder (prescribed 100% dose). The inverse planning optimisation minimised the dose deviation across the targets whilst fulfilling dose-volume histogram (DVH) constraints--based on what could be achieved with conformal RT (CRT)--for both the normal tissues and the targets.
Results: Fourteen of the 30 radical therapy candidates (47%) and 10 of the 15 RT patients (67%) were suitable for a boost. The 20% boost could be obtained while maintaining target coverage with at least one IMRT plan in 9 of 10 cases with wide margins and for all 10 cases with narrow margins. Using wide margins, all 3-field plans were unacceptable, the 5-field plans were acceptable for 5 of 10, and the 7- and 9-field plans for 9 of the 10 patients. The normal tissue volumes receiving doses >100% were on average reduced by a factor of 3-4 compared with CRT. The normal tissue volumes receiving intermediate doses (73-88%) decreased slightly, whereas volumes receiving the lowest doses (30-48%) increased with the number of beams. The use of narrow margins resulted in markedly lower normal tissue irradiation.
Conclusion: This study has shown bladder tumour boosting to be both clinically relevant and technically feasible using IMRT. This approach is ready for clinical implementation, although further improvement could be expected if integrated with target localisation techniques.